Simple Telrad dew heater

[M110]

Hey guys, Here's a simple dew heater i knocked up for my telrad - first test was last night seems to work really well, cleared it from completely dewed up in around 10mins .

It's just a 10ohm 2watt resistor connected to two rechargeable AA's then taped onto the telrad! Using two 2700mAh high capacity batteries I get approx 5hrs of heat till the voltage drops away too much to be of any use...

Total cost was around £10 (the expensive bit was the batteries which cost just under £9)

Next stage is to run the wires inside and run both the heater and the telrad itself off the same set of batteries.

Anyway some pics:

[tom.yates]

Great idea

[beamish]

A neat little mod -bet it'll be even better once you've connected up to the internal batteries

[M110]

Right ho it's all internal now. The telrad and dew heater both run off the same set of AA's. The telrad is still on it's variable resistance switch and the heater had it's own on/off switch and LED to show it's on. Not bad for under a tenner

Finished pic and circuit diagram:

[Doc]

Looks really neat. Wish I could do something like this. My electronic skills are Zilch.

[Psychobilly]

Yep agreed .. nicely done indeed...

Peter...

[Jarndyce]

Very neat.

Does the resistor just get mildly warm?

[M110]

Very neat.

Does the resistor just get mildly warm?

More 'reasonably hot' I'd say - you can touch it without burning yourself but it is pretty warm. Works out it gives around 0.58 Watts of power from fresh batteries.

regards,

Alex

[beamish]

That IS now very neat- just wish I knew what all the symbols in the circuit diagram meant and I'd do it I think.

Karlo

[Bigwings]

Very good,

My Telrad will soon be getting one but I think I will use the space inside the telrad for a second set of batteries. There seems plenty of room and if push comes to shove I can revert to the Mk1 demister...mucky finger and a rub round. That way if the heater batteries fail I can still use the Telrad

[Mark at Beaufort]

Well done Alex you clearly know your electronics. I can see that you will be giving that talk at the Hereford Astro meeting in July.

See you soon

Mark

[M110]

Very good,

My Telrad will soon be getting one but I think I will use the space inside the telrad for a second set of batteries. There seems plenty of room and if push comes to shove I can revert to the Mk1 demister...mucky finger and a rub round. That way if the heater batteries fail I can still use the Telrad

Yep there does seem room - although with 4 batteries higher resistance resistors would have to be used (otherwise you'd just get twice the heat output and the batteries would still last the same amount of time!) Or could try running it off 2 of the larger 'C' cells...

Alex

[drummerp]

Nice one alex, make me one up for the rigel quickfinder I gonna order, if i try it'll only end up like the auto focuser i tried to do last week, unfinished and rubbish.

[M110]

That IS now very neat- just wish I knew what all the symbols in the circuit diagram meant and I'd do it I think.

Karlo

All you really need is a simple switch and 10ohm resistor - like this (the 10R one about 2 thirds of the way down) some wire and a soldering iron... It's not as hard as it looks

Alex

[Rusty Strings]

Great idea Alex, have to install one on mine, no issue with batteries as I fitted an external power socket after forgetting to turn it off too may times.

John

[Moonshane]

Superb idea. I am going to make one with 330 ohm resistors and run off a dew controller I think. might be good to put heat shrink tubing round the whole thing too.

[sbooder]

Could this idea be carried over to work with a secondary mirror?

[Bizibilder]

Yes - if you google "telescope dew heater" there are a couple of sites that show how to make a "resistor" dew heater for your secondary (basically solder a couple of longish wires to the resistor and, after gluing the resistor to the secondary, running them out along the spider supports to a switch and battery that is atached to the scope tube, use a variable resistor in series to adjust the heat - job done!!)

[sbooder]

Thanks.

[Moonshane]

there seems to be endless uses for this sort of heater. I bet you could even sew a few into your gloves and make heated gloves!

[pleiades]

Those are also a good solution

and this one . I use the first and I am satisfied .

[sbooder]

Looking at the first link, you could probably use one of those plastic square backed desk filers cut to size.

[m4sh]

Right ho it's all internal now. The telrad and dew heater both run off the same set of AA's. The telrad is still on it's variable resistance switch and the heater had it's own on/off switch and LED to show it's on. Not bad for under a tenner

Finished pic and circuit diagram:

Thanks for this.

I copied your solution.

It works splendidly.

[Joe12345]

Sorry for reawakening an old thread!

I was wondering, if I wanted to make one of these for, say, a 12v 8ah lithium power pack, what resistor should I use? Sorry if it's a silly question, but I'm not a big fan of that electrical 'You've left the motor on too long' smell, and I would rather my evening was cut short by something other than a fire and £250 worth of damage!

[Sporadic Dobstronomer]

10 minutes ago, Joe12345 said:

Sorry for reawakening an old thread!

I was wondering, if I wanted to make one of these for, say, a 12v 8ah lithium power pack, what resistor should I use? Sorry if it's a silly question, but I'm not a big fan of that electrical 'You've left the motor on too long' smell, and I would rather my evening was cut short by something other than a fire and £250 worth of damage!

You want about 1W of heat so a resistor of 150 Ohms is about right.  If you use a 100 Ohm resistor you will get around 1.5W.  The formula is Power= Volts^2 / Ohms

The power rating needs to be a bit higher than the power you are using so it does not overheat -say a 2W resistor.  A larger power rating will do no harm; it will just be a bit bigger.

The power rating of the resistor has no effect on the heat generated but does affect whether you get that smell...