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Old 27th July 2010, 11:41 AM   #1
john65b is offline john65b  United States
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Default Simple (?) Capacitor question

Hey all, on a simple rectifier - cap power supply, is it strange to see AC voltage on the top metal casing of a electrolytic capacitor wrt ground? I have a single electrolytic cap on a bad LCD TV Power Supply that has this AC voltage - all other cap show no voltage on casing wrt ground...

I have seen this before on an Adcom Preamp, where a electrolitic capacitor had an AC voltage (again, on top metal casing) with respect to ground, and turned out to be the upstream rectifier had gone bad, but unfortunately never checked to see if that AC voltage remianed on the top of cap after replacing the rectifier.

Can this mean anything?

I will be replacing the cap and rectifier tonight (hopefully) but wonder if anyone knows if this is a sign of a cap or rectifier going bad...

Any other way to test if a cap is going or gone bad? I have an LC meter, but only goes to 200uf, and really don't want to pull all caps to test.

Last edited by john65b; 27th July 2010 at 11:47 AM.
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Old 27th July 2010, 01:24 PM   #2
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I've seen lots of twist-loc cans with voltage on the can. It's just tied to the negative terminal and if the cap floats, so does the can (may not always be true- some cans may float independently and can be grounded regardless of the terminal voltages). Always a good idea to cover the can in HV supplies. BTW, if you have a known cap of some value near the max of your meter (measure it with the meter!), you can extend the range of your meter by just measuring the (smaller) series combination of that and the unknown, then calculating the unknown. Somewhere I have the formula to back out the unknown dissipation factor as well.
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Last edited by Conrad Hoffman; 27th July 2010 at 01:26 PM.
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Old 27th July 2010, 01:47 PM   #3
john65b is offline john65b  United States
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Quote:
I've seen lots of twist-loc cans with voltage on the can. It's just tied to the negative terminal and if the cap floats, so does the can (may not always be true- some cans may float independently and can be grounded regardless of the terminal voltages).

But reading an AC voltage on the can after a rectifier? Its 120VAC into rectifier and seeing 30VAC on can....30VAC ripple? Just sounds goofy to me...

OK, I can see the measuring the Cap in series, then backcalculating the unknown series value...neato...I guess I can use same method to calculate resistance value for resistors under 1 ohm via parallel back-calc?

Sorry for being a electronics lamo - I am a Mechanical Engineer and not a EE...

Last edited by john65b; 27th July 2010 at 01:52 PM.
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Old 27th July 2010, 05:35 PM   #4
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I'm the lamo for not seeing "AC". That much only seems possible if the can is floating and you're just getting coupled pickup.
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Old 27th July 2010, 06:43 PM   #5
benb is offline benb  United States
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Do you have a schematic? Is the capacitor case SUPPOSED to be connected to ground?
Quote:
Originally Posted by john65b View Post
But reading an AC voltage on the can after a rectifier? Its 120VAC into rectifier and seeing 30VAC on can....30VAC ripple? Just sounds goofy to me...
I don't think that's a ridiculous amount of ripple for the DC input of a SMPS. Modern switching power supplies operate over a wide range of input voltages (I recall seeing like 85V-250VAC input voltage), and the regulation is plenty fast enough and runs on a wide enough input voltage to to "regulate out" substantial input ripple. This allows the filter caps to be lower capacitance than would otherwise be needed.
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Old 28th July 2010, 06:48 AM   #6
Mooly is offline Mooly  United Kingdom
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It's what the ripple component is measured across the cap terminals that matters... forget the case.
Cap meters are useless for testing electroylitics as the main failure mode is an increase in E.S.R. which shows up essentially as an increase in ripple.

Main reservoir caps do occasionally fail in SMPS but more often it's the smaller low voltage caps on the rails and error sensing network that dry out and cause trouble.
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Old 28th July 2010, 11:34 AM   #7
john65b is offline john65b  United States
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OK...not a crazy ripple amount. I have redirected my sights elsewhere on the PSU...

Don't have the schematic - but its a Mitsubishi LT-37131 LCD TV with a flashing blue LED that does not go into standby/startup. These LCDs have a "Bootup" stage that is required on connection of power.

I have bought about seven dead LCD televisions and first four were simple cap replacements - evidenced by leakage or bulging. Two of the four were the main 150uf 450V cap, another was on inverter board, while the other was a random cap on the PSU. The caps were replaced and all four LCDs fired right up (even impressed the wife and kids, which seldom happens these days).

The last two LCD tv are different story all together as the caps all look fine.

Is there an easy way to test Caps in situ? I have an LC and multimeter (and a scope I don't yet know how to use)
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Old 28th July 2010, 11:43 AM   #8
AndrewT is offline AndrewT  Scotland
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the main storage cap inside a ClassII (double insulated) TV will very likely be directly on line to the mains. No isolation is potentially very dangerous

You would need to insert an isolation transformer between the mains and the TV before starting to probe inside.
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Old 28th July 2010, 11:19 PM   #9
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You would need to insert an isolation transformer between the mains and the TV before starting to probe inside.
If your scope is mains powered, what he said above. Don't be tempted to float the scope (disconnect the earth wire), because not only will you turn the scope's case into an electrocution risk, but also the measurements will be unreliable. On my scope, the trace bounces a lot if i power it from an unearthed outlet, making measurements impossible.
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Old 29th July 2010, 03:40 AM   #10
gootee is offline gootee  United States
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Quote:
Originally Posted by john65b View Post
OK...not a crazy ripple amount. I have redirected my sights elsewhere on the PSU...

Don't have the schematic - but its a Mitsubishi LT-37131 LCD TV with a flashing blue LED that does not go into standby/startup. These LCDs have a "Bootup" stage that is required on connection of power.

I have bought about seven dead LCD televisions and first four were simple cap replacements - evidenced by leakage or bulging. Two of the four were the main 150uf 450V cap, another was on inverter board, while the other was a random cap on the PSU. The caps were replaced and all four LCDs fired right up (even impressed the wife and kids, which seldom happens these days).

The last two LCD tv are different story all together as the caps all look fine.

Is there an easy way to test Caps in situ? I have an LC and multimeter (and a scope I don't yet know how to use)
You can usually test electrolytic caps for high ESR in-circuit, if you have a scope and an adjustable square/pulse generator (or even just a simple 555 timer circuit). But you kind of have to learn how a good one should look (on the scope display), before you can spot bad ones. But once you do that, it works very well most of the time. I have repaired a LOT of equipment that way. I have a description of the method, that I posted years ago, still on line at TESTING ESR OF ELECTROLYTIC CAPACITORS, without an ESR meter . And if you keep the voltage low-enough, it won't even turn on any p-n semiconductor junctions, which makes it a little more idiot-proof (which I always needed).

Cheers,

Tom
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