0
\$\begingroup\$

This video says I should see ohmmeter deflection with an NPN transistor, placing positive probe on the base, and negative probe on collector or emitter.

A BC547 NPN is giving me opposite results - ohmmeter deflection with negative probe on the base, and positive probe on collector or emitter.

I tested a simple rectifier diode with the same ohmmeter. I got meter deflection with positive probe on cathode, but no deflection with positive probe on anode. Does that help?

What am I doing wrong?

enter image description here enter image description here

\$\endgroup\$
  • 1
    \$\begingroup\$ How are we supposed to see what you are doing to tell you what you are doing wrong. Obviously something is wrong with your test setup, or you are mistaking the pin-out of the tranny. \$\endgroup\$ – Trevor_G Nov 15 '17 at 15:19
  • 4
    \$\begingroup\$ Not all meters have a positive voltage on the positive probe when on the ohms range. Traditional analogue meters were reversed. \$\endgroup\$ – Kevin White Nov 15 '17 at 15:19
  • \$\begingroup\$ Also be aware BC parts are pinned out differently from 2N parts. YOu are likely looking at the collector base junction. \$\endgroup\$ – Trevor_G Nov 15 '17 at 15:22
  • \$\begingroup\$ @KevinWhite that's interesting, and surprising. I'm using a new GB meter, GMT-319. \$\endgroup\$ – johny why Nov 15 '17 at 16:43
  • 1
    \$\begingroup\$ @johnywhy: Test your meter on a diode to confirm Kevin's point. \$\endgroup\$ – Transistor Nov 15 '17 at 17:34
2
\$\begingroup\$

Give @KevinWhite a biscuit / cookie for being first to point out that 'Traditional analogue meters were reversed'.

Your meter manual hasn't got a schematic but if you do an image search for 'analog multimeter schematic' you should be able to trace out the battery positive going to the - socket in most of them.

enter image description here

Figure 1. A 'simple' analog multimeter with the typical devious circuitry. Note that there are two batteries for the resistance ranges and both head towards the '-' (black) socket. [Click for full resolution.] Source: Vesselyn.com.

It's weird, but when you think of it the meter deflects towards full-scale when current comes in on the red lead. Therefore the current must come from the black lead.

I tested a simple rectifier diode with the same ohmmeter. I got meter deflection with positive probe on cathode, but no deflection with positive probe on anode. Does that help?

Your results from your diode test confirm that the black lead is more positive than the red on resistance measurement ranges.

\$\endgroup\$
  • \$\begingroup\$ Thx for great answers! \$\endgroup\$ – johny why Nov 16 '17 at 22:59
1
\$\begingroup\$

You likely are actually checking the base collector junction.

BC parts have a different (backwards) pin-out from 2N parts.

enter image description here enter image description here

\$\endgroup\$
  • \$\begingroup\$ thx for answer. In both types, pin 2 is the base. So,with either type, if i put the pos probe on pin 2, then i should get meter deflection with neg probe on either of the other 2 pins. Correct? \$\endgroup\$ – johny why Nov 15 '17 at 16:42
  • 1
    \$\begingroup\$ @johnywhy depending on the meter... \$\endgroup\$ – Trevor_G Nov 15 '17 at 17:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.