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### Topic: Help with base resistors for half-bridge circuit (Read 1 time)previous topic - next topic

#### BetterSense

##### Apr 27, 2011, 05:32 am
I'm building a 5-phase stepper motor driver with 5 of these half-bridge circuits, one for each stepper motor coil:

I'm using TIP111 darlington for the NPN and TIP116 darlington for the PNP. I have them hooked up as shown, but I don't know what resistors, if any, that I need to use  on the bases of the transistors so that I can hook up my arduino pin and have it work. To be honest I don't understand why I need resistors at all, because it seems like switching both transistors "on" completely would be the goal.

Also, it seems that I will have to drive the base of the PNP tranistor all the way up to 5V in order to turn it off. If I'm running my motor on 5V, that is, if my positive rail is 5V, can I switch this setup with the 5V output of the Arduino satisfactorily? What if I want to run my motor on 12V; is there some way I can make it work?

#### MarkT

#1
##### Apr 27, 2011, 12:10 pm
Firstly that circuit has a problem - shoot-through.  Darlington's take a long time to switch, several us, and thus when you switch the output there will be a time when both NPN and PNP devices are switched on - this will cause a current-spike which may cause problems.  If you can drive each transistor separately you can adjust the timing to prevent this.

Secondly the base resistor needs to be low enough to drive the transistor into saturation - this you get by taking the minimum hFE value at the current levels of interest - ensure the base gets more current than the hFE value suggests by a factor of about 2.  So for example if hFE min is 500 and the output current is 3A, you'll get 6mA to just turn on the device, so set the resistor to provide 12mA - ie about 270 ohms (remember  Darlington's base will be about 1.6V above the emitter).

You will have a lot of loss in that circuit - the supply is 5V, each Darlington may drop about 1.0V when switched on, so the motor will only see 3V or so (only 60% efficiency)  - Using ordinary power transistors instead of Darlingtons would be better (but you might then need more current amplification to drive them (but not as a Darlington!)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### BetterSense

#2
##### Apr 27, 2011, 03:05 pm
Thanks for the info on the base resistors. Both the NPN and PNP have the same hFE, so should I use the same calculation for the base resistor for both?

I have regular NPN and PNP transistors I can use, but I was worried about being able to switch them completely on directly from my arduino. I think the arduino will only put out a bout 20ma on a pin.

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