Transistor time delay off circuit - Page 1
Forum section: electronics / beginners

author: FenderBender
author_rank: Super Contributor
timestamp: December 31, 2012, 03:28:15 am
content: I am designing an "automatic" capacitor discharger. As part of it, I want to be able to switch off a relay if the temperature of a the load resistor gets too hot.Originally, I thought of using a circuit like this:Power supply v+ --> PTC --> Potentiometer (to adjust cutoff temperature) --> Base of NPN transistor --> (Gnd)The Collector-Emitter junction would be in series with the relay coil circuit. When the PTC heats up, the resistance increases, and eventually it is enough to limit the coil current to the point where the coil disengages. This coil is also part of a polarity checking circuit. In that circuit, if the polarity is incorrect, it does not connect the capacitor to the discharge circuit. Then in the discharge circuit, a comparator monitors the capacitor voltage, and when it falls under a threshold value, it lights an LED (and maybe sounds a buzzer). I've drawn it out on paper. I attached the circuit. It might not all be correct. Just initial thoughts.------Now back to the problem... When the PTC cools down enough, the relay engages, then the PTC gets too hot, so the relay disengages, and then it oscillates forever until the operator removes the capacitor. Or at least, this is what my brain is telling me...So here's what I want. At a specific temperature, cut off the relay for a set time (say 30 seconds), and while the relay is open, light a red LED to note there the temperature overload. This would give the resistor time to actually cool off.I have an idea on how to do it but this is my first “official” circuit I’ve ever designed by myself (or atleast my own design with external advice). I've thought about using a 555. But it seems that you need to have just a pulse to activate the timer. In my case, the input would be low until the PTC cools down. But I want it to continue to stay low AFTER it has cooled down. How can I do this? How would you set it up so that transistor stays off for a certain amount of time (and thus the relay open)?Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: December 31, 2012, 04:46:13 am
content: Thermal mass of the resistor and PTC will give you that 30s delay for sure, just use a 10W resistor and epoxy the PTC to the side of it.

author: FenderBender
author_rank: Super Contributor
timestamp: December 31, 2012, 05:10:08 am
content: I would have to play around with it. But even if the temperature drops 0.01*C that still could be enough to switch the relay and vise versa. Though I guess by the time the PTC heats up...there will probably still be some heat in the resistor that has not been transfered yet and thus, the temperature will be hot for a while. But when it's "cool" it still may be very close to the threshold temperature. That's the dilemma.If it doesn't work how I like, I may implement a circuit similar to this. Took me a while to find one. This looks about right.http://easy-electronics4u.blogspot.com/2012/02/simple-dc-timer-using-mosfet-onoff.htmlHow about the circuit itself? I haven't put in many values. Does it look okay? I know it's hard to follow.Thanks alot!

author: SeanB
author_rank: Super Contributor
timestamp: December 31, 2012, 05:50:02 am
content: relay has hysteresis in it and will not chatter. Thermal mass will take a while to heat up to switch off, and will take time to cool off enough. It will not chatter.

author: FenderBender
author_rank: Super Contributor
timestamp: December 31, 2012, 06:05:50 am
content: Thank you!