[Diy_efi] A 2N2222A can handle up to 50mA

[bcroe at juno.com]

A few more comments here:   
   While I pay 80 cents for FETs in 100s, I don't think a 
couple quarters are the issue here.  I am going to 
assume the relay coil may be on the plus or minus side 
of the coil driver to allow the most common parts.

As mentioned, bipolar and darlingtons are what we had 
to use before FETs came along.  My 80 cent FET will get 
me a huge improvement in current gain (infinite), voltage 
rating (I NEVER go below 100V in auto circuits), and 
saturation voltage.  The low sat also fixes heat problems, 
and sometimes the FET can just replace the RELAY 
outright.  The IRFI530N has an insulated tab that may 
be bolted directly to any heat sink, 0.15 V drop at 1 A 
(thats .15W), 100V rating (use 630 for 200V), and no 
drive current req.  

Assuming we have the 339 driving an N channel FET 
with a 47K pullup to the 339 supply, let's address another 
problem.  The 7808 V reg is wide open to transients coming 
from the battery (the starter can generate 300V for 
microseconds, just long enough to take out V regulators).  
We only need 2 ma for the 339, and maybe another 1or 2
for 47K pullup and input circuit.  So replace the $1   7808 
with a $0.02  390 resistor from the battery to a $0.11  10V 
zener diode to ground.  

The zener will clip any spikes that come along, even 
reverse polarity.  The 390 will limit current for those brief 
overvoltages to prevent any damage.  

You can get a max of 10 ma off the zener, all you need 
with the drive requirement solved.  You will need 
some kind of clipper to protect the FET output; perhaps 
a couple more 10V zeners source to drain, plus a 
1N4004 diode across the coil ought to do it.

Bruce Roe

17 Feb 2003 David Gravereaux <davygrvy at pobox.com> writes:
> bcroe at juno.com wrote:
> 
> >Personally I'd use a good FET output, easily available with 
> >200V rating and an insulated tab.  Infinite current gain, you 
> >can reduce the power supply to a zener diode.
> 
> 11 cents (in bulk) for a TO-92 2N2222A vs. ~$2.50 for a TO-220 FET 
> output
> driver.  Arrange it in a darlington if you want more base gain and 
> stay
> with the cheap stuff.  It's just a switch and either it's off or
> saturated.  There's minimal heat from a saturated transistor -- aka
> class-D.  It's running it linear where you need to concern yourself 
> about
> vce drop equaling heat dissipation (mostly).
> --
> David Gravereaux <davygrvy at pobox.com>

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