tranformers again and again

Since the CT is at ground, you cannot use it to power anything. D2 and D4 power U2, U3, U4 and U5 which are positive regulators; D1 and D3 power U6 and U7 which are negative regulators. From the CT, each side of the transformer is rated at 12.6V AC RMS "nominal" meaning as if the line voltage was 110V AC RMS. In most places in the US, the typical line voltage is 123V AC RMS. Furthermore, with a large capacitor, the average DC voltage at the rectifiers will be "close" to the peak which is 1.4 times the RMS, translating to a max near 17V and a crude estimate of the DC average near 15V. That is not enough of "headroom" for a three terminal 12V regulator, but plenty for an LDO (low drop out) type of regulator. If any desired regulated voltage needs to be larger, then consider a switcher; for 5V and lower - especially at large currents, then again consider a switcher for better efficiency.

I don't follow this? The potential difference between CT the positive is ~12V and hence can power stuff...

All of the voltage regulators are negative regulators... I'm not sure what you are getting at?

The "ground" at U2-U5 is ~12V and the power is at -12V which gives me ~-12-12 =-24V to use to regulate with.

U6 and U7 work from G-P ~= -12V and is used to regulate the 8V and 5V supplies(U7 actually goes from N to G but the potential difference is the same with it being -12-0 = -12V)

Why isn't it enough? if its near 15V then its 3V above the 12V which means that it is good enough from what I have read?

At this point in time I just need a PS that can supply +-12V and +-5 so I can work with my MCU's. But Since its just a matter of adding some extra regulators(and I have many) I might as well add some other fixed and 2 variable just incase I need them...

Eventually I might make or just buy a SMPS but for now I need something to work with that is decent... I'm not to worried about efficiency but I figure that I might as well do what I can do make it a little more efficient.. (i.e., using 12 volts across the 8 and 5 regulators instead of 24V to save about 13W+12W = 25W of wasted power).

Thanks, Jon

AS i read it, the CT *is* ground. Therefore, D2 and D4 give a *positive* voltage, and powers U2, U3, U4 and U5 which must be positive regulators. And, D1 and D3 give a *negative* voltage, and powers U6 and U7 which must be negative regulators. Yes, according to the spec, 3V is the minimum that 3-terminal regulators work at. So, say there is a heavy load; that will cause a larger ripple on the input capacitors to the regulators than a lighter load. The low part of that ripple might go below the minimum for regulation.

hmm... thats in volts.. *2A to get 50W wasted ;)

So? Also CT is not earth ground but it shouldn't matter. For the schematic both sides of the transformer must be grounded so it can use probes and stuff. (since its done in a simulator)

D2 and D4 might give a "postive" voltage but it is used as the ground of the regulator.

no.

Potential is relative. A negative regulator only needs its In terminal to be lower than its ground terminal... since I used the "positive side" of the bridge as the ground I get 12V on it... the negative terminal as -12 for a total of -12-12 = -24V that the negative regulator see's...

if I saw them around and use a positive regulator I'll get 24V (but then I can swap its output with its relative ground to get -24V again.

Negative and Positive regulators can be used interchanabe... just requires you to swap GND with In if you want to swap the polarity of it.

Maybe... I'm not sure... hopefully the regulators work quite well over there maximum current rating. I don't plan on using any heavy loads I think.. I'll have to do some testing to find out how well it works though.

I'm sure that having heavy loads on each regulator can cause some major problems but I doubt I will be using more than 2 regulators at a time.

Jon

If you want to run both positive and negative regulators from the same transformers, and use the resulting posive and negative outputs simultaneously, _all_ the regulator ground terminals _must_ be connected to the transformer center tap - otherwise you will get short circuits when you try to connect the "ground" of the positive supply to the "ground" of the negative.

If you just want a single supply that you can use as either negative or positive, you just need one regulator - either polarity will work - then call whichever end you wish "ground".

So the schematic you show is incorrect? That the CT is not ground? *IF* the regulators were as i stated and connected as i stated, then there would be no problems, as the load on each regulated polarity could be connected to ground, instead of floating. I suggest that you make a monochrome schematic and make it complete and accurate (include all loads and multiple level regulators).

One trick I use is a voltage doubler circuit so a 12 VAC tap on a transformer will give about 30 VDC. You use one electrolytic cap C1 with negative to the transformer, and use a diode D1 cathode to C1(+), and D1 anode to CT. Then another diode D2 anode to C1(+), and D2 cathode to another capacitor C2(+). Tie C2(-) to the CT, and you will have about 30 volts to CT (gnd). The inverse of this circuit on the other end of the transformer gives you -30 VDC. As you load it down, the voltage drops fairly quickly, so you just use capacitors that are just large enough to keep the VREG input above its limit at full load. It also tends to be fairly forgiving for overloads and short circuits. It will draw fairly high current surges from the transformer, however, so check for overheating at heavy loads. The true RMS(AC+DC) current will be much more than what you read on an ordinary meter.

If you really just need +/-12 VDC and +/-5 VDC for MCUs, why not find an old junk PC and use its switching power supply? You can probably get all you need for free or for a couple dollars at a swap meet or computer recycler.

Paul E. Schoen

Not according to your schematic.

Now I have the 7912 which

Ignoring your supply for the moment and just looking at the concept: the DC output from the rectifier will be the ac peak voltage - about 1.4 times the AC rms voltage - minus two diode drops in the bridge.

Series is OK, where a higher Vout regulator is used to provide Vin to a lower voltage regulator, as long as there's enough Vin "headroom". Parallel is not good.

But your transformer is limited to 2A - so the scenario you paint above is transformer limited, not "cascade limited". You can get a total of 2 amps out, however you divide it. In theory, you could cascade the 24,18,15, all to feed into the 12, and you could pull 2A from it. You could not pull

All of the regulators shown except U7 use positive as the common. The powered circuit will have its ground connected to + by regs 1 - 6. That's fine. But - if you supply the same circuit simultaneously with U7, you have the CT connected to the + which is not good. Essentially, you are shorting the CT to the + output of the bridge through the regulator and the powered circuit ground. I would suggest that you avoid connecting the CT to anything, bite the bullet and dissipate the heat. And you can cascade to distribute the heat across multiple regs instead of just one.

If you want to use the transformer to make a + gnd - supply, and the kind of supply (minus U7) in the existing diagram, put a switch in there to totally isolate it from the circuit you drew and feed it to a + gnd - regulator designed for the purpose. Otherwise, I fear you will "clever" yourself into a state of confusion.

Ed