Transistors and DMM

[simonrichie]

My DMM has holes for transistors. I've tried several (2N3904, 2N5088, 2N5089) get readings of either 6, 7, or 8. Shouldn't it be more like 210, 330, etc.?

[nick d]

 You need to know which leads on the transistor under test correspond with the C B E  sockets on the DMM , otherwise any readings are meaningless .

[PRR]

> 6, 7, or 8.

That could be reasonable if the E and C are swapped. (Forward hFE is maybe 200, reverse hFe is much lower, 1 to 10.)

[simonrichie]

Double-, triple-, quadruple-checked. Those were the readings. (The holes are in the CBE order, so it'd be really hard to @#$% it up.) Later in the day I tried some other trannies that had regular readings. I'm attributing it to the DMM being a cheap POS.

[PRR]

> holes are in the CBE

C and E are easy to switch.

Many Japanese transistors have B on one end, not in the middle.

[nick d]

 Are you sure the tranny is the right way round , i.e. EBC or CBE ? I just plugged a 2n3094 in at random , got an hfe of 2 . Rotate 180, it reads 183 . Give it a shot .

[simonrichie]

Yup.

I put it the other way (this is an MPSA18) and I get 812. I put it the "right" way and I get 7.

Is this because it's a Tayda product, or is this fairly common? Should I be checking every transistor on the DMM for orientation?

-SR

[simonrichie]

Nope. Wrong again.

I was following a vero layout that had the orientation of the tranny reversed.

Sheesh.

[nick d]

    The supplier ( Tayda or whoever ) is irrelevant , they just sell them .
    If I come across a transistor I've not seen or used before , I look up the pinout on the data sheet ,
    and write it down in my little notebook for future reference .
    I've seen quite a few layouts posted with reversed pinouts , as if  " pin view " and " top view "
    had been swapped .
     
     So , if in doubt , check , then check again . Twice .

[PRR]

> I get 812

"Right" answer for MPSA18 is 500 to a thousand (depending what current and voltage you test at).

So 812 is right down center field.

Datasheets never give the reverse beta, so your reading of 6 to 8 is possibly good-to-know for analyzing odd or mis-wired (mis-documented) circuits.)