Super Capacitor on Pin 21

[G Herzog]

If I were to use an In-rush protection scheme, would a 1 farad, 5.5 volt super capacitor on Pin 21 [the regulated +5V]be risky?

I am thiinking of a Schotty Barrier diode [rated at least 100ma] to allow output with minimal voltage drop and a 2K ohm resistor in parallel to eliminate the high in-rush which might damage the voltage regulator and possible create general havoc else where.

It seems like that would be about 3ma peak input and 100ma peak output.

[Edit: corrected the pin number, was 22]

[dkinzer]

Are you expecting to use the on-board regulator to charge the capacitor? If so, you should be aware that the regulator is limited to 100mA total output - that includes the current required by the other components on the ZX-24. There is only perhaps 40-50mA to spare for off-board use, less if any of the I/O pins are sourcing current.

You may be able to get a longer run time on the capacitor by charging it with an external supply that is higher than 5 volts. During an outage, the capacitor could supply current to a low drop out regulator that supplies 5 volts to the ZX and other circuitry. The approximate run time could be computed as

[G Herzog]

I see that you are using a 78L05, but I was hoping to limit the in-rush to less than 5ma to avoid problems with it.

The Super Capacitors are rated at 5.5volts max, so the 7 volts required by the regulator makes me assume that I must use Pin22.

From what I can gather, I may get 10-30 seconds on time from 1 Farad. That would be enough to hot swap batteries or supply without a reboot.

[dkinzer]

The brown out detector on the ZX-24 is programmed to reset the CPU at 2.7 volts. That will give you more operating margin. A 1 farad capacitor can supply 100mA for 20 seconds while discharging from 5 volts to 3 volts.

You can probably devise a constant current charging circuit that will charge the capacitor from a higher voltage supply up to 5.2 to 5.3 volts. Then, the Schottky diode could supply current to the 5 volt input of the ZX when the normal supply voltage drops out. This is the same general idea as is used for battery backup.

[G Herzog]

I understand what you are saying about charging at 5.2V - 5.3V, but it seems odd to me to have one power supply path that is directly through the voltage regulator and a second supply path in parrallel that goes around the voltage regulator.

I had not planned on including any switching. Your scheme seems to imply such. I can understand being wary of have the super cap downstream from a 100ma regulator. That is why I posted this. I am seeking pro and con.

Since you quote a range from 5V to 3V and the actuality of my scheme would be 4.75 to 2.7, I suppose it would be simplier [just two components and not switching] to have the super capacitor charge from Pin 22 as well as supply via Pin 22.

[G Herzog]

There is an obvious error. I should be refering to PIN 21 throughout , which is the 5 Volt pin.

Seems that I should have the documents in front of me, rather than rely on memory. Pin 24 is 7volts or More DC input ; Pin 23 is Ground; Pin 22 is RESET; and Pin 21 is 5volt regulated supply input/output.