Can I run more power with relays then bipolar transistors?

[Jean Perret]

Hi folks,
I'm still looking how to boost some few RC cars using a lipo 3S instead of the standard 9.6V nicd/nimh packs and my last attempt on different cars resulted on a fried NPN transistor on the circuit board of one of the cars. Then I decided to do a little more research and it seems that RC toy cars using relays on their board instead of bipolar transistors could take more power and maybe run safely on the lipo 3S, correct or not? And also why do some cars use 3 relays? forward, rear and steering
Thanks!

 

[Harald Kapp]

Your question can't be answered in such a general way.
There are powerful transistors and weak relays and vice versa. A relay is muuuuuuch slower in switching than a transistor. Therefore, when switching speed matters a transistor is the component to choose.
A fried transistor ist a clear sign of overload. You need to look not only at the voltage and current ratings of the transistor but also at the power and thermal ratings. Typically you need a heatsink to cool a power transistor. Without heatsink it can easily blow even if the voltages and currents are all within specification.

If the transistor was on the original PCB and worked with the original battery, it may have been overloaded by the LiPo. On the other hand: a change from 9.6V to 11.1V is only 1.5V.I doubt that this small increase was the cause for the transistor's death. That would mean it had been designed absolutely marginally for this circuit. Without any safety margin. I think the transistor may have been damaged by some other mechanism.

As for the number of relays: 1 relay per function? Doesn't sound implausible, does it?

 

[MicroMe]

Just a thought, but as said 9.6V to 11.1V doesn't sound much, but assuming a resistive load the power being controlled (and the power dissipated in the transistor) is proportional to V squared so 9.6x9.6 compared to 11.1x11.1 is 92 to 123 which is about a 34% increase!
There may even be a drop off in switching efficiency adding further to the power being dissipated in the transistor, so it depends how close to the limit you were in the first case.

 

[c131frdave]

I have over seven years of electric RC experience, but mostly in the airplane arena. Are the motors brushless or brushed? A 3S lipo is 12.2-12.4 volts fully charged, by the way. They drop under load, obviously, but not down to 11.1 until they are about half spent. If I were you, I'd simply get a brushless motor from hobbyking and use a ESC. Twenty bucks and problem solved.