At 03:14 PM 6/17/02 -0400, you wrote:

>Yeah, MOS is what I was thinking originally but it seems that most of these
>transistor/relay circuits are BJT.

Nothing wrong with BJTs, they might be a bit cheaper than a MOSFET, and
they work to rather low Vcc if that is a requirement (not in an automotive
application, most likely). For more than 100mA the MOSFET starts to look
better, but at 100mA or less (roughly) a 2N4401 does an excellent job for
pennies.

> > There is no such diode in the TIP120, sorry.
>
>http://www.fairchildsemi.com/ds/TI/TIP120.pdf
>
>That's the datasheet for a TIP120.  There is a diode, however it appears to
>be in the wrong direction to shunt current from a collapsing field in a
>relay coil that would be above this device (connected at C).  When base
>current disappears, both the BJTs go off, but nothing can flow to ground
>through that diode... no?

Right. The voltage will increase limited by the breakdown voltage of the
transistor or the parasitic capacitance in the circuit. The parasitic
diode shown in some data sheets doesn't conduct at all. This can cause
SOA failures of transistor (typically "on"). Some MOSFETS are
avalanche-rated and you don't need the diode if the energy of the
relay coil is low enough.

>Shrug!

;-)

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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