How Do I Test A Negative Power Supply Using Dave's "Current Sink Dummy Load"? - Page 1
Forum section: electronics / general

author: DaveFan
author_rank: Contributor
timestamp: January 29, 2014, 05:49:54 pm
content: I'm developing a small automated tester with 4 e-Loads to characterize some Low Voltage Power Supply Boards which have both positive and negative power supply outputs.I want to be able to apply loads to all 4 outputs simultaneously and load each output accordingly via software control, and measure the output voltages for each load.I want to be able to use a current sink dummy load to test both negative, as well as positive power supply outputs, but am not sure how to go about doing this.I've combed through tons of posts regarding the Current Sink Dummy Load and watched many Youtube videos demonstrating how it works, but not one post on how to test a negative power supply. I assume that due to seeming lack of interest or activity on this subject there must be a way to use the same device to test both positive as well as negative power supplies, but I just haven't discovered it yet.My first thought would be to connect the negative power supply backwards to keep the current flow in the same direction as for the positive power supply, but this would connect the negative power supply directly to ground would it not?Additionally, I would like to use the same circuit (current sink) as a constant current selector, preceding the adjustable voltage regulators for both the positive and negative sources.This will be DAC controlled via OpAmp comparision of the differential voltage across the sense resistor. This will require two OpAmps, one as a differential amp for measuring the voltage across the sense resistor which is fed back to the second OpAmp which compares the DAC voltage and adjusts the output current. It's the same exact current source circuit as Dave's except instead of measuring the voltage across the sense resistor referenced to ground, I will take a differential measurement across the sense resistor.My second question is, I already know how to do this for a positive supply, but how is it done for the negative power source input to the regulator?I assume the answer to the first question will lead to a solution for the second.Please see the two rough sketched schematics to get an idea of what I am trying to do. I can add gain in the feedback OpAmps if necessary, and I know these will work for a positive supply, both for the constant current configuration as well as the dummy load. Problem is, I don't think it will work for a negative constant current & negative dummy load. Please give me some guidence on this...Can someone please help me with this? THANK YOU!!!!!!!

author: fcb
author_rank: Super Contributor
timestamp: January 29, 2014, 07:21:44 pm
content: You could build a bipolar current sink, or you could float a standard layout current sink and connect to it with a bridge-rectifier.

author: DaveFan
author_rank: Contributor
timestamp: January 29, 2014, 07:51:37 pm
content: @ fcb: Regarding the bridge-rectifier, not sure I understanding what you mean. Can you please explain? Thanks!

author: fcb
author_rank: Super Contributor
timestamp: January 30, 2014, 12:27:44 am
content: Take a bridge rectifier (BR).Connect your bipolar power-supply under test to the AC terminals on the BR.Connect your unipolar (floating!) dummy-load between the + and - terminals on the BR.The potential disadvantage of doing this is that you will add approx. 1.2V to the minimum voltage that you can be sunk by your dummy-load. e.g. if your duumy-load is designed to operate down to 2V, then it will now operate down to 3.2VYou'll also have to design your dummy load to float - but this might not be a bad idea anyhow.

author: DaveFan
author_rank: Contributor
timestamp: January 30, 2014, 12:50:10 am
content: Now that's just plain cool! I get it, pretty smart idea indeed. And, since I'm using a differential amplifier to read the voltage across the sense resistor I don't actually need the ground reference for the sense OpAmp. What if I use a DPDT relay to connect the two power supply outputs to the load? Would I be able to go down to the minimum without the diode drops?If I use the relay and leave the load floating will I need to connect the reference ground of the Gate drive OpAmp to the power supply somehow, and how would I go about doing this. I am assuming (I need to draw a schematic to get a bigger picture) with the diodes the ground will take care of itself?Thanks!

author: DaveFan
author_rank: Contributor
timestamp: January 30, 2014, 02:47:33 pm
content: Are there any other methods for doing this? Any input would be greatly appreciated...

author: Rerouter
author_rank: Super Contributor
timestamp: January 31, 2014, 01:36:26 am
content: If you isolated your DAC's comms or used a current dac you could hook it up negative to negative positive to ground and provide it with a supply rail however many volts below ground to test it, (positive voltage regulator would be good here, gnd terminal goes to the negative rail, input goes to ground and output goes to the op amp, this ensures its always powered with X volts above the negative rail,)

author: DaveFan
author_rank: Contributor
timestamp: January 31, 2014, 07:31:37 pm
content: Rerouter, sorry, I don't understand what you mean. Would you please upload a schematic to show me what you mean? Thanks!

author: Rerouter
author_rank: Super Contributor
timestamp: January 31, 2014, 09:54:33 pm
content: I admit using a positive regulator on a negative rail is not immediately clear to all unless like you use that negative rail is your reference point,The current DAC allows you to shift it down because it directly reproduces a voltage across Rset in respect to that negative rail,

author: DaveFan
author_rank: Contributor
timestamp: January 31, 2014, 10:18:46 pm
content: Rerouter, thanks! I think I get what's going on in this circuit, but I will study it over the weekend and simulate it to get the full picture.

author: DaveFan
author_rank: Contributor
timestamp: February 03, 2014, 05:11:54 pm
content: Is there an equivalent circuit to the one Dave designed which works on a negative rail and doesn't require shifting the regulator to the negative rail? The problem is I don't want to have the load supplying power to the regulator and OpAmp since I will have to also know the current they are drawing from the load in order to get the true current draw from the Load.I prefer to not power the OpAmp from the Load, and using a voltage regulator will certainly draw extra current from the Load.Are there some isolation techniques for connecting the driving OpAmp to the FET without connecting it directly to ground?Please, if you have some ideas I would really appreciate if you share them. Just to make my question clear:1. I want to be able to develop a dummy load for negative power supply, just like the one Dave shared with us.2. It doesn't have to work for both positive and negative power supply loads, one of each would be fine and I can simply use one for testing positive loads, and the other for negative.3. If it is possible to do this with the same circuit, but without having to rely on the Load to power a voltage regulator and the OpAmp, can you please share it with me?THANKS!!

author: rizoma
author_rank: Newbie
timestamp: May 20, 2014, 08:42:37 am
content: Hi there, I'm interesting too for split power supply testing, but I dont need dac or any microcontroller, I just need a simultanyous test on both rails, can I built two dummy load circuits, use a br to one on the negative rail? the only drawbacks it's that I add a 1.2 voltage drop doing that. I need to test 5v, +-12v & +-15v, for what I undestand that will work, right?thanks to anyone would help meRegards

author: Rerouter
author_rank: Super Contributor
timestamp: May 20, 2014, 12:32:15 pm
content: There are a few ways you can attack it, the biggest hurdle you have to overcome is that your op amp supply will have to exceed the voltage of whatever your testing, as otherwise you can exceed the input rating on the op amps,Next up is to measure the current differentially, so that it gets presented to the op amp controlling the mosfet as a signal referenced off its own supply,Third up is to feed in a control signal in reference to its supply negative, there are many ways to do this, in your case likely a stereo potentiometer, with each wiper referenced to each supply range, e.g. the positive load would be in reference to ground,get that done and you can easily enough make up a symmetrical loadonce again stressing that your supply rail has to be wider than the supplies your testing for it to not be powered by the load (this is to reply to the last 2 posts at once)

author: rizoma
author_rank: Newbie
timestamp: May 20, 2014, 09:45:30 pm
content: Now I really don't get it, but I will study a bit then I'll post something, I'm really newbie in Electronics but I'm learning fast.Thanks a lot for the advices