Stumped on a circuit, ideas to replace relay??? - Electronic Circuits Projects Diagrams Free

fat.tony · 29th March 2008, 04:19 PM

Ok, well what does my circuit do? its a camera flash taken from a disposible camera and has been slightly modified so only 2 contacts have to touch for it to trigger. I built a 555 timer circuit which i used to seperate from the main flash circuit (Prevent shocking myself....again) and its connected to a relay which triggers the flash. The problem is that this relay is switching at about 7Hz so its pretty noisy and i want a device to replace the relay.
The 2 contact on the flash circuit carries around 400V so it destroys any sort of transistor placed on it...and so i have no idea what to do now.

http://aycu15.webshots.com/image/511...8736906_rs.jpg

I want to replace the relay with something that will do the same but is solid state and doesnt mind being subjected to high voltages...oh and it completely opens and closes the circuit on each switching,

im not sure if ive explained that very well but any questions please ask

thanks,
fat tony

crutschow · 30th March 2008, 05:11 AM

A high voltage SCR may work for this. Typically an SCR can be used to fire a flash unit since the current will momentarily drop to zero after the flash which turns off the SCR for the next pulse.

For example the Littlefuse Q6004D3RP device available form Mouser Electronics has a 600V 4A rating which should be adequate.

fat.tony · 31st March 2008, 02:05 AM

I did wonder about using one of these little devices but would the current through the anode to the cathode need to be exactly 0? or can there be a insignificant amount of current which will "reset" the thyristor?
i apologise for all the questions but ive looked so much into it i confused myself too much

thanks for the reply

fat.tony

crutschow · 31st March 2008, 05:12 PM

Actually I gave you the part number for a Triac which would work, but an SCR would probably be better. An example is the Littlefuse S6008LS3 which is rated at 600V and 8A.

The minimum anode-cathode current for the SCR to stay on is IH on the data sheet, which for the S6008LS3, is 8mA at room temperature. If the current drops below that the SCR "may" reset to the off state. To insure reset, the current should probably be less than 1mA.

The gate trigger current for this device is 0.5mA minimum. A good value to insure triggering would be 5-10mA so select a gate resistor to give that current. The gate looks like a diode with a 0.8V maximum (~0.7V typical) gate-cathode voltage at room temperature.

The gate pulse should be fairly short so that the SCR doesn't stay latched on by the charging current after the flash fires. The typical required gate pulse is 5us so 50-100us should be adequate.

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30th March 2008, 05:11 AM	(permalink)
crutschow	A high voltage SCR may work for this. Typically an SCR can be used to fire a flash unit since the current will momentarily drop to zero after the flash which turns off the SCR for the next pulse. For example the Littlefuse Q6004D3RP device available form Mouser Electronics has a 600V 4A rating which should be adequate.

31st March 2008, 02:05 AM	(permalink)
fat.tony	I did wonder about using one of these little devices but would the current through the anode to the cathode need to be exactly 0? or can there be a insignificant amount of current which will "reset" the thyristor? i apologise for all the questions but ive looked so much into it i confused myself too much thanks for the reply fat.tony

31st March 2008, 05:12 PM	(permalink)
crutschow	Actually I gave you the part number for a Triac which would work, but an SCR would probably be better. An example is the Littlefuse S6008LS3 which is rated at 600V and 8A. The minimum anode-cathode current for the SCR to stay on is IH on the data sheet, which for the S6008LS3, is 8mA at room temperature. If the current drops below that the SCR "may" reset to the off state. To insure reset, the current should probably be less than 1mA. The gate trigger current for this device is 0.5mA minimum. A good value to insure triggering would be 5-10mA so select a gate resistor to give that current. The gate looks like a diode with a 0.8V maximum (~0.7V typical) gate-cathode voltage at room temperature. The gate pulse should be fairly short so that the SCR doesn't stay latched on by the charging current after the flash fires. The typical required gate pulse is 5us so 50-100us should be adequate.