Bench PSU Fault. - Page 1
Forum section: electronics / beginners

author: orbiter
author_rank: Frequent Contributor
timestamp: August 23, 2010, 08:14:33 pm
content: OK my friends, here's one for ya..I have a dual 30v bench PSU which has CV & CC mode options. The PSU is in one box but there are two transformers inside with separate displays and controls for each side.The problem I have is that on one side of the PSU, the CC mode doesn't workWhat happens is.. I can select CC mode just fine.. But once I try to set the amount of current using one of the 'up' buttons the display shows the current increasing, but once I release the button the current drops back to zero and the display goes back to CV mode.Now considering that I am a noob, Please don't laugh when I say.. Could It possibe that there could be a defective diode on the control side allowing the current to leak back through it resulting in me getting zero's in the display once it's dropped to nothing? The output is dead once this happens so it's not a dodgy display reading.The other side of the PSU works fine, I just set CC mode and it stays set.RegardsJohn

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 23, 2010, 08:56:10 pm
content: Just tell us the brand and the model , so the other buyers to avoid it ..If you have any warranty left , use it ..No one can troubleshoot the problem with out schematic , and the informations that you offer falls sort.Not all bench PSU's are build alike , so how is possible , some one to offer an useful tip !!

author: Simon
author_rank: Unknown
timestamp: August 23, 2010, 09:09:36 pm
content: sounds like a fault in the control circuit to me

author: veryevil
author_rank: Unknown
timestamp: August 23, 2010, 10:05:24 pm
content: Would the power supply happen to be the circuit specialist CSI3003X3 (http://www.circuitspecialists.com/prod.itml/icOid/3506) or the 5 Amp version.If so the schematics are on that page so people on here can help.

author: orbiter
author_rank: Frequent Contributor
timestamp: August 23, 2010, 10:47:02 pm
content: Thanks Simon & veryevil..Yes I would sort of assume the control board too (only seeing it from a beginners eyes though.)Yes it is a Circuit specialists model CSI3005XIII. The schematic is below if you guys would like to take a look... I don't expect definative answers here my friends... Just some opinions or ideas would be welcome and helpful. It all goes towards my gradual learning curvehttp://www.circuitspecialists.com/pdf/3005X-III_schematic.pdfThanks fellasJohn

author: sigxcpu
author_rank: Regular Contributor
timestamp: August 24, 2010, 02:20:33 pm
content: do you have a load connected when setting CC mode? i mean, while pressing the buttons, you set up the desired current. when finished setting up, it displays current value, which, without a load, is 0.

author: orbiter
author_rank: Frequent Contributor
timestamp: August 24, 2010, 02:41:06 pm
content: Quote from: sigxcpu on August 24, 2010, 02:20:33 pmYes that's the way CC mode works on this PSU. I just put a resistor across the terminals and set whatever CC value I require. As I mentioned though the other side of the PSU works fine when I do this, it's just the right hand side that looses the CC setting.. It's just leaks away to zero even with the load connected.

author: armandas
author_rank: Frequent Contributor
timestamp: August 24, 2010, 04:57:43 pm
content: Why don't you contact the seller? By trying to fix it yourself, you'll loose any warranty you might have. Leave DIY repair as the last option.

author: orbiter
author_rank: Frequent Contributor
timestamp: August 24, 2010, 05:14:45 pm
content: Quote from: armandas on August 24, 2010, 04:57:43 pmIt doesn't really bother me that much to be honest as both the CV modes work just fine, and if I did require a CC mode I've got one that works. I was just interested in what you guys thought.

author: Unknown
author_rank: Unknown
timestamp: August 24, 2010, 08:08:59 pm
content: It's not clear to me what the symptoms are. Is it just the display reading zero, or is there actually no current flowing (check with DMM in series with the resistor)? Is the resistance low enough that the required amount of current can flow without exceeding the voltage you set? Does current limiting work in CV mode?I'm surprised that you can select CC mode. Normally, you set both a current and a voltage limit. The power supply will limit both, and if one of them exceeds the limit, it will regulate that. If it limits current (eg. with terminals shorted), it will be in CC mode. If it limits voltage (terminals open), it's in CV mode. Not something you switch between yourself. Does CV mode mean that the current is limited to the max, and CC mode that the voltage is limited to the max? That doesn't seem very useful.Looking at the schematic (top part control board, but the other one appears the same), it appears to me that the regulation is performed by IC12/13. D7 and D10 form an OR (assuming the anode of D10 is connected to the base of Q5), both can sink current from Q5, which decreases the base current through Q1-4.I would probably check the input of IC12. If my guess is correct, I would expect to see the voltage around R15 change when you change the current limit, and the voltage of IS increase when any current is flowing (regardless of CV/CC mode). This problem could be caused both by the regulation around IC12 and the control, eg. IC10/11.IC12 appears to responsible for current limiting, and is set to max. current when in CV mode. In CC mode, the current limit is limited by digipot IC10. IC13 appears to be for voltage limiting, although I'm not sure where CN7-3 comes from.The CV light appears to be controlled by IC6D(?), which shows which of the two op-amps is actually sinking current, so it's not controlled by the buttons, but by the actual regulation.

