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    HV Probe Suggestions

    I need to get a HV probe for working on some CRTs... mainly for safe discharging purposes at this time.

    I am on a budget but I also want to get a well made unit, so I am thinking used may be the way to go.

    So far the cheapest on fleabay has been around $40 shipped.

    My questions are:

    1. is there any place cheaper I can get a decent probe?

    2. are there any things I should specifically look for?
    sigpic

    (Insert witty quote here)

    #2
    Re: HV Probe Suggestions

    I think the magic word here is the "safe", i worked in a service centre where we used to discharge with 2 long shafted screwdrivers because of "cost versus use" until a manager thought he would try, guess what, a new discharge wand and the manager stayed in his office from then on.

    Comment


      #3
      Re: HV Probe Suggestions

      Originally posted by Sparks88 View Post
      I think the magic word here is the "safe", i worked in a service centre where we used to discharge with 2 long shafted screwdrivers because of "cost versus use" until a manager thought he would try, guess what, a new discharge wand and the manager stayed in his office from then on.
      You See!
      They CAN learn!
      .
      Mann-Made Global Warming.
      - We should be more concerned about the Intellectual Climate.

      -
      Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

      - Dr Seuss
      -
      You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
      -

      Comment


        #4
        Re: HV Probe Suggestions

        Originally posted by ratdude747 View Post
        My questions are:

        1. is there any place cheaper I can get a decent probe?
        Make one.
        Screw driver with insulated braided high amp wire jumper [like a car battery strap but not an heavy as for a car] and alligator clip attached.
        Or two screw drivers with a resistored jumper between them. [Is what I do now.]
        ~~ Not two screw drivers with no jumper though...
        ~~ Too hard to control that way.
        -
        Or get a copper bar at home depot and make your own wood handle.
        Still need the wire because you can't always reach both terminals with a bar.
        Glass is better than wood but good luck threading a glass bar..
        -
        If you don't like sparks add a resistor.
        -
        You can get fancy like this.

        I think I'll do a couple like that [with different resistors] only using a meter probe on one end and an alligator clip on the other end.

        Originally posted by ratdude747 View Post
        2. are there any things I should specifically look for?
        Yes, one with the insulated jumper wire & clip [and a resistor if you don't like excitement].

        ~
        Side points.
        Big caps can partially recharge from residuals so once discharged attach a jumper across the leads for a while.
        We used some that were so large [about 60 pounds] they actually came with metal bars to bolt across the terminals when they weren't in a circuit.
        .

        [edit: uploaded pdf in case the link dies]
        .
        Attached Files
        Last edited by PCBONEZ; 02-11-2012, 12:15 AM.
        Mann-Made Global Warming.
        - We should be more concerned about the Intellectual Climate.

        -
        Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

        - Dr Seuss
        -
        You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
        -

        Comment


          #5
          Re: HV Probe Suggestions

          I see.

          I always thought it was better to have a measurable probe so one could tell if it was discharged or not. is there any merit in that?

          Also, I read that w/o the resistor, there is a good chance of bombing caps and whatnot...
          sigpic

          (Insert witty quote here)

          Comment


            #6
            Re: HV Probe Suggestions

            This is the one I purchased years ago:

            http://www.mcmelectronics.com/product/73-520

            It was 75 bucks when I got it prices have risen.

            Seeing the rate of discharge is cool but it's only vital if you have to measure anode voltage. Keep a high resistance there for 10-15 seconds and you're done you'll also hear static pops when the voltage is almost completely depleted.

            Make sure the probe reads up to at least 40Kv I've seen cheaper 20Kv max probes around the Internet I a color CRTs normal anode voltage is around 32Kv.

            Don't forget to make a jumper wire with alligator clips on each end and attach it from the anode to the ground strap after discharging the anode. It can come back up to a few kV and give you quite a surprise when reinstalling the anode cap. The high voltage regained is released from the glass I'm told.
            Last edited by Krankshaft; 02-11-2012, 05:03 AM.
            Elements of the past and the future combining to make something not quite as good as either.

            Comment


              #7
              Re: HV Probe Suggestions

              Originally posted by ratdude747 View Post
              I see.

