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    #41
    Re: Constant current load for testing power supplies (guidelines)

    That is the Voltage follower circuit. not Constant Current SOURCE that AUTOMATICALLY maintain the output current to be constant with the variable load, the key word is 'CURRENT SOURCE'
    https://www.allaboutcircuits.com/tex...tage-follower/
    If you add zener to the circuit then it become Voltage regulator.
    http://www.circuitstoday.com/voltage-regulators
    http://www.radio-electronics.com/inf...ent-source.php
    http://www.learningaboutelectronics....ent-source.php
    Last edited by budm; 10-03-2017, 09:18 AM.
    Never stop learning
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      #42
      Re: Constant current load for testing power supplies (guidelines)

      Originally posted by budm View Post
      That is the Voltage follower circuit. not Constant Current SOURCE that AUTOMATICALLY maintain the output current to be constant with the variable load, the key word is 'CURRENT SOURCE'
      https://www.allaboutcircuits.com/tex...tage-follower/
      Well, that's if you connect your load, Rl, between the transistor's Emitter and ground, as is shown in the above link.

      However, if you connect you load between the power supply positive bus and the transistor's Collector in my schematic, and the resistor Emitter Re is a constant value, then the current going through Rl would be dependent on the voltage you set across Re... so in other words, the circuit *will* work as a constant current source.

      Originally posted by eccerr0r
      Now here you consider a slightly different different topology, I had not considered this topology, but this topology still needs some reference whether a CVVS or a diode/zener drop. This definitely requires a Darlington, the base current would otherwise be a problem.
      Yes, a diode or Zener reference would definitely be required for accurate voltage/current regulation.

      You don't need a Darlington transistor for that circuit if doing it for low current. That said, even for high current, if you don't have a Darlington, you can build your own with two regular transistors.

      Comment


        #43
        Re: Constant current load for testing power supplies (guidelines)

        Sorry for digging up such an old thread, but I just thought of something: all this time we've been discussing how to maintain a constant voltage drop across a sense resistor - which is how this whole project got started - but what if the resistance of the resistor drifts during operation ? Resistors DO change their value with heat I reckon. The op-amp circuit I originally came up with would not be able to compensate.

        I thought of this: say I have a 1ohm sense resistor and set the reference at the NI input at 1v. The op-amp will maintain 1v across that resistor so I'd get a 1a current draw. 1v/1ohm=1a.

        Now imagine the resistor heats up a bit and its resistance goes up to 2 ohms (ok, let's keep it round). The op-amp won't know to crank the voltage to 2v to still get 1a, since it's still set at 1v, despite the current now being only 0.5a. 1v/2ohm=0.5a. Is this assumption right ?
        Last edited by Dannyx; 01-07-2018, 01:41 PM.
        Wattevah...

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          #44
          Re: Constant current load for testing power supplies (guidelines)

          Temperature Coefficient of Resistance (TCR), you want very low for precision current sense.
          Current sense resistors are low TCR parts, special wire or metal film, foil construction.

          If you used a plain 1R 5W wirewound resistor, like Yageo SQP500, temperature coefficient is high ±300ppm/°C.

          So the 1R resistor at 20°C heated to 70°C (shift of 50°C), gives 1,500ppm to give 0.015 ohms shift or a change of 1.5% just due to temperature.

          In reality, power resistors have hot spots and their temperature drift is hard to know exactly.

          Also, you need 4-wire Kelvin-sense to prevent voltage drops at the resistor measuring point (leads connection) from giving wrong values to your op-amp.

          Comment


            #45
            Re: Constant current load for testing power supplies (guidelines)

            Originally posted by Dannyx View Post
            Sorry for digging up such an old thread, but I just thought of something: all this time we've been discussing how to maintain a constant voltage drop across a sense resistor - which is how this whole project got started - but what if the resistance of the resistor drifts during operation ? Resistors DO change their value with heat I reckon. The op-amp circuit I originally came up with would not be able to compensate.

            I thought of this: say I have a 1ohm sense resistor and set the reference at the NI input at 1v. The op-amp will maintain 1v across that resistor so I'd get a 1a current draw. 1v/1ohm=1a.

