# Can't get H bridge working properly

Hello everyone ,
I am trying to build a H bridge for a DC motor of a toy car that I have found from my childhood ,
motor is 6 V I think and I have used BC547 NPN bipolar transistors.

I connected transistors in darlington so I can get current amplified to get the motor running and used two resistors ,
R1 = 120 ohms and R2 = 8k ohms because without them transistors were getting really hot.

For controlling transistors I use two digital output pins

So I am encountering really strange problem for example when I connect battery as on the picture above I get no motors moving one the one darlington pair of the transistors and on the other ones I get motor going but it's just really slow ,
but If I short circuit collector and base electrodes of the any transistor I get motor running at full speed, but since C-E gets forward biased it destroys the transistors eventually.

This is what I have measured with the voltmeter , where transistor 1 is getting input from the microcontroller here is the circuit:

Vce2 = 5.6V , Vbe2 = 0.71V
Vce1 = 6.2 V , Vbe1 = 0.7V
and VRB = 2.36 V ,

where VRB is voltage drop across the base resistor
from the base emitter voltage it seems like both transistors are forward biased,
and base current of the first transistor is Ib1 = VRB/RB = 2.36 / 820 = 2.88 mA which is pretty high current and it should get amplified twice with both transistors.

What is more confusing is when I take the battery and connect it to the opposite sides the motor gets going with very low speed, and I measure voltages:
Vce2 = 0-82 V, Vbe2 = -0.19 V
Vce1 = -1.39 V , Vbe1 = -0.87V

I can even disconnect microcontroller pin to feed the base electrode of the first transistor and the motor is still running since the BE is reversed biased.

Why are you using darlington configuration ?

The top half of your bridge is using NPN transistors.

They need to be PNP .

EDIT

What is U1.

Does it have suitable drive output for NPN transistors.

The BC547 has an absolute max current rating of 100mA, making it unsuitable for driving motors at all.

Do you know the stall current of your motor?

Boardburner2:
Why are you using darlington configuration ?

The top half of your bridge is using NPN transistors.

They need to be PNP .

EDIT

What is U1.

Does it have suitable drive output for NPN transistors.

I am using darlington configuration because with single transistor I couldn't get motor moving so I hoped I can get it going smoothly with darlington transistor. And I thought I can achieve H bridge with only NPN transistors since those are the only ones I have at home.

Not quite sure about U1 , I guess you thought about the picture taken with my phone and the text next to the yellow wire it says "uC" = microcontroller if that's what you meant.

MarkT:
The BC547 has an absolute max current rating of 100mA, making it unsuitable for driving motors at all.

Do you know the stall current of your motor?

I hoped with darlington transistor I can get that 100mA amplified.

Managed to find some details about the motor:

voltage：3-6V
Rated Voltage：3v
reference current ：0.35-0.4A
Test:
3vrotate speed： 8000 R/M

I measured resistance across the Motor with my voltmeter and it's ~2 ohms.

So do what transistor configuration do you suggest me to use?

The top transistors require a high side driver a uP cannot bias them correctly.

I do not know where you found this circuit but it is totally inappropriate.

Google on H bridge, its possible to make your own but pricewise buying one off the shelf will be much cheaper.

Boardburner2:
The top transistors require a high side driver a uP cannot bias them correctly.

I do not know where you found this circuit but it is totally inappropriate.

Google on H bridge, its possible to make your own but pricewise buying one off the shelf will be much cheaper.

Well I had Power electronics in my college but we were studying how H-bridge works and some other charachteristics , we didn't bother too much what transistors should be used , instead of transistors professor used "switches" in all the circuits.

So I didn't know that I need PNP for the upper pair of the transistors. We just mentioned which transistors are good depending on the motor power and for what switching frequencies are BJTs/MOSFETs/triacs/IGBTs good.That is why I've made the circuit myself and yeah it's not quite good as I thought it would be because I hoped to make it using only NPN transistors , and basically I should get two pair of PNP for the upper pair of the transistors as you said.Another problem that BC547 are amplifier transistors and they really overheat when I use them for my circuit

Of course I can buy some chip as 'L293D'(which I have ordered actually 10 days ago) or any other chip but I would like also to build my own H bridge because there I can practice the theoretical knowledge that "I have" plus it way more fun to do it.

[ BTW I can't make any sense of your circuit, did you make it up?]

Here's a circuit topology that I've used in the past, a shared driver stage for diagonally opposite
switch transistors (note in my circuit the H is twisted round, you might want to redraw it more
conventionally - I arranged it that way to make the bridge-rectifier easier to fit in. I use the
bridge-rectifier as the 4 flyback diodes to save parts and board area, don't be distracted by that).

The key resistors are R5 and R6 which set the max driver stage current and thus the max
base current to the main transistors. The driver transistors both act as level shifters and turn
on the diagonally opposite low-side switch.

This circuit can't do all the decay modes since either 0 or 2 switches are on at any one time.
Don't drive both inputs high at once or all 4 will turn on and instantly fry! Leave a little
dead-time in fact between driving the forwards and the reverse input.

SimpleThings:
Well I had Power electronics in my college but we were studying how H-bridge works and some other charachteristics , we didn't bother too much what transistors should be used , instead of transistors professor used "switches" in all the circuits.

Fair enough.

Commonly mosfets are used effectivley as switches, they are better than darlingtons as they drop less volts.

Something else to consider is what the output stage of your uP is like, source/sink current, it may or may not be symmetrical , digital outputs will normally drive a mosfet,but not always bias a bipolar transistor correctly.

What you guys think about Jones NPN Transistor H-bridge

http://homepage.cs.uiowa.edu/~jones/step/circuits.html

Instead of BD679 just place Darlington with BC547s, is there any hope for this?

No, the top transistors need to be in a switching configuration, not emitter followers. Or else you'll be
limited to low currents and waste a couple of volts as heat in the top transistors.