View Full Version : Dangerous/incorrect usage of transistor?

04-30-2007, 01:53 AM
I'm using a Board of Education with a Basic Stamp II. I'm powering the board using a 9v battery.

I have a NPN transistor that I'd like to use to trigger a relay. The dc adapter I'm using for the relay puts out about 16v.

I've attached (as a test) the VDD to the base of the transistor, the relay's ground to the collector pin, and the adapter's ground to the emitter.

My understanding is that for transistor to work, it needs to also connect back to the base's ground (i.e., the Board's vss)... I'm feeling a bit nervous about mixing the adapter's grounding with the battery's. Is this ok to do? Would I put a resistor going from the emitter to vss (in addition to the emitter connecting to the adapter's ground)?

Many thanks!

Post Edited (SimEnzo) : 4/29/2007 8:19:38 PM GMT

PJ Allen
04-30-2007, 02:54 AM


04-30-2007, 03:16 AM
Thanks for the response, but the schematic and the pdf doesn't really answer the thrust of my question... which has to do with using a battery for the base (and needing, as I understand it, ground from the emitter). Is this safe in combination with an adapter? My concern being that the ground of the load going into the battery and doing something nasty there.

04-30-2007, 03:24 AM
Could you provide a schematic as PJ's drawing is how it's done most of the time.

- Stephen

04-30-2007, 03:43 AM
My schematic skills are a bit rough, so apologies in advance.

Here's the difference, I believe the schematic that PJ provides depends on the the base and the load to be using the same power source... right? If the base is using a different power source, then it needs grounding too.

Note, the power is coming from the VDD and ground in the VSS of my board, but the schematic has the 9v battery just to be explicit as to what is powering the board.

PJ Allen
04-30-2007, 04:20 AM
The Stamp and the transistor circuit and the relay supply should all share/have the same Ground. If they don't have the same Ground (Reference), then the circuit will not work.
Full Stop.

04-30-2007, 04:30 AM
Hmm... well, it works when I wire it up per my schematic (though I'm still wanting to know if this is dangerous and will cause the battery to explode or something)... and doesn't when I don't include a grounding from the emitter to the battery (via vss).

Do you suppose the reason it works in the setup I'm describing is that, in effect, I'm connecting the stamp's vss to the power supply's ground?

In anycase, my main question is still out there: Is this safe?

Post Edited (SimEnzo) : 4/29/2007 9:35:33 PM GMT

04-30-2007, 05:09 AM
In your case it is safe. and you don't need/want the resistor in the ground connection. All the grounds need to be at the same potential and then the 9v is powering the base in reference to ground. (which, as you said, works)

- Stephen

04-30-2007, 05:10 AM
Great, many thanks Franklin!

04-30-2007, 06:28 AM
You need to connect the grounds together, yes.

You definitely need to keep the + voltages separate.

The "common ground" insures that both supply voltages are referenced to the same ground. That's why you connect the grounds.