Partialy Discharging Capacitor

[nebojsa]

There are schematics of some flash units on the web. Search for the schematic of a Metz 45ct1.

I'm still not sure how they work

OK. Thanks for advice about METZ45.
I googled it and found this circuit: https://sheepofdoom.files.wordpress.com/2012/02/45ct1_schaltung.jpg.
In low right part of image is switch with capacitors and resistor. Could that be a regulator for flash power?

 

[(*steve*)]

It's a light sensor with variable sensitivity. It essentially integrates the light over time and triggers the rest of the circuit to stop the flash once sufficient energy has been "seen" by the sensor.

 

[nebojsa]

It's a light sensor with variable sensitivity. It essentially integrates the light over time and triggers the rest of the circuit to stop the flash once sufficient energy has been "seen" by the sensor.

Yes, it is . That light sensor (photo transistor) I have replaced with 1,5k resistor on vivitar 3500 flash and after that flash works with manual control. I was wondering could part of this circuit (with switch, capacitors and resistor) be integrated in circuit of flash I linked first?

 

[nebojsa]

The easy way is what I said, use a thyristor (SCR) to short the capacitor. This is not the most efficient solution since it still requires full recovery time on a shortened flash, but it is the simplest to implement. I believe this is the way older units worked back when I was into photography. As Steve said, newer units may use a more efficient method that preserves the remaining charge.

Bob

Even it is not the most efficient solution I want to try it. I have added thyristor and resistor in circuit, but I dont know is that correct way. Advices, critics, anything is appreciated.

 

[(*steve*)]

If it we're that simple everyone would do it.

The circuit we've seen to control the flash duration is significant more complex, yours won't do it at all.

 

[nebojsa]

If it we're that simple everyone would do it.

The circuit we've seen to control the flash duration is significant more complex, yours won't do it at all.

I agree It wont. I have roamed around net and couldnt find something useful...
But, today i found a tip. According to it I drawn this circuit. I must say againg it is assumption, but I dont give up so easily. In pink rectangle should be a power control switch.

 

[(*steve*)]

Perhaps if you gave us a link to the original rather than your own interpretation we might be able to comment

 

[nebojsa]

Perhaps if you gave us a link to the original rather than your own interpretation we might be able to comment

http://www.bristolwatch.com/ele/

 

[BobK]

Your latest schematic just shows a flash tube with an SCR trigger. It does not change the duration of the flash.

For the third time, the scr has to go across the capacitor CS in you latest drawing to dump the charge after a time. And it must be driven by a monostable circuit triggered at the same time the flash is triggered in order to trigger the scr after a delay.

Do you understand how a flash tube works? You give it a high voltage pulse on the trigger input to ionize the gas, which then conducts all the charge from the capacitor through the two main terminals.

Bob

 

[nebojsa]

Your latest schematic just shows a flash tube with an SCR trigger. It does not change the duration of the flash.

For the third time, the scr has to go across the capacitor CS in you latest drawing to dump the charge after a time. And it must be driven by a monostable circuit triggered at the same time the flash is triggered in order to trigger the scr after a delay.

Do you understand how a flash tube works? You give it a high voltage pulse on the trigger input to ionize the gas, which then conducts all the charge from the capacitor through the two main terminals.

Bob

OK. I am beginner at this and my assumptions are somtimes a little hasty, sorry for that. Just to clear ' SCR across the capacitor' thing... Does this mean this?

 

[(*steve*)]

Yes. You cause the SCR to fire a short time after you trigger the flash tube. The remainder of the charge goes through the SCR rather than the flash tube. This causes a shorter flash with less total power.

Be careful that your SCR can handle a very short, but very high current pulse.

Also be aware that your wiring (especially to the SCR) needs to be low inductance.

 

[AnalogKid]

For a standard low cost flash, almost 100% of the capacitor energy is dumped into the flash tube every time. An SCR switching device works well for this because it is both a switch and a latch. It stays in the on state until the cap voltage is so low that the tube extinguishes itself and resets the circuit.

For a controlled flash the switching device cannot be a simple SCR, because it must be able to be turned off under circuit control. A GTO (gated turn-off) SCR can do this, as can a power MOSFET. The idea is that the capacitor voltage is regulated so it always starts with the exact same amount of total energy. Then the switching device is turned on for a specific period of time, and this translates into an amount of total illumination.

One of the things you get when you buy a flash is its calibration. It would be nice if you just decreased the on time by 50% to reduce the total light output by 50%. But the physics of a flash tube are much more complex than that because, among other things, the capacitor voltage is changing continuously during the on time.

