Electrolytic Capacitor ESR - Page 1
Forum section: electronics / beginners

author: Gassan
author_rank: Contributor
timestamp: June 06, 2020, 02:31:04 pm
content: I'm trying to repair a camera flash unit. Inside the device, I found a bunch of huge electrolytic capacitors fixed on a board that's other than the main power supply board. There was a little bit of a swell on top of one cap. It's 800uf, 330wv (I have no idea what wv stands for. I presume it's voltage rating in a different fashion). I removed that cap and tested its capacitance. It was perfectly around 800uf range. But from my electronics lessons, I know that bulged caps need another test called ESR. Some even say there's no point in testing a swollen cap. But to my own reassurance, I went went ahead with the test. My ESR meter is called MK-328 TR\LCR\ESR Tester. This capacitor showed an ESR of 0.27. Is my capacitor good or bad?

author: kripton2035
author_rank: Super Contributor
timestamp: June 06, 2020, 03:14:09 pm
content: 0.27Ω is ok for a 800µF cap.but if they are swelling or bulking you've better change them !

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 06, 2020, 03:14:55 pm
content: You can find a datasheet for that cap, or very similar, and make a decision.

author: radiolistener
author_rank: Super Contributor
timestamp: June 06, 2020, 03:24:15 pm
content: it looks too high for 300V 800uF, but I think it may be due to loss in your measurement wires.

author: helius
author_rank: Super Contributor
timestamp: June 06, 2020, 03:25:33 pm
content: I can say that your phone camera is definitely bad as a blurry 12 megapixel file is in no way necessary to show what is on your ESR meter.Flash units require special capacitors (they are sold as Photo Flash capacitors) with unique characteristics: They need to be very compact and tolerate rapid discharge into very low impedance for the flash tube. Nippon Chemi-Con notes:QuoteW.V. is Working Voltage, the normal DC voltage to which the capacitor should be charged. There is also a Surge Voltage which it must withstand without exploding for occasional spikes etc, usually twice the WV. The ESR and loss tangent (Vloss) appear normal, but you can tell that this tester uses a low test frequency. The formula\$ \mathrm {ESR} = \tan \delta \cdot {1 \over 2 \pi f C} \$tells us thatfmust be 27 Hz! ESR depends on the frequency, and a frequency of 120 Hz or higher would be more useful. Additionally, this type of tester is not able to report leakage, which is important for capacitors used to store energy in this way. The cap may have high leakage and bleed the charge from the others, which would explain its bulging from overheating.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2020, 03:44:31 pm
content: A picture of the cap itself would be helpful. Sometimes there is an insulator disc on top that can bow up and look like swelling when it isn't. In addition, if your flash unit is not working at all, I seriously doubt the cap as you have measured it would be the cause.

author: Gassan
author_rank: Contributor
timestamp: June 06, 2020, 06:29:02 pm
content: Thank you for the thorough explanation. I've done more testing and my ESR meter wasn't consistent. It's a very cheap instrument I bought for hobby works. I guess it's time for me to go for a better one.

author: Gassan
author_rank: Contributor
timestamp: June 06, 2020, 06:37:55 pm
content: I don't know if the shows the problem. I placed two caps side by side. The one on the left is not bulged. The one on the right is raised little bit on the top. I hope you can see the difference.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2020, 06:42:13 pm
content: That bowing is of a plastic insulator disc, not the capacitor can. It may very well be completely OK and I strongly suspect it is not your problem. You could try measuring the other cap just to compare. I wouldn't worry about your cheap ESR meter, measuring caps is tricky even with good instruments.b/t/w, what is the problem with the flash?

author: Gassan
author_rank: Contributor
timestamp: June 06, 2020, 07:31:39 pm
content: To be honest, I don't know what the problem is. It belongs to a friend of mine who operates a small photo studio. He said the device died with a strong odor of burning. When I read the label on the outside of it, I came to know this device was designed to be plugged into 100 - 130 volts AC supply. Our country has 230 volts. What surprised me is that they used it for more than two years without blowing the fuse. Bad design, maybe, but robust enough to tolerate extremely high voltage.There was no visual sign of burning in the inside. I checked the input area thoroughly and was able to confirm that all diodes were good. The power supply didn't use a transformer. It was a capacitor + resistor type power supply with which I'm not familiar at all. I removed the brown capacitor and checked its value. It was perfect. There was a white color 5 watt resistor right at the AC input area. It resistance was within the correct range. The fuse also blow.I don't think even my friend can tell exactly what happened as he has not used it for a long time.This device is very unlikely to be completely dead. The problem is, I can't test it live right now as I don't have 110 volts AC. I might be able to test it this week. I should come back here after live testing. Thank you! Your help as really helped.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2020, 09:59:18 pm
content: Double voltage is bad! Send some clearer pictures of both sides of the circuit board so we can determine how it is set up. It is still possible that the cap has internal leakage at higher voltages, but that would take a different test. Are there two power diodes or four? It also might be possible to redesign the unit for 230 volts.

