help on dual dc supply

I just got some things im not sure about, sorry I don't have any pictures/shematics, my phone can't upload So I want to build a decent +/-12v supply I've got two 120vac to 12vac 5amp transformers which I plan on using instead of one center tapped one, will this work the same? I don't remember off hand if they had dots on them but I would have the dot side of one connecte to the non dot side of the other, and that would be the ground reference right? Can I connect this to actual ground? I've had a hard time finding a good shematic for rectifying the +/-, one I saw had one full bridge rectifier another had two but it seemed sketchy what's the difference and is one better/possible than the other ? any links to a good design would be great

Yes, two 12V transformers can be wired to act as a center-tapped 24V transformer.

Full-wave rectifiers will produce lower ripple than half-wave rectifiers.

You'll need +12V and -12V regulators (if you want regulated voltage) and filter capacitors.

But all I need is one full wave rectifier right? I got these nice ics that I forget the name of, basically lm317's but rated for 5amps, which I imagine will be my max output anyway I just gotta get a negative regulator I guess

One full-wave rectifier per power supply so you need two.

johnwasser:
One full-wave rectifier per power supply so you need two.

Usually you’d use a bridge-rectifier, with + and - outputs going to your two smoothing capacitors. Transformer centre-tap is 0V.

Yeah that's the confusing part, I've seen shematic for with one and with two, any reason to use two not one?

For centre-tapped circuits one bridge rectifier = two full-wave rectifiers = four diodes = sufficient.

Now if I were to use two positive regulators would I need to change my setup? I got my two tranformers, one full bridge and like +/- 15v unloaded, almost exactly 12.1v +/- with a load, but I need it to be 12v all the time : /

Yes, you'd need those two separate transformer windings and 2 bridge rectifiers and connect the output +ve side of one regulator to ground on the other... Considerably less satisfactory arrangement than one regulator of each polarity.

If you are saying the output without a regulator is dropping to 12.1V under load, then you haven't enough voltage to support 12V regulators (typically they need 1.5 to 2V of "headroom". It might be that your smoothing capacitors are too small, it might be the transformer windings are too low a voltage or it might be the transformers need a higher power (VA) rating.

I measured that before. I had the the bridge actually I forgot to mention, unfortunetly they are 12v transformers from highhats actually and are meant to supply 12vac up to 5 amps now that I think about it I guess I can't get 12v, gonna definetly have to get adjustable regulators and put it to like 10v But then again maybe after the caps itll handle it since that 12v ac is almost 16vdc, and that's under load, ill get probably 18/19 volts not loaded and just a related question to the regulators, is it better to have big caps before or after the regulator?

is it better to have big caps before or after the regulator?

Both are good but the biggest caps go before the regulator.

Ok, I had some bigger caps and smaller ones, I put the big ones on already and I didn’t do the regulator yet so that works out,

About the mains ground and my "ground", is it safe to connect these two? In the future I wanna put my supply in some sort of housing and am gonna ground it from the mains if I can I looked at my supply with my oscope and I get a sine wave unless I attack the probe ground to my ground so I guess obviously its safe since I believe the probe ground is the mains ground as well?

this is a good solution

Double power.pdf (213 KB)

Personally I would not do this because it restricts how you can use your power supply. Leaving the output floating allows you to do things like use it as a negitave supply. I do not think it is any safer.

Each country has its own regulations about earthing. I'd earth unless there is a definite reason not to for a home-brew power supply. If your transformer is certified double-insulated then I might relax that rule.

You can connect the 0V rails to mains earth, but do you really need to?

General-purpose bench power supplies usually have floating outputs and a separate front panel protective earth terminal (connected to mains earth & chassis) so that you can strap earth to either rail if you choose.

A dual power supply with floating outputs gives you some useful options:

You can use it as a positive and a negative supply with a common 0V, by strapping the -ve of PSU A to the +ve of PSU B.

You can use it as a dual positive supply by strapping the -ve of PSU A to the -ve of PSU B.

You can use it as a dual negative supply by strapping the +ve of PSU A to the +ve of PSU B.

You can put them in series (to increase the voltage) by strapping the +ve of PSU A to the -ve of PSU B.

You can put them in parallel (to increase the current) by strapping the +ve to +ve and -ve to -ve. (But there are a few caveats to this configuration, though)

Additionally, you might want to connect an oscilloscope between two points of the load that are not connected to the common 0V rail. You can do this with an isolated supply, but you can't with an earthed supply.

If none of the above matters (but I think it does!) then there is probably no reason not to connect the negative supply rail to mains earth.

John.

Yeah those aspects are less important as I really wont be re configuring it too often, and when in oscoping its annoying to have to attach the ground clip to get a meaningful result

and when in oscoping its annoying to have to attach the ground clip to get a meaningful result

No. A scope is measuring a voltage. A voltage is a potential difference between two points. Measuring anything on a scope using the ground return of the instruments is always going to give you a false result. Sometimes it might be close to the real result but it is always false. There is a lot of rubbish talked about grounds being safe. Grounds can be just as lethal as high voltages under some circumstances.

So it would be a good habit to always attach the ground clip to the circuit being probed?