Reduce flash trigger voltage

Thread Starter

danmih83

Joined Nov 30, 2020
5
Hello,

I am trying to reduce the trigger voltage of a soviet flash, like this one:



The internal schematic is this, a bit different meaning that the trigger closes the contact via a resistor to GND (other designs connect the transformer directly to the GND):


On the hot shoe it has 33v. I found this voltage reduction circuit:


http://www.photomacrography.net/forum/viewtopic.php?p=55625




I have used a MAC97 triac and inserted the circuit like in the picture.


Anode2 of triac at C1, Kathode of Zener before the switch, negative at the GND of the flash. I got 63v on the hot shoe !!!. I connected the negative of the voltage reducer to pin 4 of the flash transformer, but it does not discharge the flash.

Is there something that can be done?
 

drc_567

Joined Dec 29, 2008
963
It looks like the 5V zener diode may be creating excessive reduction of the flash voltage.
Capacitor C1 is being discharged through the transformer and creating the flash firing pulse when the switch is closed.
Maybe replacing C1 with a smaller value, say 400 μF, would produce an acceptable firing voltage.
... A guess ... no more, no less.
 

Thread Starter

danmih83

Joined Nov 30, 2020
5
Thank you, but replacing C1 is not an option because i can`t find at ease flash capacitors. It is a simple circuit and i don`t understand why it doesn`t work.

I have made a mistake describing how i connected the schematic.
Anode2 of triac at C2 (not C1), Kathode of Zener before the switch, negative at the GND of the flash.
 

Dodgydave

Joined Jun 22, 2012
9,848
I would use an optocoupler like the MOC3023 series, just put a 330ohms resistor in series on the Led side and use a 5V supply to trigger it, pins 4,6 go to the Flash Trigger.


unnamed.jpg
 
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Thread Starter

danmih83

Joined Nov 30, 2020
5
Well, i would not like to add an external battery to the flash. I would't have where to place it. Not even a lithium 3v battery.
I am puzzled because many people that built the circuit were satisfied. Thought they used it with a flash that triggers via connecting the transformer directly to GND (not via a resistor).
 

MisterBill2

Joined Jan 23, 2018
7,697
If the flash operation is OK and the only issue is the trigger then changing the trigger circuit is the solution. What I see is that the trigger capacitor is charged to some randomly high voltage through two 4.3 megohm resistors so that the camera body is sort of isolated from the charging voltage, which might be a mains connection. If there is not a battery in that flash unit, then what is the power source??
There are a lotof good trigger circuits that put a much lower voltage and current on the trigger switch than this one. A very simple one adds an SCR to carry the high current pulse, and the trigger switch sees only a few volts and about 5 milliamps. The one thing we need to know before being able to answer is the voltage across C1 when the unit is ready to flash.

The unit looks like a 220 volt mains transformer-less power supply system that was available in the late 1960's era. Very effective but not completely safe, and hard on the shutter's trigger contacts.
 

Thread Starter

danmih83

Joined Nov 30, 2020
5
Thank you for your replies.
1. The x1 supply voltage is 220v AC, as you said transformerless power supply. The flash was made in 1987 on 1960s technology, but it`s a nice vintage object.
Optionally, the flash can be powered from a 4 x D size battery pack that delivers 300v DC. Just for fun the schematic of the pack is here:


2. The voltage on C1 is 306v (though the capacitor is rated for only 300v :) )
 

MisterBill2

Joined Jan 23, 2018
7,697
Using an SCR of suitable rating will not take up much additional room inside the flash unit and would not add to the power requirements.
But the question is, would this flash be used with a camera? A modern camera, that might have a more delicate trigger circuit?
An SCR triggered part will need to deliver a low power positive pulse to the SCR gate, and while the circuit to provide that can be simple and small, neither side should be tied to the camera frame through the "hot shoe" mounting, because of a possible shock pulse when the flash is triggered.
 

Dodgydave

Joined Jun 22, 2012
9,848
Well, i would not like to add an external battery to the flash. I would't have where to place it. Not even a lithium 3v battery.
I am puzzled because many people that built the circuit were satisfied. Thought they used it with a flash that triggers via connecting the transformer directly to GND (not via a resistor).

Can you measure the Hot Shoe on your camera for Voltage and Resistance when pressing the shutter, and when in normal mode.
 

MisterBill2

Joined Jan 23, 2018
7,697
DD, measure the voltage and resistance relative to whatt??? We have the published circuit already and it is easy to see what the voltage across the trigger switch will be, if the trigger capacitor has no leakage resistance. And we know that when the shutter contacts are closed the resistance will be quite low, and when they are open the resistance will be quite high.

And now it popped into my head that simply adding a shunt resistor across the trigger connection will reduce the voltage at the switch. The challenge will be to find a value that will still allow an adequate trigger. So bit of experimentation is in order. I suggest one megohm for a start.
 

Dodgydave

Joined Jun 22, 2012
9,848
I was on the understanding that the flash gun was sat on top of his camera , and he didn't want to damage his camera due to the high voltage from this flag gun.

That's why I asked to measure his camera shoe, I think most cameras just short the shoe terminals together when taking a photo, thus making the Slave flash work.

Yes a resistor across the flash trigger terminal will lower the voltage.
 

MisterBill2

Joined Jan 23, 2018
7,697
Of course, which is why I commented as I did. At least one side of that "Hot Shoe" connection is common to the camera frame, and thus also connected to the user's hand. That is why it is important to not connect any higher potential to the trigger circuit.
Actually, it is quite likely that a zener diode can be used to clamp the trigger terminal voltage to some lower level and still fire the flash tube. And that addition would mean adding only one zener diode to the internal electronics of the flash gun. And even adding an SCR would not require any additional power source. There is plenty of power available in that package.
 

Thread Starter

danmih83

Joined Nov 30, 2020
5
Thank you for your explanations. I found that the Zener diode was shot. Replaced it with another Zener and the same result.
Until i will figure out what is wrong with the first circuit, i decided to build this one with a SCR. I chose C106. It works perfectly. I have 3.1 volts on the flash contacts.

 
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