            Now imagine the resistor heats up a bit and its resistance goes up to 2 ohms (ok, let's keep it round). The op-amp won't know to crank the voltage to 2v to still get 1a, since it's still set at 1v, despite the current now being only 0.5a. 1v/2ohm=0.5a. Is this assumption right ?
            Yes, resistor resistance can drift with heat, but it's never going to be that bad. Why? Because of tolerance. As long as you are using a resistor within its rated power level and rated operating temperature, its resistance will never drift below or above its tolerance.

            So for a 1 Ohm 5% resistor, it's resistance may go up to 1.05 Ohms or dip to 0.95 Ohms when really cold. With these resistances and your example above, your current may go as low as approximately 0.9524 Amps or as high as 1.0526 Amps.... which as you can see, is also only about 5% off (and remember, that is worst case). If you use a resistor with 1% tolerance, then your current will vary accordingly (even less).

            If the above wasn't true, then none of the electronics we have today would work at all.
            Last edited by momaka; 01-08-2018, 02:11 AM.

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              #46
              Re: Constant current load for testing power supplies (guidelines)

              So what you're saying is I shouldn't bother to create some super complex setup to compensate for the resistance because it's not critical.

              The reason I'm asking is because I was thinking of making my own resistor out of nichrome wire or to use a lightbulb as one (given the high power it can), though that's silly really because once that thing starts glowing red, the resistance is all over the place so anything BUT constant.....scrap the idea - stupid.
              Wattevah...

              Comment


                #47
                Re: Constant current load for testing power supplies (guidelines)

                Originally posted by Dannyx View Post
                The reason I'm asking is because I was thinking of making my own resistor out of nichrome wire or to use a lightbulb as one (given the high power it can), though that's silly really because once that thing starts glowing red, the resistance is all over the place so anything BUT constant.....scrap the idea - stupid.
                Don't scrap the idea yet.

                Yes, an incandescent or halogen light bulb will NOT work due to too much change in resistance. I've actually tried it before (used a low-resistance halogen bulb in place for a low-resistance resistor in a self-built amplifier) and got wonderful oscillations.

                The Nichrome wire *could* work, though. But in order for it to work, you will likely have to keep the Nichrome wire at lower temperatures - i.e. not glowing. The less of a temperature variation you put on the Nichrome wire, the more stable its resistance will be.

                Of course, you will have to experiment with it to determine if the resistance variation meets the criteria of your circuit. If it doesn't then you may need to put several more series and parallel strands of Nichrome wire together to make each Nichrome wire strand run cooler (and thus exhibit less of a resistance variation).

                The best way to do that is to connect your Nichrome "resistor" to a stable voltage source and measure the current draw (with a multimeter) when its cold and when it's warm. By knowing the voltage and the current, you can determine the cold and hot resistances. For example, if your Nichrome "resistor" has a cold resistance of 2.5 Ohms and you connect it to a 5V power supply, then the cold current will be about 2 Amps. Leave it connected to the 5V supply for a few minutes and note the current again. If the current has gone down to let's say 1.5 Amps, then that means the resistance has gone up to 5V/1.5A = 3.3333 Ohms... i.e. that's a 33.3333% increase in the resistance from the original 2.5 Ohms.... i.e. the current in your circuit could vary as much as 33% from what you set it to when it's cold... i.e. not very good. However, if the current only goes down to 1.8 Amps, then that means the resistance has gone up to about 5V/1.8A = 2.778 Ohms... i.e. a 11% increase in the resistance, which is not that bad.

                Thus with Nichrome wire used as a resistor, you will find that the higher the current you put through it, the more of a variation you're going to get between cold and hot resistances (and thus currents). So inevitably, you will have to correct for that on your home-made CC source until the Nichrome wire reaches a more stable temperature.
                Last edited by momaka; 01-08-2018, 09:38 AM.

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                  #48
                  Re: Constant current load for testing power supplies (guidelines)

                  That was my original thought, but I figured I'd compensate for large "jumps" in resistance electronically by building a circuit that can crank higher or lower depending on the resistance. A CV AND CC supply IMO....at the end of the day, isn't a CV supply something thar maintains a constant voltage across a load regardless of the load itself ? I'd need to couple that with a CC section so the current doesn't drift down as resistance goes up....pretty complex
                  Wattevah...

