Power supply current limiting

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  • lti
    Badcaps Legend
    • May 2011
    • 2547
    • United States

    #21
    Re: Power supply current limiting

    I actually disconnected R13 and R14 from the shunt. I did that so I could do some more testing of the difference amplifier. Disconnecting Q1 would have the same effect.

    Also, I just remembered that I can see the current limit LED dim as I turn the current adjustment potentiometer. I guess the LM324 doesn't work well as a comparator.

    Comment

    • rievax_60
      Badcaps Veteran
      • May 2012
      • 897
      • australia

      #22
      Re: Power supply current limiting

      The opamp output might not be able to swing all the way down to the 0v rail. This is a common limitation of opamps.
      Maybe the inverting input is 0.4v higher than the other input.

      Comment

      • lti
        Badcaps Legend
        • May 2011
        • 2547
        • United States

        #23
        Re: Power supply current limiting

        It should go down to the negative rail since the BA10324 is really just an LM324 made by Rohm Semiconductor.

        Comment

        • budm
          Badcaps Legend
          • Feb 2010
          • 40746
          • USA

          #24
          Re: Power supply current limiting

          "It should go down to the negative rail " but there is no negative rail, as shown in the diagram, not +/- supply.
          Last edited by budm; 08-25-2014, 10:12 PM.
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

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          Comment

          • lti
            Badcaps Legend
            • May 2011
            • 2547
            • United States

            #25
            Re: Power supply current limiting

            The output will get close to the voltage on the negative supply input, which is grounded since there is no split power supply.

            Comment

            • ben7
              Capaholic
              • Jan 2011
              • 4059
              • USA

              #26
              Re: Power supply current limiting

              This thread just caught my attention, I've had a few thoughts about trying to make a power supply recently too!...

              I just cobbled this together.
              Built it on a breadboard and indeed it works!
              Voltage control only, though.
              Definitely some room for improvements, but it is just a simple test, for now.

              Yes, there are no fuses either... wouldn't trust this thing running without supervision... :P
              Attached Files
              Muh-soggy-knee

              Comment

              • lti
                Badcaps Legend
                • May 2011
                • 2547
                • United States

                #27
                Re: Power supply current limiting

                This appears to be the same problem, but with an LM358:
                http://www.eevblog.com/forum/project...rrent-sensing/

                I think that isn't a good circuit. I just need to figure out how to make it work with the parts I have. I think I'll have to add a second reference and connect R15 to the reference instead of ground. That will increase the minimum output voltage so weird stuff like this won't happen. I haven't seen a difference amplifier circuit that has that resistor connected to ground and a single power supply.
                Last edited by lti; 08-28-2014, 05:17 PM.

                Comment

                • lti
                  Badcaps Legend
                  • May 2011
                  • 2547
                  • United States

                  #28
                  Re: Power supply current limiting

                  I also need to figure out the best way to add a constant current indicator LED. What I had in the schematic doesn't work very well, and I might need that op-amp.
                  Last edited by lti; 08-28-2014, 07:06 PM.

                  Comment

                  • rievax_60
                    Badcaps Veteran
                    • May 2012
                    • 897
                    • australia

                    #29
                    Re: Power supply current limiting

                    It will help if the gain is set higher to something like 10 or more. The feedback resistors should be higher in value to reduce the current that the opamp's output needs to sink.
                    A load resistor between the opamps's output and ground will help to get its output voltage much closer to 0 volts.

                    Comment

                    • lti
                      Badcaps Legend
                      • May 2011
                      • 2547
                      • United States

                      #30
                      Re: Power supply current limiting

                      I switched to 47.5K ohm resistors and the output voltage dropped to 0.65V. I haven't tried anything else yet. Unfortunately, those resistors aren't as closely matched as the 6.81K ohm resistors I was using earlier. Their values are really close to both ends of their +/-1% tolerance range. One measures 47.8K, one measures 47.3K, and the other two measure 47.5K.

                      Comment

                      • lti
                        Badcaps Legend
                        • May 2011
                        • 2547
                        • United States

                        #31
                        Re: Power supply current limiting

                        I've used a diode as the reference voltage source. It's buffered using an op-amp so it will be a little more stable than just putting a diode in series with R15. It seems to work, but I'll need to do some more testing.

