Huge voltage drop across transistor H bridge

Hello everyone
I had a try at buiilding my on h bridge using this tutorial

Instead of the transistors he used, i used 2 BC327 PNP transistor rated @ 800ma, 2 BC338 NPN rated at 800ma. I am driving a small dc motor whose stall current is 400mA
Everything else is basically the same, but the problem is, i am getting a large voltage drop through the h bridge. There is no heat produced or smoke and i am getting 3.4v instead of 12.

What could be the cause of this? I have to admit that during soldering i may have spent more time on the legs of the transistors as the solder was not sticking properly and the transistors did get warm from the soldering.

Could it be the soldering or incompatibility of the transistors? Thank you

Basically, that's a useless design, nothing to do with the transistors or soldering. The design has an inherent voltage drop of around 1.5V. Worse, when driven from 5V Arduino pins, even if you use a supply voltage greater than 5V you will not get more than about (5 - 1.5)V out of it. The only way to get a higher voltage out of that design is to level-shift the inputs to 12V using another 2 transistors. You could get about 10.4V from it that way if you used a 12V supply.

Find yourself a good H-bridge design, or (preferably) use a single-chip H-bridge instead. For that motor, I would use a SI9986, assuming that you don't need to go above 12V.

For BC327 PNP transistor you will need about 50mA to drive to saturation, but the circuit used 1K base resistor can only supply about 1.5mA, so the transistor was not in saturation therefore high voltage drop.

Find yourself a good H-bridge design, or (preferably) use a single-chip H-bridge instead. For that motor, I would use a SI9986, assuming that you don't need to go above 12V.

if i used this pair of mosfets (using that schematic) would it work fine without the extra two transistors?? i was just using those littletransistors to get a better understanding of H bridges. The real deal i want to make later has a current capability of 80A (to control 4 big motors)

The real deal i want to make later has a current capability of 80A (to control 4 big motors)

The below is an interesting read concerning making an H-bridge. Your best bet might be to check ebay for an H-bridge that fits your needs.

http://forum.arduino.cc/index.php?topic=53425.0

tawandapro:
The real deal i want to make later has a current capability of 80A (to control 4 big motors)

To build an 80A H-bridge, you need to use four or more N-channel mosfets, with specialised gate driver chips that provide a floating gate drive for the upper mosfets, and provide an idle time (when both upper and lower mosfets are off) to avoid shoot-through. Good circuit layout will be very important. This is most definitely not a project for a novice.

You may read this circuit-Using the high-low side driver IR2110 to drive the MOSFET

To build an 80A H-bridge, you need to use four or more N-channel mosfets, with specialised gate driver chips that provide a floating gate drive for the upper mosfets, and provide an idle time (when both upper and lower mosfets are off) to avoid shoot-through. Good circuit layout will be very important. This is most definitely not a project for a novice.

so i cant make a h bridge using 2 n and 2p channel mosfets, just like with bjt transistors?

tawandapro:
so i cant make a h bridge using 2 n and 2p channel mosfets, just like with bjt transistors?

You can, but for 80A you would be better off using all N-channel mosfets. This is because it's easier to get N-channel mosfets that have good characteristics for high-current switching.

You can, but for 80A you would be better off using all N-channel mosfets. This is because it's easier to get N-channel mosfets that have good characteristics for high-current switching.

Thanks for your help sir.

Is it possible for you to link me to a recommended circuit diagram or show me how to set up a system with all n channel mosfets?
Thank you

tawandapro:
Is it possible for you to link me to a recommended circuit diagram or show me how to set up a system with all n channel mosfets?
Thank you

The circuit show at reply #6 was using all N channel MOSFET.

The circuit show at reply #6 was using all N channel MOSFET.

I may have overlooked that one. I will have look. With the mosfets i have chosen i will need 10v to fully switch them on. I think i will have to use BillHo's idea. I will do some more research into this.

High side driver setup. Note that on some N high side H-bridges, the instructions say to limit PWM to 95% to apparently to operate a charge pump circuit.

zoomkat:
Note that on some N high side H-bridges, the instructions say to limit PWM to 95% to apparently to operate a charge pump circuit.

Very true when the H-bridge itself is part of the pump circuit. If you were to go 100% on the pump stops working and the capacitor soon runs out of steam and the gate capacitor on the high-side MOSFET will discharge and suddenly your bridge shuts down. Been there :frowning:

As to the first circuit shown by the OP, the transistors are in the wrong places. The NPN's should be lowside and the PNP's should be highside.

VikkiFord:
As to the first circuit shown by the OP, the transistors are in the wrong places. The NPN's should be lowside and the PNP's should be highside.

I wouldn't say that the circuit is wrong, it's using the transistors in the emitter follower configuration, which makes it safe from shoot-through. But this comes at the cost of higher voltage drop. Using NPNs lowside and PNPs highside, it is certainly possible to get lower voltage drop, but then the drive circuitry is more complex and precautions need to be taken against shoot-through.