[time-nuts] Temp control of LPRO

WB6BNQ wb6bnq at cox.net
Wed Sep 10 14:30:33 EDT 2008

Hi Neville,

You said ". . . I mounted a12V  1 watt 40mm brushless DC ball bearing fan, . . "

Damn, is that thing turbo charged ?


Neville Michie wrote:

> Hi,
> I have just commissioned a temperature control for my LPRO rubidium
> oscillator.
> When I read the specs I noticed that at 18V supply and about 40*C the
> unit only requires about
> 8 watts. That is getting close to what my stand-alone power source
> can maintain.
> Running at a higher temperature takes a little load off the ovens on
> the lamp and the detector,
> but it does have a penalty in MTBF. At 40*C I thought the trade-off
> was worth it.
> Now the local crystal oscillator is not temperature controlled and
> its control voltage drifts widely.
> So I bolted the LPRO to a plate of aluminium about 1/4 inch thick. On
> the other side of the aluminium plate
> I bolted heat sink fins about 3/4 long over the whole length.
> The whole unit mounts inside a piece of about 3X4 aluminium
> extrusion. At one end in a little plenum
> chamber I mounted a12V  1 watt 40mm brushless DC ball bearing fan, in
> the AL plate I drilled a
> deep well in which a bead thermistor lives.
> The whole thing is insulated in 1/2 inch plastic foam sheet.
> When the fan running on 12V the temperature is held to 4 *C above
> ambient. At 8 volts this rises to about 7*C.
> The fan is quite unobtrusive on 8 V.
> The thermistor is in a bridge circuit and switches the fan in a PWM
> control mode with a 50 second cycle time.
> The surface of the plate has less than 0.1*C variation over a day
> with no sign of the temperature cycling.
> The oscillator control voltage now rises on startup and then becomes
> rock steady.
> The next step is to get on with the building of the gear to measure
> how well the LPRO now performs
> at constant temperature.
> Cheers,
> Neville Michie
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