Bipolar Plugpacks?

[krivx]

I want to build a few projects that require bipolar 12V or 15V supplies. For projects where only single supplies are needed and I want to do things at a low cost I would source a wallwart/plugpack power supply and either regulate or filter. Do bipolar versions exist, possible with a concentric 3-pin barrel connector? This would be much easier than an inverting charge pump or SMPS solution.

[SeanB]

If you use a 12VAC wallpack and then 2 half wave rectifiers you can just do the job with a single 2 pole connector. The rectifiers are in the equipment, and will provide a pair of low current rails. A 1A transformer will be able to supply a pair of rails that draw around 300mA each with not too much difficulty. The capacitors though would have to be around 3300 -4700uF 25V, and will need a regulator on them afterwards to reduce ripple, which will be quite high. The diodes will have to ne 1N540x diodes though as the peak current through them will be rather high.

[krivx]

That would work but wouldn't it be less efficient? As you say I would be shelling out for a 1A transformer to get 2x300mA output. If I could get a a similar transformer with a center-tap then efficiency would be higher and the power supply could be cheaper... this could be worth pricing out though. I like that a 12VAC wallpack can just use a normal 2 pole barrel connector.

[SeanB]

This is often done to just use the cheaper transformer, power lead as well as a common powerpack across a range. Efficiency is lower, as there is more loss in heating diodes and wiring due to high peak currents, but overall the power loss in either method is around the same. You just use a cheaper more readily available connector and cable, along with using the cheaper to wind transformer.

[krivx]

You're right, it looks like using a 12VAC or even 15VAC lets me leverage economy of scale, transformers are very cheap. I think I'll evaluate a nice one and a really cheap one and try this out.

[digsys]

You're right, it looks like using a 12VAC or even 15VAC lets me leverage economy of scale, transformers are very cheap. I think I'll evaluate a nice one and a really cheap one and try this out.

Actually, there's another small issue with using a centre-tap. Volumetric Energy efficiency ie Each half of the transformer ONLY delivers energy
on successive half cycyles, so your transformer is ~60-80% larger than it needs to be. A better plan would be to split the centre-tap (if possible),
or buy a dual 12/15VAC and make 2X separate supplies. That way you can then configure for (assume 2x12V 1A) ... +/- 12V 1A, +12V 2A, +24V 1A etc
Most power supplies have all outputs floating, so you can reference any OP or none to real GND. Also you get better ripple due to full wave rec.

[Zero999]

You should also try to make the DC 0V current as low as possible, as a lar DC current in the secondary of the transformer could lead to core saturation and overheating. In other words, the average current drawn from the positive and negative 15V rails should be pretty similar.