Need advice on my first circuit. >Transistor motor controller<

[Zoxks]

Hey guys!
I just finished making my first circuit, which takes and input of +- 5V and amplifies it +- 12V to output to a motor. Since this is my first circuit design ever, I would like to avoid blowing up my house by showing it all to you guys, so you can point out all the problems with it (hopefully none)

I have a voltage source in the top right that can vary from -5v to + 5v.
Thanks guys!

 

[Minder]

Personally I would have used a 'standard' H-Bridge with Opto isolators to transition from the 5 to 12v.
Run it through a simulator like LT spice and see the results of your set up!
M.

 

[Gryd3]

Hey guys!
I just finished making my first circuit, which takes and input of +- 5V and amplifies it +- 12V to output to a motor. Since this is my first circuit design ever, I would like to avoid blowing up my house by showing it all to you guys, so you can point out all the problems with it (hopefully none)

I have a voltage source in the top right that can vary from -5v to + 5v.
Thanks guys!

View attachment 25985

Right off the bat... The +-5V Source is in series with the right-hand 12V source, and motor... that motor will have between 7v, and 17V across it before you consider the transistors...
Good thought, but you need to revisit your drawing.
It also helps to draw, similar to how you write, from left to right, or top-down.
The first items should be the 5V source, and the last should be the motor.

Keep in mind that this design and H-Bridge designs will allow you to cause damage if you try to go forward and backward at the same time.
Try out another design, then visit google and look up an H-Bridge

 

[Zoxks]

Yeah. I tried the circuit and fried my 5v source. :/ The reason I wasn't going to use a H-Bridge is the motor draws 10 amps. So yeah.
I will keep at it though. Thanks guys!

 

[Zoxks]

Well this escalated quickly!

 

[Gryd3]

Well this escalated quickly!
View attachment 25994

You can greatly simplify this circuit
In the neutral state, the 12V source is NOT connected to the motors. Scrap the rectifier, resistor, and transistor that 'connect' the 12V source based on the 5V control voltage. (If neither relay is connected, it does not matter if the 12V is on or off) . Keep the voltages separate though. That's what a relay is good for. There should not be ANY common connection between the 12V or 5V sources with this circuit.
You can also rely on using 1 Diode per relay input instead of the two you have. Remember the diode will prevent current flow for the entire 'loop'.

Please also note that all of the methods so far will offer a 'full-forward' or 'full-backward' action for the motor, if you want speed control, it's a whole other can of worms

 

[Gryd3]

If I had to use relays, this would be my design:

2 Diodes, 2 Relays, and the +-5V Source for control.
Motor and 12V Source for the output.

Control and output are completely isolated, so as long as the variable source has enough current to drive the relay, you could control smaller or much larger motors.
It's a very simple cirtcuit though. The motor will start FAST, and keep going full-tilt.
There is not speed control with this method.

If you want speed control, look into building an H-Bridge out of transistors. That is the first step.

 

[Zoxks]

Thanks for the advice!
I should have clarified though, the 5V source varies from 5v to -5v So the voltage can be anywhere in between. I put the transistor in there to control the speed of the motor; the amplitude of the signal. The relays just control the sign.As for the diodes, I knew they were redundant, but two of them helped me visualize the flow my bad really.

The transistor is a 10 AMP NPN that I got that should handle the current.

Thank you VERY much for all your help! I hope I cleared things up a bit.

 

[Gryd3]

Thanks for the advice!
I should have clarified though, the 5V source varies from 5v to -5v So the voltage can be anywhere in between. I put the transistor in there to control the speed of the motor; the amplitude of the signal. The relays just control the sign.As for the diodes, I knew they were redundant, but two of them helped me visualize the flow my bad really.

The transistor is a 10 AMP NPN that I got that should handle the current.

Thank you VERY much for all your help! I hope I cleared things up a bit.

You did, thank you.
Good job on the updated circuit then, and please keep in mind that the rectifier for the +-5V Adj supply will drop 1.4V... Then you need to reach/exceed the threshold for the transistor, so the speed control may not actually begin until half-way through the adjustment.
Be careful with linear control of high current devices as well. I'm not aware of the current draw of the motor, but the speed control transistor will drop a lot of 'power' , even if it's rated for 10A, the heat build-up could very quickly kill it. ie... 12V @ 2Amps would be a whopping 24Watts! You need a heat-sink on there, and if that does not work, you should look into 'pwm' speed control.

 

[Zoxks]

Thanks!
I thought about the rectifier drop. Not sure what I would do about that.
So I took your advice and looked up transistor H-Bridges, and got of this video:

I think this would work well for me. Would it not?

 

[Arouse1973]

Well this escalated quickly!
View attachment 25994

Hello
You have your emitter of the transistor connected to +12 volts, is this correct?
Adam

 

[Zoxks]

Yes. I found that as long as the voltages were independent of the transistor, that current would flow in either direction.

 

[Arouse1973]

It will anyway but the gain of the transistor is much reduced connected the other way.
Adam

 

[Zoxks]

Really! That is very good to know! Thanks!

 

[Arouse1973]

Yes look at how a transistor is made and look at the levels of dopant used for each terminal of say an NPN for example. You also have operating voltage limits less than used the correct way around.
Adam