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banditsammy · Jan 19, 2009, 07:47 PM

Need help building a dual power supply. Can anyone tell me how to do it and tell me where a schematic could be found.

KISS · Jan 19, 2009, 08:57 PM

Take two 9-v batteries and connect the + and - together. You now have a +-9V power supply.

DanielF · Jan 20, 2009, 05:32 AM

Sam,

Your question is very vague! Do you want two fixed output voltages? What voltages and current capacities? Or do you want two variable outputs? Ranges? Current limits? Tracking or non-tracking?

What about overload protection, foldback limiting, and all that stuff?

If you take the trouble to tell us what you actually want to achieve, we just might be able to help you. If you don't know what supply specifications you need, tell us what device you want to power (in detail) and we'll try to help.

Daniel

Stratmando · Jan 20, 2009, 07:06 AM

More info as Daniel mentions would help.
Here's some kits:
http://www.electronics123.com/s.nl/it.A/id.1715/.f

banditsammy · Jan 20, 2009, 05:41 PM

I have to build this for school. I need it to be 5v and 12 v. I have a step down transformer 115v to 12.6, bridge rectifier, and capacitor. I have the power supply together I just need to make it variable.

Stratmando · Jan 20, 2009, 06:35 PM

Try this:
http://www.electronics123.com/s.nl;j...ge%20regulator
This may be better, just use 2 of them (less than 10 parts)

DanielF · Jan 20, 2009, 07:05 PM

Sam,

You still haven't told us much about your requirements! First you say '5V and 12V', and then you say 'variable'. Do you mean two variable outputs, one 1.2-5V and the other 1.2-12V? Or is it 0-5V and 0-12V? At what maximum load current? If they need to go down to 0V, you'll need a negative supply as well as the positive one you've already built. For that, a centre-tapped transformer (e.g. 12-0-12V) would be better than a single secondary winding.

If this is a school project, I would have thought they'd issue you with a 'requirements' or 'specification' document. Can't you copy-and-paste the spec here so we know exactly what's required?

If it's actually a school 'student project' (as opposed to something you're simply doing to help out the physics lab, or whatever), I would have thought a kit approach was not what's required. Usually for a school project you're meant to learn something, and you don't learn much from building a kit!

If you really want to do it from scratch, download this datasheet:
http://www.national.com/ds/LM/LM117.pdf
and have a look at the 'Typical Applications' section for some ideas.

Since you haven't told us the requirements for your project, I'm assuming a linear regulated supply will be OK. A switchmode would be more efficient, but more complex. For linear regulators, one of the most important things to cater for is the heatsink. The size/type of heatsink required depends on the maximum expected load current and the maximum expected ambient temperature. Knowing those factors you can easily calculate the required heatsink - it's a simple calculation, not a black art!

Let us know if you need more help.

Daniel

KISS · Jan 20, 2009, 07:15 PM

As Daniel said, we have NO IDEA what you need or what you have. Yes, that's the exact reson why I said two 9V batteries. I satisfied your requirement of a dual power supply.

Power supplies have all sorts of specifications:
CV or constant voltage
CC or constant current
Foldback current limiting
Tracking
A ripple specification
A current specification
A voltage specification
If switching, what topology
Input voltage
Input frequency

In your case, what parts are allowed?

Tracking can be done the hard way or the easy way.
Easy way: Reference is used as a setpoint; e.g. +5 and -5
Or true tracking where there is always symmetry in the rails. i.e. the lowest number wins. If one rail of a +-12 V supply drops to 11.9 volts, the other rail does too.