              I always thought it was better to have a measurable probe so one could tell if it was discharged or not. is there any merit in that?
              Only if you dunno how to use a DMM.
              Frankly I wouldn't trust a meter on something like that to last [or work if I haven't used it before].
              Meters and large current surges don't generally play well together.
              .
              The standard one in the Navy was a bent 1/4" or 5/16" diameter copper rod set in a glass handle with the lead/clip I talked about attached to the rod near the handle.
              [The insulation was clear so you could visually check the braided cable before use.]
              They didn't have a resistor. [Their thinking was that you can't visually check a resistor and it might be open.]
              .... And they didn't work that way.... These are for HV caps not like 25v. NAV considers HV as 30v or more.
              The jumper goes to ground.
              You short the cap with the copper bar across both terminals/leads. [Not bar to jumper,,, bar to bar with a grounded bar.]
              Procedure was to short the cap 3 times then check for volts with a DMM.
              .
              I managed to find a NAVEDTRA manual online that has a pic. [attached]
              .
              Attached Files
              Last edited by PCBONEZ; 02-11-2012, 06:07 AM.
              Mann-Made Global Warming.
              - We should be more concerned about the Intellectual Climate.

              -
              Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

              - Dr Seuss
              -
              You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
              -

              Comment


                #8
                Re: HV Probe Suggestions

                I see. my multimeters max out at 600v (digital) or 400V (analog). mess up and bye-bye multimeter.

                However, I think I may know how to make a kinda crude 1000:1 probe:



                the Idea is to make a voltage divider using a 100Ω resistor, either 2 50K resistors (shown) or a single 100K resistor, and a 10K pot to tune the probe. I plan to put the circuit on a PCB in a small plastic project box. I also plan to use 1/2W resistors as a safety measure. for the probe, I will likely get a cheap screwdriver at HF and solder my wire to that.

                Look good?
                Attached Files
                sigpic

                (Insert witty quote here)

                Comment


                  #9
                  Re: HV Probe Suggestions

                  Originally posted by ratdude747 View Post
                  I see. my multimeters max out at 600v (digital) or 400V (analog). mess up and bye-bye multimeter.
                  You trust some probe or a resistor and don't double check with a meter and it's bye bye YOU instead.
                  .
                  Most meters have fuses and meters are replaceable even if the fuse doesn't work.
                  Not to mention you can pick up a cheapo 'burn meter' to use for that for like $2.
                  - Use a meter.
                  .
                  Last edited by PCBONEZ; 02-11-2012, 10:00 AM.
                  Mann-Made Global Warming.
                  - We should be more concerned about the Intellectual Climate.

                  -
                  Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

                  - Dr Seuss
                  -
                  You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                  -

                  Comment


                    #10
                    Re: HV Probe Suggestions

                    its current not voltage that trips fuses.

                    current isn't the issue. voltage is. the fuse wouldn't blow.

                    we are talking thousands of volts here. when a CRT is runnign you usually see around 24,000V going to the anode wire. 24,000v vs a 400/600v meter = the flaming meter from the fluke video. granted, it has set for a few weeks since last ran, but there still could be some serious charge in it.

                    the whole Idea is to step down the votage to levels that are safe for meters to read.

                    most probes with meter hookups use a 1000:1 stepdown, which is what I am designing mine to do.

                    I've redesigned my probe circuit to use a 100K 1/2 w resistor hooked to the probe. in series with that is a 1K pot and a 100 1/2w resistor. the meter hookups are between the 100 and the pot.

                    some math:

                    voltage diveder equation (for series circuits):

                    V = E *(Rv / Rt)

                    *E is the voltage source*

                    in our case, we will assume a 1000V input (E) will be read as 1V (V):

                    1 = 1000 * (( 100 + pot)/ (100000 + 100 + pot)

                    pot = 0.1 ohm

                    not much, but the pot is also to acommodate tolerances in the resistors.

                    I will calibrate it using a 12v bench PSU I have.

                    any questions?

                    also, any input from anybody else?
                    sigpic

                    (Insert witty quote here)

                    Comment


                      #11
                      Re: HV Probe Suggestions

                      You need to study you physics a little more.
                      Voltage + a path is the cause of current.
                      You can't have current without voltage.
                      Thus you can't blow a fuse without voltage.
                      .
                      If you have a little bitty voltage you will have a little bitty current and you won't blow the fuse.
                      .
                      If you have a great big voltage [on the same path] you will have great big current and blow the fuse.
                      .
                      Thus voltage does blow fuses.
                      .
                      .
                      .
                      And no - you are NOT talking about much voltage AFTER you have shorted the cap.
                      .
                      OMFG: If you can't tell if you shorted a cap or not then you shouldn't play with electricity.
                      LEAVE THE CRTS ALONE.
                      .
                      It's just not that hard to tell if you connected two points together. - JEZZZ!
                      If you connect a copper bar across 10kv and you can't tell if there is voltage there from the noise, sparks and the smell then you REALLY shouldn't be playing with electricity.
                      LEAVE THE CRTS ALONE. - By what you've said you have no idea about what you're playing with.
                      ..
                      Last edited by PCBONEZ; 02-11-2012, 01:01 PM.
                      Mann-Made Global Warming.
                      - We should be more concerned about the Intellectual Climate.

                      -
                      Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

                      - Dr Seuss
                      -
                      You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                      -

                      Comment


                        #12
                        Re: HV Probe Suggestions

                        I'll tell you what.
                        Go discharge a 12v car battery with a wrench and let me know if you can tell when it's safe to connect a meter without blowing the fuse.
                        .
                        That of course is ignoring the fact connecting across 12v won't blow the fuse to start with.
                        .
                        So here's your clue:
                        If you connect a bar across two terminals and don't get sparks then it's safe to connect a volt meter.
                        .
                        Last edited by PCBONEZ; 02-11-2012, 01:18 PM.
                        Mann-Made Global Warming.
                        - We should be more concerned about the Intellectual Climate.

                        -
                        Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

                        - Dr Seuss
                        -
                        You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                        -

                        Comment


                          #13
                          Re: HV Probe Suggestions

                          Originally posted by PCBONEZ View Post
                          I'll tell you what.
                          Go discharge a 12v car battery with a wrench and let me know if you can tell when it's safe to connect a meter without blowing the fuse.
                          .
                          That of course is ignoring the fact connecting across 12v won't blow the fuse to start with.
                          .
                          So here's your clue:
                          If you connect a bar across two terminals and don't get sparks then it's safe to connect a volt meter.
                          .
                          you don't quite understand it.

                          they require each other to exists, yes, but they are inversely related:

                          power = voltage * current.

                          so, for a given amount of power, the more voltage you have, the less current.

                          lets go to basics:

                          voltage is the potential difference between two points.

                          current is the flow of electrons


                          a car battery is a HIGH current, LOW voltage example.

                          spark plugs on the other hand are a HIGH voltage, LOW current situation.


                          anyway, fuses (generally) do not care about voltage when they are not blown. Fuses are blown by high amounts of current. the reason fuses have voltage rating is because when they do blow, there is a certain voltage where the two blown ends of the fuse will arc. But as long as the current stays within the limits of the fuse, the fuse will stay intact at any voltage.

                          CRT tubes are more like spark plugs than batteries. They use a very high voltage (from the flyback transformer) and low amount of current to create the image on the screen.

                          what this boils down to is that if you overvolted a multimeter, the fuse would survive but the other parts (transistors, diodes, etc) would go boom and there would be heavy arcing...

                          my probe on the other hand steps down the voltage to a safe level that the mutimeter can read w/o increasing current (like a transformer would).
                          sigpic

                          (Insert witty quote here)

                          Comment


                            #14
                            Re: HV Probe Suggestions

                            No, you don't understand it.
                            .
                            ""they require each other to exists, yes, but they are inversely related:""
                            .
                            WRONG.
                            .
                            The 12v exists in the battery but with no path current doesn't exist.
                            ... Current requires voltage to exist...
                            ... But voltage does not require current to exist.
                            And the voltage -is- energy regardless of there being current or not.
                            .
                            .
                            That fuse has a resistance.
                            You need a particular voltage across it to get enough current through it to blow it.
                            .
                            Your analogy is fucked.
                            Spark plugs [and plug wires] have a high resistance.
                            A wrench doesn't.
                            Try shorting the coil with the wrench and let me know how that works for you.
                            ~ That would make it like shorting a HV cap. The plug doesn't.
                            .
                            .
                            As my example should have made clear a direct short on -just- 12 volts causes significant sparks.
                            Pretty fucking hard to miss 'em even with only 12v.
                            Thus if you over volt a meter after discharging a cap that means you aren't bright enough to discharge a cap properly [which should be done safely but IS REALLY FREAKIN' EASY] so you should stay the hell away for all things electrical so you don't hurt yourself or someone else.
                            .
                            .
                            If you are even remotely worried about blowing the meter then you don't understand enough about what is involved to be messing with it.
                            - You are in effect admitting that you can't tell if a cap is charged to over 400v [your most limiting meter] with a shorting bar.
                            If you are really that inept then leave it alone until you have a clue.
                            .
                            Last edited by PCBONEZ; 02-11-2012, 03:49 PM.
                            Mann-Made Global Warming.
                            - We should be more concerned about the Intellectual Climate.

