using a SD1555 transistor which supports currents up to 10A, I am trying to set up a high current relay in order to control an electric magnet. When I connect my external voltage source directly to the base of the transistor (Vcc to base and GND to emitter), the magnet receives sufficient power to work correctly and uses a current of approximately 4.5A. But if I now connect the external source to the transistor collector and add a wire from an Arduino digital pin to the transistor base and add one from the Arduino GND to the emitter (as you can see in the circuit image which I have attached), the magnet does not receive enough power anymore to function.
Can anyone tell me why and what I have to do in order to control the magnet using an Arduino?
Firstly that's a completely wrong transistor for low voltage switching, its an ultra-high
voltage part and is incapable of saturating at a low enough voltage for 6V switching.
You've built an emitter follower, this is wrong, all switching is done with the common-emitter
circuit - emitter to ground, base via a resistor to the controlling pin, collector to the load, other side of load to +6V. You will not be able to power this transistor directly with an
Arduino since the base needs about 600mA, twenty times more than the Arduino can provide.
Here you have an inductive load so you must protect against inductive voltage spike, and
this is normally done with a flyback diode connected across the coil.
Since you didn't use a diode in the circuit shown it is quite possible you've destroyed the
transistor. But its the wrong device anyway...
Use a logic level n-channel MOSFET with an Rds(on) of 0.025 ohms or less. Make sure
its logic level, and doesn't have a stupidly high voltage rating (60V would be a sensible
upper limit). The 2SD1555 is not going to hack it.
Don't forget a flyback diode capable of handling pulses of the current you use.
I am trying to set up a high current relay in order to control an electric magnet.
Hi, the relay is missing from your diagram and what are it specifications?
What is the coil voltage and current or resistance?
The 2SD1555 is only 5A from my source but until we see the spec for your relay that is not relevant.
