Story Problem

[kl7aj]

Snag Glandsearcher, NN%NN, has picked up a grab bag of unmarked transistors at the local hamfest. While listening to the local 75 meter net in the background, Snag sets his Fluke meter to the diode position, and starts checking junctions on his newly acquired transistors. At least he can identify whether they're PNPs or NPNs. Or at least he thinks he can.

All goes well for the smaller transistors; there seems to be a mix of small signal PNPs and NPNs...he sorts them accordingly, and puts them in his parts bins.

He runs into problems though, when he starts checking some large power transistors. From all appearances, they seem to be 2N3055s, or their complements. However, Snag reads no conductivity in any direction, from any pair of terminals. They seem to be new devices, quite unlikely that they'd be burned out. And, most transistors show SHORTS when they go bad, anyway...not opens.

What ARE these things?


Eric

[ve2nsm]

I have seen regulators in TO-3 cases but they have to show some kind of junction somewhere.

[VK3PA]

well not zeners, as they have a short between two terminals..es between those two act as a normal zener

[ab1ga]

Could they be some of them new IGBT thangs?

[K9YLI]

ha broken meter lead.....

[kl7aj]

Quote[/b] (ab1ga @ April 30 2007,19:26)]Could they be some of them new IGBT thangs?
Bingo!

You need a few volts to forward bias them enough for any kind of reading...which is why they handle high voltages so well. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

eric

[n2jso]

Quote[/b] (kl7aj @ April 30 2007,16:44)]What ARE these things?
Triodes. # With burned out filiments. http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif

[kb2vxa]

Hi all,

This is where the "check mark checker" comes in. Using an old 6 or 12V filament transformer and about a 1K series resistance put AC across the junction and read it with a scope. You can tell a lot more than polarity if it's calibrated, for high voltage zeners crank up the voltz.

Here's an interesting note, I just knew I was looking at a bad diode but it seemed fine on the resistance scale. On a hunch I switched to conductance and sho'nuff it was crapped out.

[ve2nsm]

Quote[/b] (kb2vxa @ May 01 2007,15:53)]Here's an interesting note, I just knew I was looking at a bad diode but it seemed fine on the resistance scale. On a hunch I switched to conductance and sho'nuff it was crapped out.
That's why you NEVER check a diode on the resistance scale http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif

[ka0gkt]

Quote[/b] (ve2nsm @ May 01 2007,12:57)]Quote[/b] (kb2vxa @ May 01 2007,15:53)]Here's an interesting note, I just knew I was looking at a bad diode but it seemed fine on the resistance scale. On a hunch I switched to conductance and sho'nuff it was crapped out.
That's why you NEVER check a diode on the resistance scale #http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif
Unless you are using an old Simpson 260 or equiv. analog VOM.

Lots of the diodes which I check at woek require a megger to get an accurate fix on whether they're good or bad.

40 KV @ 5A DC Beam Supplies. Once you get past the smell of the high napheletic transformer oil, they're really neat!

73 DE KAØGKT/7

--Steve