SparkFun Forums

[jdraughn]

I am designing a little portable bench power supply that will give me numerous outputs, while I am still undecided, and I don't want it to be too overkill, I am thinking +-3.3v +-5v +-12v +-15v plus +-adj x 2.

Or maybe just 3 or 4 +-adj with switches and/or knobs to adjust each output. I do lots of pic programming hense the 5v and 3.3v need, plus there are numerous 12v circuits, and I just started playing with op amps which require the +-, not to mention some audio amps which also require the +-.

Anyway, one of the other must have features was for it to be portable meaning it can run off batterys, probably just use some nimh AAAs, but I have started thinking about some small sealed lead acid batterys. I am trying to keep the weight down though and the transformer is already heavy enough as it is.

I see in the LM317 datasheet an example where there is a switching regulator circuit. Does anyone have any idea how much this changes the circuit as far as effeciency goes? I would like to possibly have as much as 25+ volts available, but if I use a 30v+ transformer then when I want 5v I am creating a ton of extra heat.

I have considered using a PC power supply but the very very low current outputs of the negative rails have me concerned that they may not be enough.

[waltr]

This will be a handy item.
I'd suggest either designing it to be powered from the AC mains or from a battery, not both.
From AC mains use a step-down transformer with multiple outputs or more than one transformer.
I don't see much need for +-3.3V so suggest outputs of +3.3V, +5V, +adjustable then +/-5V, +/-12 or +/-15V and/or Adjustable +/-5 to 20V.
Have the option of having each set of voltages isolated or on a common ground. Of course a voltmeter on the adjustable supplies is essential and current meters are handy.

I've used lots of LM317's and work fine (do put all the protection diodes on as in the data sheet) but not in a switcher configuration. From my design experience with switchers I wouldn't use that combination. Its old technology and there are plenty of newer easier to use parts.

For an AC main powered unit some heat sinks and maybe a small fan would be all that's needed. I wouldn't be too concerned about efficiency since it is powered for the mains. Just use fixed voltage regulators for the fixed voltages. Linear Tech makes some good ones.

Now for a battery power unit things change. The easiest is just enough batteries to get all the voltages needed directly. Crude but it works and I do this at times. Keeping all the batteries properly charged is problematic however.

To use a single, say 12V, battery, I would used DC-DC converters. The technology of these devices has improved greatly over the past few years. Again Linear Tech has some nice easy to use DC-DC chips. The LT3501 is a 3A dual step-down supply where the output voltage is set with a resistor divider. This would be prefect for your +3.3 and +5V outputs.

An idea: Make each of the voltages modular and start with the minimum needed. Then you can add voltages as needed or replace a module if you come up with a better one or if one fails.

Good luck and ask more questions.
walt

[jdraughn]

Waltr,
Thanks for your response. Sorry it has taken me so long to get back to you, I have had a lot going on in my life and have not been able to work with my electronics lately.

I made a quick temporary supply by using two 18v printer switching adapters but wiring them so I had -+18v by connecting the positive lead of one to the ground of the other.

It works ok but would like to make it variable. I don't have the negative equivilent of the LM317 and would rather not pay 6.00 of shipping for one chip.

Would it be possible to wire two LM317's in series like I did with the switching adapters to get a negative rail? Would it require a different ac transformer? It seems to easy to me, why would they make the negative voltage equivalent if thats all you have to do? I don't quite understand the priciples well enough yet.

[waltr]

Yes, you can use two LM317 to get a +/- supply but with a caution.

The LM317 inputs (transformer outputs) must be isolated, as in two separate transformer windings or two separate transformers. Draw it out and examine the current paths.

Here at my employment we have a couple of products that are done exactly this way. You were able to successfully connect your two 18V supplies due to their outputs are isolated.

[jdraughn]

Thanks Waltr, I think I will try using those two 18v adapters. With a 3v voltage drop I should hopefully be able to get +-15v on down with two LM317s. And actually, I just did receive two of the 5 amp equivilents. I have some laptop adapters that do around 19v at 3+ amps. This could make a pretty compact and powerful bench power supply that does not weigh 30lbs.