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From: Fred Bloggs
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Subject: Re: How to get CMOS counter to drive a relay?
References: <0001HW.B9CEF278023A90B81662EAD0@news.covad.net> <3DA9D6CC.email@example.com> <3DA9D9E1.firstname.lastname@example.org> <3DA9EE2A.B7CFD919@juno.com> <3DAAF96D.email@example.com> <firstname.lastname@example.org> <3DAC2583.email@example.com> <firstname.lastname@example.org> <3DAD689F.email@example.com> <3DADBB67.4917CD48@bellatlantic.net> <%Cpr9.64373$XF.firstname.lastname@example.org> <3DAFF367.email@example.com>
Date: Sat, 19 Oct 2002 18:07:07 GMT
NNTP-Posting-Date: Sat, 19 Oct 2002 11:07:07 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> I'm not too good at ASCII logic diagrams - so I scanned some partial
> Here's how the transistor pair did it --
> Logic on left was 74LS154, first transistor was part of CA3081 transistor
> array, final transistor was TIP102 equivalent. Coil diode was located on
> coil. This method worked quite well but sometimes load was too great for
> TIP102 - they'd get pretty hot sometimes and those solenoid coils took a
> heft chunk o' current.
>>Was there something else that accompanied the transition to
>>MOSFET drive like direct microprocessor I/O control?
> Here's how the MOSFET's did it --
> Simpler, no two stage. In this case, coil diode was located on board.
> These MOSFETS seldom fail.
Okay- if you say a TIP102 then that's a Darlington with Vce,on=1V on a
good day. So this device was dissipating 1W per Amp of coil current. The
12N10L has an Rds,on of ~0.2 ohms which makes for a much more efficient
switch at intermediate values of coil current. For example, at 2 Amps,
the FET is dissipating 4 x 0.2= 800mW and the Darlington is dissipating
2W. This is a substantial difference. In the case of this type of
application where the power elements are subjected to an ON/OFF stress
on a time scale that is large compared compared to the thermal time
constant of the die, the major failure stress is most likely thermal and
this goes as the 9th root of temperature excursion per stress cycle.
Using the 2 Ampere example and assuming equal thermal resistances and
time constants, this would put the transistor lifetime at 90% of that of
the FET. At 1 Amp of coil current this is still 85%, so I don't see the
lifetime differences as being tremendous due to static dissipation. The
answer must be in the transistor Pmax at turn-off due to those Miller
capacitors which were added to limit the solenoid line dV/dt -probably
for EMI suppression.
> In both cases, transistor failures were usually caused as a result of coil
> rectifier failure.
Hmmm...same thermal stress issue there if they are not derated properly
and the construction did not allow good thermal conduction out the leads.
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