                  Comment


                    #49
                    Re: Constant current load for testing power supplies (guidelines)

                    If you want accuracy and still want to use a light bulb or unstable resistance load (instead of letting the transistor be the power sink), you could still cheese it and use a lower wattage low resistance resistor in series as part of the stack, and using just the bona fide low drift current measurement resistor to measure current (and by choosing the topology carefully, the current through the lamp and measurement resistor are the same). Unfortunately this means the voltage across the resistor will be lower and you'll need a higher quality, lower noise, lower offset op amp to complete the feedback loop.

                    For me I don't particularly care how accurate the load is; I accept that on startup the light bulb's resistance will be lower and thus more current will draw upon first connection (though the cold resistance will need to be measured carefully to know how much current it will draw when cold). On the positive side, it will also tell me if the transient will cause the PSU to oscillate or shutdown...

                    I recall some type of nichrome changed around 1.5% or so every hundred degrees Celsius ...

                    Comment


                      #50
                      Re: Constant current load for testing power supplies (guidelines)

                      Originally posted by eccerr0r View Post
                      I recall some type of nichrome changed around 1.5% or so every hundred degrees Celsius ...
                      I assume it heavily depends on the length and cross-section and stuff like that. I've got a few hundred meters of nichrome on my hands to play around with
                      Wattevah...

                      Comment


                        #51
                        Re: Constant current load for testing power supplies (guidelines)

                        Originally posted by Dannyx View Post
                        A CV AND CC supply IMO....at the end of the day, isn't a CV supply something thar maintains a constant voltage across a load regardless of the load itself ? I'd need to couple that with a CC section so the current doesn't drift down as resistance goes up....pretty complex
                        Well, you will be controlling the current electronically by measuring the voltage across the (known) resistance of your current shunt (i.e. the Nichrome wire) and adjusting the voltage accordingly with an active device (BJT, MOSFET, etc). Meanwhile, the CV section can be done by a simple linear regulator with a pass-transistor to allow you to draw heavier currents - at least that's how I would build the thing (I've actually been thinking about it a lot lately).

                        Originally posted by Dannyx View Post
                        I assume it heavily depends on the length and cross-section and stuff like that. I've got a few hundred meters of nichrome on my hands to play around with
                        Yes, exactly.
                        More heat in less surface area = hotter Nichrome wire. So you will likely want a few long parallel strands to keep the surface area high and the resistance low. Think of it as building a low-resistance, high-power resistor out of many high-resistance, low-power resistors.

                        Comment


                          #52
                          Re: Constant current load for testing power supplies (guidelines)

                          It all makes sense in my head but building the thing is a whole different story I know how to build a CV supply (op-amp and feedback loop) at its most basic, but its voltage would have to be adjusted up/down based on the current draw. If the resistance drifts up/down (which is what gave me the idea of using the lighbulb as a load, albeit a variable one), the supply would also need to compensate with more/less voltage from the supply under test via the pass element...that's how I see it at least.
                          Last edited by Dannyx; 01-19-2018, 06:40 AM.
                          Wattevah...

                          Comment


                            #53
                            Re: Constant current load for testing power supplies (guidelines)

                            Originally posted by Dannyx View Post
                            It all makes sense in my head but building the thing is a whole different story I know how to build a CV supply (op-amp and feedback loop) at its most basic, but its voltage would have to be adjusted up/down based on the current draw. If the resistance drifts up/down (which is what gave me the idea of using the lighbulb as a load, albeit a variable one), the supply would also need to compensate with more/less voltage from the supply under test via the pass element...that's how I see it at least.
                            That's because you are probably using ground as the reference point for your CV. You must use the high-side of the current shunt resistor of your CC as the ground reference for your CV, if you want the voltage across the DUT to be correct.

                            Comment


                              #54
                              Re: Constant current load for testing power supplies (guidelines)

                              Originally posted by momaka View Post
                              That's because you are probably using ground as the reference point for your CV. You must use the high-side of the current shunt resistor of your CC as the ground reference for your CV, if you want the voltage across the DUT to be correct.
                              I originally conceived the reference by putting a pot between GND and VCC (let's say 5v for argument's sake). It would also work with a zener diode for a fixed reference, but that's beside the point (since I'm trying to put what you suggested into perspective). The pot method works, but I have to "manually" adjust it if the resistance changes, because that reference would stay the same regardless of the load, so what you're suggesting is using the feedback from the shunt as the ref ? Sorry if I didn't get it - I'm a bit rusty right now having left electronics projects behind for a brief period
                              Wattevah...

