Street amp build log

[PRR]

> it doesn't seem to say.

"ELECTRICAL CHARACTERISTICS (C1=0.33µF, CO=0.1µF, Tj=25 deg C)"

This is a test-spec, to show that it performs well with small (lo-cost) caps. In practice we scuff a couple 5 or 10 uFd from the crack under the workbench and use those.

[PRR]

1971 puff-sheet for the ICs which became 78xx:
http://www.ti.com/lit/an/snva512b/snva512b.pdf
Page 9 discusses the caps.

[Rob Strand]

"ELECTRICAL CHARACTERISTICS (C1=0.33µF, CO=0.1µF, Tj=25 deg C)"

http://www.mouser.com/ds/2/308/mc7800-d-303284.pdf

Look on page 1.   The original statements made for these caps were:
1) "Cin is required if regulator is located an appreciable distance from power supply filter."
2) "CO is not needed for stability; however, it does improve transient response. Values  of less than 0.1 uF could cause instability.

My experience you should obey both.   

For (1): One day I found a datasheet which specified what "appreciable distance" meant ; depending what data 2 to 4 inches from the cap.   These things will oscillate without input caps.   It is true if you have a large cap > 0.33uF close to the regulator you can often get away without the 0.33uF but it is good practice to put a 0.1uF there because when the electrolytic cap ages generally that's enough to stop oscillation.    If you look at professional test gear they tend to add the extra input cap.    If you look at some HP equipment they even use 0.33uF when the regulator is close to the filter cap.  Some datasheets show a current booster using a PNP transistor - this circuit is often shown with a 1uF input cap.

For (2):  I definitely recommend adding at least 100nF.  Larger values do help transient response. It must be close to the output and ground pins.  The silly thing about the MC datasheet (and some others) is it implies the 100nF is not required by it may oscillate if the capacitance is < 100nF;  so if you don't use the output cap,  the 1pF stray capacitance to ground causes oscillation!?   I have found circuits with no output caps often oscillate.

When you design power supplies it is often the case the ESR of the output cap needs to be taken into account for stability.  Low ESR often a problem.   I've never tried putting some crazy low ESR cap on those regulators to see if they oscillated.

FYI:  the negative regulators require large "minimum" caps.

In practice we scuff a couple 5 or 10 uFd from the crack under the workbench and use those.

Yes.


[Edit: looks like I'm not the only one]
http://forum.allaboutcircuits.com/threads/7805-voltage-regulator-oscillation.31651/

[Ben79]

Thanks guys, building it up now.

[tubegeek]

Junkbox esthetic taken to great heights!

There's a lot to like about this project but I want to give an extra shout-out to that badass carrying handle.