Capacitor testing

Servicewiz_3

Can you test a capacitor on the work bench with a digital ohm meter?

Ironman

If it has a capacitor scale.

unclejohn

We used to use the analog Simpson meter with the roll top case.

EBEBRATT-Ed

I don't beleive a digital ohm meter will work. If it has a capacitor setting you can. With an old analog meter when you attach the meter leads it should initially show no resistance and then gradually show resistance. Discharge the cap first

mattmia2

it work with the ohmmeter on a dmm. it on;y tells you if it is a capacitor or not, not if it has lost say 90% of its capacity.

HomerJSmith

A DMM usually will test in the Pico to the lower Micro-Farad range, if it has a capacitance test port. But, they don't test for leakage. I have had capacitors meet the rated capacitance but had leakage beyond the acceptable rating for that capacitor type. My meter will test leakage up to 600 volts. Some capacitors, such as, electrolytic capacitors may need reforming before being put into service.

Whether it will work or not depends on the capacitor type and rating. A DMM is all right for a quick test but it is not definitive.

To accurately test a capacitor, it must be removed from the circuit.

ChrisJ

It's still not being tested for leakage etc.

No digital multi meter does that to my knowledge.

JUGHNE

Old school analog meters would give you a "go" or "no go" result.
I believe on ohms setting the needle would raise up and then slowly fall back. Had to be sure cap was discharged first.

25-30 years ago never worried about caps unless physical damage was evident. They seldom failed and lasted the life of the machine. Just checked some on a 1982 system and they are still fine. When scrapping out old units I keep some of the original caps for back ups.

Now check all on PM and replace many. Chinese OEM caps might make it 2 years.

mattmia2

JUGHNE said:

Old school analog meters would give you a "go" or "no go" result.
I believe on ohms setting the needle would raise up and then slowly fall back. Had to be sure cap was discharged first.

I have done this many times with a DMM. It also works with the ohmmeter in a DMM. With either an analog or digital DMM it will only tell you it has some relatively large capacitance, not it it has degraded to a point where it has lost a great deal if its value.

JUGHNE said:

25-30 years ago never worried about caps unless physical damage was evident. They seldom failed and lasted the life of the machine. Just checked some on a 1982 system and they are still fine. When scrapping out old units I keep some of the original caps for back ups.

I replaced the caps in a lot of things as a kid. They would last maybe 10-15 years or more but many would eventually fail, usually window air conditioners. Some of it may have been significant loss of capacity combined with undersized wiring that was not able to provide proper starting voltage.

ChrisJ

I have a spare Titan HD cap on hand for my AC.

JUGHNE

I have that and many others in stock but a new combo of dual caps seem to pop up all the time.
For instance today 40/5 440 VAC.
Often have to temp in single caps until the dual arrives.
Probably should just change all the dual caps to 2 singles, but mounting room and clamps do not always allow it.

Big Ed_4

I stock all the sizes in dual with the fan micro ferret of 5 , I can add to the fan if need be .

ChrisJ

I hear turbo caps are where it's at for service calls.
They're also said to be really good quality as well. Just expensive.

SuperTech

> @ChrisJ said:
> I hear turbo caps are where it's at for service calls.
> They're also said to be really good quality as well. Just expensive.
>
>

The Amrad Turbo 200 and 200X are definitely the best capacitors I've used. They come with a five year warranty. I've only seen one, maybe two fail. They are more expensive than other capacitors, but they are worth it.
I also am in favor of using separate single caps for the compressor and fan. Dual capacitors don't seem to last as long.

mattmia2

a dual cap will concentrate the heat from both caps in the same can...

ChrisJ

mattmia2 said:

a dual cap will concentrate the heat from both caps in the same can...

Dual and triple caps were extremely common in the 1950s-1970s in electronics. I never noticed any issues specific to them if they were good quality caps sized appropriately.

Obviously if you run a cap near it's rated voltage or current you'll be pushing your luck. I suppose under those conditions individual caps will fair better.

Sadly, in my specific case I no longer have room for two separate caps in my A\C unit. The fan controller takes up a large chunk of real estate.

ratio

The builder's special model I've got has the cap mounted in a hole, extending into the airstream inside the condenser. A screw captures the rolled lip of the cap. Takes up a lot less room & should keep the cap cooler as well. I don't know why everyone doesn't do that.

JUGHNE

Reem/Ruud was doing that in the 80's.
Was handy, easy to test, easy to change.......but if cap swelled from failure it may not come out easily.
Also if you want to up grade to 440 VAC might not fit in the original hole.

ChrisJ

In the 1960s Fisher was putting cement power resistors in between the leads of the bias supply chassis mounted capacitor. I guess they felt it didn't get hot enough on it's own so they put a heater under it. They fail..........often.

Here's one with a cap someone put in underneath.
You can see the cement resistor under the cap on the right side and that resistor ran hot enough to burn you.

mattmia2

People put modern replacement caps under the chassis to keep the look of the old caps in the can(and/or as a quick fix for a cap that failed open). Electronic filter caps see a whole lot less average current than a motor run cap, dissipation of heat generated within the cap usually isn't an issue.

ChrisJ

mattmia2 said:

People put modern replacement caps under the chassis to keep the look of the old caps in the can(and/or as a quick fix for a cap that failed open). Electronic filter caps see a whole lot less average current than a motor run cap, dissipation of heat generated within the cap usually isn't an issue.

I understand that, I used to restore Fisher's from the 1950s-60s..

That said, it completely depends on the circuit. I designed a power supply using capacitors rated for 14A of ripple current and under full load the circuit needed every bit of it.

EdTheHeaterMan

In the past several years or so I was checking the capacitance of dual caps while the unit was operating using the voltage and amperage of each. Then calculating the capacitance using this formula. Start Winding Amps x 2,652 ÷ capacitor voltage = microfarads.

I have found this to be easy and accurate on tune-up or maintenance calls. Easy to sell a cap at a lower price "While I'm already here" in comparison to an emergency "NO AC" call when there is a possibility of overtime and long wait time.

EdTheHeaterMan

I also stopped using OEM caps and stocking the truck with Turbo caps. Fewer items to stock, always have the right size, no trips to the supply house then returning that job. 5-year warranty.

Most of the cost of a service call is labor.

A $6.00 part or a $60.00 part does not change the price of the customer pays by much in the big picture. If you have to return with the correct part later and the customer pays for all that time then the more expensive part is actually less costly and a better value.

mattmia2

But would the mars capacitor of the single value be of equal quality or is there some line of mars that is constructed the same?

SuperTech

> @EdTheHeaterMan said:
> In the past several years or so I was checking the capacitance of dual caps while the unit was operating using the voltage and amperage of each. Then calculating the capacitance using this formula. Start Winding Amps x 2,652 ÷ capacitor voltage = microfarads.
>
> I have found this to be easy and accurate on tune-up or maintenance calls. Easy to sell a cap at a lower price "While I'm already here" in comparison to an emergency "NO AC" call when there is a possibility of overtime and long wait time.

Yes, I was shown this method a couple years ago and its definitely saved my butt a few times.
I've found capacitors that test in the acceptable range when out of the unit, discharged and being tested by a multimeter can test outside of acceptable range when the method Ed described is used. Our company has pretty much eliminated dual capacitor failure on the systems we maintain and check annually.

I wish I could convince my boss to use only Turbo 200 and 200X dual capacitors. I know the added expense of it, but then we no longer need to worry about not having that oddball 25-3 or 80-10 dual capacitor. Not running out to the supply house saves labor costs and gets customers equipment running faster. And that makes everyone happy.