[SOLVED] I need to identify if trans is NPN or PNP

[sdowney717]

I drew out the circuit schematic.
Item in question is an SMD transistor either 3D or 3B, a hole has partially burned out the ID!
3B is NPN
3D is PNP

The 2SC3205 is an NPN transistor

more images of the Inverter damaged board

This is from a 3000 watt MSW inverter

 

[BradtheRad]

The buzzer is normally off. I think the 324 generates pulses to make it beep.

You marked the buzzer with + and - terminals, but did not show the + wire going anywhere. I imagine it goes to supply positive?

It suggests that the unknown transistor is NPN. Its collector will pull current down through the buzzer.

This might be confirmed if you measure the 324 output as normally being low.

It also would help confirm if you were to find that is a ground wire between the emitters of the two transistors.

OTOH if the 324 output is normally hi, then it suggests the unknown transistor is PNP.

 

[sdowney717]

The buzzer is normally off. I think the 324 generates pulses to make it beep.

You marked the buzzer with + and - terminals, but did not show the + wire going anywhere. I imagine it goes to supply positive?

It suggests that the unknown transistor is NPN. Its collector will pull current down through the buzzer.

This might be confirmed if you measure the 324 output as normally being low.

It also would help confirm if you were to find that is a ground wire between the emitters of the two transistors.

OTOH if the 324 output is normally hi, then it suggests the unknown transistor is PNP.

Hi, thanks, I am thinking it is NPN also.
Instead of using a 3B smd, I think I will solder tiny wires to board and attach a leaded transistor like a 2N918 NPN
shows some subs and I called Allied http://www.marsport.org.uk/smd/mainframe.htm

If I used the wrong one PNP instead of NPN, would it just not function or would it destroy more components?
I was thinking buy both 2N918 NPN sub for 3B and a MPSH81 (NTE395) PNP sub for 3D. Just try both and see which one works.

 

[BradtheRad]

You also need to figure out what caused the destruction of that transistor. Isn't that a jagged edge at the left? Is some of the transistor gone?

A surge of current had to go through it. Also through the nearby copper trace to vaporize it.

 

[sdowney717]

yes, it was me. I destroyed the Inverter by adapting the on-off switch to a remote on - off switch. I accidently sent 120vac into the on - off switch with a slip of the screwdriver.
I destroyed 1 power mosfet, 2 electrolytic caps exploded and vaporized some traces.

yes the transistor has a hole in it. Cant really tell if it is a 3B or a 3D

So I will replace some parts and hope it runs again. 3000 watt 6000 surge msw.

 

[BradtheRad]

Ah yes, the old slip-of-the-screwdriver gotcha.

I think if you install the wrong type of transistor, you'll know from the way it makes the buzzer sound all the time (even when nothing is wrong).

Then the buzzer will shut off when something goes wrong.

 

[sdowney717]

I am pretty sure now it is NPN. I checked the bat neg power terminal to that emitter point on the circuit board and get 0.2 ohms.

I was thinking of using a NTE199 NPN trans since I have one. Likely many NPN could work here, if all it does is turn the buzzer on and off as a switch.

Since it is SMD, I plan to solder small wires to the board and drill 3 holes in a bare unused part of the board to mount the transistor.

 

[Syncopator]

I destroyed 1 power mosfet, 2 electrolytic caps exploded and vaporized some traces.

You're having a lot more fun than I am

 

[Tahmid]

It's got to be NPN. I'll tell you how I reached that conclusion.

Emitter of the 2SC3205 is connected to the emitter of the unknown transistor. So, it's an NPN transistor, whose emitter and the emitter of the 2SC3205 are connected to a common point - ground.

The collector is connected to the -ve terminal of the buzzer. If it was a PNP transistor, the collector would be connected to the +ve terminal of the buzzer. If instead, the -ve terminal of the buzzer and the collector were grounded (in which case, it could have been a PNP transistor), the base should have had a pull-down (or pull-up) resistor, which it doesn't have.

And you checked the common emitters are connected to ground. So, that proves that it is an NPN transistor.

Hope this helps.
Tahmid.

 

[sdowney717]

Thanks.
I have several ideas on a replacement for a 3B smd

currently have these
2SC3205 (all i can find on this is for 3206)
NTE199

would have to buy this 2N918 which is supposedly more direct cross

How well would the NTE199 or 2SC3205 work compared to the 2N918?
A 2N918 has lower emitter base voltage of 3v versus 5v
would that affect it turning on and off?

Any other better choices to use? 2N918 is 4 leads and it may be obsoleted.

OK, I found one for c3205