12V regulator

[N4NV]

I have an old mill that has a switching power supply with outputs of +5, +15, -15 and +24V. I need to get 12 volts. I purchased a 78m12 voltage regulator. When I test it on my bench I get 12.06v with an input between 14 and 16V. When I connect it to the Deltron power supply +15v, the output is 11.6V. I looked at the output with a scope and with the time set at .5uS I can see two saw tooth waves with min and max peaks about +- 1.5V. When I look at the +15 signal, it looks smooth. What is causing this? What is the best way to get 12v from this Deltron power supply?

Thanks

Vince

[Al_The_Man]

Did you put a cap. on the input and output of the regulator?
Al.

[N4NV]

Originally Posted by Al_The_Man Did you put a cap. on the input and output of the regulator? Al.

No, what size cap?

Vince

[Al_The_Man]

Usually the spec sheet has recomendations, the minimum is usually .22µf on the input and .1µf on the output, I generally put a 1~10µf on the output.
Al.

[pastera]

You need at least some output capacitance, especially if you are driving a load with a switching regulator. Anything from 10uF to 47uF would be acceptable if all you want is to reduce the oscillations.

As long as the regultator is mounted close (<<6") to the power supply who shouldn't need any input capacitance but it won't hurt to add.

Going to a low dropout voltage regulator (IE: lm2940) with proper capacitors would give you a cleaner signal.

Aaron

[N4NV]

Thanks, I will try a capacitor and see what happens.

Vince

[N4NV]

I installed a 33uF cap to the output and a 22uF to the input of my regulator and the signal looks much better.

Thanks for all your input.

Vince