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newby needing help

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smokebelch

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Firstly, hello all, this is my first post, so go gentle !!!
I will point out at this stage, what i know about electronics is what i've self learnt this week !!, so please dont flame me if my terminology is wrong, and very basic
Right, i've made some heaters for my telescope, using 330k 1/2w resistors in parallel running from 12v, I've connected a SPST switch into the mix, and it all works fine, BUT it gets too hot too quick. I was told that I would have to put a potentiometer in to control the voltage going to the resistors, and therefore controlling the heat. I inserted a 1k potentiometer, (all wired correctly), and now my heater doesn't work at all !!!.....i've checked it all out with a meter, and everything is fine, am i correct in assuming that the pot creates too much resistance for enough current to heat the resistors?

Can anyone tell me what i need to put into the circuit to be able to control the voltage, and the heat generated?

Apologies for the length of the post.

many thanks in advance
 
smokebelch said:
Right, i've made some heaters for my telescope, using 330k 1/2w resistors in parallel running from 12v, I've connected a SPST switch into the mix, and it all works fine, BUT it gets too hot too quick.

330K at 12V generates only a tiny 0.00044W each. Are you sure that you have got the resistor value correct? Not by any chance 330 Ohms?

If the value is really 330K, then a 1K variable resistor will definitely work if you have connect it correctly. But it won't work if the resistors are 330 Ohms.

If you wants to control the heat from a group of resistors from a source of 12V, the easiest method is to use a LM317 voltage regulator to provide variable voltage to the resistors. You can change the output voltage from 1.2V right up to about 9V, giving a really big range of adjustment.

We need to know how many resistors are there. Can you post a photo?
 
hi, thanks for your reply, the rsistors ive used are stock # M300R from maplin, if you'd care to take a look to see what info you can gleen from there, ive used 20 resistors on the heater band....HTH
 
smokebelch said:
hi, thanks for your reply, the rsistors ive used are stock # M300R from maplin, if you'd care to take a look to see what info you can gleen from there, ive used 20 resistors on the heater band....HTH

Ok. I have checked the value is actually 300R. You have twenty in parallel and that makes the combined resistance 15 Ohms and it is taking 0.8A from a 12V supply.

This fits the current ouput requirement of LM317. As I said you can varies the LM317 output from 9V down to 1.2V, you might found at 9V the present resistors can't give out sufficient heat. The solution is to change to resistors of smaller value. Twenty 168 Ohms resistors in parallel will give out the same amount of heat at 9V. If you can't get 168 Ohms, the closest value smaller than 168 will work.

Is the proposal sounds good?

But if the resistors are now properly installed and you do want to change them, you would need to use a variable voltage dropper to drop off excess voltage and that would gives you a range of 12V to 0V.
 
I was answering Smokebelch's same post on another forum.

I don't think a bunch of heating resistors need a variable voltage regulator. A power darlington transistor can be used as a follower and fed from a 1k pot. A small heatsink will be needed for the transistor.
 
thanks for your replies gentlemen, but as i said, im a complete newb.......could any guys point me in the direction of suppliers(uk only) and part numbers please
Will this be easy for me to integrate within the circuit i already have?
many thanks again for bearing with my lack of knowledge
 
Nigel Goodwin said:
audioguru said:
A small heatsink will be needed for the transistor.

Use the transistor to heat the telescope as well! - no point in wasting the heat!.
This has a big advantage. The resistor dissipation is a parabolic function of the voltage across the resistor. The dissipation of the resistor and the transistor is a linear function of the voltage across the resistor.
The down side is that it is probably difficult to distribute the heat evenly, because the transistor power is also parabolic, but it is approximately zero at both ends of the pot, and maximum in the center.
 
could 1 of you gentlemen be as kind as to draw me a very very basic circuit diagram for the above please?.......i know its an ask, but i really do need to sort this asap, as i can't take my 'scope out for very long before its frosting up!!
As i said, the more basic the better.

Many thanks in advance

Dave
 
Here it is:
 

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many many many thanks...........just got time to go get the bits, put them together before tonights observing.......many thanks again
 
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