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I'm trying to modify a Nikon SB-26 camera flash to use AC power instead of batteries. I have a setup similar to this with two dummy batteries connected to wires that lead out of the battery housing. Instead of hooking up the wires to an external battery pack though, I've connected the dummy batteries to this variable voltage 300mA AC adapter. My results with different voltages are as follows:

  • 1.5V: nothing
  • 3V: the unit turns on and all of the menu options work, but the flash itself doesn't charge
  • 4.5V: the unit does not turn on but beeps about once per second
  • 6V: the unit beeps at about once per second and also quickly turns on and off at the same interval.

I was about to call this experiment a failure when I decided to hook the flash up to some of the outputs on this Radio Shack electronics learning lab (which is powered by AA batteries.) Sure enough, when I plugged it into 4.5 volts, the flash was powered and worked normally (although the flash charged a little more slowly than normal)

This leads me to believe that I'm using the wrong AC adapter. My ideal solution would be charging the flash much faster than it does with the electronics learning lab and closer to how quickly it charges normally. Does anyone have any recommendations for what adapter I should be using instead? Even if you don't have a specific answer, any information on a good troubleshooting/honing path to proceed with would be extremely helpful.

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2 Answers

up vote 3 down vote accepted

The problem is that your "AC adapter" can't supply enough current. It is only rated for 300mA, which way to wussy. I don't know about the SB-26 stobe in particular, but other strobes I have looked at will happily draw 1 A or more after a strong flash.

Get a proper power supply rated for at least 1A. Don't play games and experiment with the voltage if you value your strobe. You can safely apply about 1.5 V times the number of AA cells the strobe is intended to take. If it's two cells, then 3 V. If 4 cells, then don't exceed 6V. 1 A may not be enough for short periods right after a flash, but is probably enough to get thru the tough charging phase a little slower to where it will draw less current after a few seconds. A little voltage sag is OK since the strobe has to be designed to at least not get hurt by low batteries. I would probably get a 2 A 6 V power supply (assuming 4 AA batteries) to get good quick recharge time.

 

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I do thinks like this all the time.

There are two issues:

  1. You need approximately the same voltage. Too high will cause too much current to pass through the device and something will fail... usually. Doubling the voltage will cause double the current and quadruple the heat. Sometimes you can get away with a little less or more. A volt extra is usually ok.

  2. You must have ATLEAST enough current. If the device draws 300mA then you need a power supply that can supply 300mA. BUT!!!! Batteries can generally supply an amp or more very quickly!! (depending on the type... LI can do a several amps or more in some cases)

For you camera, chances are you are charging the flash too slow or the circuitry that charges the flash simply won't work with the voltage or current you've given it.

It seems that the device runs about 3V to 4.5V. Maybe you need to give it 4V instead of 4.5V. It seems like it has overvoltage protection circuitry in it that prevents it from working if the power supply voltage is too large. This is a good thing. BUT at 3V it may be enough to power some of the camera but not all of it. You might need 3V for the camera but 4V for the flash.

My suggestion is to try voltages in between 3 and 4.5V. You can use a resistor to reduce the voltage a little but it might be hard to make this work. Another way is to use a diode in. If you only have 4.5V and a diode then you can get down to about 3.8V which might be what it needs.

Diodes are ok to use but waste power.

There is always the issue of polarity but I imagine it wouldn't work at all. There is also the possibility that you simply need a higher current supply. 300mA may mean that the flash takes too long to charge. Maybe try it like you have at 3V but leave it on for a while?

My best guess is that you probably need around a 4V supply. 3V might not be enough to activate mosfets for charging the flash if it uses them.

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