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    Ohms Law Calculation

    Hello,

    The question is below if you want to skip the background.

    Background:
    I built a make-shift power supply that I use for a vaping box mod while I'm sitting at my desk so I don't have to worry about charging batteries etc. The way I did it was quite simple... I used a decent sized transformer (because this is a fairly high wattage application). So the voltage is stepped down to roughly 30 volts AC. From there, I feed a packaged bridge rectifier which then gives me roughly the same voltage DC. THEN, I connect the DC output of the rectifier to a gizmo that I bought from Amazon for $12 (rated at 15 amps at 24 volts DC), that allows me to step down DC anywhere from like 2 volts to something like 95% of the input voltage. So I step the voltage down to 6 Volts DC and feed that to my Vape mod.


    Question:
    When I put my Fluke in line with the 6 volts and check the current as I'm using the vape, I read 12.2 amps, which is 73.2 watts.

    THEN, I read the current on the AC side from the wall to the transformer (which is of course 120 volts AC) and the amperage being drawn from the wall is 1.8 amps while i'm running the vape, which comes out to 216 watts (yes, I did make sure the Fluke was dialed on AC not DC ).

    216 - 73.2 = 142.8 watts MISSING...

    ...to me, this means that I'm losing 142.8 watts in the transformer, the rectifier and the voltage step down device, but this is more than double the wattage that I need for the vape ... is it possible that these components are really that inefficient? Or is my math wrong somewhere?

    It's been many years since I took electronics, and I'm trying to understand what I'm seeing, so hopefully someone much more in the know than I am can help me make sense of this.

    Thank you,

    Mike

    #2
    Re: Ohms Law Calculation

    OK, so do you have filter cap at the output of the bridge? Cap for 15A load will be large and it depends on what the acceptable AC ripple is.
    30AC feeding the bridge will have DCV output of about 40VDC, it sounds to me you do not have filter cap at the output of the bridge .
    Check the AC current feeding the bridge to see what you have.
    Attached Files
    Last edited by budm; 09-25-2016, 03:40 PM.
    Never stop learning
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    Comment


      #3
      Re: Ohms Law Calculation

      How are you measuring current, are you reading apparent/peak current or real/RMS current/power? Sometimes a quality AC wattmeter is needed and you can't simply measure current with a multimeter.

      How many amps flowing on the secondary? (guessing around 7.2A)

      What's the documentation of this "gizmo"? Probably some sort of switching power supply it looks.

      If you're really losing that much wattage, it should be as a significant amount of heat. Is anything generating a lot of heat other than the load?

      Comment


        #4
        Re: Ohms Law Calculation

        Are you measuring ac Amps on the 12Vac side and dc Amps on the dc side?

        Comment


          #5
          Re: Ohms Law Calculation

          Originally posted by EasyGoing1 View Post
          Hello,

          The question is below if you want to skip the background.

          Background:
          I built a make-shift power supply that I use for a vaping box mod while I'm sitting at my desk so I don't have to worry about charging batteries etc. The way I did it was quite simple... I used a decent sized transformer (because this is a fairly high wattage application). So the voltage is stepped down to roughly 30 volts AC. From there, I feed a packaged bridge rectifier which then gives me roughly the same voltage DC. THEN, I connect the DC output of the rectifier to a gizmo that I bought from Amazon for $12 (rated at 15 amps at 24 volts DC), that allows me to step down DC anywhere from like 2 volts to something like 95% of the input voltage. So I step the voltage down to 6 Volts DC and feed that to my Vape mod.


          Question:
          When I put my Fluke in line with the 6 volts and check the current as I'm using the vape, I read 12.2 amps, which is 73.2 watts.

          THEN, I read the current on the AC side from the wall to the transformer (which is of course 120 volts AC) and the amperage being drawn from the wall is 1.8 amps while i'm running the vape, which comes out to 216 watts (yes, I did make sure the Fluke was dialed on AC not DC ).

          216 - 73.2 = 142.8 watts MISSING...

          ...to me, this means that I'm losing 142.8 watts in the transformer, the rectifier and the voltage step down device, but this is more than double the wattage that I need for the vape ... is it possible that these components are really that inefficient? Or is my math wrong somewhere?

          It's been many years since I took electronics, and I'm trying to understand what I'm seeing, so hopefully someone much more in the know than I am can help me make sense of this.

