H-Bridge delivering half voltage, what's up?

Blake

Hi,
I have created an H-Bridge using four darlington (TIP120) transistors to power a motor. The bases of the four pins are connected to a BS2. The transistors, when activated in pairs, supply 12V to the motor, either in a forward or reverse direction. The problem is that they only seem to be supplying right around 5 Volts. I have checked the collector emitter of each transistor and it indeed registers 12V.

Why would the motor only be supplied with around 5 Volts?

Thanks,
Blake

allanlane5

You COULD be in the "linear" region of the TIP120's, in which case you need less resistance in the 'base' leads.

You COULD be measuring the voltage incorrectly -- an H-Bridge needs to be measured across the motor, not from one lead to ground. Though that shouldn't matter.

You COULD be controlling the H-Bridge incorrectly.

You COULD be using too small (too little current available) in your 12-volt supply, and the motor current is pulling down the voltage.

Blake

This is what I have. With 24V switching power supply I get 4.7V delivered to the motor.

Blake

by the way this is my code:

LOW 4
LOW 11

main:

HIGH 4
PAUSE 5000
LOW 4

PAUSE 5000

HIGH 11
PAUSE 5000
LOW 11

PAUSE 5000

GOTO main

Phil Pilgrim (PhiPi)

Your upper transistors, being NPN are operating as emitter followers. You'll never get a higher voltage on the emitters than you've applied to the bases. To remedy this, you must either use PNPs for the upper transistors or provide a base voltage to the NPNs that's at least your desired drive voltage, plus 1.2V (for Darlingtons).

-Phil