Power Supply Circuit Help 1

[scottm8211]

This is probably a simple question, but I'm more of a computer guy with some experience in logical circuit design (i.e., electricity is not my strong point). I need to know how to build a power supply circuit for a project I'm working on, and here is a description of it: We have a basic circuit with a microcontroller that interfaces with an LCD screen and a keypad (nothing fancy, low power draw). We also have four motors that interface with the microcontroller via relays. These motors each draw upwards of 1.8 amps or so when turned on.

Here is my problem: I need to build a power supply that can handle powering the basic circuit and the four motors. When I power all of it using the same power line (even with a supply that is more than capable of providing the current), the motors won't run. I'm assuming there is some kind of current bottleneck in my circuit somewhere. When I use completely different power supplies though (one for the circuit and one for the motors), then they work. How would I go about building a power supply with completely separate power lines then? Is it enough to have one transformer with separate voltage regulators, or would I need separate transformers too? Does any of that make sense?

Thanks for your help!

 

[sisif]

It looks like the motors are reseting your micro when turned on... Separate supply lines would be nice! While the motors don't mind of supply fluctuations (do they really need a regulator?), the micros need good quality supply: filtration, regulation, no spikes/dropouts, etc.
You didn't say anything about the supply voltages of the micros and motors: same? you mean the motors run at 5V (or less), drawing 2A?!
Basically, if you use a separate regulator for the micro/lcd, with good filtration AND use separate ground lines for the electronics and motors, you should be ok.

 

[itsmoked]

I agree with sisif.

You should have a supply big enough to run the motors that is unregulated. Then heavily filter some of this unregulated power and regulate it. Send this only to the micro.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[scottm8211]

Thanks for the responses sisif and itsmoked! Yes, I forgot to mention that the motors do run on 5-6V. They run under a pretty heavy load (specifically as part of a peristaltic pump), so they draw a lot of current. It does seem like a pretty low voltage, but they work really well; the motors are geared, so I don't know if that makes a difference. The microcontroller also runs on 5V.

 

[itsmoked]

That's a typical setup. Motor of that type usually don't care about the voltage +/- a few volts. So I would set up the unregulated supply to be 6.5~7.5V

Then you have some decent head to run the 5V regulated output.

Normally one runs motors of that ilk with transistors not relays. Relays wear out! Also you can't do speed control or mechanical shock limiting. With transistors you can PWM the power to the motors and have them start smoothly and in a controlled manner and run them at any voltage you want. Transistor would likely cost less than relays too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

You could isolate the micro supply with a resistor and a large value cap of 1,000uF or more. Micros usually don't draw too much and can get away with 37-100 ohms without much of a voltage drop. Even lower resistance will work with enough capacitance. The cap would maintain the power to the micro on a momentary sag. There are better ways, but this will work if much of the hardware is already established.

 

[scottm8211]

Thanks a lot for everyone's help! I have kind of a follow-up question to this then. I'd like to use a transformer to build an internal power supply (along with bridge rectifier, capacitors, voltage regulators, etc.). My question is, how do I compute the necessary power rating for the transformer? That is, how do I figure out the necessary AC volt-amps given that I have four motors using about 1.5-2 amps of DC current?

Thanks!

 

[itsmoked]

Actually what you're asking is non-trivial since it depends on the "form factor", which is how the transformer has to deliver current to the output circuit.

It can range from about 1.2 x the load VA to 2.4 x the load VA.

If you are just using a rectifier and caps that's about as bad as it gets.

If you are just doing a one-off figure out your maximum power needs and just get a transformer that's 2x what that comes to.

Note you can also just buy an unregulated supply that has all this figured for a maximum current rating.

Keith Cress
kcress -

http://www.flaminsystems.com