MOSFET ratings

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Making a timer to switch off a soldering iron in a telephone exchange after
an hour. Irons are 50V Wellers fed from exchange battery. Timer prototype is
working fine but I'm hoping to get away from use of a relay to switch the
iron on and off, mainly because inexpensive small relays with contacts rated
at 50VDC are scarce. Every dollar counts when I want management to blow the
moths out of the petty cash tin and pay for parts for half a dozen copies
for half a dozen exchanges when the prototype is finalised.
I thought a power mosfet was the way to go so I have been experimenting with
a $2.00 MTP3055 rated at 60V and 12A. The current rating is well within
requirements but 60V seems to me to be uncomfortably close to the actual 54V
of the exchange battery. The exchange battery voltage  is, at least, well
regulated and filtered. How robust are mosfets generally? How conservative
are manufaturers when they quote ratings? Should I go for a more expensive
100V type like IRF50?
PH



Re: MOSFET ratings



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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

 



Re: MOSFET ratings



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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



Re: MOSFET ratings



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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




Re: MOSFET ratings


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.




Making a timer to switch off a soldering iron in a telephone exchange after
an hour. Irons are 50V Wellers fed from exchange battery. Timer prototype is
working fine but I'm hoping to get away from use of a relay to switch the
iron on and off, mainly because inexpensive small relays with contacts rated
at 50VDC are scarce. Every dollar counts when I want management to blow the
moths out of the petty cash tin and pay for parts for half a dozen copies
for half a dozen exchanges when the prototype is finalised.
I thought a power mosfet was the way to go so I have been experimenting with
a $2.00 MTP3055 rated at 60V and 12A. The current rating is well within
requirements but 60V seems to me to be uncomfortably close to the actual 54V
of the exchange battery. The exchange battery voltage  is, at least, well
regulated and filtered. How robust are mosfets generally? How conservative
are manufaturers when they quote ratings? Should I go for a more expensive
100V type like IRF50?
PH




Re: MOSFET ratings



"Peter Howard"
Quoted text here. Click to load it


**  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





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