I was talking about this 50 volt soldering iron timer project September/October 2005 and now have four test prototypes installed in facilities belonging to a "large Australian telecommunications firm". Two have relays doing the switching and two use a power MOSFET and the idea is to see which version quits first. I prefer the MOSFET version myself. A circuit diagram is posted at alt.binaries.schematics.electronic. The diagram is of the 4.4 version. The 4.3 version has only one driver transistor for the FET, a BC557. When the 4060 counter IC is counting, its output pin is low. Base current flows from the PNP transistor, which turns on and applies voltage to the MOSFET gate which turns on and conducts current to the soldering iron. When output of counter goes high, BC557 turns off and the 10k load resistor pulls the MOSFET gate low which turns the MOSFET off.
In this 4.4 version, only breadboarded so far, I've added an NPN BC547. (see circuit at a.b.s.e) Idea being that it conducts when counter output goes high and pulls MOSFET gate low faster (maybe) than if I just relied on gate charge leaking away through 10k resistor.
I've seen this arrangement a number of times in designs on the 'net and in magazines where MOSFETS are used for switching in simple SMPS's.
In practice, the 4.3 versions already installed are working fine with a single transitor. The MOSFET is, after all, switching off once an hour, not
30,000 times a second so there's not much opportunity for overheating due to slow switching.However, I'm trying to sell management on this idea and with luck, the design will eventually be looked over by a real engineer. That's if the company has any left given the present mania for outsourcing. I wonder which would impress the most? The simplicity of a single transistor with a pull-down resistor or the greater "elegance" of the fast turn-off arrangement?
Any opinions from real engineers out there?
PH