Hi Tom,

First, verify that the inputs are logic level, not more than 5 or 6 volts.
Second, R1 and R2 are 47 ohms. That puts a lot of drive to the power
transistors and allows the bridge to drive 10 amps of output. However,
at 48 volts I would only rate the bridge at 4 amps. So you could
increase R1 and R2 to 100 or 220 ohms and your driver transistors will
thank you for it. Definitely while testing under light loads 220 ohms will
allow you to debug with less chance of trouble.

Cheerful regards,

Bob



On 2 Apr 2003 at 20:10, RADAR Engineering Team wrote:

> Bob,
> I appreciate your prior help and I am coming to you again for assistance.  I am still trying to use your H-bridge design (with very few changes).
> I have exchanged the 2n2222 switching transistors with 2n2102 and I have exchanged the TIP120/125 with TIP140/145.
>
> The 2n2222 have a breakdown voltage of 40VDC and the 2n2102 have a breakdown voltage of 65VDC.  I am using the H-bridge with a 48VDC geared motor (at 300Hz and a max of 450rpm).  currently I can easily control a 12VDC motor from 0-100uty.  When I increase the voltage and use a 48VDC motor the
2n2102 transistors heat up and my scope shows very little change in speed when the duty cycle is changed.  I have simulated the circuit and all component voltages are within spec.  I am not sure where to go from here.  Will biasing the circuit differently be useful (or is there any other easy
fix)?
> Thanks for your time,
> Tom

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