author: orbiter
author_rank: Frequent Contributor
timestamp: August 24, 2010, 11:04:07 pm
content: Hi alm,I can switch between CV & CC mode. For CC mode I just have to select it, then short the + - terminals and turn up the control knob which increases the current.I've uploaded a video of what's happening instead as I thought It would be easier than trying to explain it..

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 25, 2010, 02:43:18 am
content: I have the same power supply (it works fine). But it will exhibit the same behavior as yours if I keep the "down" button depressed while raising the current with the "up" button. As soon as I stop clicking "up", the current decrements back down to zero. So the first thing I would look at is to see if the "down" switch is stuck closed. Check if CN8 pin 4 is always 5V. There is a clock circuit (IC8, IC9) that generates pulses when the buttons are held down, so this would constantly be decrementing the digital pot (IC10).

author: orbiter
author_rank: Frequent Contributor
timestamp: August 25, 2010, 07:26:49 am
content: OK thanks MH, I'll check that out. Although the buttons do feel as though they're working properly with each giving a proper 'click' when pressed. However that doesn't mean that there's not a faulty contact or something I guess, or a fault with the switch.Thanks again mateJohn

author: veryevil
author_rank: Unknown
timestamp: August 25, 2010, 09:52:23 pm
content: I have the same supply and my constant current doesn't work at all. It stays in CV mode and the up and down buttons don't do anything. The CV light comes on but that's about it.Sometimes If I have a high voltage the on something that is drawing a bit of current say 10v and ~ 300ma the I can press constant current mode and if I hold down the current down button for ages eventually it will decrease the current but it doesn't work anything like your working channel.

author: orbiter
author_rank: Frequent Contributor
timestamp: August 25, 2010, 11:39:58 pm
content: Quote from: veryevil on August 25, 2010, 09:52:23 pmI assume you are shorting out the + & - whilst trying to set your CC mode? If so I guess we may have found a PSU supplier who is selling stuff that is not too great.As far as the PSU goes for CV mode it works fine on both sides, and I suppose I knew when I bought it that it was a low end model. It's a pain in the butt though when they stick features on things that don't work properly.John

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 26, 2010, 03:56:29 am
content: For the record, mine seems to work pretty well. I've used it quite a lot and have been satisfied with it. I even use the CC/CV mode to charge NiCad batteries, if I'm around to monitor. The only complaint I have is that the multi-turn pots are not smooth and cause the voltage to jump around while adjusting.One thing for owners of this power supply to note: The digital potentiometer that adjusts the current regulation, has 100 steps. That is the reason it increments by about 50mA per click. (5A limit / 100 = 0.050 A) It also has non-volatile memory, so the position of the virtual "pot" is kept through power off/on cycles. This can make you think it's not working when you engage the CC/CV mode. It can take quite a few clicks to get down to where the current limit will engage. I try to remember to return mine to the bottom end of the range after use, so that next time I can start adjusting with 0 volts and 0 amps, and go up from there. IMHO, a real pot and a real knob would have made a better user interface...

author: veryevil
author_rank: Unknown
timestamp: August 26, 2010, 09:12:54 pm
content: Quote from: orbiter on August 25, 2010, 11:39:58 pmIn my test I was using a 15 Ohm power resistor. I'm going to take mine apart tomorrow when I get home from work. See what I can find.

author: orbiter
author_rank: Frequent Contributor
timestamp: August 26, 2010, 10:16:29 pm
content: QuoteOK.. Keep us up to date with your findings please mate

author: veryevil
author_rank: Unknown
timestamp: August 27, 2010, 09:22:45 pm
content: Hey, I've had a look at my supply and I'm a bit stumped. I've traced the constant current buttons through the logic on the control and it all seems fine.I am however very confused about what IC5A is doing as it seems to just provide the Chip Select for the Digital Potentiometer (IC10). Since it doesn't take any variable inputs it just sits there at about -5v which obviously is zero on the other side of the diode.The trimmer pot RP3 which sets the Vhigh pin on the Digital Pot has been calibrated to give ~300mv which I assume is right for my 3A version. given that the pot had 100 steps the output then ranges from 0 to 300mv in ~3mv steps.I'm not 100% sure how the output stage of the control board works either (analogue isn't me thing). The ADJ out obviously sets the voltage out and must ether be the set voltage via the pot in CV mode or presumably use some sort of current feedback based around the Digi Pot setting and the current reading? (complete guess)Any one have any ideas what to check?