                        I just need to figure out how to add the constant current indicator LED. I guess I could connect another transistor to the output of U3, but I don't think that will work very well.

                        Comment

                        • lti
                          Badcaps Legend
                          • May 2011
                          • 2547
                          • United States

                          #32
                          Re: Power supply current limiting

                          It still doesn't work most of the time. The output voltage will usually drop to zero as soon as the current limit is reached and slowly increase to the set voltage. The output voltage is sometimes not affected by the set current limit at all. The current limiting did work properly for a few seconds. There might be a bad solder joint on the current set potentiometer, and the slow increase in output voltage is caused by the cap I added between the wiper and ground for no reason.

                          At least it isn't stuck in constant current mode now.
                          Last edited by lti; 09-02-2014, 09:45 PM.

                          Comment

                          • lti
                            Badcaps Legend
                            • May 2011
                            • 2547
                            • United States

                            #33
                            Re: Power supply current limiting

                            The current limit seems to work after removing the cap and putting a jumper across the connector I was using to connect both potentiometers.

                            However, the regulation is horrible. A 400mA load at 5V makes the output drop to 4V. It appears that there is enough leakage through Q1 to drop the set voltage. The voltage at the voltage set pot is correct (2.5V for 5V output), but the voltage at the collector of Q1 is 2V. The transistor I'm using is an S8050.

                            Also, the output from U2 is still a little too high. It outputs 0.89V, while the reference I added is 0.83V. The voltage does increase as the current increases, but it means that there is a big dead spot in the current adjustment pot where the voltage is between 0.83V and 0.89V.

                            Comment

                            • lti
                              Badcaps Legend
                              • May 2011
                              • 2547
                              • United States

                              #34
                              Re: Power supply current limiting

                              This circuit oscillates just like it did in the simulation. I can hear it squealing. Also, the output of U3 doesn't drop low enough to turn Q1 off. I don't think I can make it work.

                              Comment

                              • ben7
                                Capaholic
                                • Jan 2011
                                • 4059
                                • USA

                                #35
                                Re: Power supply current limiting

                                Originally posted by lti
                                This circuit oscillates just like it did in the simulation. I can hear it squealing. Also, the output of U3 doesn't drop low enough to turn Q1 off. I don't think I can make it work.
                                I've fiddled around with mine some more (well, like a week ago) and added current limiting. The way it controls the pass transistors looks similar to how tom's works. Mine does have a negative rail, so the op amps can go down to 0V on their output.

                                I can paste the updated circuit diagram if you'd like.

                                Interestingly, my circuit doesn't need any added capacitor to make it stable. Not even a capacitor on the output. Perhaps the breadboard has enough capacitance to do that?
                                Muh-soggy-knee

                                Comment

                                • SteveNielsen
                                  Retired Tech
                                  • Jun 2012
                                  • 2327
                                  • USA

                                  #36
                                  Re: Power supply current limiting

                                  Yeah, I think it's a good idea to post the schematic.

                                  Comment

                                  • budm
                                    Badcaps Legend
                                    • Feb 2010
                                    • 40746
                                    • USA

                                    #37
                                    Re: Power supply current limiting

                                    It seems to me if you look at the SCH, when the over current happens, the pass transistor will be shutdown by the small transistor connected between Gate and Ground, once that happen, there will be no current flowing through current sensing resistor, so the sensing circuit will turn off the transistor connected to the gate and allow the pass transistor to conduct again, but once the too much current flows through the sensing resistor again (due to too much load still connected to the output of the power supply), it will go into shut down, so the cycle will just keep repeating in the loop. Over current shut need to be latched and manually clear out after the overload is removed.
                                    It is acting the same way as many people here have seen the ticking of the SMPS power supply with its output fluctuating up and down due to shorted output or shorted load.
                                    Last edited by budm; 09-06-2014, 10:30 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

                                    • ben7
                                      Capaholic
                                      • Jan 2011
                                      • 4059
                                      • USA

                                      #38
                                      Re: Power supply current limiting

                                      Here is the updated schematic.

