Hello, I need to control 6 DC motors, direction and speed. H-bridge would be great, but i would need 24 transistors. I found simple schematic, with one relay for controlling direction and one transistor for controlling speed. I have one question, in H-bridge there are protective diodes on transistor, but in this schematic with relay I've got no idea how to connect it or if I have to connect it at all. (only on transistor for controlling speed). I appreciate all answers.

 

A diode across the relay coil and an MOV across the motor.

 

I have diode across the coil, and MOV is?

 

You could also use a r/c snubber across the motor.
Max.

 

I have diode across the coil, and MOV is?

https://en.wikipedia.org/wiki/Varistor

 

It doesn't matter about polarization in circuit?

 

What is the motor voltage and maximum current rating?

 

Voltage maximum 5V and current peak up to 1.5A

 

It doesn't matter about polarization in circuit?

MOVs are bidirectional which is needed in your circuit as the voltage across the motor can be either polarity.

 

You might consider one of these. Some of them are dual-bridge, so you would only need 3 of them.
You might also consider PWM for controlling the motor speed using a 555 timer. It's more efficient, dissipating little heat, so shouldn't require a heat sink.

 

Yes that's true I didn't notice that, thank you very much

 

I'll be control motors wih arduino, so i will have PWM from it

 

How do you intend speed control/PWM with a relay?
Why not use a mosfet to switch the relay, 2n7000.
Max.

 

Relay will just be switching between directions, MOSFET will be controlling speed of the motor

 

MOVs are bidirectional which is needed in your circuit as the voltage across the motor can be either polarity.

Yes, but the diode should be placed across the two common contacts of the relay, through which the current flows unidirectionally!

And in answer to an earlier question, yes - polarity IS important! The diode must be reverse biased in the normal direction (ie diode -ve to the +ve side of the supply) otherwise you will quickly begin sending smoke signals with your transistors!

Finally, what's wrong with using 24 transistors? I suspect they would be cheaper than six relays! Or use the IC H-bridges mentioned by Crutschow.

 

G

 

What's wrong? That 24 transistors willl cost me arround 24€ and 6 transistors plus relay arround 10€ and I found IC for max 1A and that's too low for me, so i can add diode across the transitor like in normal h-bridge?

 

I think you will spend 5x of 24€ in labor trying to reinvent the wheel. My suggestion is to start with a proven design like this https://www.pololu.com/product/2503 and go from there.