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 27, 2010, 10:06:39 pm
content: According to the data sheet, the digital pot stores its current wiper value in non-volatile memory when CS goes high, so the IC5A arrangement must pull it up at the last second when the power goes off, based on the way that the +8V and +5V supplies come down. That's a guess.Try checking that the wiper voltage is indeed going up and down with the control buttons, and getting through the 4066 over to IC12. Run it all the way down to 0. Even without a load, the power supply output should go down near 0V at that point. In CC/CV mode, of course.

author: veryevil
author_rank: Unknown
timestamp: August 27, 2010, 10:27:07 pm
content: Hey, nice call on the IC5 circuit just checked it your right. On shutdown it goes high to save the position, good call.Checked and the 0 - 300mv signal gets though to IC12 ok.Will play some more when I've got more time.Any more thoughts?

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 27, 2010, 11:40:00 pm
content: We're getting over to where the circuit is hard to follow with all the on- and off-board connectors and such. And my PDF reader only zooms to 400%.Other thoughts: The current sense resistor R2 is 0.1 ohms, so +IS should be 100mV per amp of output current. That makes sense. However I'm having trouble with where the ground reference for the "control board" is tied. Looking at CN4 pins 2 and 3, it seems backwards, making +IS negative. Also, I can't find a source of base current for Q5.Is this schematic accurate or are my eyes just tired?Edit: Never mind about Q5, I re-read what alm said. R68 is really connected to Q5's base, it's just that the dot is missing on the schematic.

author: veryevil
author_rank: Unknown
timestamp: August 28, 2010, 09:07:12 am
content: Quote from: alm on August 24, 2010, 08:08:59 pmCN7-3 goes to the pin 2 on the main voltage pot and CN7-2 goes to pin 1 as listed on the pot connector.The CV light is latched by IC9A and Q6.

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 28, 2010, 11:56:06 am
content: The CV light alm was referring to is the "CV" indicator on the LCD panel meter, not the LED that comes on in CC/CV mode. Alm is correct that those CV and CC indicators are controlled by the actual regulation, ie. it tells you which parameter is currrently being limited, since in CC/CV mode, both limits are in effect, but only one will be active.

author: lwi004
author_rank: Contributor
timestamp: November 10, 2016, 07:46:28 pm
content: I know this topic is 5 years old, but just had EXACTLY the same issue.The problem was the 100nF debounce cap next to the down button. I had a dual output and both CC exhibited the same fault. Changed Both debounce caps and all working normally.Just thought i'd leave the comment here if anyone else has the same issue.

author: orbiter
author_rank: Frequent Contributor
timestamp: November 10, 2016, 08:10:34 pm
content: Thanks for that. I've still got that PSU in the garage somewhere with the original fault. I'll dig it out sometime and see if it's the same thing.Thanks again

author: ProBang2
author_rank: Frequent Contributor
timestamp: November 11, 2016, 08:36:50 am
content: Quote from: lwi004 on November 10, 2016, 07:46:28 pm

author: macboy
author_rank: Super Contributor
timestamp: November 14, 2016, 03:52:10 pm
content: Have you considered that the problem might be that you turned the output voltage down to zero prior to connecting the load and setting the current? To operate any supply in constant current mode, the output set voltage must be higher than the expected voltage across the load for a given current. Zero isn't.Consider that a bench supply has two essentially separate control loops which will limit the output: a voltage control loop and a current control loop. The voltage control loop allows the outputcurrentto increase until the setvoltageis reached, then it restricts the current. The current control loop allows current to increase until the set current is reached. Both operate simultaneously, and the first one to reach its limit is the one which regulates the output and thereby determines the mode of operation of the supply at that given instant. For CV operation, the current limit must be set higher than the load will draw at the set voltage. For CC operation, the set voltage must be higher than the voltage drop across the load at the desired current.