          Thank you,

          Mike
          So what type of gizmo thing did you buy that is rated at 15 amps and 24volts that allows you to step down between 2 volts to something of 95% of the input. 95% of 30 volts is 28.5 volts which is higher then it is rated for at 24 volts.

          Comment


            #6
            Re: Ohms Law Calculation

            OK ... WOW ... I wasn't expecting this kind of interest ... I was really just trying to see if it was logical to get the results I was getting ... something like "yes, that makes sense" or "no, something isn't quite right" etc...

            As far as filter caps on the output, I'd have to say yes and no ... the "gizmo" that ultimately supplies the power to my vape is this:

            DROK® DC Car Power Supply Voltage Regulator Buck Converter 8A/100W

            And as soon as my iphone is charged up enough to power on, im going to take some pics and post them.

            On the output cap ... im powering a vape mod ... this make-shift power supply is literally connected to each end of two metal coils connected in parallel for a total of 1/2 an ohm. When I hit the button, 6 volts at 12 amps flows through the coils and makes them very hot which vaporizes my e-jiuce which I then inhale... lol ... thats all im doing... worrying about clean DC power in this application almost seems silly... right? Unless that capacitor would affect my readings?

            As far as how I'm taking my readings, I'm using a Fluke 115 "True RMS Multimeter"

            Comment


              #7
              Re: Ohms Law Calculation

              Originally posted by EasyGoing1 View Post
              OK ... WOW ... I wasn't expecting this kind of interest ... I was really just trying to see if it was logical to get the results I was getting ... something like "yes, that makes sense" or "no, something isn't quite right" etc...

              As far as filter caps on the output, I'd have to say yes and no ... the "gizmo" that ultimately supplies the power to my vape is this:

              DROK® DC Car Power Supply Voltage Regulator Buck Converter 8A/100W

              And as soon as my iphone is charged up enough to power on, im going to take some pics and post them.

              On the output cap ... im powering a vape mod ... this make-shift power supply is literally connected to each end of two metal coils connected in parallel for a total of 1/2 an ohm. When I hit the button, 6 volts at 12 amps flows through the coils and makes them very hot which vaporizes my e-jiuce which I then inhale... lol ... thats all im doing... worrying about clean DC power in this application almost seems silly... right? Unless that capacitor would affect my readings?

              As far as how I'm taking my readings, I'm using a Fluke 115 "True RMS Multimeter"
              That SMPS needs good clean DC input for it to run properly, the small caps (2) on the input side is not going to be good enough when lots of current is drawn by the load. Right now the you are probably feeding the circuit with high ripple Voltage (with 120Hz frequency) due to not enough filter caps after the bridge which can easily destroy the board.
              What are the value and the Voltage rating of those caps on that SMPS board?
              Last edited by budm; 09-26-2016, 09:31 AM.
              Never stop learning
              Basic LCD TV and Monitor troubleshooting guides.
              http://www.badcaps.net/forum/showthr...956#post305956

              Voltage Regulator (LDO) testing:
              http://www.badcaps.net/forum/showthr...999#post300999

              Inverter testing using old CFL:
              http://www.badcaps.net/forum/showthr...er+testing+cfl

              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
              http://s807.photobucket.com/user/budm/library/

              TV Factory reset codes listing:
              http://www.badcaps.net/forum/showthread.php?t=24809

              Comment


                #8
                Re: Ohms Law Calculation

                So basically you are running this power supply at 0.2 amps above it's maximum peak rating? Actually your available input DC wattage from the 120 VAC and 1.8 Amp ac is 174.96 Watts. The supply at 12 Volts and 8 Amp is 91 percent efficient. If we take that number, although you are using it at 6 volts output, now makes the available DC output wattage at 158.34 Watts. So now you have the 158.34 - 73.2 = 57.66 missing
                Last edited by keeney123; 09-26-2016, 12:57 PM.

                Comment


                  #9
                  Re: Ohms Law Calculation

                  I did a experiment which isn't quite the same but may be similar (also a nonlinear load).