Another method to produce variable total light output is to fire the flash tube for a very brief time period, multiple times. For a simplified example, have a flash circuit that makes a 1 millisecond flash for the least amount of light, and fire it 10 times in a row over 20 ms for 10 times more light. Now the total light output can be controlled by a digital counter acting as an integrator instead of an actual analog integrator.

ak

 

[BobK]

OK. I am beginner at this and my assumptions are somtimes a little hasty, sorry for that. Just to clear ' SCR across the capacitor' thing... Does this mean this?View attachment 34161

Yes, that is what i am talking about.

Bob

 

[(*steve*)]

Another method to produce variable total light output is to fire the flash tube for a very brief time period, multiple times. For a simplified example, have a flash circuit that makes a 1 millisecond flash for the least amount of light, and fire it 10 times in a row over 20 ms for 10 times more light. Now the total light output can be controlled by a digital counter acting as an integrator instead of an actual analog integrator.

Flash units which use this method typically do it in order to make a longer duration flash so as to allow for a shutter speed faster than the sync speed for the camera (we're talking final plane shutters). Perversely this longer duration flash results in a fraction of the power being available to expose the sensor (or film, for those of us old enough to remember it) as the shutter is a moving slit which may take between 4ms and 15ms to pass in front of the sensor (the speed is fixed for any camera, the width of the slit varies).

For a controlled flash the switching device cannot be a simple SCR

I don't understand the circuits, but SCR's can be used. There is some magic in charging another capacitor or discharging one in such a way that the voltage across the flash tube falls to zero, quenching the flash.

Feel free to figure it out and explain it to me

 

[nebojsa]

For a standard low cost flash, almost 100% of the capacitor energy is dumped into the flash tube every time. An SCR switching device works well for this because it is both a switch and a latch. It stays in the on state until the cap voltage is so low that the tube extinguishes itself and resets the circuit.

For a controlled flash the switching device cannot be a simple SCR, because it must be able to be turned off under circuit control. A GTO (gated turn-off) SCR can do this, as can a power MOSFET. The idea is that the capacitor voltage is regulated so it always starts with the exact same amount of total energy. Then the switching device is turned on for a specific period of time, and this translates into an amount of total illumination.

One of the things you get when you buy a flash is its calibration. It would be nice if you just decreased the on time by 50% to reduce the total light output by 50%. But the physics of a flash tube are much more complex than that because, among other things, the capacitor voltage is changing continuously during the on time.

Another method to produce variable total light output is to fire the flash tube for a very brief time period, multiple times. For a simplified example, have a flash circuit that makes a 1 millisecond flash for the least amount of light, and fire it 10 times in a row over 20 ms for 10 times more light. Now the total light output can be controlled by a digital counter acting as an integrator instead of an actual analog integrator.

ak

There is one intresting thing about circuit that controls level of capacitor discharge, flash power or whatever is that (I have done this on vivitar 3500 - https://www.flickr.com/groups/71917374@N00/discuss/72157629108782589/ - replacing photo transistor with resistor and getting manual control). Resistor of 3K produces full power, 1,5K produces 1/2 power of flash, 750ohm produces 1/4 power etc. If wires stays unconected (without resistor) flash fires full power. If resistance is lower the output power of flash is lower.

 

[(*steve*)]

Without a circuit diagram that tells us nothing we don't already know.

 

[nebojsa]

Without a circuit diagram that tells us nothing we don't already know.

Maybe something like this...
I found it on some russian site, it is circuit board marked EPT260-01B.
As I mentioned before in this circuit replacing photo transistor with fixed value resistor(example 1,5k) will result in level control power in steps: Orange 1/2, Yellow 1/8, Blue 1/32.

 

[(*steve*)]

There is no trigger in that circuit. You'll need to post the entire circuit diagram.

That circuit simply provides a variable delay that is partially controlled by the incident light.

 

[nebojsa]

There is no trigger in that circuit. You'll need to post the entire circuit diagram.

That circuit simply provides a variable delay that is partially controlled by the incident light.

Why are we constantly mentioning 'trigger'?
Maybe there is no trigger in solution for circuit I am looking for.
Anyway here is the links: http://impulsite.narod.ru/hanimex/tz2/03ede5bc3296.png
http://impulsite.narod.ru/hanimex/tz2/28794178.png

 

[(*steve*)]

Why are we constantly mentioning 'trigger'?

Because you need one, and also a method of stopping the flash.

That circuit controls the timing, that's all. Without the other two parts it's about as useful as a soluble fish.