Power Supply kit - current limit function upgrade - Page 1
Forum section: electronics / beginners

author: tlovie
author_rank: Contributor
timestamp: May 28, 2021, 05:06:03 pm
content: I've looked around here on eevblog and this power supply kit with the red PCB seems to have been discussed at length.https://www.eevblog.com/forum/beginners/question-on-power-supply-diodes/I recently bought and assembled this kit, and its working fine, but the current limit function is a little inconvenient to use, in that you cannot enter a desired setpoint, ahead of time. You'd need to setup a dummy load, and adjust the limit current before applying it to your real circuit. I'm mostly a complete beginner, but looking to learn - so my question is if it would be possible to upgrade this power supply to be able to output the current limit set point? So you could see the set point before you hooked up your load. Would that involve characterizing the voltage output of the current limit pot, and converting this to an output current or would there be a better way?Atttached an image of the power supply unit in question.

author: ledtester
author_rank: Super Contributor
timestamp: May 28, 2021, 06:27:46 pm
content: QuoteGenerally you build the kit with a volt- and ammeter. To set the current limit you just short the output and read the current from the ammeter.I usually turn the voltage down first to avoid sparks when I short the output terminals.

author: floobydust
author_rank: Super Contributor
timestamp: May 28, 2021, 06:56:44 pm
content: Yes you could look at the output voltage of the current-limit potentiometer. With the kit's 0.47R 5W+/- 5% sense-resistor it's scale is about 0.47V per amp.You could add an op-amp to multiply the voltage there by 2.13 to get 1V per amp which can go to a digital panel meter and display the current-limit setting.Another method is to label the dial so you have a rough idea of your setpoint. Or add switches to limit the potetiometer to say 0.1A or 1A full-scale

author: tlovie
author_rank: Contributor
timestamp: May 28, 2021, 08:37:32 pm
content: Thank-you both floobydust and ledtester for your replies. I didn't initially realize that it would be ok to short the terminals, even though lab power supplies are generally allowed to do such things. I guess I wasn't sure that this was actually a proper lab power supply - mine didn't come with a manualI was looking for some useful project to work on, but it seems that its not all that necessary in the long run. I might still try to do it.

author: tlovie
author_rank: Contributor
timestamp: May 29, 2021, 02:26:01 am
content: I'm going to ask another question here - similar topic, hopefully someone will help me understand. The LED for this power supply stays on all the time, but I think its supposed to be related to the current limit function. Someone on a different thread had posted a link to the manual, which had a schematic for this power supply,R19 is 2.2K, R20 is 10K. I've looked at pin6 of the opamp U3 - when the power supply goes into current limit mode - this pin is pulled to less than 0V (-20mV), but when it is not in current limit mode, this pin is high (around 35V). From what I understand, R19 and R20 would form a voltage divider for the Q3 base, and what I measure is 0V at the opamp pin 6, 35.6V at the Q3 base, and 36.4V at the +ve rail in current limit mode. When I'm in voltage mode, I measure 26V at pin 6, 35.8V at Q3 base, and 36.5 at the +ve rail.I guess I don't understand what the LED and the Q3 transistor are supposed to be doing. If they wanted the LED to be on all the time, I would have thought that there would be an easier way to accomplish that.What should I try next to diagnose what is going on?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 29, 2021, 02:46:11 am
content: The LED is an indication that the power supply is in constant current mode.If pin6 of U3 going to 35V is not enough to close the transistor and turn the LED off, you may have made a mistake while soldering the circuit, or your transistor may be defective.If you have over 36+V at the power rails, you're really stressing the opamps. Note that the negative supply of the opamps is connected to the charge pump which goes to -6V or so, so the total supply voltage for the opamps is over 40V. This has been discussed several times before in other threads.If you build a box for this thing, then adding a simple graded scale for the current limit