250v electrophoresis power supply troubleshooting, very unstable output? - Page 1
Forum section: electronics / repair

author: XOIIO
author_rank: Super Contributor
timestamp: July 20, 2014, 06:40:17 am
content: First video on my new channel strictly for electronics stuff, unfortunately the audio cable I use to record got damaged in the move so there is a feedback sound on and off throughout, when I was going through it usually was somewhat quiet though, but still annoying as hell.There will be another tomorrow with more troubleshooting, testing transformer voltages, etc, so this one is a little behind, I am just impatient to hear what you guys think because I want to get this working. Audio has been fixed in that one.Anyways, this is a Gibco BRL/Life electronics model 250 power supply, various voltages up to 250 volts, and up to 500ma current (I thought it was 250 but was wrong).Anyways, if you don't feel like watching the video (or don't have time right now) I will describe the issue here.Basically, the output voltage fluctuates drastically, I demonstrate this in the first bit of the video so you can see, and the voltage meter on it was off, but that is not a huge issue, I can work on that later. I have traced everything back, tested the transformer out of circuit, it is stable, bridge rectifier diodes are all fine, reseated chips, -5v and 5v rails are fine, switch is functioning properly, yet the voltage at the outputs fluctuates like crazy.22v on the PS reads between 21 and 25 or so, and that escalates, where it should be 250, it is in between 270 and 300v, and the voltage basically sweeps up and down like crazy. It is much easier to see what I mean in the video itself.It seems like this should be a simple fix but I am completely stumped as to what could be causing this, except for the quad comparator if it somehow affects the output voltage. I have a couple ideas as to what it could be in the second video as well I believe, but like I said I am just impatient.

author: XOIIO
author_rank: Super Contributor
timestamp: July 20, 2014, 08:54:47 am
content: There seems to be a considerable amount of AC getting to the output somehow, I think I might need to reverse the schematic to find out why but that seems to be the big issue. I am thinking perhaps a diode across the output might cancel it out though, but there is not one like that on the board, but I don't see why this would put out so much DC, for example if it is set to the max, I get the varying dc, but for ac I get 148v, and with ac+dc true rms I get 326 volts.On the lowest setting I get 20-25v dc fluctuating, with 11.7v ac, and 26v ac+dc truerms, so it seems that this AC component getting through is the issue, I should have thought of that sooner.

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 12:57:52 am
content: Parts 2 and 2.5I was hoping someone would have replied with an idea about this by now but maybe there was not enough info in the first video(not many watched it though it seems)

author: Zucca
author_rank: Unknown
timestamp: July 21, 2014, 01:19:17 am
content: Hi XOIIO,don´t give up, there are people watching your videos. Also you are right, you need a schematic. I tried to google the user manual or the service manual but no hope.Do you have a scope?Z

author: Unknown
author_rank: Unknown
timestamp: July 21, 2014, 01:20:03 am
content: Quote from: XOIIO on July 21, 2014, 12:57:52 amTo be a little blunt: Why sit through a 20 minute video in the hope that it may provide a clue, when the relevant info could all be summarised in a paragraph or two?At this stage, without watching the video, we have no idea of even the basic topology of the supply. Linear, boost, buck-boost, ... ?Quote from: XOIIO on July 20, 2014, 06:40:17 am

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 01:47:09 am
content: That's why there is a video, to provide all that information.@Tac Eht XilefThe voltage meter is off of what it should be reading, not turned off.I tested the diodes to see that they are good, providing the correct voltage dropAll of the chips, and I will work on a schematic later5v is from a regulator, I am not sure where -5v is coming from as of now.The voltage selection switch.The other IC is a 7 segment driver.All it does is switch the different transformer taps to the bridge rectifier, and that leads to the output.Quote from: zucca on July 21, 2014, 01:19:17 amYeah, I can't find a thing on this, I emailed life technologies but still no reply. I am going to trace out the schematic to the best of my abilities today.I do not have a scope, looked around loads to get one locally but no luckAnyways the issue really just seems to be that a lot of AC is getting through to the output though I am not sure how/why when all the components seem fine, I am thinking it has to do a bit at least, with how the shunt is placed. Tracing the schematic for that part should help to figure that out. (though I will need to take off the transformer again, bleh)Like I say in the short video though everything visually is fine, but I can't imagine that it would have been designed to let AC through to the output, it's strange. Maybe it was made to let AC through for some reason, if so then I want to modify it so it's just DC

author: Smokey
author_rank: Super Contributor
timestamp: July 21, 2014, 02:34:57 am
content: I'm a terrible person, and I didn't read all the previous posts, but I have an electrophoresis supply I scored off ebay. Here is what I found out about that supply.... it needs some sort of load to regulate. If there is nothing hooked up to the output it's WAY off. A multimeter is essentially nothing.If that turns out to be irrelevant or already been said or something like that.. sorry.

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 02:42:01 am
content: Quote from: Smokey on July 21, 2014, 02:34:57 amAlright I'll give that a try as well, I really need to make a programmable constant current load, I'll just toss a resistor on it and hopefully it won't draw too much.

