yet another dumb question (multi meter)

[freeradical24]

diodes and resistors im clear on.trannys and caps i am not what am i looking for with these(testing for faults).what function should i use?

[Paul Marossy]

Here's a little page that tells you how to test transistors:
http://www.reprise.com/host/tektronix/workbench/transistor.asp

Hope that helps...

[niftydog]

caps are difficult to test using a DMM.  Some meters have a capacitance measurement feature, but it's usually for caps smaller that 20uF or so, and it's not very accurate, particularly when you're looking below 1uF.  It also doesn't tell you anything about high ESR, which is the most common fault, especially in electros.

As for transistors, it depends on the type as to what you'll find.  Basically, for an NPN you should find a "diode action" or a relatively low resistance from base to emitter.  You'll possibly find the same from base to collector, but for many transistors, the forward bias is too high for the DMM.

For all other combinations you'll find no diode action and high resistance.

Some DMMs also have transistor testing settings, usually they have a hfe measurement.

There was a thread about this just yesterday or the day before.  Some useful links were provided, I suggest you search for that thread and visit the links for more info.

[freeradical24]

thanks for the help.so how should i go about testing capacitors?

[niftydog]

Try this link.

It has good suggestions like;

-building a 555 timer circuit and substituting the suspect cap into the circuit to see if it behaves like it's supposed to.

-Using the RC time constant to check sus caps.  A resistor, power supply, a DMM and a stopwatch will at least give you a rough idea of capacitance.


or you fork out for an RCL meter (not cheap by any means)

or learn about resonant circuits.  With a decent selection of inductors of known values and a wide ranging signal generator you can get reasonably accurate results...
still, you need a good signal generator!

I have one of these kits, very, very, very useful for electros, not much use for the smaller caps.

Between my DMM and the above kit, I get by.  But, I also have a AUD$9000 RCL meter on my bench at work so...  sorry!

[niftydog]

ESR meters;

http://www.ozemail.com.au/~bobpar/esrmeter.htm
http://www.awiz.com/cwinfo.htm

[Paul Perry (Frostwave)]

There are only 2 ways an electro can fail, either it shorts out (which you can tell by checking its lreakage resistance, takes a while fot the meter to stabilise) or else it loses its capacitance, and you can check this by putting a known good cap of similarr value across it.
That gets you the answer nearly all the time, at least for FX.

[mr_winter]

HI everybody!!!

in my case, I did learn to test caps for this way:

All knows that the cap "charge" any signal in his plates...
ok, if you have AC the cap charge and discharge ever, but if you have DC, the condenser, only one time are charged & if you invert the polarity, you can dischage it and re-charge again...
ok, when you use the digital multimeter to test for example, resistors, the multimeter send current & DC voltage...
then, I use the multimeter in mode "resistor test" to see if the cap is open, in short or works well.

if is open, I do not have any read in the DM.
if is short, should me shows me low ohm read or nothing of variation in the same read...
if the cap is ok, I can see how up the read from a low ohm to infinite...

this is a old trick that I learn many years ago, and to me... works!

Saludos desde Argentina!