"Crosby, Jay" wrote:

> Now that the list has slowed down discussion on relay drivers with low
> voltage supplies, I need some ideas on the opposite problem, too much
> voltage on my supply.
>
> I need to drive a solenoid on and off from a logic level output.  The
> solenoid has 28ohms of coil resistance, and a 500mA operating spec.  The
> only available power I have to drive it must come from the 80VDC supply.  If
> I had a lower voltage on the power supply, say 60V or less I could use an
> off the shelf regulator design to give me a reasonable 14V output in which
> to drive the solenoid.  Of course the simple solution is to use a power
> resistor to 80V, in series with the solenoid, but this wastes way to much
> power.

ANY linear solution will waste just as much power!!!
(Including the one listed below!)
Skip the regulator, you can get cheap 100 volt FETs and a 50 ohm 15 watt
resistor.
(Believing 28 ohms and .5 amp is 56 volts to the coil!)

>   I am thinking about using a hexfet as the switching device and of course
> apply diode protection across the coil, and the needed current limiting
> resistors to obtain 500mA.
>
> The LM317HV looks like it could provide some solutions, but I don't think I
> can exceed 60V from input to output.  Is there a way to cascade this device?
> Here is a rough idea of what I had in mind.
>
>                           80VDC Supply
>                                |
>                                |
>                        Regulator Solution
>                                |
>                                |
>                              +14VDC
>                                |
>                                |
>                   Solenoid with diode protection
>                                |
>                                |
>                   50ohm   ||---.
>       Logic Out---\/\/\---||<--,  N Channel HEXFET
>                           ||---|
>                                |
>                                |
>                               Gnd
>
> Priority of tradeoffs:
> 1) Reliability (room temp. operation only)

WATTS are WATTS, you won't save any heat.

> 2) Needless Power loss

Use a switching converter! (Expensive!)

> 3) Number of components (complexity)

1 resistor, 1 FET

> 4) Price (don't want to spend more than $30)

$5 for the resistor, $1 for the FET

>
> 5) Size (not much of a factor)
>
> Thank you in advance for your help,
> Jay

--
Thomas C. Sefranek  WA1RHP
ARRL Instructor, Technical Specialist, VE Contact.
http://www.harvardrepeater.org
http://hamradio.cmcorp.com/inventory/Inventory.html