                            -
                            Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

                            - Dr Seuss
                            -
                            You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                            -

                            Comment


                              #15
                              Re: HV Probe Suggestions

                              In your little gadget if R1 opens the cap will look like it's discharged when it isn't.
                              .
                              That's why you use a shorting bar and suck up the fact it will spark.
                              .
                              It will keep you alive....
                              .
                              Mann-Made Global Warming.
                              - We should be more concerned about the Intellectual Climate.

                              -
                              Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.

                              - Dr Seuss
                              -
                              You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
                              -

                              Comment


                                #16
                                Re: HV Probe Suggestions

                                Originally posted by ratdude747 View Post
                                I see. my multimeters max out at 600v (digital) or 400V (analog). mess up and bye-bye multimeter.

                                However, I think I may know how to make a kinda crude 1000:1 probe:



                                the Idea is to make a voltage divider using a 100Ω resistor, either 2 50K resistors (shown) or a single 100K resistor, and a 10K pot to tune the probe. I plan to put the circuit on a PCB in a small plastic project box. I also plan to use 1/2W resistors as a safety measure. for the probe, I will likely get a cheap screwdriver at HF and solder my wire to that.

                                Look good?
                                Correct theory, but incorrect implementation. Standard resisters have a voltage rating of 200 to 600 V, You would need many more resistors in series to be sure the high voltage wouldn't arc internally and present a much higher than expected voltage across your meter.

                                Assuming 600V resistors, we're talking about 40+ resistors in series. These resistors should be placed in a PVC or similar tube, and the tube should be filed with RTV sealant.

                                Of course ebay is a good source of modestly priced high voltage resistors, assuming a 5KV resistor, you could get away with using 4-10 resistors in series, and a WAY shorter tube.

                                Comment


                                  #17
                                  Re: HV Probe Suggestions

                                  Originally posted by PCBONEZ View Post
                                  You need to study you physics a little more.
                                  Voltage + a path is the cause of current.
                                  You can't have current without voltage.
                                  But you can have extreamly high current with very low voltage. Your car battery example is is just that. A wrench across the car battery will drop the voltage down to millivolts, but you'll have hundreds of amps flowing.

                                  Thus you can't blow a fuse without voltage.
                                  No, but you can have thousands of amps, with millivolts of voltage.

                                  If you have a little bitty voltage you will have a little bitty current and you won't blow the fuse.
                                  Fuses have millohms of resistance, so millohms of voltage can push multiamps of current through them, and not just blow them, but vaporize them, assuming you're voltage source can push that current, a CRT, can't. Assuming an non powered TV, the only source of that voltage is a few hundred PF of charged capacitance. Enough to ruin your day, but not enough to push any real current through a fuse.

                                  If you have a great big voltage [on the same path] you will have great big current and blow the fuse.
                                  .
                                  Thus voltage does blow fuses.
                                  Assuming we're talking about standard fuses, ONLY the heat of it's internal resistance (caused by current flowing through it) blows them, although you would certainly need voltage to push that current through the fuse

                                  .
                                  .
                                  .
                                  And no - you are NOT talking about much voltage AFTER you have shorted the cap.
                                  .
                                  OMFG: If you can't tell if you shorted a cap or not then you shouldn't play with electricity.
                                  LEAVE THE CRTS ALONE.
                                  .
                                  It's just not that hard to tell if you connected two points together. - JEZZZ!
                                  If you connect a copper bar across 10kv and you can't tell if there is voltage there from the noise, sparks and the smell then you REALLY shouldn't be playing with electricity.
                                  LEAVE THE CRTS ALONE. - By what you've said you have no idea about what you're playing with.
                                  ..
                                  Now you're just living up to your "grumpy old fart" motto.

                                  Comment


                                    #18
                                    Re: HV Probe Suggestions

                                    Originally posted by LLLlllou View Post
                                    Correct theory, but incorrect implementation. Standard resisters have a voltage rating of 200 to 600 V, You would need many more resistors in series to be sure the high voltage wouldn't arc internally and present a much higher than expected voltage across your meter.

                                    Assuming 600V resistors, we're talking about 40+ resistors in series. These resistors should be placed in a PVC or similar tube, and the tube should be filed with RTV sealant.

