Hey all
I'm thinking about making my own pedalboard PS, but I'm running into the problem of different voltages. If I had 2 transformers that were 18-0-18 and 9-0-9, could I make supplies for +18, -18, +9, and -9v, right? I'd think I'd have to regulate the + and - of each sepereately, so I could pull more current from one side than the other. But how would I wire the two tranformers' primary windings to the mains outlet? Would I wire them in parralel or in series?
Cheers
Ben
primary side wired in parallel.
Yes, you'll need separate regulators for each supply. It is possible to use all 78xx regulators, consult a data sheet for circuit examples.
QuoteIf I had 2 transformers that were 18-0-18 and 9-0-9, could I make supplies for +18, -18, +9, and -9v, right?
Almost, but not quite. You might enjoy reading "Power supplies basics" at GEO. Probably also the "Spyder Power Supply" series.
The primaries go in parallel.
Please be sure you know how to wire AC power line safely before you try. Effects are not worth dying for.
Ben:
Read R.G.'s article about the Spyder power supply on GEO.
Cool, thanks guys. I don't have time now (it's bedtime), but I'll cehck ik out in the morning :)
Quote from: R.G.Quote from: BenIf I had 2 transformers that were 18-0-18 and 9-0-9, could I make supplies for +18, -18, +9, and -9v, right?
Almost, but not quite.
Huh??
I have to ask why not? I would would answer Ben's question with an unqualified yes.
If you use bridge rectifiers on each winding, followed by a couple of good caps, followed by any decent regulator pair (7809 / 7909), and finally some output caps, it works great.
As long as the transformer sources enough current, the 9 volt windings will give a little more than enough to get stable output from a 7809.
Take care,
-Peter
But will I be able to draw more current from the +9 than the -9? I think so, they're independantly regulated, but just checking.
Quote from: Peter Quote from: R.G.
Quote from: Ben
If I had 2 transformers that were 18-0-18 and 9-0-9, could I make supplies for +18, -18, +9, and -9v, right?
Almost, but not quite.
Huh??
I have to ask why not? I would would answer Ben's question with an unqualified yes.
I was thinking the same thing, but I think he meant to express that you CAN get those voltages, but it's not as straight forward as 9VAC=9VDC. I beleive RG was hinting that 9VAC will tranlate into something more like 11.5VDC after rectification and filtering.
But if you want to regulate that, you'll need that much to drop accross the regulator... and have some wiggle room.
Smoguzbenjamin, you can draw different current amounts from the +ve and -ve supply. How much current do you require? There are some practical limits to simple 78xx regulator circuits.
QuoteI have to ask why not?
You and I know instantly that by rectifying you get square-root-of-two times the AC voltage for DC, and that would be sufficient for the headroom for a regulator. However, based on several years of observing the average level of expertise that this question implies, I usually assume that the asker has decided that 9Vac = 9Vdc and so on, which is not true. Worse, it can get in their way to believe that as they come up the learning curve later.
As Nifty said, it's to draw a clear distinction between the AC RMS value and the DC peak value that you absolutely have to know to do a decent job of understanding power supplies.
If I shot under Smoguz, it was unintentional. It's just that my experience with people is that if they don't understand what transformer for what, they also don't understand rectification.
The odds are pretty good that the 36vct transformer you have (18 + 18) has more than enough current to supply a pedalboard of any reasonable size. The same is likely true of the 18vct transformer. (One rarely sees either of these with less than 200ma of current capability and that is plenty for anything that isn't attempting to power the entire lseries of Line ? modeller pedals all at once).
If you rectify the 9-0-9, you will end up with about 12 and a half volts DC on each "side". This can be further regulated down to +/-9vdc. The typical 3-pin regulator wants at least 2 volts more at its input than it will be expected to deliver at its output, so 12.5 v meets those requirements.
A +/-12.5V supply and a +/-9vdc supply will be fine for the majority of line-level or instrument-level devices. The majority of analog synth modules you'll find posted around the net will also work off +/-12v to +/-15v.
Alternatively, consider using the 38vct (18-0-18) transformer in the same manner with double or even triple regulation. 18v, once rectified, will deliver a bit over 25v. That can be regulated down to 18v with a standard 3-pin regulator, and the output of that can be regulated down to 15v with another of the same, then down to 12 and/or 9. The advantage of double or triple regulation is that each stage of regulation effectively reduces the ripple at the output. By the time you've turned 25 into 9v, it oughta be pretty ripple free, at least on paper. Ideally, what you want is something that is sufficiently well-regulated that the hum is inaudible. The extra regulation will push you more in that direction.
Thanks for the explanation R.G. :) Now I understand why you said that.
It's worth pointing out that the span of voltage where you can use a transformer of XX VAC to get XX VDC using a 78xx regulator is only from about 7.5V to 20V. Below that 7.5 you don't have enough extra voltage to get stable regulation and above 20 you end up putting too much voltage into the regulator for proper operation.
Take care,
-Peter