Daisy Chaining Voltage Regulators

Hi,
Quick question:

How many voltage regulators (LM317, 7815, etc.) can be daisy chained off each other? I don't know how to explain it that well, so check out the Gyraf SSL power supply: http://www.gyraf.dk/gy_pd/ssl/ssl_sch.gif

See how the 78L12 is getting its Vin from the Vin of the 7815 in series through a 10 ohm resistor? How does one go about figuring out the series resistor value? If I wanted +15, +12, AND +5, could I used the same arrangement, maybe a larger series resistor for the +5V regulator??

Thanks,
Ian

[quote author="Ian MacGregor"]How many voltage regulators (LM317, 7815, etc.) can be daisy chained off each other?[/quote]
A lot.

could I used the same arrangement, maybe a larger series resistor for the +5V regulator??

Click to expand...

Yes. size the resistor so the 7805 gets 8V in at full load.

Best regards,

Mikkel C. Simonsen

Ahh... That simple, eh? Cool, thanks Mikkel.

Ian

If I have bi-polar 18 and want to daisy chain a lm7815volt reg to power some leds. Im planning on tapping into thepos line before the 317 18v reg.

Do I need to also add a lm7915 even if the the negative rail isnt used?

Helsing

I know what I`ve written below doesn`t apply to the circuit that your talking about but thought it might be useful.

Daisy chaining implies that you are taking the regulated output of say an 7818 & putting it in to a 7812. This is not a good way to do it because the 7818 has to cope with extra current that the 7812 is using/supplying. It is better to supply both regulators from same unregulated point then you are stressing the 7818 less.

depends what you're trying to accomplish... in the GSSL, the 12v supplies are pulling a tiny amount of current, hence the low power parts, so they don't really contribute too badly to the beefier 15V supply's current draw. If they were fed off the unregulated supply, there would be more headroom in the regulator, which transfers directly to quiescent current and heat, and reduces the output current before heat-death of the device

[quote author="Rob Flinn"]Daisy chaining implies that you are taking the regulated output of say an 7818 & putting it in to a 7812. This is not a good way to do it because the 7818 has to cope with extra current that the 7812 is using/supplying. It is better to supply both regulators from same unregulated point then you are stressing the 7818 less.[/quote]
I have to disagree. You often use a 78xx regulator to supply circuits that need much less than 1A, so there's no problem. And you should get a better ripple reduction by daisy chaining the regulators - assuming the noise of the regulators themselves is low.

Best regards,

Mikkel C. Simonsen

have to disagree. You often use a 78xx regulator to supply circuits that need much less than 1A, so there's no problem. And you should get a better ripple reduction by daisy chaining the regulators - assuming the noise of the regulators themselves is low.

Click to expand...

I don`t disagree with what your saying it just depends on the current your drawing. If you are drawing close to 1A then what your saying does not hold true.

One other thing that came to mind!

These regs need a few volts in hand to actually regulate. If you had a 7818 daisy chained to a 7815 then you may not have enough in/out voltage difference for it to actually regulate.

Used the wrong terminology

I am powering two mic pres with +/18v and want to use chain off a lm78xx to power the "power" and"phantom" indicator leds. I was planning on tapping into the unregulated line, before the the 18v reg.

Again my question is this: Do I need to use a negative regulator in conjunction with the positive? Most schemes Ive seen including the GSSL and John Hardy's have a pos and neg reg.


Thanks

Helsing

a) no you don't need a negative regulator if all you want is positive voltages
b) if you're just powering LEDs, just use an appropriate resistor for each one which limis current. The LED will only drop a fixed amount, size the resistor so that a sane amount of current flows, say 10mA maybe (use ohms law!)

Again my question is this: Do I need to use a negative regulator in conjunction with the positive?

Click to expand...

Something else comes to mind, is this what you want to know Helsing?
Given the two modes of connection discussed
in parallel before reg. unregV-7818 and unregV-7812
or in series after reg unregV-7818 then 7812

what happens when you have a significantly higher load on the positive leg than you do on the negative leg of a dual rail supply.

Does this have negative effect on audio circuits (provided one side isn't maxing out the supply)?

Sleeper