On Tue, 8 Jan 2008 11:51:41 -0800 (PST),
wrote:


Thanks again for the info. This gives me another possible scenario for
why I toasted the solenoid.

I was 'driving' the gate with 5V+ a resistor and a switch. I didn't
have a pulldown resistor. I gather that it may be possible that the
field still existed on the gate even after I opened the switch.

---
Not just possible, that's exactly what happened. :-(
---

So I
guess with a pull down resistor on the gate I'd be more safe.

---
Of course.
---

What do you think?

---
Since you're driving the 12V, 100% duty cycle coil with 24V, that
mandates a 25% (or lower) duty cycle so, assuming it's going to be
working 100% of the time, I think a good idea would be to drive it
with something like a 555 wired like this:

+24----------------------------------------+
|
VCC--+--------------------------------+ |
| | +------+
[20k]R1 +---------+ | | |K
| |_ | | [COIL] [1N4001]
+-------------7-O|D Vcc|-8---+ | |
| R2 | _| | +------+
+---[82k]---+--6-|TH R|O-4--+ |
| | |__ | | D
+-[1N4148]-+-2-O|TR OUT|-3---|--G NCH
| | GND | | S
[C] +----+----+ [0.1µF] |
| |1 | |
GND--------------+---------+----------+----+

Note that a pot can be substituted for R1 and R2, so you can adjust
the frequency, as required, to accommodate the inductance of the
coil, by choosing the end-to-end resistance of the pot and then
choosing the duty cycle by cranking the pot.

Also, if you can't find a pot with the end-to-end resistance you
need, you can always do this:


V---+
|
[R1]
|
[POT]--
|
[R2]
|
GND--+

Since we don't know the inductance of the coil it's impossible to
determine the pulse repetition frequency but, assuming it's 100
millihenrys:

Version 4
SHEET 1 880 716
WIRE 160 -32 -752 -32
WIRE 256 -32 160 -32
WIRE 160 16 160 -32
WIRE 256 16 256 -32
WIRE -272 176 -624 176
WIRE 48 176 -48 176
WIRE 160 208 160 96
WIRE 256 208 256 80
WIRE 256 208 160 208
WIRE 160 224 160 208
WIRE -624 240 -624 176
WIRE -576 240 -624 240
WIRE -464 240 -496 240
WIRE -272 240 -464 240
WIRE 16 240 -48 240
WIRE -464 304 -464 240
WIRE -432 304 -464 304
WIRE -320 304 -352 304
WIRE -272 304 -320 304
WIRE 112 304 -48 304
WIRE -16 368 -48 368
WIRE -464 400 -464 304
WIRE -416 400 -464 400
WIRE -320 400 -320 304
WIRE -320 400 -352 400
WIRE -320 464 -320 400
WIRE 16 464 16 240
WIRE 16 464 -320 464
WIRE -624 496 -624 240
WIRE -16 496 -16 368
WIRE -16 496 -624 496
WIRE -752 512 -752 -32
WIRE -624 512 -624 496
WIRE -320 528 -320 464
WIRE -752 640 -752 592
WIRE -624 640 -624 592
WIRE -624 640 -752 640
WIRE -320 640 -320 592
WIRE -320 640 -624 640
WIRE 48 640 48 176
WIRE 48 640 -320 640
WIRE 160 640 160 320
WIRE 160 640 48 640
WIRE -752 688 -752 640
FLAG -752 688 0
SYMBOL nmos 112 224 R0
SYMATTR InstName M1
SYMATTR Value IRF7468
SYMBOL ind 144 0 R0
SYMATTR InstName L1
SYMATTR Value .1
SYMATTR SpiceLine Rser=20.7
SYMBOL voltage -752 496 R0
WINDOW 0 42 56 Left 0
WINDOW 3 42 84 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 24
SYMBOL diode 272 80 R180
WINDOW 0 -43 35 Left 0
WINDOW 3 -101 -5 Left 0
SYMATTR InstName D1
SYMATTR Value MURS120
SYMBOL Misc\\NE555 -160 272 M0
SYMATTR InstName U1
SYMBOL voltage -624 496 M0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 0 -73 53 Left 0
WINDOW 3 -65 76 Left 0
SYMATTR InstName V3
SYMATTR Value 10
SYMBOL res -448 288 M90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 58 VTop 0
SYMATTR InstName R1
SYMATTR Value 8e4
SYMBOL res -592 224 M90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 2e4
SYMBOL cap -304 528 M0
WINDOW 0 -33 32 Left 0
WINDOW 3 -39 58 Left 0
SYMATTR InstName C1
SYMATTR Value 1e-6
SYMBOL diode -416 384 M90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName D2
SYMATTR Value 1N4148
TEXT -610 666 Left 0 !.tran .2 uic


--
JF