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Old 18th May 2015, 08:08 PM   #1
dbx01 is offline dbx01  United States
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Default Building my own Voltage Regulator

Revising from original post - may have contained too much information.

I plan to build a few AC to DC voltage regulators for various audio projects. Each project will be built around a linear power supply, and using the 78XX voltage regulator. The other various components for each project will use a diode bridge rectifier, and filter capacitors.

The voltages for each project will range 5 to 12V, and the amperage for each project will consume less than 1A.

The only measuring equipment I have at my disposal is a DMM, and a LCR Meter.

The question is, what would be the difference if I made my own power supply built around the 78XX voltage regulators, versus purchasing a kit from AMB which does not use a 78XX voltage regulator: The σ11 regulated power supply?

Is there any advantage of using an AMB Power Supply Kit over a 78XX regulator?

If there are better voltage regulator substitutes than the 78XX, what would they be?

Thanks everyone for your time!!!

Last edited by dbx01; 19th May 2015 at 01:46 PM.
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Old 19th May 2015, 06:44 PM   #2
kward is offline kward  United States
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A 7812 is going to be less expensive and probably provide as good or better regulation than a discrete build. I can't see a downside to using the 7812 if you don't mind building the frontend of the power supply yourself, and can work within the safe area of operation of the device. It will likely need to be heat sunk in your application at 1A current draw. On the other hand, a discrete build could potentially handle a lot more current and heat and can offer custom features that an off the shelf three terminal regulator cannot.

In the 3 term regulator camp, unless you need ultra low noise and ripple rejection performance, there may not be a need to use a newer generation device like the LT3080. The tried and true LM317 is quite good for general stuff.
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Old 19th May 2015, 06:51 PM   #3
Mooly is online now Mooly  United Kingdom
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I suspect that for what you have in mind, that the 78 series regs will be just fine. They are not the last word in performance (but they are very good).

And consider this... no matter how perfect or good the regulator is, its only that good at its output terminals (ignoring the advantage of 3 or 4 wire sensing types). That means that by the time you have connected a circuit to it via a cm of wire, and introduced a few cm of PCB print and so on, that perfect voltage is much less so at the destination end of the wires.

The 78 series are excellent all rounders. Use them properly and follw the data sheet recommendations for any local decoupling.
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Old 19th May 2015, 07:54 PM   #4
dbx01 is offline dbx01  United States
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KWard, Mooly - thanks much for your responses, and for anyone else who wishes to chime in as well.

My amperage for each of these projects should amount to a fraction of 1 Amp, and my voltage needed for each project will be 5 or 12 volts (hence 7805 or 7812 voltage regulators). I plan to get a 9V output toroidial for my 5V projects, and perhaps a 15V output toroidial for my 12V project - if the output voltage should be more, please chime in. Would I need to worry about the toroidal transformer saturation with these (low?) figures? I just discovered transformer saturation today by going through this forum, but I have not absorbed the meaning of it as of yet.

Another question - I plan to use Diodes for the voltage rectifier. Is there a formula I should use when picking a diode when it is configured as a voltage rectifier? I understand that a part of the formula should be the toroidal Voltage Out * SQRT(2) /2 /4. Is this formula correct? Does the formula apply to the voltage handling side of the diode? And how do I figure the load (or peak load) of the amperage that the diode needs to handle?

Thanks again for everyones input regarding this!!!
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Old 19th May 2015, 11:15 PM   #5
kward is offline kward  United States
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Dropout voltage for the 78xx is about 2V, and the voltage drop across an individual diode in a full wave bridge is anywhere from .6V to 1V (let's assume 1V), so you need about 4V minimum overhead. So yeah, I'd say 9V and 15V secondaries, respectively, are about the smallest you want for 5V and 12V regulated outputs. I probably wouldn't go larger than that, either, as that will just generate excessive heat in the regulator device itself.

I like to size diode steady state current requirements at least 3x to 5x of steady state load.

Peak inverse voltage requirement per diode if connected as a full wave bridge (4 diodes) is Vsec*SQRT(2), where Vsec is the RMS voltage rating of the secondary.
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Old 20th May 2015, 06:54 AM   #6
Mooly is online now Mooly  United Kingdom
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The universally used 1N4002/3/4/7 series diodes are suitable for your use.

Small transformers tend to have poor regulation (its just the way they are) which means that at light loading the secondary voltage can be quite a bit higher than expected. Look at the specs, it should be given as a percentage. The quoted secondary voltage is at full load. You add the regulation figure to that to get an idea of the no or very light load voltage. I'm not sure what you are looking at when mentioning saturation.

The DC voltage across the main reservoir cap is approximately the transformer secondary voltage times 1.414. The diodes drop a little bit, but give or take its near enough.

Your choices look fine.

Diodes are so cheap that it makes no sense to go for marginal specced parts. Although the 1N4001 would be fine (50 volts P.I.V.) you should aim for the 1N4002 or above. The P.I.V. is just the max voltage the diode can stand in the blocking direction.
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Old 21st May 2015, 02:36 AM   #7
dbx01 is offline dbx01  United States
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It turns out that one of my voltage regulators requires up to 2 Amp Input. If I were to use a voltage regulator that can output 2 Amps, yet the load needs 500mA or less, with a power supply that outputs around that much for the load, would it be OK to use a 2A voltage regulator safely for my project?

Would the 2 Amp Voltage regulator put out more heat than a 1 Amp Voltage Regulator or require a larger heatsink for the same project?

Thanks again everyone!!!
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Old 21st May 2015, 06:19 AM   #8
Mooly is online now Mooly  United Kingdom
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I think you are misunderstanding the regulator specs here. The regulator itself (with no load) draws only a couple of milliamps to keep itself powered. If your load is 500ma then a 1amp, a 2 amp or even a 5 amp reg would all dissipate the same power and all need exactly the same heatsink.

The power the reg dissipates is simple the current flowing through it (which is the load current) multiplied by the voltage lost across the regulator.

So a 12 volt regulator delivering 500ma and having a voltage on the unregulated side (the reg input) of say 23 volts would dissipate (23-12)/0.5 which is 5.5 watts.
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