Isolate voltage regulators

[armdnrdy]

I'm working on the design for a bipolar adjustable voltage (1.2v to 25v) bench power supply using a LM317 and LM337.

The supply is to be fed with a 12va, 28-0-28 transformer into a full wave bridge rectifier, then the usual filtering before the regulators.

Now this is where the gray area begins...

I'm also incorporating two voltmeters (one for positive and one for negative voltage) based on the ICL7107 IC. The 7107 calls for a bipolar 5 volt supply.

If I had the need for more than one voltage in a build I would usually tap the lower voltage regulator downstream from the first, higher voltage regulator. The first regulators in this case are adjustable so this is not do-able.

I plan to tap the second set of regulators (5 volt) from the DC source at the full wave rectifier.

The 7107 contains a clock and all kinds of other digital goodies... so what is a quick and easy way to isolate the input of the 5 volt bipolar supply feeding the 7107 from the input of the adjustable power rails? Ferrite beads? I've seen 10-12 ohm resistors. I'm not too sure how to approach this!

[brett]

Hi
Not sure that I'm answering your main questions, but...
Ferrite beads aren't seen here very often. They are good at keeping RFI out, but lower (audio) frequency noise causes lots of problems, too. Often, you'll see a large electro cap (e.g. 470uF) then a small film cap (0.1uF, for it's low ESR) across the power supply to filter out noise. If necessary, you might add a small resistor (0.33 to 3.3 ohms) in series with the supply (upstream of the caps) to make the caps work better (a deeper one-pole filter).
cheers

[PRR]

> voltmeters ...based on the ICL7107

(IMHO) Some things are not worth DIY-ing.

Get the whole thing with power issues figued out.

http://www.circuitspecialists.com/digital-panel-meters

LED, Jumbo, grounded 5V supply:
http://www.circuitspecialists.com/pm-1029b.html
http://www.circuitspecialists.com/images/PM-1029A&Bspecs.pdf

> 28 transformer

This gives 40V raw DC. You are pushing the 35V rating of the regulators. The 5V reg is often rated less. IAC, it is a long way down from 40V to 5V, much power wasted. (Especially if, like me, you prefer LEDs.)

I'd be real inclined to find a 9V-12V 200mA transformer just for the meters.

[Jdansti]

I did the same thing a while back and ran into a problem with the voltmeter module (I used one voltmeter and a switch to toggle between the pos and neg supplies).  The meter gave unaccurate readings when I tried to tap off of the power that I was using for the ower supply.  I used a cheap store bought meter module that might have had a 7107 or similar IC.  If I remember correctly, it added or subtracted 9V from the actual output voltages.

Here's what didn't work:
(The drawings just show the basic components.)



Here's how I solved the problem:



I just thought I would mention this in case you run into a similar problem.

[Ronan]

I'd be real inclined to find a 9V-12V 200mA transformer just for the meters.

Definitely the easiest and best approach IMO, been there myself. There are other ways around it but simply not worth the extra effort. That ICL7107 works very well despite its age You can buy modules with that IC and a nice large LED display built-in from ebay for peanuts.

[Jdansti]

Maybe something like this at Amazon: