Help with low power h-Bridge

[Rajagopal87]

I have a few doubts with the following circuit:

**broken link removed**

consider the one at the bottom without any inverter..
There are no protection diodes in the circuit!!
Is it ok to use this or Must the diodes be added? Is a 1n4007 across the four transistors will do?

Also to connect its i/p to an at89c51 Uc, is the following modification ok(included in attachment)? Is 1K resistor ok?

Also what about the base terminal of the mct2e transistor? can it be left open? What about the NC pin? Can it be left open or must be tied to high or low?

Thank You.

 

[vortechs2k]

Diodes to protect against a reverse voltage....a good idea.
The resistor size is going to depend on the amount of current needed throught the base of your h-bridge BJTs. You should find that out and then use Ohm's Law to figure out the rest.
I don't see any reason why the attached circuit won't work, except that you will need a resistor in series with the diode of the photocoupler in order to limit the current.
As for connecting the base of the photocoupler, there is no base connection on a photocoupler. The base is optically connected to the diode which is why it's a photocoupler. And, NC stands for No Connect so you don't want to connect that to anything.
I'm not sure what you're talking about with the inverter, but I don't see one anywhere.

 

[Rajagopal87]

Base terminal..

But the datasheet shows pin 6 as the base of the phototransistor???

 

[audioguru]

There are no protection diodes in the circuit!!
Is it ok to use this or Must the diodes be added? Is a 1n4007 across the four transistors will do?

The inductance of the author's motor must have been small so the voltage spikes weren't big enough to destroy his transistors. Diodes are cheap protection and one should be added across each transistor to keep the spikes within 0.7V of the supply voltage. Make sure that the supply has a bypass capacitor.

1N4007 diodes are 1000V and cost more than 1N4002 which are 100V. These are very slow rectifier diodes that won't work if the motor has high frequency PWM for speed control.

Also to connect its i/p to an at89c51 Uc, is the following modification ok(included in attachment)? Is 1K resistor ok?

A 1k resistor provides only 4.3mA of base current to the NPN transistors. If your motor current is 50mA or less then 2N3904 transistors would saturate fine. Reduce their value for higher motor current.
Add a second resistor in series with the base of each PNP transistor.

Also what about the base terminal of the mct2e transistor? can it be left open? What about the NC pin? Can it be left open or must be tied to high or low?

Don't connect anything to the base nor to the NC pin.

 

[Rajagopal87]

Doubts...

Thank You audioguru for the reply.

If I use a 220 ohm resistor the current is about 22mA (sorry the supply is 6v and not 5 as indicated) , will it be ok (collector current upto 0.5 A max)?
Also what value should I use for the byepass cap? On what basis it should be decided? Also why should a series resistor be added to the pnp and of what value?

In the input side of mct2E will a current of 10mA suffice? What should I take the drop in the led as?

Thank You Once again.

 

[audioguru]

If I use a 220 ohm resistor the current is about 22mA (sorry the supply is 6v and not 5 as indicated) , will it be ok (collector current upto 0.5 A max)?

Some of those little transistors have a max current rating of only 200mA. The BC337 or BC338 NPN and BC327 or BC328 PNP are rated for a max of 800mA. If your motor draws more when starting or when stalled then these little transistors will blow up.
Their current gain is low when they have a 500mA load but the 22mA from the 220 ohm resistors should drive them well.

Also what value should I use for the byepass cap? On what basis it should be decided?

I use 100uF as a bypass capacitor for low frequencies and add a 0.1uF ceramic disc capacitor for medium frequencies. I add a 1000pF ceramic disc capacitor for RF frequencies. I don't know if you have high frequency PWM motor speed control.

Also why should a series resistor be added to the pnp and of what value?

You don't want the NPN opto-transistor trying to apply unlimited current into the base of the PNP transistor. The opto-transistor will fry since it would have high current with 5V across it. Also the unlimited high current into the base of the PNP might POP it. Use 220 ohms in series with the base and 1k to +6V.

In the input side of mct2E will a current of 10mA suffice?

Didn't you look at its datasheet? Its minimum current transfer is only 20% so 10mA in gives only 2mA out. Its max input is 100mA which still isn't enough and the Uc can't supply such a high current anyway. You need an opto-darlington coupler that has a 100% minimum current transfer rating.

What should I take the drop in the led as?

It will be listed on the datasheet of which one you choose.

 

[Rajagopal87]

Please explain...

Quoted from your reply...
You don't want the NPN opto-transistor trying to apply unlimited current into the base of the PNP transistor. The opto-transistor will fry since it would have high current with 5V across it. Also the unlimited high current into the base of the PNP might POP it. Use 220 ohms in series with the base and 1k to +6V.


Sorry but I cannot understand it. Can you explain it again?
When the phototransistor has 5 volt across it it implies the point A is at 5v. If it is so then willnot the pnp transistor be OFF?

 

[audioguru]

The base of the PNP will be at about +5V (when its emitter is at +6V) when the NPN photo-transistor conducts, and nothing limits the current unless a resistor is in series. It should be wired like this:

 

[Rajagopal87]

Got it..

Thank you audioguru, I have got it..