TIP125 - Newbee Question

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
Hi all;

Just venturing into some basic electronics, and not at all familiar with transistors like TIP125.

In diagnosing a printer logic board, I was checking continuity on pins of a TIP125 transistor (see photo). With the TIP125 still soldered into the board, and with the board un-powered, should I be reading continuity (a short) between the TIP125 center screw and EACH of its three pins?

Or, is this indicating a TIP125 that is shot?

Tim
 

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Lestraveled

Joined May 19, 2014
1,946
That is one of the fundamental trouble shooting methods. Don't unsolder it unless it looks at you the wrong way. Dead shorts are a dead giveaway.
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
That is one of the fundamental trouble shooting methods. Don't unsolder it unless it looks at you the wrong way. Dead shorts are a dead giveaway.
Sorry - just to be clear. A normal, functioning TIP125 would NOT be reading a short between its center screw and each of its 3 leads when still connected into the board?
 

Lestraveled

Joined May 19, 2014
1,946
Sorry - just to be clear. A normal, functioning TIP125 would NOT be reading a short between its center screw and each of its 3 leads when still connected into the board?
First the center pin and the tab are common, they should read a dead short. If the outside pins read very low resistance to the tab, this would be cause to remove the part from the board and retest.
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
First the center pin and the tab are common, they should read a dead short. If the outside pins read very low resistance to the tab, this would be cause to remove the part from the board and retest.
Okay, great. That's certainly the case. I'm reading NO resistance between pins 1 and 3 and the tab. Thanks very much for the guidance! I really hope I have found the troubled component here. It's on the logic board for a DECWriter III terminal printer which won't line feed. If it's the TIP125 at fault, I'm going to be ecstatic.

Best regards,
Tim
 

Lestraveled

Joined May 19, 2014
1,946
@treehouse3911
Do the following test. In the ohms scale of your meter, one setting should have a diode symbol next to it. Select that setting. Place the negative on the left pin and the positive on the right pin. You should read about 1.4. Next, move the positive to the center pin. You should read .65.

Meters measure resistance with voltages that are too low to forward bias transistors or diodes. That setting with the diode symbol is for semiconductor devices. Use it when testing transistor or diodes.
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
@treehouse3911
Do the following test. In the ohms scale of your meter, one setting should have a diode symbol next to it. Select that setting. Place the negative on the left pin and the positive on the right pin. You should read about 1.4. Next, move the positive to the center pin. You should read .65.

Meters measure resistance with voltages that are too low to forward bias transistors or diodes. That setting with the diode symbol is for semiconductor devices. Use it when testing transistor or diodes.
OK, (while still soldered into the board) between left and right pins, I'm measuring 0.56 on the diode setting on my DMM. Between left and center, I'm reading 0.544.
 

Papabravo

Joined Feb 24, 2006
13,930
I think what is measured on the "diode" setting is the forward voltage drop of the junction and any other components in parallel with the two pins. Those values are lower than I would expect in view of the TIP125 being a Darlington connected transistor. The Vbe drop of the BE junction should be closer to 1.4V without anything else connected to it.
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
I think what is measured on the "diode" setting is the forward voltage drop of the junction and any other components in parallel with the two pins. Those values are lower than I would expect in view of the TIP125 being a Darlington connected transistor. The Vbe drop of the BE junction should be closer to 1.4V without anything else connected to it.
Looks like I need to get it out of the board and do some new measurements. Will report back tomorrow.
Thanks all, Tim
 

ramancini8

Joined Jul 18, 2012
473
I would test it powered up. Short the B-E junction and the collector voltage should rise. Connect a 1K resistor from V+ to the base and the collector voltage should drop. If it fails either of these tests unsolder it and use the resistance test.
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
I would test it powered up. Short the B-E junction and the collector voltage should rise. Connect a 1K resistor from V+ to the base and the collector voltage should drop. If it fails either of these tests unsolder it and use the resistance test.
Thanks for the diagnostic tip, ramancini8.... I'll give that a shot and report back!
Tim
 

Thread Starter

treehouse3911

Joined Feb 6, 2016
10
Hi ramancini8 - just thought I'd report back that your diagnostic tip has helped me to identify a failed TIP125. Thanks so much.

Now, with the component desoldered from the board, further tests reveal a short between its base and collector:

Between B (Pin 1) and C (Pin 2) : 1.6 Ω
Between B (Pin 1) and E (Pin 3) : 3.78 KΩ
Between C (Pin 2) and E (Pin 3) : 3.78 KΩ
There is continuity between B (1) and C (2).

I'm happy to have found the failure, but haven't yet uncovered why it failed, so we're going further back in the circuit now to see if there is problem with the 7406 that the TIP125 is connected to.

Thanks to all who pitched in.
Tim
 
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