run more power with relays then transistors

[jean-karim]

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!

 

[kripacharya]

a LiPo 3s would give you around 12v-13v, which is higher than your 9.6v.
This may be one reason for the burnout.

No doubt a relay would likely handle it better, but even cars with relays would probably have some circuitry, and that circuit might be sensitive to this higher voltage.

 

[KlausST]

Hi,

Racing RC cars may draw current in the region of 100A. Surely only for seconds...
A BJT may have too much Vce sat. Try to use low ohmic mosfets.

Klaus

 

[kripacharya]

100amps !! wow. And this is at sub 12v operation ?

With a bjt in saturation the typical power dissipation would be around 20-30watts, with a typical Vce(sat) or 0.2 - 0.3v, or would it be higher at those currents ?

However to achieve better than this, a MOSFET would have to have an Rds(on) of less than 3 milliohm !