electrically testing a condenser

[FrankWest]

I have and analog multimeter and 2 digital multimeters.
How do I electrically test a condenser?
There only appears to be volts and amps on the meters not ohms?
Even though I loved physics, I always hated electronics
C = Q V

[alexiskai]

1. Using a meter with a capacitance function, measure the capacitance. Ideal spec is 200-250 nF; up to 350 nF is probably OK if it's stable.

2. Using the resistance function, measure the resistance. You should see the readout gradually increment up until it reaches infinity. If the readout plateaus at some number of ohms, that indicates leakage.

3. Place the condenser and the multimeter in series with a battery and measure the amperage. You should see the amps start at some number and drop to zero. If the amps don't reach zero, that indicates leakage.

4. Heat the condenser gently until it reaches about 150° and repeat tests 1-3.

[alexiskai]

Note: 200-250 nF is for A/B condensers. V8 condensers will be higher.

[rockfla]

IF doing the above test 1-3 all check out....UNTIL you do test #4 you will not "truly" know IF the condenser is "good".

[glennpm]

https://www.electricaltechnology.org...h-digital.html

[tubman]

Sponsored Links (Register now to hide all advertisements)

Most older multimeters don't have a capacitance test function; mine didn't, so I bought one of these on Amazon. It only tests capacitance and seems to be very accurate and doesn't break the bank. https://www.amazon.com/dp/B0036FQ3FW...fed_asin_title.

[PeteVS]

This spring our AC wouldn’t run. A family friend who’s an HVAC guy stopped by and suspected the capacitor (condenser). You need an ANALOG (not digital) volt ohm meter (VOM). He set the VOM to “resistance” and touched the leads to the capacitor. This pushed a small charge into the capacitor. The meter will swing to the right but then slowly return to zero as the charge builds. He then disconnected the meter, switched to “voltage” and connected back to the capacitor. The needle will quickly go to the right indicating that there is a charge and then drop back to zero as that charge drains. Failure of the charge to build and then drain means the capacitor is shot.

[alexiskai]

Originally Posted by PeteVS

This spring our AC wouldn’t run. A family friend who’s an HVAC guy stopped by and suspected the capacitor (condenser). You need an ANALOG (not digital) volt ohm meter (VOM). He set the VOM to “resistance” and touched the leads to the capacitor. This pushed a small charge into the capacitor. The meter will swing to the right but then slowly return to zero as the charge builds. He then disconnected the meter, switched to “voltage” and connected back to the capacitor. The needle will quickly go to the right indicating that there is a charge and then drop back to zero as that charge drains. Failure of the charge to build and then drain means the capacitor is shot.

For the record, you can test a condenser/capacitor with a good digital multimeter, but I agree that if you're picking between a cheap analog VOM and a cheap digital VOM, the analog VOM will give you a more nuanced readout when performing the test as described here.

[Ronnieroadster]

Originally Posted by PeteVS

This spring our AC wouldn’t run. A family friend who’s an HVAC guy stopped by and suspected the capacitor (condenser). You need an ANALOG (not digital) volt ohm meter (VOM). He set the VOM to “resistance” and touched the leads to the capacitor. This pushed a small charge into the capacitor. The meter will swing to the right but then slowly return to zero as the charge builds. He then disconnected the meter, switched to “voltage” and connected back to the capacitor. The needle will quickly go to the right indicating that there is a charge and then drop back to zero as that charge drains. Failure of the charge to build and then drain means the capacitor is shot.

That's how I easily test condensers before I use them. I have found a digital meter will work fine as well.
Ronnieroadster

[Bored&Stroked]

The above tests are pretty good for some basic/initial tests. I also have a vintage "condenser tester" that plugs into 120V AC and charges the condenser and tests for insulation breakdowns (which is very common - especially on older condensers!).

Here is a good article that covers ALL you need to know . . . read it in detail!

Part2_Condenser_Testers_And_Testing_Correctly.pdf

[Juergen]

A little correction to PeteVS's explanation. With the resistance test, the charge builds until the capacitor is fully charged and the current goes to zero which the meter interprets/displays as infinite (not zero) resistance.

[rotorwrench]

Capacity, leakage, and resistance both out of the box and with normal operating temps as was mentioned, are all the test parameters. The meggar test can check resistance but an ohms meter is likely not that sensitive. Most condensers for ignitions are around .15 to .35 Micro Farads depending on coil design. Ford had some different coils with the way they developed the distributors for the V8 engines, These used the higher capacity range where the can coils used the lower end of the range.

A good condenser won't spark much when a person manually opens the breaker points with a connection to the primary and a secondary coil connection to a spark plug on a common ground plane. The engine can idle very slowly with a stable condenser with the spark retarded and can also function well during all road speeds with full advance. This is how most folks tested them with no meters.Now days a person has to purchase two or three to get a decent one.