                                    Of course ebay is a good source of modestly priced high voltage resistors, assuming a 5KV resistor, you could get away with using 4-10 resistors in series, and a WAY shorter tube.
                                    that schematic is outdated. I since modified it (been too lazy to redraw it).

                                    as it is now, a 100K resitor drops the high voltage and a pot/100ohm resistor drop the rest.

                                    I saw no voltage ratings on the pack I bought... stupid me for thinkign it was irrelivant.

                                    with this one CRT, I need to rivet metal to the frame (the back plastic failed on it) and it was more for safety. the next is an emac that needs some work around the cdrom done. the last is a mitsubishi monitor that has an unknown issue.

                                    what I don't get is all the homebrew guides I saw showed the use of standard resistors...

                                    the part that will be dropping the high voltage is one of these:

                                    http://www.radioshack.com/product/in...lue=RadioShack

                                    which says it is a 350V resistor. I wish they put that info on the package...

                                    from what you made it sound like, what I built will have to be chucked away... it is in a plastic project box on a PCB... I designed the HV part so the traces were 1.25" away from each other at least.

                                    so, what to do?

                                    A. replace the existing 100K resistor with a pice of wire and and add an external battery of HV resistors.
                                    B. chuck the existing setup and build another probe setup
                                    C. chuck the existing setup and buy a name brand probe in a few months

                                    right now, I am thinking of either choice C (the battery of resistors bit sounds like a massive PITA)

                                    Last edited by ratdude747; 02-11-2012, 06:08 PM.
                                    sigpic

                                    (Insert witty quote here)

                                    Comment


                                      #19
                                      Re: HV Probe Suggestions

                                      Originally posted by ratdude747 View Post
                                      that schematic is outdated. I since modified it (been too lazy to redraw it).

                                      as it is now, a 100K resitor drops the high voltage and a pot/100ohm resistor drop the rest.

                                      I saw no voltage ratings on the pack I bought... stupid me for thinkign it was irrelivant.

                                      with this one CRT, I need to rivet metal to the frame (the back plastic failed on it) and it was more for safety. the next is an emac that needs some work around the cdrom done. the last is a mitsubishi monitor that has an unknown issue.

                                      what I don't get is all the homebrew guides I saw showed the use of standard resistors...
                                      Not every home brew circuit you find on the net was designed by an electrical engineer.

                                      the part that will be dropping the high voltage is one of these:

                                      http://www.radioshack.com/product/in...lue=RadioShack

                                      which says it is a 350V resistor. I wish they put that info on the package...
                                      With rat shack parts it's lucky you get ANY useful info.

                                      from what you made it sound like, what I built will have to be chucked away... it is in a plastic project box on a PCB... I designed the HV part so the traces were 1.25" away from each other at least.
                                      With voltages as high as a CRT uses, it's a whole other world. You need special high voltage wire with insulation rated to 30 or 40 KV, super rounded solder joints so sparks, arcs, and corona discharges don't occur, etc. Or you just pot everything up in high voltage RTV or epoxy.

                                      so, what to do?

                                      A. replace the existing 100K resistor with a pice of wire and and add an external battery of HV resistors.
                                      B. chuck the existing setup and build another probe setup
                                      C. chuck the existing setup and buy a name brand probe in a few months

                                      right now, I am thinking of either choice C (the battery of resistors bit sounds like a massive PITA)

                                      Assuming you're just worried about discharging the CRTs, and don't need to accurately measure the voltage, a 5KV or 10KV resistor of just about any value soldered to a piece of 10 or 12 gauge copper wire as a probe on one end, and a piece of wire with an alligator clip on the other end should be more than good enough. Stick the resistor in a piece of 1 or 2 inch diameter PVC or ABS about 8 inches long, and fill it with RTV. Attach the alligator clip to the CRT ground, hold the probe by the PVC handle, and CAREFULLY touch the copper probe to the anode cap's metal clip.

                                      Actually, all you really need is the wire and the handle, it's not like discharging an electrolytic cap. where you have to worry about discharging it to quickly. It's essentially a giant PF range cap with a glass dialectic. The insulated handle is the only critical part so you don't wind up with a Flock of Seagulls hairdo.

                                      Comment


                                        #20
                                        Re: HV Probe Suggestions

                                        I am an electrical engineering technology student if that counts... but I am only in my second semester and all the HV stuff is in classes I take next semester.

                                        I will probably save up for a proper probe...
                                        sigpic

                                        (Insert witty quote here)

                                        Comment

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