                  Hardware:

                  P3 Kill-A-Watt
                  BK Precision 4½ Digit True RMS DMM (2833)
                  Load: old 12V on line open frame 50W max SMPS and 12V car tail lamp

                  Input: 120VAC. Measured 30W (from Kill-A-Watt), 380mA (BKP and Kill-a-Watt match), 45VA (calculated on Kill-A-Watt or just do math from BKP)

                  Output
                  12VDC 1.9A => 23 Watts

                  Real Wasted = 30-23W = 7 W
                  Efficiency = 23/30 = 76%

                  Apparent Wasted (VA) = 45-23W = 22W
                  Apparent Efficiency = 23/45 = 51%

                  So it looks like current measurement is still not enough... Looks like you do need a true wattmeter after all.

                  Comment


                    #10
                    Re: Ohms Law Calculation

                    Originally posted by budm View Post
                    That SMPS needs good clean DC input for it to run properly, the small caps (2) on the input side is not going to be good enough when lots of current is drawn by the load. Right now the you are probably feeding the circuit with high ripple Voltage (with 120Hz frequency) due to not enough filter caps after the bridge which can easily destroy the board.
                    What are the value and the Voltage rating of those caps on that SMPS board?
                    BudM when they are talking about 300KHz operation on this supply is that on the input, output or both?

                    Comment


                      #11
                      Re: Ohms Law Calculation

                      Originally posted by eccerr0r View Post
                      I did a experiment which isn't quite the same but may be similar (also a nonlinear load).

                      Hardware:

                      P3 Kill-A-Watt
                      BK Precision 4½ Digit True RMS DMM (2833)
                      Load: old 12V on line open frame 50W max SMPS and 12V car tail lamp

                      Input: 120VAC. Measured 30W (from Kill-A-Watt), 380mA (BKP and Kill-a-Watt match), 45VA (calculated on Kill-A-Watt or just do math from BKP)

                      Output
                      12VDC 1.9A => 23 Watts

                      Real Wasted = 30-23W = 7 W
                      Efficiency = 23/30 = 76%

                      Apparent Wasted (VA) = 45-23W = 22W
                      Apparent Efficiency = 23/45 = 51%

                      So it looks like current measurement is still not enough... Looks like you do need a true wattmeter after all.
                      Usually when one drives an output to its max and has more input the rest is usually give off as heat. Of course this is old knowledge on linears. The new stuff I have read and learned and forgot because I do not repair them on a regular bases.

                      Comment


                        #12
                        Re: Ohms Law Calculation

                        Originally posted by budm View Post
                        That SMPS needs good clean DC input for it to run properly, the small caps (2) on the input side is not going to be good enough when lots of current is drawn by the load. Right now the you are probably feeding the circuit with high ripple Voltage (with 120Hz frequency) due to not enough filter caps after the bridge which can easily destroy the board.
                        What are the value and the Voltage rating of those caps on that SMPS board?
                        Budm, the filter caps will lower the high ripple voltage?
                        -- Law of Expanding Memory: Applications Will Also Expand Until RAM Is Full

                        Comment


                          #13
                          Re: Ohms Law Calculation

                          Example of how to calculate the filter cap value, capacitors have to supply the current during the off time of the diodes.
                          http://www.electro-tech-online.com/t...acitor.106374/
                          Attached Files
                          Last edited by budm; 09-26-2016, 09:20 PM.
                          Never stop learning
                          Basic LCD TV and Monitor troubleshooting guides.
                          http://www.badcaps.net/forum/showthr...956#post305956

                          Voltage Regulator (LDO) testing:
                          http://www.badcaps.net/forum/showthr...999#post300999

                          Inverter testing using old CFL:
                          http://www.badcaps.net/forum/showthr...er+testing+cfl

                          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                          http://s807.photobucket.com/user/budm/library/

                          TV Factory reset codes listing:
                          http://www.badcaps.net/forum/showthread.php?t=24809

                          Comment


                            #14
                            Re: Ohms Law Calculation

                            Originally posted by budm View Post
                            Right now the you are probably feeding the circuit with high ripple Voltage (with 120Hz frequency) due to not enough filter caps after the bridge which can easily destroy the board.
                            I just bought a little oscilloscope kit that I assembled the other day ... I'll take a look.
                            Originally posted by budm View Post
                            What are the value and the Voltage rating of those caps on that SMPS board?
                            All 5 of the green caps are 330 micro farads at 50 volts.