author: helius
author_rank: Super Contributor
timestamp: June 07, 2020, 07:33:20 pm
content: A capacitor dropper has higher reactance at lower mains frequency, partially compensating for the difference between 120V 60Hz and 230V 50Hz, although, that gets you 1.6 times the DC instead of merely 1.9 times...Depending on the exact circuit it may shut off charging at the same DC level and just work with a faster cycle time, but some components can be overstressed if not designed for that. I would check any zener diode, thyristor, and the bridge rectifier diodes.

author: Gassan
author_rank: Contributor
timestamp: June 07, 2020, 07:39:02 pm
content: I'm gonna come back with some nice pictures. It still do suspect the capacitor as I'm not able to press down its bulge with my thumb. It might not be only insulation issue.

author: Cubdriver
author_rank: Unknown
timestamp: June 08, 2020, 10:14:29 pm
content: Quote from: Gassan on June 07, 2020, 07:39:02 pmIf it's solid and won't compress at all when you push on it with your thumb, then I'd be more inclined to suspect that the cap is in fact swollen. If so, I'd plan to replace it, as it should be flat. Seeing more detailed photos of the board will help to clarify things and get you more/better suggestions.-Pat

author: Gassan
author_rank: Contributor
timestamp: June 14, 2020, 07:27:14 am
content: Quote from: helius on June 07, 2020, 07:33:20 pmDid you mean that the DC output will be less when the AC input voltage is high? For example, in my case, with 230V 50Hz input I get 1.6 times DC instead of 1.9 times DC which I'll get from 120V 60Hz input?

author: Gassan
author_rank: Contributor
timestamp: June 14, 2020, 10:43:46 am
content: Here is a photo of the power supply board. I checked all suspective components. Diodes and capacitors look good. The board seems to produce higher DC output voltage than expected. It produces 400 VDC while the photo flash capacitors are rated at 330 volts. I actually blew an 800uf capacitor.

author: Gassan
author_rank: Contributor
timestamp: June 14, 2020, 10:54:15 am
content: This is the capacitor board. You can see that the top of one capacitor is gone. It is the one that exploded during the live test.

author: Gassan
author_rank: Contributor
timestamp: June 14, 2020, 11:17:41 am
content: This photo shows the rear of the device where all the switches and controls are located. The red swtich is the main power switch. Underneath it is the fuse holder. The knob at the center shows a range of voltage that can be adjusted. The range is AC100 - AC130. This is why I assume this device is meant to be used with about 110 volts. Below the knob is the mains input port. On the right there's a black switch which I don't know about its purpose. Below that switch you can see a port for 3mm jack that says SYNC. This one connects this device with the camera. On the top left there's an empty hole that says TEST. This button is attached to the PCB. The two empty holes on top right is for LED indicators that are attached to the PCB.0

author: Cubdriver
author_rank: Unknown
timestamp: June 14, 2020, 04:36:38 pm
content: Quote from: Gassan on June 06, 2020, 07:31:39 pmThe problem appears to be that unless it was being run on a step-down transformer, your friend was operating a flash that was clearly marked for and built to operate at 110-130 volts at nearly double that. I don't think you can say that a device has 'bad design' if it manages to operate for years while being supplied with nearly double the AC mains voltage it was meant to operate on. I'd say it was a damned good and tolerant design!Can you reverse engineer and draw a schematic of the power board with the blown cap? That would help us to understand how it's designed and what might have blown out. If it's a capacitive dropper as it seems to be from your description, it *might* be possible to reconfigure it to operate at the higher line voltage in your country.The slot on the fuse holder in your photo of the control panel appears to be a bit chewed up - was the fuse frequently blowing when they were using it? And the switch on the right labeled 'model' is probably to turn on an incandescent or LED modeling lamp in the flash head. This permits the photographer to see what the lighting will look like by projecting a steady (but obviously much dimmer) version of the flash pattern so the flash can be at least roughly aimed and adjusted without needing to take a bunch of photos to do so.-Pat

