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Telrad Heater - Here goes!!

Bart

By Bart
in DIY Astronomer

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Bart

Bart

Posted
Posted

Hi all

After buying the kit some time ago, finally got around to knocking this up.

Got the 9V battery connection, a mini switch ( a casualty unfortunately!!), electrical wire, and a resistor.

After buying a soldering kit, first mistake was melting the mini switch!!!

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Bart

Bart

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Mike

I'd say you know more than me. Hopefully you won't be reading a story at some point in the future about an amateur astronomer and his scope being vaporised and joining the great cosmic smorgasbord....

Barry

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Lonestar70

Lonestar70

Posted
Posted

Hi Bart,

I applaud you for having a go a making your own Dew Heater, however, I would like to add a note of caution before you use it.

Looking at your pictures, it would appear that you are using a 10 ohm resistor of around the 1 Watt or 2 Watt size.

My apologies if this is not the case, however, if it is this value then at 9v this will dissipate 8.1 watts and will consume 0.9amps from the battery... Hence the resistor will overheat and either melt your Telrad, or worse it could catch fire. :eek:

You only need around 1 watt of heat to keep the Telrad screen clear so a resistor of 68 ohms will do that using only 0.132amps from a 9 volt battery.... the resistor should be a 2 Watt device.

Be sure to leave about a 1/8" (3mm) gap between the resistor and the plastic body of the Telrad.

This will run for much longer on your battery and will not create a fire risk or damage your Telrad.

I hope this helps prevent an unfortunate accident.

Best Regards.

Sandy. :grin:

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Bart

Bart

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Sandy

Thanks a million for your advice, I'll take all the advice I can get in this area, as I am clueless in this regard. Electricity is something I just don't get, apart from wiring a plug

I've used it twice and while it has been very effective, it eats the battery. No sign of melting, but that might be because it's been so cold!!

I'll get the resistor you recommend. Again, many thanks indeed, very much appreciated

Best regards

Barry

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NEM

NEM

Posted
Posted

Actually Barry, I think your circuit is OK.  The resistor looks like a 5 band type so its value is 100 Ohms (if the colours are brown, black, black, black, brown).  That's slightly higher than Sandy's suggested 68 Ohms, but if it's working for you there's no need to change.  I used this site to look it up, http://www.digikey.com/us/en/mkt/calculators/5-band-resistors.html

I guess Sandy (like me) is more used to the older 4 band types.

Noel

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Lonestar70

Lonestar70

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Hi Barry,

Like Noel stated... if the colours are Brn Blk Blk Blk Brn then it is indeed a 100 ohm resistor of 1% tolerance and will not require changing since this will give you 0.81watts at 90mA current using a 9 volt battery.

My concern was that I could not be sure of the 4th band colour on my Rubbish monitor (my high res one is on the blink at the moment) and thought it might have been Gold... which would have made it 10 ohms.

I thought it best that you checked before using it.

My apologies if I alarmed you.

Noel, thanks for stepping in and correcting my observation and putting Barry's mind at ease, you obviously can resolve the colours more acurately than I can at the moment... I really must get my high res monitor sorted. :lol:

Keep Happy.

Best regards.

Sandy. :grin:

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Bart

Bart

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I quite like your original design of the Telrad/Nosewarmer to be honest although I can understand the health and safety problems with it if you started making them commercially Barry. :)

You're not joking Mike! I'd expect I'd be taking my clients out one at a time in a rather grizzly fashion. Watch this space...

Barry

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Bart

Bart

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Gents

Any ideas why, after installing a toggle switch, the resistor appears to continue to produce heat whatever way the switch is turned?

Should the wires be attached to the other terminals of the switch, I.e the other way around? Help!!

post-11876-138875647363_thumb.jpg

Thanks

Barry

Barry

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DanLXIX

DanLXIX

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I know this is bringing back a slightly older thread, but would there be room in the telrad for a PP3 battery? I wondered if this could be used to combine the power source for the dew heater and the LED inside the finder - in fact could the resistor that would be needed to knock the voltage down to 3v be used as the dew heater?

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Moonshane

Moonshane

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could someone please put a formula on this thread to work out the appropriate resistor for e.g. 1x AA and 2x AA batteries?

It would be nice to use the current AA batteries and create a circuit for the telrad itself and the dew heater.

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Bart

Bart

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Good questions, for someone electrically minded. That is to say, not me. Definitely worth considering. Use it every time I'm out now. Mind you, the way thats going, the same 9V battery will last me 3 years........

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