MOSFET ratings

Peter,

I applaud your enthusiasm and I'm assuming the project has a safety slant to it besides saving energy. Seriously though, one would think that a few dollars spend on such a worthwhile project would not be beyond the budget of the big T? Having said that I'm not surprised you are having trouble convincing management to come up with the $$$ to fund a small project.

Several years ago when I was still with the Big T, I developed (in my own time) an inexpensive device to interface with a specific piece of test equipment and a laptop PC. The Big T has well over 1000 of these test units in use in the field and a lot of the field techs had laptops on which to run the software the Big T had purchased from the test equipment manufacturer at a huge cost. What they couldn't do was get the laptops to talk to the test equipment. I offered them a prototype for nothing but management had closed eyes and weren't interested at the time. Later I was approached by someone within the Big T to supply a few units for evaluation. Well the rest is history because since then I've sold them hundreds of units at a nice tidy profit. I still get orders for units from time to time. :) Stupid thing is they could have had it for nicks. Their loss = my gain :)

Nothing wrong with using the 60V MOSFET, but the 100V type just gives you that extra margin. Not that a drain-source short would be a big deal. It would just leave the soldering iron on - and aren't they already that way?

Cheers, Alan

I had a 12V Weller iron, the transformer went open so I got another transformer and put it in a computer case power supply it looked rude but with stick on rubber feet and the iron spring on top worked OK,

Then I found a mechanical timer like cheap microwaves have, that timed out after 20 minutes or so, it was a nuisance sometimes if you went to use the iron a few minutes after the timer had switched the iron off, Then the element went open after a few months of heavy use due to metal fatigue from heating up and cooling down.

So I think what you should have is one that switch off after a hour you last use it, not a hour after you start the timer ticking, but then the project gets a bit tricky then.

100V type like IRF50? PH

"Peter Howard"

** The 100 volt rated IRF520 or IRF530 would be good choices - only a couple of dollars each from Farnell in One.

Mosfets are sensitive to voltage spikes created by inductive loads so it is wise to include a zener across the drain and source pins. In your case, a

62 volt one like the BZT03-C62.

.......... Phil

Alan, I'm amused by the war story, especially when it rings so absolutely true. I always think that Big T management to workforce communications are like a diode. The threats, exhortations and useless bumf flow in one direction only. Try to communicate a good idea to higher authority and you can see the manager thinking "Will there be any comeback or will my career be affected if I choose to ignore this liney?" I find it better to commit a submission to writing so it can be binned straight away. Saves time for everyone. My timer idea is for safety and also to save wear and tear on the soldering irons. Much as I'd like to see it done on a national basis with a properly developed custom PC board, the reality is that it will only happen, if at all, in our local patch and I'll be flat out getting funds for the parts, let alone work time to assemble them. P.A. pointed me in the right direction, to Farnells. I was gratified to see that the Farnell website and search facility is lots better than it was and I found a suitable 100V mosfet for a mere 37c more than the one I can source locally. Just to be on the safe side. Only problem with Farnells for me is the $10 overnight satchel delivery so I bought up lots of other bits to make it worthwhile. PH

Good to hear you are making some headway with the idea and can source the FETs you are after. Lots of luck convincing management though.....

Cheers, Alan