author: Gassan
author_rank: Contributor
timestamp: June 14, 2020, 05:42:02 pm
content: I don't think you can say that a device has 'bad design' if it manages to operate for years while being supplied with nearly double the AC mains voltage it was meant to operate on. I'd say it was a damned good and tolerant design!Can you reverse engineer and draw a schematic of the power board with the blown cap?The slot on the fuse holder in your photo of the control panel appears to be a bit chewed up - was the fuse frequently blowing when they were using it?-PatYes. I shoudn't have said the design was bad. I understand your point clearly now. The design was actually damned good. But I thought the fuse should blow immediately when its plugged into double voltage. Anyway, it's too technical for me to discuss.Although I have never attempted before, reverse engineering is a possibility. I have no idea how to approach it. Should I do it with pen and paper or a free software? It'll certainly take me a lot of time to do so, but I'll come back with my best effort.I'm not sure about what happened to the fuse holder top. It's clearly disfigured. I asked the owner if the fuse used to blow when he used it, to which he said no. He took this to a few other techs before it came to me. All failed attempts. The fuse was holder was already like that when I saw it. But I also noticed that the plastic was unusually soft and delicate. I myself managed only to make it look worse.By the way, I'm not doing this for money. I love electronics repair and I'm just hungry to learn more. And the owner of the flash head is a childhood friend of mine.

author: bdunham7
author_rank: Super Contributor
timestamp: June 14, 2020, 05:52:45 pm
content: We need to see the underside of the capacitor board. If the caps are in series, it may be using a voltage doubler which could be reconfigured to simply rectify since your voltage is already double. Also, looking at the other board, I have no idea how this thing operates. You typically need a HV pulse transformer and I don't see any inductors anywhere. Are these two parts all there is?

author: james_s
author_rank: Super Contributor
timestamp: June 14, 2020, 11:56:49 pm
content: The fuse is there to prevent a fault condition from starting a fire, it is not there to protect the unit from excessive line voltage. I would not expect the fuse to blow until damage has already occurred to the device resulting in excessive current draw.

author: Cubdriver
author_rank: Unknown
timestamp: June 15, 2020, 01:15:19 am
content: Quote from: Gassan on June 14, 2020, 05:42:02 pmReverse engineering the cap board should be pretty easy, as it appears to be a single sided board with a relatively simple circuit on it. Probably the easiest way to do it would be to scan the board's solder side, then print out a relatively low density copy of the scan (so what you draw on it will show) and sketch the component symbols in the places they go on the board. Alternately, you can draw the traces on a piece of paper (a bit more confusing because you must either mirror them to get a top through view of the board, or translate the components from top to bottom) and add the symbols. Another alternative is to sketch the components, then draw in the traces to show the interconnections. You can then translate the interconnections into a normal schematic.Here are some pictures of a board from an old DVM that I've been in the process of reverse engineering, using the last method I mentioned above:Board:Component location sketch (I initially missed one of the resistors when making the sketch, but noticed once I started tracing things):Partially drawn layout (note that is is mirrored right-to-left from the actual bottom view of the board - this can make keeping things straight a challenge):Solder side of board from which the layout above was derived:-Pat

author: Gassan
author_rank: Contributor
timestamp: June 15, 2020, 02:28:21 am
content: I have forgotton to tell part of the story. The capacitor exploded while the device was operated with 110V. I used a stepdown tranformer for testing. It took around 30 secs for the cap to blast. Before that, I was able to measure the voltage across the cap. It was 400 VDC. The caps were rated at 330 working voltage. This indicates a problem with the power board. The power board was producing too high voltage.Anyway, I'll come back with the diagram.

author: AG6QR
author_rank: Frequent Contributor
timestamp: June 15, 2020, 05:43:07 am
content: That looks like a small studio flash unit, sometimes called a monolight. These are typically used in a photo studio. They are fairly simple, with a manual control for the flash output intensity, which works by controlling the voltage used to charge the main capacitor bank. That's the only significant control. When the flash is triggered, all the capacitor charge is sent to the xenon flash tube. This is much less sophisticated than modern battery-powered camera-mounted flash units, which normally charge the capacitor to a constant voltage, and adjust the output power by using thyristors to interrupt the flash output in the middle of the flash as soon as sufficient light has been produced. Those small camera-mounted units have a lot of automatic smarts in them, but the manual studio units produce much more light output per dollar, and often come with "modeling lights", which continuous lamps that allow the photographer to preview the effect of the flash. In a studio, the lights are typically set up on stands, and adjusted manually to produce the desired amount of light. They're rated in watt-seconds or joules (those units are equivalent), with ratings of around 200J to 1000J being common. The capacitors are normally charged up to a few hundred volts when producing maximum light output.I'd expect to see some sort of voltage multiplier circuit capable of rapidly charging the main capacitor bank up to a few hundred volts, with the charging voltage controlled by that pot on the control panel. There should also be some triggering circuitry, which produces a brief pulse of a significantly higher voltage -- the high voltage is used to ionize the xenon gas and get the arc going.Be very careful around those capacitors! At several hundred volts and hundreds of Joules, they can store lethal amounts of energy. Even when "discharged", they can bite painfully (ask me how I know).