# Current constant across h-bridge load

Discussion in 'General Electronics Chat' started by kevin0228ca, Jun 16, 2015.

1. ### kevin0228ca Thread Starter Member

Jun 5, 2015
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0
I am testing how to use h-bridge to allow current go both directions across load.
My schematic

I can get current going both directions as I see current square wave in simulation.

How to design a h-bridge that will produce constant current across Rload?
Or is there any simple IC?
My I source will be constant less than 10 mA.

Thank you.

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
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1,456
With the circuit shown there is shootthrough if Rload is >100 Ohms, i.e Q3 and Q5 conduct at the same time and rob the load of current.

3. ### kubeek AAC Fanatic!

Sep 20, 2005
4,851
859
The bigger problem is that the current goes straight through the BE diode of Q6 into BE diode of Q4 and straight to ground. You need to swap top and bottom transistors.

4. ### Sensacell Senior Member

Jun 19, 2012
1,494
394
Base current in the transistors steals some of the current.

Change it to a complimentary MOSFET pair and things will get better.
You will still have some charge injection when the gates switch, but at DC it will work perfect.

OR... put your current regulator inside a rectifier bridge- at the output of a normal H-bridge.
This would only work with a 2- terminal current regulator.

5. ### kevin0228ca Thread Starter Member

Jun 5, 2015
30
0

My V1 and V2 has a phase so that when V1=5v V2=0v.

From what I read, Q5Q6 are activated by 5V and Q3Q4 by 0V.
So when V1=5 V2=0, Q4Q5 activate, so current goes source->Q5->load->Q4->ground
when V1=0 V2=5, Q3Q6 activate, so current goes source->Q6->load->Q3->ground

Is my circuit correct?
How should I modify my circuit?
And why is Rload varying current?

Sorry I am new electric engineering which is why I use a simple example.

6. ### DickCappels Moderator

Aug 21, 2008
3,833
1,042
It might require a transformer to couple power to the circuit, but you could put a full wave bridge rectifier in series with the H-Bridge output (connect one AC input to the bridge output and the other to the load). Connect a circuit that draws a constant current between the + and - bridge terminals. I know it can be done because am doing this myself at 240 VAC 50 Hz.

7. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,800
I'm going to guess a little bit because you don't show the most important part - the current source. But I bet it must use 5 volts or you could not always turn off the top transistors. If so. the most voltage at it's output is probably around 4 volts, so if it all went thru your 500 ohms that would only be 8 ma. But it does not all go there 4 ma goes to base drive current for the top transistor leaving only about 4 ma for your load.

8. ### crutschow Expert

Mar 14, 2008
16,217
4,336
That circuit has the PNP bases directly connected to the NPN bases so there is nothing to limit the base-emitter current and both transistors will always be on (remember the base-emitter junction looks like a forward biased diode).
Place a separate resistor from each base the square-wave drive source to prevent that.