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Partialy Discharging Capacitor

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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It depends on what you mean by "discharge xx%". However if you can restate that as "discharge to xx Volts" then the answer may be as simple as using a suitable value resistor in series with a zener diode.

For example, if the capacitor is charged to 300V and you wish to discharge it to 200V, you can use a string of zener diodes which as up to 200V. Let's say you use 4 zener sides of about 50V and rated at 1W.

1W at 50V is 20mA, so the resistor needs to be chosen to limit the current to 20mA at the full capacity voltage. So that's 100V and 20 mA, so the resistor should be 5k at 2W.

Connecting these to the capacitor must be done with care because 300V is easily enough to overcome your skin's resistance giving you a kick you'll never forget. If you get that shock across your chest, it could be the last thing you ever do. (Working with one hand in your pocket is not a silly thing to do)

I would also recommend you connect a multimeter across the capacitor to monitor the voltage across the capacitor.

One sensible option is to connect the discharge circuit across the meter leads at the meter end and then to apply the leads to the capacitor. At this voltage it's safe to rely on the insulation of the probes (assuming they are rated for at least 300V) but if you can connect one and then the other using just one hand you'll be even safer.

Be very sure you have the zener diodes connected around the right way. A simple test is to connect it across a capacitor charged to something less than 100V (to protect the resistor if you made a mistake). If it's connected correctly the calcite should not discharge. Repeat at 200V, and then you should be right to go.
 

nebojsa

May 13, 2017
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It depends on what you mean by "discharge xx%". However if you can restate that as "discharge to xx Volts" then the answer may be as simple as using a suitable value resistor in series with a zener diode.

For example, if the capacitor is charged to 300V and you wish to discharge it to 200V, you can use a string of zener diodes which as up to 200V. Let's say you use 4 zener sides of about 50V and rated at 1W.

1W at 50V is 20mA, so the resistor needs to be chosen to limit the current to 20mA at the full capacity voltage. So that's 100V and 20 mA, so the resistor should be 5k at 2W.

Connecting these to the capacitor must be done with care because 300V is easily enough to overcome your skin's resistance giving you a kick you'll never forget. If you get that shock across your chest, it could be the last thing you ever do. (Working with one hand in your pocket is not a silly thing to do)

I would also recommend you connect a multimeter across the capacitor to monitor the voltage across the capacitor.

One sensible option is to connect the discharge circuit across the meter leads at the meter end and then to apply the leads to the capacitor. At this voltage it's safe to rely on the insulation of the probes (assuming they are rated for at least 300V) but if you can connect one and then the other using just one hand you'll be even safer.

Be very sure you have the zener diodes connected around the right way. A simple test is to connect it across a capacitor charged to something less than 100V (to protect the resistor if you made a mistake). If it's connected correctly the calcite should not discharge. Repeat at 200V, and then you should be right to go.
Yes, I was thinking "discharge to xx Volts". Circuit in image is part of flash for camera. I want to find a way to manualy control the power of light that flash produce. So, my idea is to discharge capacitor through lamp F.T. to 165V (1/2 power), 240V (1/4 power), 285V (1/8 power), 310V(1/16 power) ... That is my idea, I dont know is it posible...
Can you give me an example of discharging to 280V (1/8 power)?
 
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Audioguru

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Your understanding of voltage and brightness is backwards.
When the main voltage for an xenon tube is halved then the power and brightness are 1/4 times. If the main voltage is doubled then the brightness and power are 4 times.

Your trigger circuit must not have its voltage reduced when the main voltage is reduced.
 

nebojsa

May 13, 2017
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Your understanding of voltage and brightness is backwards.
When the main voltage for an xenon tube is halved then the power and brightness are 1/4 times. If the main voltage is doubled then the brightness and power are 4 times.

Your trigger circuit must not have its voltage reduced when the main voltage is reduced.
O.K. But,how to avoid reducing voltage on triger circuit while reducing it on flash tube?
 

Audioguru

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O.K. But,how to avoid reducing voltage on triger circuit while reducing it on flash tube?
Make a steady 350V for the trigger transformer circuit and make a variable main discharge voltage. I do not know if you can stepup the 12V to thousands of volts with one trigger transformer circuit.
 

nebojsa

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Your understanding of voltage and brightness is backwards.
When the main voltage for an xenon tube is halved then the power and brightness are 1/4 times. If the main voltage is doubled then the brightness and power are 4 times.

