Capacitor discharge

[zsbreaker]

Hi to all.

I have some simple circuit from disposable camera.
It is for flash control, with trigger switch.


I want to control of time of discharge of that big 300 uF capacitor. If the time for full discharge is T, I want to have times like T/2, T/30 or even T/100

I read somewhere that it should be made with thyristor, but have not find any solution.

Regards

 

[DerStrom8]

I wonder if it would work to discharge it through a resistor? Well, a few different resistors if you want the option to discharge for different lengths of time.
I don't really see how that could be done with a thyristor, though. Maybe a special circuit?
Der Strom

 

[Nigel Goodwin]

I presume that what he wants, is shorter flash durations - you do this by only partially discharging the capacitor to get a shorter flash. Obviously to do this you need to disconnect the flash tube from the capacitor, presumably thyristors are involved?.

 

[DerStrom8]

I presume that what he wants, is shorter flash durations - you do this by only partially discharging the capacitor to get a shorter flash. Obviously to do this you need to disconnect the flash tube from the capacitor, presumably thyristors are involved?.

Oh, I'm sorry. I'm thinking backwards
I was thinking he wanted longer durations, but I didn't even think that that would be T MULTIPLIED, not divided.
Sorry for my mistake.
Der Strom

 

[zsbreaker]

yes, I want shorter flashes

but how can I do it ?

 

[Veraxis]

connecting a resistor in parallel with the flash tube would probably make the capacitor discharge faster, as long as the resistor were not such a low value that the flash tube would not go off at all.

 

[zsbreaker]

pls, give me some numbers, because there is a lot of voltage in this circuit, and I m not a pro.

 

[Veraxis]

Unfortunately, I don't have a relative sense of how much resistance the flash tube is adding when the capacitor discharges, and it is not something that can be easily measured, because the resistance of the tube is going to change over time as the capacitor discharges. However, because of how much energy is being released as light/heat, I would guess that the resistance is pretty high. Perhaps the more experienced members on this site can give a more exact answer, but all I can recommend for now is that you try a resistor with a value of something like 1 mega-ohm (1,000,000 ohms), and then experiment from there. If the flash time seems to be the same length, try a lower resistance value, and if the flash does not go off at all, try a higher resistance. If you have any potentiometers, (variable resistors) those would work excellently for tuning the resistance to what you want.

 

[crutschow]

One method is to add another SCR to discharge the cap to ground shortly after the tube fires. The normal discharge probably takes about a millisecond so you would need an adjustable delay of perhaps a few hundred microseconds before you fire the SCR. This could be done with a 555 configured as a one-shot.

 

[zsbreaker]

Crutschow, pls give me schematic for this SCR and 555

 

[crutschow]

Crutschow, pls give me schematic for this SCR and 555

Sorry, I don't have time to do the detailed design for you.

You need an SCR rated for at least 50% over the voltage on the cap with probably at least a 5A capability.

There are zillions of 555 circuits on the web, which you can use.

 

[Veraxis]

One method is to add another SCR to discharge the cap to ground shortly after the tube fires. The normal discharge probably takes about a millisecond so you would need an adjustable delay of perhaps a few hundred microseconds before you fire the SCR. This could be done with a 555 configured as a one-shot.

pardon my asking, but are 555 timers rated for that kind of trigger voltage? also what is being used for Vcc? these flash circuits usually run off of single 1.5V batteries, which is less than the minimum voltage requirement of a 555 timer (4.5V). other than that, it sounds like it would work.

 

[crutschow]

pardon my asking, but are 555 timers rated for that kind of trigger voltage? also what is being used for Vcc? these flash circuits usually run off of single 1.5V batteries, which is less than the minimum voltage requirement of a 555 timer (4.5V). other than that, it sounds like it would work.

An SCR only takes a volt or so to trigger.

But the 555 timer will obviously require more than 1.5V to operate.

 

[colin55]

Here is a simple circuit that will create a repeated flash at the maximum rate that the capacitor will charge:


**broken link removed****broken link removed**

 

[zsbreaker]

This is what I have, from dispoabe camera:

There was a switch for charging somewhere near transistor but I made short connect on that, becouse I want to be charged all the time.

**broken link removed**

 

[mel8030]

colin55, could you explain how the transistor oscillates.

 

[colin55]

[FONT=&quot]The oscillator made up of the transistor and transformer is called a BLOCKING OSCILLATOR.
When power is applied, the transistor is turned ON slightly via the 220R resistor. This causes current to flow in the collector-emitter circuit and the primary winding of the transformer. The expanding flux produced by the primary also cuts the turns of the feedback winding and this winding produces a voltage that adds to the supply voltage to increase the current into the base of the transistor. This causes the transistor to turn on more and the whole action increases until the transistor is fully turned on. At this point maximum current is flowing in the primary of the transformer but it is not expanding flux and the voltage produced by the feedback winding instantly stops. This turns off the transistor a small amount and now the current in the primary winding decreases. This causes a reduction in the magnetic field and the feedback winding produces a voltage in the opposite direction.
This voltage opposes the supply voltage and the net result is to turn the transistor off more.
This continues until the transistor is fully turned off. This happens very quickly and the collapsing magnetic field collapses very quickly. The transistor is effectively removed from the circuit and the winding we are now interested in is the secondary. This winding consists of many turns and since the field collapses very quickly, a very high voltage is produced in this winding.
The energy of the magnetic filed is converted into a high voltage and it finally collapses to becomes zero. During this time it is producing a voltage in the feedback winding that is keeping the transistor turned off.
The feedback voltage finally becomes zero and now the voltage delivered from the supply rail via the 220R starts to turn the transistor on again to begin the cycle again.

[/FONT]The operation of the BD679 is even more interesting.
It is simply a transistor with a collector-emitter voltage of 80v. This means it is a transistor that will operate in a circuit with a rail voltage less than 80v.
In other words it is a device with a zener value of 80v.
The transistor breaks down completely at 80v and the voltage across it is just a few volts.
This means the circuit operates quite normally when turned on and the electrolytic gets charged to a few hundred volts.
During this time the 22n is charged via the 4M7 and when the voltage on the 22n reaches 80v, this voltage is passed via the trigger transformer to the collector of the transistor.
The transistor instantly breaks down and takes the end of the trigger transformer to the 0v rail and the energy in the 22n is passed to the primary of the transformer. This gives the transformer a spike of energy and the secondary produces about 4.5kV.
This is sufficient to ionize the gasses in the zenon tube and allow the 350v from the electrolytic to produce a discharge through the tube to produce a bright flash. [FONT=&quot]

[/FONT]