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                                                          CC PICList for
general interest.
                                                                 Nitrous
Oxide solenoid operation for rockets.

> > If you want a circuit that initially applies and holds full power and
> > maintains it for a well defined and longer period of time you could go
to
> > a simple electronic switch

> Hi Russell
> Could you forward me a copy of your solenoid driver circuit?


The circuit was in my head when I said that :-).
This is untried but is very very likely to work OK.
If people were REALLY interested and didn't have the facilities to play I
could lash one up and report.

Attached are two of many possible circuits.
Other alternatives could include a digital timer based one or a cheap
microprocessor (both probably overkill). The microprocessor one would cost
under $10 in parts cost and be able to be talked to in {{insert name of
winning protocol here}} :-).

The first is simpler. The second has more precise control of the timing
between full and reduced power at the cost of needing a cheap Schmitt
trigger IC. 1st cct should work just fine.

Circuit one is adaptable for almost any voltage (24, 36, more) whereas cct
two runs the IC supply from Vin so is limited in maximum Vin to not much
over 12v. A revised cct could allow eg 24v or 36v Vin with an IC.
The IC has 6 identical sections so could be shared between 6 identical
circuits.

Almost any N Channel MOSFET rated for continuous operation at solenoid rated
current should work.
I'm assuming 12v operation.
If size is not an issue or utter lowest cost (it shouldn't be) I'd use a
TO220 FET or a largish SMD pkg if you go that way to allow a bit of leeway
for solenoid operation. The Digikey part number RFP3055LE-ND at $US0.90 in
1's is rated at 60V 11A and in a TO220 pkg and will meet many needs. Also in
other packages at similar cost.

RHOLDx to suit solenoid.
PROBABLY has a resistance about the same as the solenoid (which will reduce
solenoid dissipation by a factor of 4) BUT YMWV depending on design,
pressure etc.

Zener diodes clamp FET gates for protection. Any 12V zener OK for most FETs.

CT1/RT1 and CT2/RT2 are timing control components.
For CT = 1 uF (microFarad) and Rt = 1 megohm delay is around 1 to 2 seconds.
RT can be a trimpot if desired (but don't adjust to zero ohms :-). Watch for
capacitor leakage currents with larger electrolytics. If a non polarised cap
is used for Ct then Rt can be 10 megohm for delays of 10 - 20 seconds.

RHOLDx controls long term hold current.

D1, D2 are anti switchoff spike protection diodes. Say 1N4007 or similar
(1N400x). Almost unneeded due to low values of Rhold.

DD1, DD2 allow the circuit to reset rapidly when turned off - without them
the circuit requires about a delay time after turnoff before it will again
give full turn on delay. Type = 1N4148 or similar or 1N400x.

Note 74C14 can run to Vin of somewhat over 12 volts. Some other logic
families cant!

FET dissipation is minimal as its on only briefly so heatsinking could be
avoided as long as operating repeat rate wasn't stupidly high.

R2 say 1k.



            Russell McMahon


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