High(er) current capacitor - Page 1
Forum section: electronics / beginners

author: tchicago
author_rank: Regular Contributor
timestamp: March 17, 2023, 04:09:41 am
content: Hi, All,I'm repairing a device which has an electrolytic capacitor that handles very frequent charges-discharges, with large voltage swings. It is a "saw"- shaped voltage which swings between 0 and 30 volts, with frequency up to 100Hz. The original capacitor was 10uF 60V, and it did not survive this kind of torture and disintegrated. I replaced it with some random 10uF 100V Nichicon capacitor and it works for now. But I'm not expecting it to last much.How do I search for a proper replacement? Clearly this should be a capacitor that can handle deep high current frequent recharges. Is there a special keyword or parameter on Digikey to look for?For reference, this is a capacitor in the amplifier circuit that forms a tuning voltage for the YIG oscillator in the HP 8566B spectrum analyzer. The tuning voltage makes fast periodic linear swings as the SA sweeps through the displayed frequency range.The original HP's spec for that capacitor did not say anything special: CAP-FXD 10uF 60V WT-SLG-TA-ELCTLT. And the this is a common problem with this capacitor in this device.The schematic diagram is in the attachment, capacitor is C7 (A20C7)

author: Psi
author_rank: Super Contributor
timestamp: March 17, 2023, 05:04:57 am
content: Quote from: tchicago on March 17, 2023, 04:09:41 amSo 10uf @ 100hz is 160 Ohms reactance.So 188mA when at 30V.I guess it depends on the ESR of the cap so you can determine how much heat it has to dissipate.Could be like 2 ohms for a 10uF 100V, so 2 ohms at 188mA = 0.376V and 70mW.Unless I have stuffed up the math. Capacitor heating is not something I calculate often.Are you sure there is not something else going on. Like maybe the saw voltage source is slightly charging the cap up backwards?If it's a common problem maybe it's just the capacitor aging and going into thermal runaway as ESR goes up heat does too and electrolyte dries out faster which raise ESR more. At 10ohm it's going to be up to 350mW peak.

author: srb1954
author_rank: Super Contributor
timestamp: March 17, 2023, 06:42:10 am
content: Quote from: tchicago on March 17, 2023, 04:09:41 amThe HP part description is actually quite a special capacitor. I would interpret the description code as WeT-SLuG-TAntalum Electrolytic. This is a pretty exotic and special capacitor and you could expect to pay in excess of $50 for such a capacitor on today's market. If you check the manufacturer's code in the HP BOM you might be able to look up the full spec for it.Failing that, I would measure the RMS ripple current through the cap when it is subject to its maximum sweep rate. Using that ripple current rating you could look up capacitor datasheets to find a suitable type that would have sufficient life, probably only after suitable derating. If you can't find a 60V capacitor that can withstand that ripple current then you can generally get slightly better ripple current ratings if you select a higher rated voltage for any given capacitance value.Lastly, I would put a scope on the capacitor and make sure that it doesn't ever see reverse polarity. If there is a fault in the driver circuitry or its input signal it could apply reverse polarity and electrolytic, particularly tantalum, capacitors don't last long under such conditions.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2023, 08:16:38 am
content: If space is not an issue, I would consider a motor run capacitor, so not an electrolytic, but a film type (usually PP). 10 µF is areasonable value that should be available. There are versions that come with a cable on can be moutned of board. Chances are they are cheaper and more reliable than the wet slug tantalums.The loss is not only from the ESR, but also from dielectric absorbtion, which is relatively high even for low ESR electrolytic capacitors.

author: BrianHG
author_rank: Super Contributor
timestamp: March 17, 2023, 08:33:30 am
content: Looking at this circuit, with all those resistor values between the supply - transistors - cap, I don't see the cap being fed excessive ripple current at all. However, having a very low predictable ESR cap as a quality signal noise filter is another thing all together. Can you elaborate on the space and size available for that 10uf cap?I seen some not too expensive non-polarized 50v and 100v 10uf film caps used in audio front ends which though in that DC filtering app, they would not be stressed as such, but their bulk size and PPx film nature means they would probably last.Something from this list should last, however, they are fat and big:10uf above 63v, film AC

author: tchicago
author_rank: Regular Contributor
timestamp: March 17, 2023, 02:24:58 pm
content: Quote from: BrianHG on March 17, 2023, 08:33:30 amThanks, I'll look up a dry nonpolar film capacitor that would fit. Space wise, this is an axial cap about 12mm long and 7mm diameter. Soldered into the PCB with another ~5mm length for legs on both sides. There is also a 1mm or so additional spacing around it.

author: tchicago
author_rank: Regular Contributor
timestamp: March 17, 2023, 02:35:06 pm
content: Quote from: srb1954 on March 17, 2023, 06:42:10 amAh, I see that explains it. I'll look up what this type is.QuoteGetting to that cap is pretty hard because the board is plugged inside an individual slot of a metal shield cage, and I don't have a riser board. But I can try just soldering a wire and driving it out of that slot to probe the voltage. The question about the negative voltage is also an interesting one. The schematic does not directly exclude that, as the upper shoulder of this discrete opamp is fed from +20V. If the output of the amp happens to go positive, that would reverse the cap.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2023, 04:22:19 pm
content: Much of the circuit is more or less a simple op-amp build from discrete parts. For the circuit the ESR of the capacitor could be important. So some 10 Ohm of series resistance could make a difference in the stability of the loop.I don't see high current, but more an issue with getting the wrong polarity.It could be a bit tricky to find a film capacitor for that size - at least it could need a relatively small form factor and may get away with a little less capacitance, as the electrolytic often have quite some tolerance.Some 50 V voltage rating should be enough.

author: jonpaul
author_rank: Super Contributor
timestamp: March 17, 2023, 06:38:31 pm
content: A dry tant should be OK.You can still find wet tants.How does the cap measure? Any leakage of electrolyte?Jon

author: tchicago
author_rank: Regular Contributor
timestamp: March 18, 2023, 03:05:26 pm
content: Quote from: jonpaul on March 17, 2023, 06:38:31 pmIt was badly discolored so I assume it leaked. Measures nothing, as the central (internal) pin literally fell out of it when I carefully desoldered it. I see Vishay (which owns former Sprague) still makes wet tantalum caps. Nothing in stock on Digikey, but Mouser has some. Grossly expensive, of course. There are some new old stock on eBay for about half the price of new ones from Vishay, but they could be fake.

author: Gyro
author_rank: Super Contributor
timestamp: March 18, 2023, 06:48:35 pm
content: Wet Tants contain Sulphuric acid and are hermetically sealed, check for corrosion if the cap has leaked. The case is Silver, hence the discolouration.Youcanfind genuine NOS wet Tants on ebay, I don't think their usage has ever been wide enough for them to be faked. Don't confuse them with hermetic axial dry Tants though. The wet ones are distinguished by being slightly less silver in colour (ie. not like Aluminium foil colour) and having a very noticable welded butt joint on the centre lead. They are much heavier too.