Re: [ATM] Kendrick Controller?

[Mike Lockwood <melockwo@uiuc.edu>]

Doug,

Doug Williams wrote:
> I was thinking of getting a Telrad Heater from Moonlight Telescopes.  My fan
> keeps my mirrors clear, so I don't need to worry about that.  Anyway, since
> the Moonlight Telrad heater only draws .16 amps, I wonder if it is worth
> bothering with a controller at all.  I also wonder why a simple
> Potentiometer or "dimmer switch" wouldn't work to control the heat output,
> rather than the $85 Kendrick controller.  I understand that the Kendrick
> controller is more advanced than this, but do I need this?  Why does the
> Kendrick cycle the power on and off rather than just reduce the voltage?

I have one of the Telrad heaters.  I have it wired in series with my 
eyepiece heater to use less power.  I don't need either on full power 
to keep them clear.  It will REMOVE dew with less power.  The heater 
can be a little tricky to keep in place (the wire may pull it out of 
position), but a little velcro may solve that.

A Bob N. pointed out, the Kendrick controller will use power more 
efficiently.  It sends out pulses of the battery voltage, which is 
more efficient than bringing the voltage down (like a resistive dimmer 
switch).

If you want to make your own controller for 12V stuff, this PWM (pulse 
width modulation) circuit would probably work well, for about $20:

   http://www.electronickits.com/kit/complete/motor/ck1400.htm

You can control the pulse width (and the power used) by turning the 
potentiometer.  This rapidly turns on and off a MOSFET (type of power 
transistor) and there isn't too much power loss.  The heat sink makes 
it more bulletproof (in the event the MOSFET is handling a lot of 
current), but I doubt it would even be warm to the touch when there's 
less than an amp going through it (powering several fans, or several 
heaters).

This circuit also makes a great speed control for 12 VDC fans (for 
cooling optics).  For set fan speeds, you would wire in a 
multi-position switch and discrete resitors in place of the 
potentiometer - just make sure you know what you're doing.

	Mike Lockwood

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