I have a flash gun which runs off 2 AA batteries. They drain very quickly. The flash unit lives in the same spot all day, so I would like to swap the AAs for a plug-in solution (the flash unit does not offer this).

I took a chance with some 'dummy AA' cells, which are basically empty shells wired to a USB plug. The USB plug internally steps 5v down to 3v.

If I plug that USB into a phone charger or battery bank, the flash unit displays a low battery icon. If I turn the unit on and off a few times, waiting a few seconds in between, it eventually charges up. It can then flash and recharge for the rest of the day, as normal.

?? I assume the USB plug is limiting the amount of current the flash can draw in order to initially fill its capacitors?

My question is: can I add a capacitor somewhere between the power source and the flash unit to offer a slightly higher current for a few seconds on the initial charge? Is there a specific calculation I should use to pick the cap? I added a 1000uF cap between the USB and the flash, but that has not helped. Maybe it is too small?

Thanks!

 

Placing just cap to output doesn’t help because the output is still overloaded.
You need charge the cap without load and after charging connect the load.
You can use a simple delay for this + mosfet for connecting the load.

 

What do you mean "between"?
The capacitor would go between the two power leads.

 

Thanks for the responses. In regard to my photo, I have a cap where that barrel connection is. If that barrel connection was unplugged, would the capacitor charge? Assuming it would, when I reconnect the barrel, the cap should be charged and feed the flash. Does that suggest the cap just needs increasing in size?

 

What do you mean "between"?
The capacitor would go between the two power leads.

Sorry, I mean where in the circuit, between which components.

 

1000uF is probably too low capacity for such high drains.

 

If the current is being limited by the 5V>3V adapter then I don't see how adding capacitance would be helpful.

 

If the current is being limited by the 5V>3V adapter then I don't see how adding capacitance would be helpful.

It can if the capacitor on the supply is allowed to charge before being plugged into the flash unit.

 

But the supply and cap are being plugged into the adapter's USB plug, not into the flash unit.

 

The flash runs off 2xAA, so I would have thought 3v from the USB is the correct voltage (but I'm not knowledgeable about such things). The only difference I would assume is that the USB is restricting the current. Therefore, in this theory, the battery bank would charge up the capacitor, then, when I plug the flash in, the energy in the capacitor can be drawn out at the higher rate the flash requires. This only has to happen once, as it seems fine one the initial charge was successful.

 

You can experiment with a series resistor to limit the current. OR maybe try a different power supply that has a greater current capability. The charge up time will be greater, I don't know if that will be a problem or not.

 

Get a new flash with an HV input and you can have sub-second cycle times even if you dump the whole thing. You can be a new-to-you used flash and the cost will be quite low.

The HV batteries/supplies are precisely for things like this.