                                      I only had a single-output transformer with an output of 18V, so I used another transformer for the negative supply. Might need more bulk capacitance also. (I have two 4700uF caps for that, not sure if it is enough though!)

                                      The pass transistor is one I had on hand. I don't have any 2N3055's, but I think those should work fine also. It needs to be put on a decently sized heatsink, and with some thermal paste too. (Maybe a fan too, that depends on heatsink size, ambient air temp and flow)

                                      The LM4040-25 is a 2.5V voltage reference.

                                      I set the circuit up to do 0-18V, and 0-1A.
                                      You might get higher or lower max V/A output readings because of component tolerances. (Also the max output voltage is actually 19.5V (Close enough to 18V xD).)

                                      Be sure to trim the current sense null/trim.

                                      You could try using other op-amps too, maybe like an LM324.

                                      Note: Whoops, looks like I have two R5s and two R9s... the program I used doesn't automatically number the components, so I must have missed those after copying and pasting... (However, the resistances are correct)

                                      Any comments/ideas, from the more experienced engineers here?
                                      Attached Files
                                      Muh-soggy-knee

                                      Comment

                                      • lti
                                        Badcaps Legend
                                        • May 2011
                                        • 2547
                                        • United States

                                        #39
                                        Re: Power supply current limiting

                                        Originally posted by budm
                                        It seems to me if you look at the SCH, when the over current happens, the pass transistor will be shutdown by the small transistor connected between Gate and Ground, once that happen, there will be no current flowing through current sensing resistor, so the sensing circuit will turn off the transistor connected to the gate and allow the pass transistor to conduct again, but once the too much current flows through the sensing resistor again (due to too much load still connected to the output of the power supply), it will go into shut down, so the cycle will just keep repeating in the loop. Over current shut need to be latched and manually clear out after the overload is removed.
                                        It is acting the same way as many people here have seen the ticking of the SMPS power supply with its output fluctuating up and down due to shorted output or shorted load.
                                        Yes, that is what is happening.

                                        I actually wanted a constant current mode like a real bench power supply. A latching current limit should be good enough for now, but the transistor always has voltage on its base, causing the output voltage to be too low. That needs to be taken care of first.
                                        Originally posted by ben7
                                        Here is the updated schematic.

                                        I only had a single-output transformer with an output of 18V, so I used another transformer for the negative supply. Might need more bulk capacitance also. (I have two 4700uF caps for that, not sure if it is enough though!)
                                        It looks like it will have the same oscillation problem. I don't know why the circuit in the video I posted earlier worked and this one doesn't.
                                        Originally posted by ben7
                                        You could try using other op-amps too, maybe like an LM324.
                                        The op-amp I'm using is similar to an LM324.
                                        http://rohmfs.rohm.com/en/products/d...ba10358f-e.pdf

                                        Comment

                                        • ben7
                                          Capaholic
                                          • Jan 2011
                                          • 4059
                                          • USA

                                          #40
                                          Re: Power supply current limiting

                                          Originally posted by lti
                                          I actually wanted a constant current mode like a real bench power supply. A latching current limit should be good enough for now, but the transistor always has voltage on its base, causing the output voltage to be too low. That needs to be taken care of first.
                                          Hm, What voltage does it have on its base? You know, the op-amp's output won't go down to '0V' unless you have a rail to rail op-amp, or a negative supply voltage (like in mine.) It's due to the voltage drop across the op-amp's output transistor.

                                          The same thing will happen with your Q1, V2 and R4. The PSU output won't go all the way to zero when in current limit (if needed (very low resistance load, etc...)), because Q1 WILL have a voltage drop across C to E, thus making U1's non-inverting input still have some voltage on it.

                                          Originally posted by lti
                                          It looks like it will have the same oscillation problem. I don't know why the circuit in the video I posted earlier worked and this one doesn't.
                                          I haven't noticed any oscillation problems with it. Though, as I expected, some of the wires/connections can pick up considerable noise when I touch them, and it shows up on the output.
                                          Muh-soggy-knee

                                          Comment

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