Power Supply : two voltage regulators fed by single full-wave bridge?

[cheezit]

I've got an existing power supply I built years ago with a Radio Shark 12v xformer which is connected to a full-wave bridge, then to a voltage regulator and filter caps---standard stuff.  I'd like to add a second regulated output so that I can have a clean 9v output and an adjustable 7.5v-12v output.

Is it reasonable to take the rectified output of the full-wave bridge and feed both regulators---basically tying the Vin's together and using a common ground?  Or is this a no-no?

[R.G.]

It'll work fine as long as you keep the outputs separated. Common Vin and ground are OK for voltage regulators.

[cheezit]

Thanks! 

The Weber Xformer that gives 8 distinct sets of windings on the secondary side---what is the advantage of that as compared to a single tap (with equivalently higher available output current) feeding 8 voltage regulators?  Perhaps the Weber wins on the reliability side---if one regulator blows up and takes the secondary winding out, the others might be spared.  Any other reason you can think of?

[zyxwyvu]

Thanks! 

The Weber Xformer that gives 8 distinct sets of windings on the secondary side---what is the advantage of that as compared to a single tap (with equivalently higher available output current) feeding 8 voltage regulators?  Perhaps the Weber wins on the reliability side---if one regulator blows up and takes the secondary winding out, the others might be spared.  Any other reason you can think of?

Separating the regulators on different secondaries isolates the different outputs. This eliminates ground loops, and removes a lot of potential hum. The Voodoo Labs Pedal Power 2 is a popular commercial power supply that uses multiple secondaries for this reason.

For an excellent explanation of this, check out this page (written by RG, of course): http://www.geofex.com/Article_Folders/Spyder/Spyder.htm

[George Giblet]

To help reduce ground loop problems you can add small value resistors from the (common) regulator ground to each of the 0V DC outputs.  A value of say 10ohm is usually a good compromise.  If you have high current effects say > 50mA then use a lower value.

[rogeryu_ph]

It'll work fine as long as you keep the outputs separated. Common Vin and ground are OK for voltage regulators.

Hi R.G.
OT, What if I make the Vout common also all three of them, what will be the output voltage using both 9v regulator. Just curious.

Thanks,
Roger

[cheezit]

It'll work fine as long as you keep the outputs separated. Common Vin and ground are OK for voltage regulators.

Hi R.G.
OT, What if I make the Vout common also all three of them, what will be the output voltage using both 9v regulator. Just curious.

Thanks,
Roger

Tying two 9v Vouts together would mean you get 9v out.  The only reasons I can think of are that you would get double the output current, and possibly some small reduction in ripple and noise.  But a LM317 is nominally rated at 1.5A output current (according to my package, haven't looked at the datasheet), and doubling that is not likely to be useful in most pedal applications.

[rogeryu_ph]

Thanks Cheezit, i'm interested most on the ripple and noise.

[R.G.]

I'm not sure I understand what you're referring to.

If you have two identical windings, each bridge rectified to DC with identical caps and each driving a separate but identical 9V regulator, then you connect two of these in parallel, you get the following results for zero or very low loading:
- voltage is the higher of the two "9V" outputs, as the highest output raises it until the lower regulator turns off. This may be a matter of only a few millivolts difference.
- ripple is the same as what one regulator lets through
- thermal noise is the same as one regulator

When you increase the load until the higher regulator sags and lets the lower regulator begin supplying current, then:
- the higher regulator continues to supply most current
- ripple is the same as what one regulator lets through at any given current
- thermal noise increases by the square root of two (1.414) as the noise combines in an RMS sense.

For more than two, much the same happens, but it gets very complicated as to which regulator starts conducting when.

[Ice-9]

Yes you can have the different output voltages from the one bridge rectifie. I recently built a similar device for a door lock system that reguired 7.5v and 2.65v I used 2 x lm317 regulators. If you use a small trim pot you can set it to give the required voltage you want at the output. I will have a look for the schematic i made for it when i'm n my other pc.

[rogeryu_ph]

I'm not sure I understand what you're referring to.

If you have two identical windings, each bridge rectified to DC with identical caps and each driving a separate but identical 9V regulator, then you connect two of these in parallel, you get the following results for zero or very low loading:
- voltage is the higher of the two "9V" outputs, as the highest output raises it until the lower regulator turns off. This may be a matter of only a few millivolts difference.
- ripple is the same as what one regulator lets through
- thermal noise is the same as one regulator

When you increase the load until the higher regulator sags and lets the lower regulator begin supplying current, then:
- the higher regulator continues to supply most current
- ripple is the same as what one regulator lets through at any given current
- thermal noise increases by the square root of two (1.414) as the noise combines in an RMS sense.

For more than two, much the same happens, but it gets very complicated as to which regulator starts conducting when.

R.G.
I'm referring to just one winding, one bridge rectifier but two regulator in parallel not series with only just one vout. Will it be more regulated? what if not, let say change to two regulator in series, will it be now more regulated? or non sense to use more than one IC regulator? just curious.
Thanks,

Roger

[R.G.]

I'm referring to just one winding, one bridge rectifier but two regulator in parallel not series with only just one vout. Will it be more regulated?

It will be the same regulated for all intents and purposes. Two regulators in parallel will at best be as good as one by itself, but with the possibility of  more current output.

what if not, let say change to two regulator in series, will it be now more regulated? or non sense to use more than one IC regulator? just curious.

You could argue that it would be "more regulated" in that the variation on the output voltage of the second regulator will be a little less. However, the first regulator needs to be specialized - it needs to be at least two volts higher than the second one for the second one to operate correctly. Double regulation schemes (that's what these are called in the biz) are used for special purposes, like when the incoming "DC" has huge ripple on it, or varies over a really big range. But in that case, it's just making life easier for the second regulator to provide precision.  In effects, there is no noticeable advantage.

[rogeryu_ph]

Many thanks R.G.

[cheezit]

R.G., that sounds right---I wasn't considering the effect of the current draw on the system.  I can imagine bad scenarios with parallel regulators as well---if the higher voltage comes from a regulator that sags a bit under load, the resulting drop could be enough to activate the other regulator, which reduces the load and activates the first one again--presto, instant cycling and hum.  Don't know if that occurs in real life.

[brett]

Hi
practical experience taught me that LM78XX regulators in parallel do not share the load equally, and you do not get a doubling of the rated power/current.
Try it.  One will run hotter.
It is possible to use some series resistance to help equalise the load, but if you want that much power, you probably shouldn't be using LM78XXs. 
For heavy duty supplies (such as power amplifiers), I use the usual discrete devices, and a rule-of-thumb: at least 2200uF per amp of filtering, preferably in multiple caps for extra current capacity and in case one fails.
cheers

[kurtlives]

To help reduce ground loop problems you can add small value resistors from the (common) regulator ground to each of the 0V DC outputs.  A value of say 10ohm is usually a good compromise.  If you have high current effects say > 50mA then use a lower value.

What do you mean by common?

Also isnt regulator ground and 0V DC output the same thing?

[brett]

Hi
you can raise the regulator's "common" or "ground" by adding resistors, diodes, etc.  Say, for example, you put an Si diode between the middle pin of an LM7809 and ground.  It will then regulate the input voltage to 9V plus the forward voltage of the diode (around 0.7V), giving about 9.7V.  Cool, huh?  Resistance also creates a voltage drop on the "common" leg.  There are several tricks you can do with regulators to change their performance.  It is also very common to see them protected from reverse polarity.

Other people will know a lot more about this stuff than I do.

cheers