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 03:20:25 am
content: Gave it a try with a small load (20ma or so, based on the built in current meter, if that's accurate at all) and it didn't change at all, so that's not it.main switch is acting up too, needs to be pushed down sometimes, so I'll have to get one of those at some point but that's not a big issue.

author: Unknown
author_rank: Unknown
timestamp: July 21, 2014, 05:05:40 am
content: Quote from: XOIIO on July 21, 2014, 01:47:09 am20 secs to skim a post to see if it contains any useful fault-finding info, vs 20 minutes sitting through a video to find out if the info is there. Assuming you actually want help fixing it, guess which wins?(FWIW, the guy sitting behind my keyboard (a) has fixed a few electrophoresis supplies (though not that particular one) and (b) usually only watches videos that are <2 mins, contain cats doing funny things, or involve people hurting themselves with fireworks...)Just to show that I'm not a complete grumpy bastard:Quote from: XOIIO on July 21, 2014, 01:47:09 amRipple isn'tsuper-critical in these things, though too much leads to poor definition in the results. Typically it should be <2%; ideally under 1%.Good luck.

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 08:42:40 am
content: Do the ones you have worked on get a lot of AC on the output? The ripple on these is more like 30%Anyways, first attempt, it's brutal, but I am pretty confident it covers all of the output stuff, and the rest of the circuit is just comparison and displaying the right value on the 7 segments, as well as adjusting the readout. It gets fairly complicated there, it wouldn't fit on this piece of paper anways, I'd have to use eagle and it would take a while. I don't know if it is needed or not.Also, not sure what the proper symbol for voltage regulators is so I just put them on face up, pin 1 on the left. Found the -5v regulator, I figured that was just a transistor but I was wrong.also, phew. Thought I had the diodes backwards for a second there but I got it right :p And ignore the black/brown notes, that just to remind me how the transformer wiring was.

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 12:47:29 pm
content: Hmm, I don't know, it just seems like either the output is meant to be measured in AC+DC, which I don't see why it would be, since this is for gel electrophoresis and that uses DC,or it is just not being rectified properly for some reason, but I can't tell why or how. All the components check out fine, and I'm no expert in circuit design but it seems fine, bridge rectification is set up properly, the only thing I can think of is the shunt somehow affecting it, but that doesn't seem right, and I tried adding a diode, in both polarities for the heck of it and that did not work.The diodes show .5v drop with a lead lifted, but is it possible for them to fail some other way that would have this effect? I don't think I have any new ones, I might be able to salvage some, I also have a small bridge rectifier (a teeny cute one styled like the ones you see on headsinks) that I could wire in the replace the diodes but I don't see why I would need to do that.I mean, this doesn't have a cap on the output to remove any ripple, but I doubt that could handle or improve ripple of over 40 volts.Hrmmmmmm

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 03:15:56 pm
content: Any suggestions on a bleeder resistor value? (and I will go beyond 1/4 watt, probably half or 1 watt)

author: Zucca
author_rank: Unknown
timestamp: July 21, 2014, 05:48:51 pm
content: I looked a little bit on the www,I slowly think there is a reason why they call themelectrophoresispower supply and not just power supply.Check also this out:http://www.instructables.com/id/Gel-electrophoresis-power-supply/I think those devices are designed to give a roughly voltage and current. It seems to be nobody is caring about any output ripple.Please see attachment for a comparison I found in the internet between different electrophoresis power supply models, again there is no mention or specification about the ripple.I am suspecting you are trying to fix something which is designed to work like this.Since it looks like you need a constant voltage, a good exercise would be stabilize the output:- CR filter- DC/DC converter- Linear power transistor as emitter follower (even better I think)It could be you are investigating a cheap way to get a high voltage power supply with a decent small ripple...Take care,

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 07:08:26 pm
content: Quote from: zucca on July 21, 2014, 05:48:51 pmYeah, pretty much done with it for now, I'll have to make/upload the last video or two (may just combine them), but I've basically got it down to a 5v ripple at high levels, .5v at low levels using that small value orange capacitor (I think that's in the previous video, to tired to recall lol).The voltage readout on the meter is not amazingly accurate, it's a bit over 10v off at the highest end but that's not a huge deal.I basically just have that across the output, nothing else. I was tempted to maybe add another rectifier in, but figured it's not worth the effort. The ripple is decently small, and I can get up to ~430 volts, with 500ma, it makes 1 ohm resistors (which I have a load of from ebay LED's) really nice firecrackers. I'll have to post a list of the voltage steps later but it's not bad to see what sort of a voltage they can handle.Still need to calibrate the current, but that will be easy.Considering it was $26 with $28 shipping, that's not bad for a high voltage supply like this I'd guess, and with that 500ma current it may be a good way to charge decently large capacitors for small coil guns and the like.

author: XOIIO
author_rank: Super Contributor
timestamp: July 21, 2014, 07:09:45 pm
content: would still be nice if there was a manual out there or something, not sure why there isn't, but oh welledit: current readout seems to be totally useless. Spent 10 minutes trying to calibrate it, it just gets up to a point where the display changes randomly, turns on and off sometimes, and reads a random value, and it affected the voltage readout as well. Turning the pot for the current (at least I assume it's current, they are marked V and I) just changes the voltage readout from what I can tell :/ Oh well.