My variant (horrible ASCII art below) has been used in many different
projects over the past 20 years or so.  This works great for larger devices
- solenoids and the like.  My other trick with larger solenoids is to diode
isolate from the DC supply, then use a AC driven voltage doubler (using a
smaller than normal pump capacitor) to charge the capacitor to double the
main supply.  This eliminates the need for the resistor (and its heat
generation).  The pump capacitor has to be able to withstand the ripple
current but that has not proven to be a problem.

Another trick that works well for smaller relays (P&B T90 series, Aromat
JS1E series) is to charge a smaller than normal capacitor from the AC
supply, then rely upon the ripple for the reduced supply voltage.  I've used
that for years also.  Its a handy trick when you have a 24Vac supply (giving
you about 33Vdc unregulated) and 24V relays - and don't want to regulate the
relay supply or generate heat from a series resistor.  Again, the capacitors
have to be rated for the ripple current - I've used Sanyo and Nichicon with
no problems.


+Volts (e.g. 12V unreg)

-----------res-----+------+------------------.
                   |      |                  |
                  cap    ||D                 |
                   |     ||D Relay           =
                   |     ||D                 ^ Diode
                  Gnd     |                  |
                          +------------------
                          |
                          |
                          |
                    [switched ground]

dwayne


Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax