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| Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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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 |
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"Keith Williams" wrote in message
... In article . com, says... 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? 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. 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 http://www.geocities.com/leon_heller |
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I read in sci.electronics.design that Leon Heller
wrote (in ) about 'Transistors Checking Troubleshooting - BJTs, FETs', on Thu, 31 Mar 2005: 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. 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. -- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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wrote in message ups.com... 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? 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. -- N |
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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. 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). |
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wrote in message ups.com... 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? 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 1k ohms between gate and source. These failed MOSFETs will also normally exhibit low resistance from drain to source as well. A good MOSFET's gate to source resistance will measure somewhere in the megohms to infinity range. There are exceptions to this however. Sometimes the failure (especially in the case of power MOSFETs used in such a way that if they fail large currents flow) is so catastrophic that internal fusing can occur. Sometimes enough transient power can be dissipated in the device to physically blow the plastic package apart, physically separating one of the leads from the die. In these types of failures the above mentioned resistance test may read infinity even though the device is definitely broken. In my experience these types of failures are relatively uncommon compared to more mundane failures which simply result in low resistance shorts between the three pins. When they do fail like this it is often obvious since the plastic package has literally blown apart or has otherwise suffered evident trauma. In one case I found a 2N7000 MOSFET that failed by means of a parametric shift. The drain to source became very leaky (comparable to a 100k ohm resistor) even though the gate was fully intact. The device still worked fine, besides the extra leakage. The leakage current could be reduced by driving the gate with negative gate-source potential, but still not completely turned off. I have no idea what might have happened to this device to cause this failure. Nevertheless I think this is a very rare failure mode which you shouldn't normally need to worry about. |
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"John Woodgate" bravely wrote to "All" (31 Mar 05 22:29:51)
--- on the heady topic of " Transistors Checking Troubleshooting - BJTs, FETs" JW From: John Woodgate JW Xref: aeinews sci.electronics.repair:44620 JW I use a wet finger between base and emitter both to identify the JW emitter and get a rough idea of beta from how much collector current JW results. Then reverse the (assumed) emitter and collector connections JW and apply the same wet finger. Less current means reverse beta JW applies. -- I use a variation of this tip to get a relative indication of transistor action. Simply connect an analog ohmmeter leads, on highest ohms range, to polarize the collector and emitter while leaving the base floating. Then, without touching the other pins, lightly touch the base terminal and watch the indication on the meter scale. The higher the gain the higher the deflection. The idea is that the base-emitter junction rectifies the ambient RF field picked up by your body and results in a collector current through the ohmmeter. Clearly not as intuitive with a DMM. A*s*i*m*o*v .... When I was your age, we carved transistors out of wood. |
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On Thu, 31 Mar 2005 13:06:38 -0500, Keith Williams
put finger to keyboard and composed: In article . com, says... 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? 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. 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 -- Please remove one 's' from my address when replying by email. |
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I read in sci.electronics.design that Charles Schuler
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. 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). 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. -- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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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? 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 -- ------------------------------------ http://www.voti.nl Webshop for PICs and other electronics http://www.voti.nl/hvu Teacher electronics and informatics |
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wrote in message ups.com... 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 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 |
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On Fri, 01 Apr 2005 14:46:47 GMT, (Wouter van Ooijen
(www.voti.nl)) wrote: 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? 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. You might consider the nifty little component tester by Peak at www.anatekcorp.com/testequipment/atlas.htm It tests all those components, figures out the lead configuration for you and performs an actual test of the device, not just continuity. John |
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In article ,
VEcom says... On Fri, 01 Apr 2005 14:46:47 GMT, (Wouter van Ooijen (www.voti.nl)) wrote: 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? 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. You might consider the nifty little component tester by Peak at www.anatekcorp.com/testequipment/atlas.htm It tests all those components, figures out the lead configuration for you and performs an actual test of the device, not just continuity. 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. http://www.harborfreight.com/cpi/cta...unction=Search 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 |
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I read in sci.electronics.design that "Wouter van Ooijen (www.voti.nl)"
wrote (in ) about 'Transistors Checking Troubleshooting - BJTs, FETs', on Fri, 1 Apr 2005: 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. Shouldn't that be 'B and C'? Resistance should be lower. Exchange E and C choice and repeat. Lowest resistance is the correct E and C. -- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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Thanks everybody for your help.
GN |
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