Datron 4000A Capacitor Replacements

[Johnny10]

My Datron 4000A Calibrator is blowing tantalum capacitors again!

So I replaced all tantalums on PC Boards 400444 and 400445 with same uF tantalum caps, I upped the voltage with Kemet tantalum caps .

Now on the 400451 Power Supply board Datron used 22uF tantalum capacitors.
Yet if I read the datasheets on the 7800 and 7900 voltage regulators correctly, they suggest anything higher than .33uF.


( A 0.33 uF or larger tantalum,
mylar, or other capacitor having low internal impedance at
high frequencies should be chosen. The capacitor chosen
should have an equivalent series resistance of less than
0.7ohm. The bypass capacitor should be mounted with the
shortest possible leads directly across the regulators input
terminals. )

Is it possible to use different value uF caps in this location as I have a good supply of 10uF tantalums available?

[TimFox]

Since the 7800 series came out in the ‘70s, I have always used 0.47 uF 50 V X7R ceramic capacitors at the input (very close to the pins) with good results.

[Johnny10]

Changed Caps on 400451 Board and now we are up and running again.

Up for 2 hours.. If I remember it takes about 7 hours for HP3457 to warm up !

[floobydust]

If it keeps eating those tantalum capacitors, they might be getting reverse-voltage on power-down.

I have seen some equipment where (for example) the +15V rail falls fast and the -15V rail falls much slower, during power down. Op-amps and load current between the -15V and +15V rail cause the tantalums to see negative voltage spikes and they eventually fail.
If it keeps happening, add reverse-diodes to the 7815/7915 outputs.

[Johnny10]

On another note. I am finding higher than spec. voltage on the 36volt rail.
Lokking at Test points 13,14 and 15 on the 430455 DC PCB Board should be 36V yet I measure 44V.

Going to check the main reference board since the LM101 opamp takes 36volts.

I believe I have 36 volts leaving the Inguard Power Supply board

[Dr. Frank]

If you wonder, why that often happens: Ta caps might blow up by 'Surge Currents', that is too fast a rise of the capacitor voltage, creating a fast rising charging current.

It's explained here, for example: https://nepp.nasa.gov/files/24745/2013_n240_Teverovsky_ESTEC_Derating_paper.pdf

KEMET also mentions that problem in its technical papers, but does not clearly emphasize counter measures.

Therefore, you need to limit either the initial rise of the voltage stabilizer, or the inrush current by placing a resistor of about 0.1 Ohm / Volt in series with the Ta cap, i.e. 1.5 Ohm for 15V.

Or you avoid these Ta caps, and replace them by MLCCs.
For latter, pay attention on stability problems, due to capacity reduction, caused by the charging voltage relative to  its nominal voltage, temperature, and also time.

Frank