How to affix a temp probs

**porkop** · 25-10-17, 03:43 PM

Hi peeps,

I have bought a temp switch to turn a fan on and off when the temp on my motorola CDM1550LS+ heatsink goes above 40C.

I am not quite sure how to affix it to the heatsink. I can fit the probe loosely between the fins of the heatsink but will need to have
it thermally coupled to some efficiency to the heatsink.

Anyone suggest a compound.method of affixing to not only be sound, mechanically, but to work as best for temp transfer.

Here are a few pics, one of the temp switch and the other of the heatsink with a red circle where I plan to fin the probe.

**Gitch** · 25-10-17, 05:04 PM

What type of temperature probe is it? Might be able to replace it with something more suitable..?

Otherwise, would the fine 'bend' enough to lock it in place and use some heat transfer paste? Dodgy as hell, but might work... Other wise, I'm pretty sure I've seen clips designed for for something very similar - but i can't remember where I saw them

**Uncle Fester** · 25-10-17, 05:34 PM

I think you are confusing something. You are not trying to dissipate heat with the sensor. It will reach equilibrium with the heat sink even if you just use superglue because the heat has no where to go on the sensor when it is stuck inside the fins.
If you crack open one of those sensor caps there will be a much smaller sensor inside covered in resin that provides far greater thermal resistance than a few micrometers of superglue.

Edit: while we are at it does this temperature switch have some kind of hysteresis?
If not, you would actually want the thermal resistance to delay the temperature measurement so the fan does not switch on and off the whole time.

**ammlione** · 25-10-17, 06:18 PM

Hi Porkop,
You may want to consider that the position you've chosen is on the cool side of the heatsink, you may be able to drill and tap a small M3 hole inside next to the chip that is attached to the heatsink and support it with a small plate or big washer there.
That way you are sensing the hot part that you want to protect from heat.

Also, you can buy a thermally conductive epoxy from just about any computer shop, they use it to affix heatsinks to cpu's, there are 2 type, you want the permanent one (IIRC it is the 2 part one), use it to glue the sensor just as if it was araldite.

good luck

**Reschs** · 25-10-17, 08:08 PM

At a setting of 40 Degrees C, the location is not important. The expected running temperature for the heatsink would be around 50 to 60 Degrees C.

You really need to re terminate that Aerial lead.

**porkop** · 25-10-17, 09:40 PM

Originally Posted by Reschs

At a setting of 40 Degrees C, the location is not important. The expected running temperature for the heatsink would be around 50 to 60 Degrees C.

You really need to re terminate that Aerial lead.

Not actual picture of my radio. Just of the net for illustration. My antenna termination is 100%

This is very true.

My application is not mission critical

The radio will TX for long periods, currently set to 20W output (can’t get it any lower, design limit).

I chose 40C because it will idle around this temp in most circumstances and I just want to assist as best I can with maintaining that temp.

On a side note, I think it is best to draw heat away (air flow direction of fan) from the heat sink allowing the cooler air to be pulled in and around the fins by the fan which will then direct flow out the back away from the heat sink.

Thoughts???

FYI.

The application is a digital voice repeater based around the MMDVM system.

It does DMR, P25 & C4FM. Can do dstar but I have not enabled that mode.

**Uncle Fester** · 26-10-17, 05:22 PM

Originally Posted by porkop

On a side note, I think it is best to draw heat away (air flow direction of fan) from the heat sink allowing the cooler air to be pulled in and around the fins by the fan which will then direct flow out the back away from the heat sink.

Thoughts???

The natural flow of heat is upwards. By placing a fan on the back of the heatsink so it sucks air through the fins is acting against the natural flow.
Therfore I would have the fan blowing cold air towards the heat sink so the natural flow of heat is assited.
Placing a fan on top of the heat sink is a different story where you have it air suck through the fins from the top but you would need a long thin fan like a tangential fan however these are designed as blowers, so not suited in that placement.

**Reschs** · 26-10-17, 07:57 PM

Have you checked the duty cycle for Transmitting. Most of these are not designed for continuous operation.

As for fan placement, cool air being forced across all circuitry then through the heatsink is the ideal.
Placing the fan under the heatsink so that the cool air is forced through the fins will work.

**porkop** · 26-10-17, 08:33 PM

The mechanics of it will only allow me to place the fan at the back of the heatsink.

I have set it all up now and the switch is doing its job.

The fan is switching on and off as expected with use of the transmitter.

I choose to draw air from the radio and out the back.

It appears to be doing the job.

As to duty cycle, as it is a networked repeater it is at the whim of the users worldwide.

I just have to wear the higher than normal duty cycle.