# DC circuit; how to use a Cap to keep power during quick interrupts?

#### TallAndFried

Joined Aug 22, 2023
6
Hi folks!

I have a 24Vdc circuit that pulls about 500ma. The power occasionally will be interrupted, and there's no way around that, but for less than a second. I want to not let it dip below 9Vdc.

Can I make this work with just simple components? Not wanting to do a battery, but a capacitor in parallel doesn't seem to do it (unless I'm massively under powering that cap, is a 10mF 50V).

Any thoughts? Tips? Thanks!

#### crutschow

Joined Mar 14, 2008
33,325
To drop no more than 15V in 1 second with a 0.5A load requires (0.5A *1s ) / 15V = 34mF which is 3.4 times as large as the one you used.

What is the source of the power for the load?

Are there any stray loads that may also draw current during the shutdown?
If so, you may need a diode to isolate them.

#### TallAndFried

Joined Aug 22, 2023
6
Right, I calculated that, only had a 10mF, used that to test, then TOTALLY forgot it was the wrong value! Oops.

That's total load, and nothing stray to worry about. The power is coming from a solid source but are brushes connecting the system I am working on to the source (this is actually for some fun lights and such on an amusement park ride), so vibration and simple corrosion causes occasional disconnects.

#### DickCappels

Joined Aug 21, 2008
10,102
12 volt battery connected to the anode of a Schottky diode. The cathode is connected to the circuit you want to keep from dropping too much. Simple.

#### TallAndFried

Joined Aug 22, 2023
6
12 volt battery connected to the anode of a Schottky diode. The cathode is connected to the circuit you want to keep from dropping too much. Simple.
Not wanting to have to include a battery along with a charging circuit in this application. That would be simple, but rather use basic components that don't involve extra circuitry.

#### LowQCab

Joined Nov 6, 2012
3,559
I wouldn't imagine that You are powering a Computer with ~24-Volts.

Is the ~24-Volts stepped-down before being applied to the rest of the Circuit ?

You could use a Super-Capacitor,
( never exceed 27-Volts with this particular suggested Capacitor )
If your Power-Supply may occasionally exceed ~27-Volts, higher-Voltage rated Super-Caps are available.

It will run your Load for approximately ~35 seconds before dropping below ~9-Volts.

A ~2-Ohm, 5-Watt, Power-Resistor should be installed at the Power-Input to your device to
limit the heavy Inrush-Current when first charging the Super-Capacitor from zero.
The "attempted" Peak-Inrush-Current, when Power is restored, will then be around ~12-Amps or less,
this is, of course, only if your Power-Supply can provide that much Peak-Current without blowing a Fuse.
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#### TallAndFried

Joined Aug 22, 2023
6
I wouldn't imagine that You are powering a Computer with......
Thank you for the detailed reply. There's a few microcontrollers that can take 5-24Vdc, and a 24V-12V bulk converter supplying a RaspberryPi based screen that needs 7-12Vdc. The system fully fails at 8.2V, so set a "must remain" of 9V. The supplied power is something I am unable to modify.

The reason I'm trying to go for the smallest capacitor possible is that it needs to be installed in a tight spot that junior techs will be regularly working in for repairs, so I don't want a dangerous capacitor installed. A super cap sounds exciting but I doubt I'd get approval nor feel safe about using one, especially as the circuit gets worked on live.

#### LowQCab

Joined Nov 6, 2012
3,559
Anything ........ Battery, Super-Cap, Power-Supply is going to have the same "dangers"
associated with them when untrained techs are working on them.
The only "danger" being, not much more than a Big-Spark,
and that's only if some goof-ball goes carelessly stabbing a Screwdriver on to the Board.

You're designing Circuits that need to be "Repaired" ?????
A properly designed Circuit will easily last ~20-years or more,
that is, if it's not hit by Lightening, or physically smashed,
and at that point, you probably have bigger problems on your mind.
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#### TallAndFried

Joined Aug 22, 2023
6
Ah cool I didn't realize electronics would never have to be serviced regularly. I'll let our maintenance team and tech team know this, they'll have a nice vacation.

#### crutschow

Joined Mar 14, 2008
33,325
I don't want a dangerous capacitor installed. A super cap sounds exciting but I doubt I'd get approval nor feel safe about using one, especially as the circuit gets worked on live.
24Vdc is safe, as far as an electrocution hazard is concerned.
Usually anything below 50V is not a concern, so it makes no difference how big the capacitor would be.

#### TallAndFried

Joined Aug 22, 2023
6
24Vdc is safe, as far as an electrocution hazard is concerned.
Usually anything below 50V is not a concern, so it makes no difference how big the capacitor would be.
It's more so that folks here with less tech knowledge see a cap that big and get scared. I meant "looks dangerous" rather than "dangerous", and I've found some caps that'll do what I need which aren't the size of a soda can.

#### LowQCab

Joined Nov 6, 2012
3,559
Ah cool I didn't realize electronics would never have to be serviced regularly. I'll let our maintenance team and tech team know this, they'll have a nice vacation.
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that is, with no cost-cutting measures employed,
( which are sometimes inevitable ).

Small Circuit-Boards are generally cheaper to replace, rather than to try to repair them.
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#### crutschow

Joined Mar 14, 2008
33,325
The power is coming from a solid source but are brushes connecting the system I am working on to the source (this is actually for some fun lights and such on an amusement park ride), so vibration and simple corrosion causes occasional disconnects.
I would expect those types of disconnects to be well less than a second.

#### MisterBill2

Joined Jan 23, 2018
16,469
I once built a power supply for a friend using a constant voltage transformer, a big rectifier bridge, and four 1000 microfarad 30 volt rated capacitors. It would power his radio for several minutes after he switched it off. So it is entirely possible to support a load with large capacitors. The ones that I used were about 3 inches diameter and about six inches tall. Not small at all.