Three-terminal regulators

Frequently asked questions regarding powering your pedal.
Post Reply
User avatar
mictester
Old Solderhand
Information
Posts: 2915
Joined: 11 Sep 2008, 20:29
my favorite amplifier: Mesa Boogie, Roost Sessionmaster, AC30
Completed builds: Hundreds! Mostly originals, a few clones and lots of modifications.
Location: Somewhat closer to Amsterdam than before!
Has thanked: 32 times
Been thanked: 825 times
Contact:
Contact mictester

Post by mictester »

A word of advice to anyone contemplating using one of these - put 100n capacitors from the input and from the output of the 78(L)XX to ground. These capacitors should be as close as possible to the pins of the IC (solder them to the track side of the PCB if you have to...

These regulators are superb but suffer from three problems -

1. They're hissy because they're a very old design (shouldn't be a problem in an audio circuit with good supply rail decoupling, but the extra capacitors will help).

2. They're prone to getting upset by proximity to radio signals (I've had them go wild when a mobile phone goes off nearby - the capacitors will tame that sensitivity).

3. They can suffer from spontaneous instability, especially when there's a step-change in the current drawn (the capacitors help to prevent this, too).

The types of capacitor used doesn't really matter, and the value isn't too critical (anything from 33n - 220n will do), but given the choice, 1950s wax-coated types should be used for maximum mojo... :wink:

I usually use small monolithic types (with 0.1" lead pitch) which will allow connection as close to the regulator body as possible. :thumbsup
"Why is it humming?" "Because it doesn't know the words!"
Top
User avatar
deltafred
Opamp Operator
Information
Posts: 1652
Joined: 06 Apr 2010, 16:16
Location: England
Has thanked: 808 times
Been thanked: 299 times

Post by deltafred »

+1

I have noticed that a lot of power supply schematics on the net miss off the small value capacitors. The manufacturers data sheet has them on there for a reason, if they were not needed they would not put them there.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012
Top
User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 120 times
Been thanked: 150 times

Post by Fender3D »

Just a side note:
the total capacity on output should always be lesser than capacity at input, to avoid blowing unit with reverse current.
You may use a diode from output to input, just in case...
Top
User avatar
Hides-His-Eyes
Tube Twister
Information
Posts: 1943
Joined: 02 Feb 2010, 12:34
Has thanked: 50 times
Been thanked: 49 times

Post by Hides-His-Eyes »

Yeah; you can easily catch yourself out by for example putting 100n each side of the power supply, and then putting some massive cap by the circuit itself.

I used to put a 220u where the power joined the board and then the two 100ns across the regulator.
Testing, testing, won too fwee
Top
User avatar
R.G.
Resistor Ronker
Information
Posts: 337
Joined: 22 Sep 2007, 02:24
Been thanked: 39 times

Post by R.G. »

Fender3D wrote:You may use a diode from output to input, just in case...
I would say you should always use a diode from output to input, as the manufacturer recommends. You *can* kill three terminal regulators by shorting the input if there is enough energy in the output capacitors. It's really not optional, just as decoupling caps at input and output are not optional.

It's kind of like running red lights and stop signs. You can often get away with it. But it *is* getting away with it, and one day, you'll get caught. With luck it will be the police. Without luck, you'll be caught by a speeding truck.
Top
Post Reply

Return to “Power”