                            Comment


                              #15
                              Re: Ohms Law Calculation

                              Originally posted by budm View Post
                              Right now the you are probably feeding the circuit with high ripple Voltage (with 120Hz frequency) due to not enough filter caps after the bridge which can easily destroy the board.
                              Here is the scope image of the 6 volts coming off the power supply (had to paste two images together) ... the flat line is idle, and the little ripple you see is me taking a hit off of my vape mod. It looks fairly clean to me... but I admit being ignorant on how much ripple is acceptable and how much is not...

                              I could not get a read on the DC input as it is around 30 volts and this OScope wont go that high.
                              Attached Files

                              Comment


                                #16
                                Re: Ohms Law Calculation

                                Here are images of the setup as requested...
                                Attached Files

                                Comment


                                  #17
                                  Re: Ohms Law Calculation

                                  Transformer outputs AC voltage.

                                  The bridge rectifier converts AC voltage to DC voltage but without some capacitance, what you get is basically a series of pulses, all above 0v.

                                  The peak DC voltage that comes out of the rectifier is equal to 1.414 x Vac - 2 x Vdiode where Vac is the AC voltage and Vdiode is the voltage drop on one diode in the bridge rectifier (typically between 0.7v and 1.1v, according to the datasheet of your bridge rectifier it's 1.1v )

                                  Vdc peak = 1.414 x 40 - 2 x 1.1v = 54.2v ~

                                  So you won't get 40v DC out of the bridge rectifier, but rather a DC voltage with a peak of about 55 volts. Also, keep in mind that transformers when idle typically can output up to 10% more AC voltage, only under load their output goes down to the rated AC voltage.

                                  This is very important, because as you can see on that DC-DC converter board, at the input you have two capacitors rated only for 50v, which means if what you say it's true, eventually those capacitors will vent or blow up or go bad, because you're exceeding their maximum voltage rating by about 5v. At the very least, you would have to replace these with 63v rated capacitors.

                                  Now of course, 120 times a second you'll have pulses of DC voltage going all the way up to around 55v and then slowly going down. Depending on how much capacitance you put after the bridge rectifier, you raise the minimum DC voltage so that it will always stay above some threshold.
                                  With such DC-DC converters, I think your input voltage should stay at least about 1v above the output voltage, but they'll be more happy with voltage varying within a narrower range. So if you want 6v out, I guess you could power it with some DC voltage from 7v to 50v, but the regulator will probably be more happy with something like 40v..55v

                                  How much capacitance you need, you can estimate using this formula :

                                  Capacitance (in Farads) = Current (Amps) / [ 2 x AC Mains Frequency x ( Vdc peak - Vdc minimum desired ) ]

                                  So for example, let's say you want 6v @ 5A from your DC-DC converter. The DC-DC converter probably converts the higher voltage to lower voltage with about 85% efficiency or higher, but let's make things easy and say 80%.
                                  This means that in order to produce 6v x 5A = 30 watts of energy, it will have to be fed with at least about 36 watts (because 30 watts would be 4/5 of the total energy used due to the 80% efficiency, so since 4/5 is 30 watts that means 1/5th is 6 watts, so then 5/5 equals to 36 watts). Anyway, just to make things easier, let's just round it up and say we want to give the dc-dc converter more than it needs and simply work with 40 watts.

                                  Now, right after the rectifier you have DC voltage with peaks up to 55v and some current (which depends on the VA rating of the transformer). You have to figure out how much capacitance you need to have there, so that the dc-dc converter will always be able to draw 40 watts from it.
                                  One option would be to always keep the DC voltage equal or above 40v, if the transformer can output 1A of current. In this case, you have that formula :

                                  C = 1A / [ 2 x 60 Hz x (55v - 40v ) ] = 1 / 120x15 = 1/1800 = 0.000555 Farads or 555uF

                                  In your case, you have two 330uF in parallel at the input so it would be more than 555uF which would work out right.

                                  Another option would be to use more capacitance to raise the minimum voltage higher, and lower the current. For example, let's say we want the minimum voltage of at least 50v, in which case we'd only need 40w / 50v = 0.8a of current to keep things running smooth

                                  So C = 0.8A / 120 x (55-50) = 0.8 / 600 = 0.001333 Farads or 1333uF, in which case it's obvious your DC-DC converter has too little capacitance at the input.

                                  Anyway, again, this is all assuming you want the dc-dc converter to have at least 40w of energy available at the input, so that you could have 6v at 5a (30w) and some reserves at the output. If you aim for lower maximum currents at the output, you can change the numbers and recalculate things using the two formulas above.

                                  btw. Your transformer is rated in VA and has an AC voltage at the output. So if your transformer says it's 40v AC and 25VA, that means the maximum AC current at the output is about 25 / 40 = 0.62A
                                  When this AC voltage is rectified using a classic bridge rectifier, the maximum DC current is not the same but rather smaller, and can be approximated using the formula I dc = ~ 0.62 x Iac , so for my numbers the peak AC current could be only 0.62 x 0.625 = 0.387A

                                  So going back to my examples above, you may very think you could use 1A or 0.8A in your calculations, but if the transformer can only provide ~ 0.39A then you're screwed, you'd have to raise the minimum voltage by using more capacitance.

                                  The 0.62 is a "magic constant" which works well for transformers in the 40-100va range, but is often a bit off for very small or much larger transformers.

                                  so first of all .. if your transformer really outputs 40v ac, then really really really those 50v rated capacitors at the input of your dc-dc converter should be changed to some rated for at least 63v.

                                  Alternatively, if your transformer has two secondary windings, you could connect those secondary windings in parallel for half the voltage but twice the current.
                                  Last edited by mariushm; 09-28-2016, 09:57 AM.

                                  Comment


                                    #18
                                    Re: Ohms Law Calculation

                                    Originally posted by EasyGoing1 View Post
                                    Here is the scope image of the 6 volts coming off the power supply (had to paste two images together) ... the flat line is idle, and the little ripple you see is me taking a hit off of my vape mod. It looks fairly clean to me... but I admit being ignorant on how much ripple is acceptable and how much is not...

                                    I could not get a read on the DC input as it is around 30 volts and this OScope wont go that high.
                                    You need to look at the output of the bridge Rectifier with full load, scope set to AC coupling.
                                    When you want to see AC RIPPLE riding on DC level, you set the scope to AC coupling.
                                    Also read post 17.
                                    Last edited by budm; 09-28-2016, 09:56 AM.
                                    Never stop learning
                                    Basic LCD TV and Monitor troubleshooting guides.
                                    http://www.badcaps.net/forum/showthr...956#post305956

                                    Voltage Regulator (LDO) testing:
                                    http://www.badcaps.net/forum/showthr...999#post300999

                                    Inverter testing using old CFL:
                                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                    http://s807.photobucket.com/user/budm/library/

                                    TV Factory reset codes listing:
                                    http://www.badcaps.net/forum/showthread.php?t=24809

                                    Comment


                                      #19
                                      Re: Ohms Law Calculation

                                      Quote from Mariushm; "So if your transformer says it's 40v AC and 25VA, that means the maximum AC current at the output is about 25 / 40 = 0.62A"


                                      When you are saying AC you are not actually meaning AC peak to peak you are talking about 1/2 the peak times 0.707 = RMS AC

                                      So in a 60 Hz AC circuit it is it is RMS wattage

                                      In a DC circuit it is DC Wattage

                                      In a pulsation DC circuit it is Wattage DC and Wattage of the ripple which now becomes 120Hz so it would be the peak ripple x 0.707
                                      Last edited by keeney123; 09-28-2016, 12:36 PM.

                                      Comment


                                        #20
                                        Re: Ohms Law Calculation

                                        Post 16 Voltage markings do not make sense, it shows 40VAC feeding the bridge and 40VDC on the output of the bridge.
                                        May be I missed the info but I do not see the VA rating of the transformer or the Volts or the AMP rating in the threads. Post 1 said 30V stepdown transformer, it looks like a transformer from and old stereo amplifier.
                                        Last edited by budm; 09-28-2016, 12:28 PM.
                                        Never stop learning
                                        Basic LCD TV and Monitor troubleshooting guides.
                                        http://www.badcaps.net/forum/showthr...956#post305956

                                        Voltage Regulator (LDO) testing:
                                        http://www.badcaps.net/forum/showthr...999#post300999

                                        Inverter testing using old CFL:
                                        http://www.badcaps.net/forum/showthr...er+testing+cfl

                                        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                        http://s807.photobucket.com/user/budm/library/

                                        TV Factory reset codes listing:
                                        http://www.badcaps.net/forum/showthread.php?t=24809

                                        Comment

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