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Electronic help/advice

Domis

By Domis
in DIY Astronomer

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Domis

Domis

Posted
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Guys,I've built a dew heater using 40, 330 ohm 1/2 watt resistors in a parallel ladder configuation with 18 gague wire and 18 gauge leads. The ladder is 30" long and the leads are 12' long. When wired to a deep cycle 12v battery the resistors get to a max temp of 60 degrees celsius. I think it should be drawing aprox. 1.4 amps but when I put the amp meter on it I'm reading 2.5 amps. Anyone have some thoughts on why it would be pulling an extra amp or am I mistaken in thinking it should be pulling only 1.4 amps?

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yesyes

yesyes

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Posted

40 of 330Ohm resistors in parallel is a total resistance of 8.25Ohm. At 12V it should draw 1.45Amp. But please do measure it with a multi meter. The standard resistors have a 10% tolerance. And these batteries usually have more like 13-14V. Both might account for the extra Amp.

Also, you will need some sort of heater controller. You can't just leave it on. 60 degrees is way too hot.

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Gina

Gina

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I think higher value resistors would be better. Lower current so less battery drain and a better on/off cycle. You only need a small temperature rise above ambient to stop dew forming. I think a 10C rise would be quite adequate - probably more than adequate. Using short pulses of high current is likely to cause interference too.

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Domis

Domis

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Thanks for responding everyone.

Martin and YesYes; I have put a meter on the ladder assembly only and am reading 9.5 Ohms. A measured voltage of 11.94 volts at the battery posts. The resistors are reading 327 Ohms each (5% Tolerance). I do intend to build a controller for it but would like to get it straightened out before I do that. 60 degree C is what the ladder get's to while "bare" but in the sheathing I'm using, the outer shell get's to 35 degree C and holds. This is still too hot, I know, but could be controlled.

Gina: I had'nt thought about interference,thanks for making mention of that. You may well be right in being better to use higher values for the resistors. I still want to get to the root of this problem before I build another though,so I don't repeat the problem.

It just does'nt make sense that the resistance using these materials would be so much higher than it should be. I've soldered many,many things in the past so I don't think it would be bad solder joints.

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Domis

Domis

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Barkis; it was a spur of the moment decision to build this. I had an unexpected day off and ran out to get something going. I could not source nichome wire so went for resistors and still had to hit 3 stores to find enough of them. There are no decent electronics supply places within 100 miles of me. I'd be forever in ectasy if they ever opened a place like Maplins near me. I intended to go a bit high on the wattage as the dew here on the east coast of the US is extremely bad. I'm right where the warmer onshore air tends to meet the cooler offshore air.

Pyscobilly; yes it should be less,as YesYes stated,it should be about 8.25 Ohms and I'm at 9.5 Ohms pulling 2.5 amps which does not make sense.

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Domis

Domis

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AAHA, i believe I've found the problem. All components were store bought except for the wire used in the ladder build,which my brother had said he had laying around and gave me two precut 30" pieces. These pieces I just went ahead and soldered the resistors to. After removing the ladder assembly from the sheathing, I noticed that it seemed rather stiff, when I straightened it back out. I clipped a piece off and checked the cut end under a magnifying glass and it appears not to be solid copper wire but rather some kind of copper clad wire, though the cladding seems rather heavy for normal clad wire. Well it looks like a rebuild is in order. Pity to have to waste the resistors but I don't think they can be salvaged at this point. Thanks for the input everyone, it prompted me to keep digging into the problem until I found this.

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Gina

Gina

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No electronics shops near here either. Nearest Maplin is over 20 miles away. I buy pretty much everything on the 'net, Maplin included, though their prices are rather high. Have been pointed to a couple of cheaper sources of components on here :) I also use eBay quite a lot and for bigger items, Amazon. Plenty of Nichrome wire on eBay.

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Domis

Domis

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Gina; I agree the net is the way to go and around here it's usually the only way to go. It's the 3 or 4 days of waiting for delivery which kills me. Patience has never been a strong attribute of mine I'm afraid. Strange how I can sit outside all night waiting for just a few moments of clear skies but have a terrible time waiting for something to arrive by mail. It must be a character defect.:glasses2:

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Chris Rowland

Chris Rowland

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I like Rapid Electronics, especially since I orderd some bits and pieces at about 4pm on the Friday between Christmas and the New Year. Next morning, about 9am a jiffy bag full of parts arrived...

Chris

PS Rapid - and I are in the UK so deliveries to New Jersey may take a little longer :-)

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Domis

Domis

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Chris; that's awesome to get an order filled, shipped and delivered that fast. It seems that the pony express is still alive and well here. Some deliveries here are 3-4 buisness days and alot are 7-10 buisness days. Unless of course you wish to pay through the nose for next day or expedited shipping.

Back in April I ordered a cam from a UK buisness and it was close to 3 weeks before it arrived here. Lot's of red tape I suppose.

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obscura

obscura

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I guess that some of the Rs weren't making contact with your buss bars. I used 330 ohm Rs and worked out fine with predictable results. Of course, its going to get hot in the open air, but wrapped around the scope the heat will be conducted away to where its needed. You can get 100 same value Rs from www.esr.co.uk for less than a £1 + p&p

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