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                                #55
                                Re: Constant current load for testing power supplies (guidelines)

                                Originally posted by Dannyx View Post
                                I originally conceived the reference by putting a pot between GND and VCC (let's say 5v for argument's sake). It would also work with a zener diode for a fixed reference, but that's beside the point (since I'm trying to put what you suggested into perspective). The pot method works, but I have to "manually" adjust it if the resistance changes, because that reference would stay the same regardless of the load, so what you're suggesting is using the feedback from the shunt as the ref ?
                                Use a Zener to get a stable regulated voltage. Connect the Zener's anode to the upper end of your shunt resistor instead of ground (that way, your Zener's reference voltage will vary with voltage across the shunt due to load.

                                Then use a pot connected between the Zener's anode and cathode and pot output connected to the op-amp reference input so that you can adjust the voltage to your DUT.

                                Comment


                                  #56
                                  Re: Constant current load for testing power supplies (guidelines)

                                  I drew a schematic but stopped at the following part
                                  Originally posted by momaka View Post
                                  Then use a pot connected between the Zener's anode and cathode and pot output connected to the op-amp reference input so that you can adjust the voltage to your DUT.
                                  So from what I understand, the pot's wiper goes to the NI input (what you call the op-amp's reference), but what about the Inverting input ? I assume that goes to the anode of the zener diode (hence the shunt's "output"). Nifty
                                  Attached Files
                                  Wattevah...

                                  Comment


                                    #57
                                    Re: Constant current load for testing power supplies (guidelines)

                                    I am still watching this post to see what you come with I would like like to build something like this as well
                                    9 PC LCD Monitor
                                    6 LCD Flat Screen TV
                                    30 Desk Top Switching Power Supply
                                    10 Battery Charger Switching Power Supply for Power Tool
                                    6 18v Lithium Battery Power Boards for Tool Battery Packs
                                    1 XBox 360 Switching Power Supply and M Board
                                    25 Servo Drives 220/460 3 Phase
                                    6 De-soldering Station Switching Power Supply 1 Power Supply
                                    1 Dell Mother Board
                                    15 Computer Power Supply
                                    1 HP Printer Supply & Control Board * lighting finished it *


                                    These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                                    1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                                    2 Hem Saw Computer Stack Board

                                    All of these had CAPs POOF
                                    All of the mosfet that are taken out by bad caps

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                                      #58
                                      Re: Constant current load for testing power supplies (guidelines)

                                      Originally posted by Dannyx View Post
                                      I drew a schematic but stopped at the following part

                                      So from what I understand, the pot's wiper goes to the NI input (what you call the op-amp's reference), but what about the Inverting input ? I assume that goes to the anode of the zener diode (hence the shunt's "output"). Nifty
                                      Still not quite there yet.

                                      Inverting input goes to a resistor divider network connected across the load. The resistor divider network basically "measures" the voltage across the load and the output of that network goes to the inverting op-amp pin for feedback. Meanwhile, the Zener-pot circuit sets the "reference" voltage on the non-inverting input of the op-amp. Essentially, the op-amp compares the "feedbacK" from the resistor divider network to the "reference" voltage. If the feedback voltage is low, the op-amp turns On the transistor to increase the voltage going to the load. And if the feedback voltage is high, the op-amp turns Off the transistor to reduce the voltage.

                                      Also, your circuit is missing the load. R1 should NOT be your load. R1 is what we will use later for implementing the CC circuit. The load will be located between R1 and your transistor's Emitter/Source (depending on if you use MOSFET or BJT... but if you're going to use an op-amp to drive that circuit, you're better off using a MOSFET, unless you can find a good Darlington BJT).

                                      Originally posted by sam_sam_sam View Post
                                      I am still watching this post to see what you come with I would like like to build something like this as well
                                      I'll see if I can build a circuit like that in the next few weeks, given that we might have some cold weather and I might not be able to do much outside.

                                      Comment


                                        #59
                                        Re: Constant current load for testing power supplies (guidelines)

                                        I shall attempt a re-draw to see what turns out
                                        Wattevah...

                                        Comment


                                          #60
                                          Re: Constant current load for testing power supplies (guidelines)

                                          I added the load this time. The resistor divider you mentioned should go across RLoad then ?
                                          Attached Files
                                          Wattevah...

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