PS: I got the decimal in the wrong place so I corrected the above micro farad ranges

[tubman]

Originally Posted by rotorwrench

... Most condensers for ignitions are around 1.5 to 3.5 Micro Farads depending on coil design. Ford had some different coils with the way they developed the distributors for the V8 engines, These used the higher capacity range where the can coils used the lower end of the range

Are we talking the same thing here? Most current condensers are in the low .20's range (.22-.25 microfarads), while the original Mallory trash cans were rated at .36 microfarads. I have a couple of NOS Mallory "Bus Condensers" (they look like a larger "Trash Can" with a stainless case and a red top) that are rated at .45 microfarads, but that's about the range I've seen.

Here's an interesting chart I found in some old Ford literature. I find the last couple of lines fascinating.

[glennpm]

Yes, good table and notes.


#3 I'm charged up, so let 'er rip! Excess voltage spike hits the points.


#4 Wait, I'm still charging here. Rotor moves on and discharge of coil is weak.


Glenn

[tubman]

Glenn, Thanks for the translation to English and explanation. I'm a little out of my depth here, but doesn't excess current (AMPS) cause the transference of material on the points, rather than voltage? Ohm's law gives us the relationship between resistance, volts, amps, (and therefore watts), but I don't quite get how capacitance fits into the whole scheme of things.

[glennpm]

This write-up is one of the best I've found explaining how the condensor (capacitor) works in a Kettering points system. I think of them as temporary voltage storage units from a mechaical engineer's perspective :-). For kick-start motorcycles, the alternator stores charge in a large capacitor. There is then enough voltage and amperage to start the bike.

"And somewhere around the time of 04/16/2004 09:10, the world stopped and listened as Don Bruder contributed the following to humanity: Actually, it's both. The condenser(capacitor) is about .22uf give or take for design differences. I don't know what the mH rating of a coil is, but the coil is a electrical device known as a auto-transformer. It's like a regular transformer, but both the primary and secondary share some of the windings.


The capacitor is wired in parallel to the points and in series to the coil. The capacitor tries to maintain a constant voltage while the coil tries to maintain a constant current. The basic design of a automotive ignition circuit is basically a series resonance circuit not unlike those used in radio.

When the points are closed, the capacitor is shorted out and current flows from ground, through the points, through the coil, and out onto the positive power rail. This sets up a magnetic field around the windings of the coil. At the moment that a spark plug is to fire, the points open which interrupts the current flowing through the coil, but the coil is charged (magnetic field), and in an attempt to keep the current going, the coil discharges right into the capacitor.

At this point, when the coil is fully discharged, there is a voltage charge on the capacitor in the range of +700-1000 volts in reference to ground. Since this is much higher than the +12V rail that the other end of the coil is connected to, the current flows BACKWARDS from +12V, through the coil, and into the capacitor to even the charge. Because there is little resistance in the coil, a high amperage pulse is generated through the coil primary, and with a winding ratio of 40:1, causes a voltage spike between 28KV and 40KV on the secondary, which is more than enough to jump the gap between the rotor and distributor cap, and the gap on the plug itself.

A leaking capacitor will dissipate some of the charge before it can be used to draw the current through the coil resulting in a lowered high voltage output and a weak, or no spark condition.

A shorted capacitor will cause a no-spark condition because the current will still flow through the coil.

An open capacitor will burn the points because the points will be forced to absorb the discharge from the coil causing them to arc, and you will get a weak or no spark.

Too little capacitance will cause the points to burn too, the same reason that a open capacitor will. But, it may or may not cause a weak/no spark condition depending on how much under the spec the cap is.

Too much capacitence will cause a weak or no spark condition because the voltage across the capacitor will not be high enough because of the larger value. The bigger the cap is, the bigger the charge on the coil has to be to get it to the 700-1000 volt range.

Generally the bad condenser will cause the contact points to burn
*unless* the condenser itself develops an internal short. Then it will ground the primary current from the coil *permanently* hence no interruption of the coil primary current flow resulting in no high voltage (spark) generation in the secondary circuit"

[glennpm]

From Wikipedia

"Natural capacitors have existed since prehistoric times. The most common example of natural capacitance are the static charges accumulated between clouds in the sky and the surface of the Earth, where the air between them serves as the dielectric. This results in bolts of lightning when the breakdown voltage of the air is exceeded.[4"
Static electricity too, from your 'charged" body to your unsuspecting partner's lips :-)


Lightening, the big flashes, are from the ground to the clouds!
https://earthscience.stackexchange.c...-the-ground-up?