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Topic: Controlling DC motor with transistors with reversing polarity (Read 6883 times) previous topic - next topic

__Mitnick__

So, I have problems with my project. I'm building motor with changing polarity. I can't use relays because they use a lot of power. I have 4 NPN transistors BD911 switched with arduino. I can't figure out how to connect external 12v power supply to this. I have these transistors in H bridge arrangement. I need 12v for motor and 5v from arduino to control transistors.
1. I can't get 12v on motor idk why
2. I have reverse current of -4v on base-emitter.

Please help, I need solution before tomorrow 10pm.

P.S. I'm reducing reverse current with diodes.

Southpark

Maybe npn transistors cannot handle the current unless you use some 'darlington pair' ones. You probably just need to do basic testing in an "H-bridge" configuration. Remove motor and temporarily limit the emitter and collector currents with power resistors. Then test transistor switching by applying suitable base voltage.

You should include a circuit diagram.

alka

Apart from what southpark mentioned about poor current handling with transistors.

You would need to be using PNP transistors for the upper switches of the bridge. For NPN's to work on the high side you would need to be supplying a base voltage higher than your input 12v.

The NPN will not conduct for the high side otherwise.

larryd

You will need a circuit similar to this (NPN and PNP), should be lots on the internet. Transistors sould be matched. Diodes to be added.




What are the motor ratings?
There are relays out there which draw very little current.
.
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alka

I have used this circuit for a number of small projects and it works very well.
Once again don't forget to add diodes as the schematic does not include them.



With this schematic or LarryD's you have to make sure that under no conditions will inputs A and B be both HIGH.




larryd

Since you want to use 12v for the motor, you will need a driver transistor for the PNPs.

Edit:
cct. from alka looks interesting.
.
No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

alka

Since you want to use 12v for the motor, you will need a driver transistor for the PNPs
.
The second schematic works in this way. The base of the PNP's are pulled down to 0.7v with the lower NPN's. Turning them on.

That circuit is just about the easiest H-bridge you can make that doesn't have shoot through issues. Its not perfect but it sure is simple.




larryd

Might be a good idea to have a short time where both A and B are off so there is no chance the two are on at the same time.
i.e. A on B off,  A off B off, A off B on, etc.

.
No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

__Mitnick__

Thanks for help guys! However I don't have any pnp transistors that will handle 2,2A motor and I don't have any shop with electronics nearby. Even if I would shop them on Internet they would arrive on friday and I don't have that much time. I don't have that low-power relays either.
I can't build LarryD's circuit because I have only 5 npn transistors, as I said before.

I've just measured voltages on everything connected except motor. Thing is I still don't have that 12v but when there is no resistor on transistors (for a while of course) I have 2,5V, when resistor is present I get 0.35V. Also when resistance is removed I have insane temperatures on second transistor when first remains cool.

alka

Do you have a schematic of what you have set up now?
And specs of the motor?

2.2 amps is asking a lot from a transistor bridge.

I know you don't want suggestions with parts you don't have but LarryD and I were basically showing some bare minimum circuits that will work. There is really no way to use all N channel transistors in a bridge without some type of high side driver / charge pump.

Do you have relays?  It may be your only option to get something together for tomorrow.

If you have relays and you have transistors to drive those relays...



__Mitnick__

I don't have any specs about motor. It has been recovered from old vhs player. I know that it work perfectly on 12v and require 2.2A. I'll try driving those relays with transistors, that won't suck so much power from batteries.

I have that schematic.

alka

This circuit as is would need pnp's on the top to work unfortunately.. and npn's to drive those.

The flyback diodes are not in the right place. D1 through D4 should be connected between emitter and collector of each transistor not between emitter and base.

D5 and D6 aren't needed, they aren't doing anything.

Take each transistor and replace with the relay circuit above and you might be ok.
You will need 8 total diodes then. Flybacks for the motor and the relay coils.

__Mitnick__

Thanks for help. I just realized that I put diodes in wrong place, on breadboard they're between emitter and collector. I think I'll stick to that idea to use transistors as drivers for relays. I have two power sources, both batteries, 9v and 12v 4Ah with L7805CV voltage regulator so that'll be okay. That relays I have requires 73mA of current to open.

Again thank you all for help.

CrossRoads

"I know that it work perfectly on 12v and require 2.2A."
And you're worried about driving the transistors from batteries? The motor will drain battery all by itself - what is the AH rating, or mAH rating of the 12V battery?
You can find 2A contact rated relays that only need 11-12mA to drive the coils. Use a DPDT relay for motor direction, and a SPST for power on/off.
2A, 12V coil
http://www.digikey.com/product-detail/en/kemet/EC2-12NU/399-11029-5-ND/4291095
3A, 12.5mA coil relay, 12V coil
http://www.digikey.com/product-detail/en/te-connectivity-potter-brumfield-relays/V23105A5003A201/PB384-ND/291020

Need a transistor to drive the coil current.

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

__Mitnick__

Thanks. On friday I'll do it with relays and then I'll buy low-power relays or pnp transistors. This will work for about 10 minutes only so, it's okay. Also, I wrote about battery earlier, 12v 4000mAh.

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