Transistors Checking Troubleshooting - BJTs, FETs

How do I check if a transistor is working using a multimeter - both BJTs and FETs? How will I know if it's shorted etc? Also, for a BJT how will I know the emitter, base and collector by using a multimeter?

Thanks, GN

Reply to
genenamg
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BJTs are easy: A BJT will look like two diodes (Base-Emitter and Base - collector). Find the common lead. That's your base. If the transistor conducts ('+' to '-') from the base to the other two, it's an NPN. If it conducts the other way it's a PNP. If it conducts both ways or neither, pitch it. Telling the difference between the emitter and collector is a tad harder. If your multimeter has a "transistor tester" the direction with the largest beta is correct.

MOSFETs are harder. ;-)

--
  Keith
Reply to
Keith Williams

A wet finger across the base-collector terminals will turn a good device on and the ohmmeter reading will drop accordingly (assuming the ohmmeter is connected across C-E with the correct polarity for NPN or PNP).

Reply to
Charles Schuler

You can generally get a good idea of a MOSFET's condition by making a simple resistance measurement between the gate and source leads. The MOSFET's gate oxide is quite thin and fragile, so if the MOSFET fails one of the consequences will be that it will normally damage the gate oxide (even if the gate itself wasn't abused by excessive voltage). A failed MOSFET will normally measure

Reply to
Fritz Schlunder

The (forward-biased) resistance of the base-collector junction is usually lower than that for the base-emitter junction. The difference is quite small, though.

Leon

--
Leon Heller, G1HSM
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Reply to
Leon Heller

I read in sci.electronics.design that Leon Heller wrote (in ) about 'Transistors Checking Troubleshooting - BJTs, FETs', on Thu, 31 Mar 2005:

I use a wet finger between base and emitter both to identify the emitter and get a rough idea of beta from how much collector current results. Then reverse the (assumed) emitter and collector connections and apply the same wet finger. Less current means reverse beta applies.

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Reply to
John Woodgate

You need a 100K resistor. Find out the polarity of your analog multimeter on ohms. Connect it correctly from collector to emitter. Now add the resistor from collector to base. Note the two readings.

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N
Reply to
NSM

try this

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regards bob

Reply to
burbeck

I read in sci.electronics.design that Charles Schuler wrote (in ) about 'Transistors Checking Troubleshooting - BJTs, FETs', on Thu, 31 Mar 2005:

Sorry, my article is screwy. THIS is what I should have written:

I use a wet finger between base and COLLECTOR both to identify the collector and get a rough idea of beta from how much collector current results. Then reverse the meter connections and apply the same wet finger between base and what you believe is the emitter. Less current means reverse beta applies.

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Regards, John Woodgate, OOO - Own Opinions Only.
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Reply to
John Woodgate

You might consider the nifty little component tester by Peak at

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It tests all those components, figures out the lead configuration for you and performs an actual test of the device, not just continuity.

John

Reply to
John Bachman

There is also the el-cheapo multi-tester from Harbor Freight. I picked up three (one for the brat, two to throw in various tool boxes) this one for $2.99 each. I just got them last night, so I haven't tried the transistor tester yet. They certainly aren't Flukes, but for $3 they seem to work ok.

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Search for 90899-1RGH. If you order it with the number 90899-1RGH from their ex-tree catalog (rather than what shows up on the search above) they're $2.99 each.

--
  Keith
Reply to
krw

Thanks everybody for your help.

GN

Reply to
genenamg

dunno for fets. for BJTs first measure the diodes to distinguish PNP and NPN. Now you should know which lead is the B. Choose E and C. Use resistance measurement between E and C. Should be high. Put your finger on B and E. Resistance should be lower. Exchange E and C choice and repeat. Lowest resistance is the correct E and C.

Wouter van Ooijen

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Reply to
Wouter van Ooijen (www.voti.nl

I am surprised no one mentioned how easy it is to test a MOSFET with a DVM. First you must have a DVM that's open circuit voltage in diode mode is greater than the Vgs(th) of the device you are testing. I have a Fluke DVM that puts out 3.55 VDC which works for all MOSFETS that I have tested. Also remember that the gate has large capacity to the other pins such that it will store the last potential that was forced on it.

P to S + N to G open P to S + N to D diode (0 .70V) N to S + P to G open N to S + P to D Ron

Harry

Reply to
Harry Dellamano

On Thu, 31 Mar 2005 13:06:38 -0500, Keith Williams put finger to keyboard and composed:

Before discarding them, be aware that some transistors have a diode between collector and emitter, while others have resistors between the base and emitter. Then there are digital transistors ...

- Franc Zabkar

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Reply to
Franc Zabkar

I read in sci.electronics.design that "Wouter van Ooijen

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" wrote (in ) about 'Transistors Checking Troubleshooting - BJTs, FETs', on Fri, 1 Apr

2005:

Shouldn't that be 'B and C'?

--
Regards, John Woodgate, OOO - Own Opinions Only.
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Reply to
John Woodgate

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