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[SOLVED] Old photo flash: DIY voltage 'reducer': some questions..

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Robin Zaalberg

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May 6, 2015
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Hello forum,

This is my first post on EDABoard since I made an account for this question specifically. Please, if I don't obey a rule of this forum or do something wrong, please correct me. Now here comes my question:

I have recently bought a new DSLR and had an old Sunpak Auto 431 flash unit lying around for it. The problem is that the trigger voltage of the Sunpak is too high; it can damage flash components in modern SLRs. I measured the voltage of the flash between 30-50 volts. My camera can accept only 12-24 volts. Although some people on the internet have claimed that the Sunpak does no harm, I still want a 0% chance of damage. I did some research and found the following circuit on **broken link removed**:


First question: Further on the page it says 'You may need to use a different value for R2 for your particular flash and SCR, since this is based on a 218 V trigger voltage.', which makes me wonder what value I need for my 30-50V. Will I need to change the value of the resistor to make this circuit work effectively? And what value will I need?

Second question: I have been looking around in several webshops and saw there are different wattages for resistors. Will I be fine with the cheapest (.25 W, carbon-film) or do I need metal-film or higher wattage?

Third question: One shop I had been looking through recommended a heat sink for the diode (SCR). Do I really need that? — Look, the .30$ won't make me poor, but I'd like to make the circuit as small as possible. And if I do, do I need thermal compound for that?

Thanks in advance for reading though my tiny problems and your solution.

Faithfully yours,


Welcome to EDABoard Robin, asking questions like this is exactly what the board is designed to help with.

The problem is because older cameras used a mechanical switch to trigger the flash, basically two metal contacts that close along with the shutter mechanism and let current flow between them. They do not care what the voltage is but on modern cameras the mechanical swithc is replace with an electronic one which has a maximum working voltage.

The flash schematic should work fine, there is no need to use the flash power pack as the voltage source, you can use any battery you like as long as it produces about 6V. The voltage isn't critical, you could for example use two coin cells in series to keep the size small. They would last for several years in normal use.

The SCR will produce some heat but only for the duration of the flash so it will start to cool down before the heat even reaches the outside of it's own body, no need to use a heat sink at all. The resistor can be even smaller than 0.25W if you can find one, again it produces very little heat and only for the duration of the flash itself. Its value isn't at all critical, I would suggest using 1K might be better.

Thanks for your fast reply, Brian!
I'm going to try it with the 1K and see what happens. If it does not work as expected, I will post about it, but for now I'll mark my problems as solved.

Now I have another question, because the shop I would have ordered the MOC3010 ran out of stock.. — Can I use a MOC3020 or another optocoupler in this circuit instead? I looked at the datasheets for the 3010 and the 3020 and they are very similar..?

Thanks in advance,


You can use almost any optocoupler with a high enough voltage rating AND is 'random phase'. Some types have 'zero crossing' circuits in them which allow them to synchronize with AC voltages, you should avoid those.

What you may find is different is the best value of LED series resistor (R1 on your schematic). You can calculate the best value easily if you look at the data sheet for the optocoupler. The formula is:
(battery voltage - Vf) / I
where Vf is the forward voltage of the emitter LED inside the optocoupler and 'I' is the current you want to flow through it. The value for 'I' is in Amps and the result will be in Ohms. The data sheet advises the current you should use.

For example, the recommended current for the MOC3010 is 15mA (=0.015 Amps) and it's Vf is typically 1.15V so if you use a 6V battery, R1 should be (6 - 1.15)/0.015 = 323 Ohms. The value isn't very critical, you can use the standard value of 330 Ohms.

If you use the MOC3020 the manufacturers quote 30mA maximum for the LED but Vf is the same so R1 becomes (6 - 1.15)/0.03 = 161 Ohms so I would suggest using 150 as the nearest standard value.

Substitute your own figures for whichever device you choose and the battery voltage you use. I suggest you keep the battery voltage higher than say 4V as there is a lower limit the LED will work at.

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