Your trigger circuit must not have its voltage reduced when the main voltage is reduced.
I havent noticed this until now, but correct me if I am wrong. Trigger circuit is suplied by c1 capacitor (rated 0.047uF 630v), so I think trigger circuit is independent from capacitor CS- flash tube F.T. circuit.
And one more question: If half original voltage is 1/4 power, how much is needed for 1/2 power? I am sorry for this, but i am new in electronics and my knowledge is barely basic.
 
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HellasTechn

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You also need to consider the voltage limits that the tube can accept. For example if it is fed with 300V then 150V may not be enough to ionize the gas within the tube and make it flash.
 

nebojsa

May 13, 2017
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You also need to consider the voltage limits that the tube can accept. For example if it is fed with 300V then 150V may not be enough to ionize the gas within the tube and make it flash.
I am not sure, but I assume It should. I have an old flash that is going from 1/1 to 1/32 power. So, I assume that I can use the same or similar principle of work.
 

nebojsa

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I am not sure, but I assume It should. I have an old flash that is going from 1/1 to 1/32 power. So, I assume that I can use the same or similar principle of work.
I dont know until I try. Control of light on old flash is regulated by little PCB which consist of several resistor, capacitor and one thyristor. So I am confused with zener diodes (*steve*) mentioned above.
 

duke37

Jan 9, 2011
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I thought that the output of flashes for cameras were controlled by altering the length of the flash rather than the intensity.
 

nebojsa

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I thought that the output of flashes for cameras were controlled by altering the length of the flash rather than the intensity.
If by 'length' you think on moving lamp back and forth I have to correct you. It is for controling range of flash depending on focal lenght of lens. On that old flash power can be controled ading a pair of resistor (link https://www.flickr.com/groups/71917374@N00/discuss/72157629108782589/) or potentiometer (link http://vivitar3500.ilyatitov.com/). Im trying to figure out circuit used on old flashes and use it on circuit I am working on.
 

Audioguru

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PWM varies the duration of each "flash" to vary the brightness. I understand that a flash duration of 30ms or more results in full brightness to our vision.
 

nebojsa

May 13, 2017
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PWM varies the duration of each "flash" to vary the brightness. I understand that a flash duration of 30ms or more results in full brightness to our vision.
Im not sure how long is duration of flash, but when it comes to camera, situation become more twisted. My camera has flash sync up to 1/200s, more advanced cameras have 1/1000s or even faster flash sync. But there is option on came called slow flash sync and there you can shoot at speed of 1, 2 or even 10 seconds. I think I am off topic right now, but lets get to what you mentioned before. Capacitor is 330v 800uF, if I want to flash at 1/2 power I have to discharge it to 100V (i found calculator (http://www.xenonflashtubes.com/schematics/) and it says 230v is for half pover). Any idea? Even with zener diodes.
 

BobK

Jan 5, 2010
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230V would be correct if the current is linear in the voltage. But a xenon flash tube curve is not likely anything close to linear.

Controlling the time of the flash is a much better option. An SCR across the capacitor can be fired anytime after the flash starts to quickly end it and dump the charge of the capacitor.


Bob
 

nebojsa

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230V would be correct if the current is linear in the voltage. But a xenon flash tube curve is not likely anything close to linear.

Controlling the time of the flash is a much better option. An SCR across the capacitor can be fired anytime after the flash starts to quickly end it and dump the charge of the capacitor.


Bob
can you give me some schematic with values?
 

(*steve*)

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The answer I have about the discharging of the cap was based on your original question which did not have any context.

Flash tubes are NOT controlled by varying the input voltage, and pwm is used in the most modern of flash units not to reduce intensity, but to increase the duration of the flash.

That thyristor you see either part of the circuit which turns off the flash. In typical old school flash units, the cap is always charged to the same voltage (around 300V) and for reduced intensity flash, the current is interrupted.

The information I don't have is how the current is interrupted. It's not done AFAIK by placing a control element in series with the flash tube.

You'll have to do some research to find out what the "right" way is.
 

nebojsa

May 13, 2017
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The answer I have about the discharging of the cap was based on your original question which did not have any context.

Flash tubes are NOT controlled by varying the input voltage, and pwm is used in the most modern of flash units not to reduce intensity, but to increase the duration of the flash.

That thyristor you see either part of the circuit which turns off the flash. In typical old school flash units, the cap is always charged to the same voltage (around 300V) and for reduced intensity flash, the current is interrupted.

The information I don't have is how the current is interrupted. It's not done AFAIK by placing a control element in series with the flash tube.

You'll have to do some research to find out what the "right" way is.
OK. At least I found the right question for my project and it is: How to interrupt capacitor discharging using thyristor? Or something similar...
 

BobK

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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
 

(*steve*)

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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 :)
 
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