I had a problem getting a 12v relay to energise.
Suppy is 12v, transistor base supply = 5v (2n2222 etc) i.e. low power GPT
Initially I had the relay coil between the Emitter and Ground. but It would not energise.
When I checked the circuit I found that the input to the coil was about 5v,
When I changed the configuration so that the load is above the Transistor i.e
12v now goes to the coil
The other side of the coil goes to the Collector
The Emitter goes to earth
It works.
I get exactly the same using a 2n7000 MOSFET
So the load under the transistor doesn't work, but above does.
I've used both configurations with 5v devices before and no problems.
Does anyone know why the load has to be connected to the Collector when using 12v (and presumably any valid voltage over 5v).
It has to do with the way the semiconductor regions are doped and the potentials they need to conduct. There are some great classes you can take that explain semiconductors.. I would start with understanding diode first.
Also on your project - don't forget to use a flyback diode for the relay. Please google.
Does anyone know why the load has to be connected to the Collector when using 12v (and presumably any valid voltage over 5v).
Yes.
Because of the way a transistor works the emitter voltage follows the base voltage when you connect it the first way. By follows I mean is the same as, minus about 0.6V. Whatever voltage you measure on the base expect about 0.6V less on the emitter. I'm sure there are loads of good tutorials out there that explain it, or do a course on basic electronics.
