Overvoltage Protection Circuit

[Beefy]

Hi All,

I've just made 3 overvoltage protection circuits for my G203Vs. The circuit is from the Gecko website and uses the TIP147 darlington transistor and a 33 ohm power transistor to "dump" current when regenerative braking causes the voltage to rise higher than that of the power supply.

My drives have the max. allowed supply of 80 volts which believe it or not can only run my rubbish motors at half speed, their inductance is huge. So rapid deceleration could possibly cause the reverse voltage to rise above 80 volts and kill my beloved G203Vs.

My question about this circuit is as follows. The resistor which dumps the current is 33 ohms and this only allows a max of about 2.4 amps to flow at 80 volts. Is 2.4 amps enough to prevent an overvoltage condition. What if you had motors which had a max phase amps allowed of 7 amps. Can a current dump of only 2.4 amps prevent regenerative braking voltage from rising too high under high deceleration.

What I don't understand is the TIP147 transistors are rated at 10 amps so could the dump resitor not be 8 ohms to give this current dump capability.

The Gecko note with the circuit says this resistor value is chosen to protect some parameter of the transistor, but I didn't understand what it is. I just looked at specs I downloaded for the TIP147 and seen 10 amp capability.

Thanks,

Beefy.