Making a warming pad for an eyepiece case

[Tannehill]

Electronics/Electrical Gurus:

I've a Peli case, and I've seen some heaters that keep the EPs warm (not hot) and this is fabulous in preventing dew. also, if one has a cardboard "tent" (Dew Box) over the case, the heat also keeps the other items under the cardboard hutch dew-free (atlases, etc). Kendrick sells a small one.

I was poised to buy a pre-made heater for the bottom of my eyepiece case....from a pet store (reptile warmer!), or a small profile warming pad. I want it to plug into a 240 v powerpoint, since i'd use this only when I have power. I have a 240 AC to 12 V 12 converter....a spare from a portable DVD player that got pinched, so it can be a 12 volt powered setup, or AC....Doesn't need to be that adjustable...but can't be too hot, obviously.

But, is this something that I can make SAFELY myself? I noted the reptile warmer used nichrome wire, which I've heard mentioned here in the context of dew heaters. I'd fuse the system for safety, and my powerboard is fused, too, for added protection (on those dew-damp nights at Snake Valley).

Any advice would be welcome.

Scott

[erick (Eric)]

Scott, key issue is to match the specifications of your heating pad to the specifications of your power supply. Do you have details on the proposed heating pad. It seems that it connects directly to 240V power? If so what is the wattage rating? Or is there some form of power supply box between 240 volts and the warmer? Also what are the specifications for the heat from the warmer - does it quote a temperature that it would reach? Under what conditions? Eg. in a house (warm) or in an outdoors situation?

EDIT:- Did a bit of looking at US sites. The photos show a power "brick" with the "reptile warmer" suggesting that they run on low voltage? The statement is made that they heat to 20 deg (F?) above ambient. Shouldn't need too much power to achieve that?

[Tannehill]

Hi Eric,

The one I saw at a pet store here in Mel had the attached converter, so i assume it racheted the 240AC to some 12-18v DC rating. As for the non-specific purpose small area heating pads I've seen elsewhere on the internet...couldn't tell.

The temperature doesn't need to be that precisely controlled, as long as it doesn't overheat the case and start smoking my foam (and EPs). I think something at or just above body heat would probably accomplish the goal, since it'll be contained under a bunch of foam, atop of which sit my EPs And too hot will just create thermals over my table and muck up any viewing that direction, I'd expect. Perhaps earn me some enmity from neighbor observers, too!

I was drawn to the reptile ones because of that very same automatic feature of heating just above ambient....a built-in dew controller so to speak! But they were NOT cheap, those that I saw in the store....$80 or more, and they didn't look all that robust, either.

Scott

[erick (Eric)]

I'm just guessing now, but I think if you got a pad that was a neat fit into your case (ie. not folded over!!) then "robust" wouldn't be a problem. The photos I saw showed it in the open with a lizard (with claws) sitting on it. However, "in the open" suggests that if you, instead, have it embedded under the foam in the case, then overheating using the standard power supply may be a real risk. I suspect then that you need to obtain a supply that doesn't overheat, or add a controller so that you can lower the electricity supply to the pad.

Given that they do use a stepped down voltage rather that mains 240V (eg electric blankets), then it wouldn't be too hard to come up with a suitable supply. For a heating application, I would expect the supplied power "brick" just transforms down to a lower voltage AC. If they are just nichrome wire, they can be heated with AC or DC - just need to get the voltage right. A bit of work measuring resistance, then using adjustable power supplies and a thermometer in the case to monitor the result of different voltage inputs until you get the temperature differential you want - then we have the voltage and current settings you need - then we source a suitable supply for that. A bit tricker if you decide that you really need some control for more heating some nights than others. However, I have the gear we need to do all this, if you want.

First step, visit the pet store again and read the box and write down the specifications that you can find - voltages, currents wattages - and let us know.

Now, where are the electronics and heating experts to correct my comments!

[Tannehill]

Eric,

Cool, thank you. I'll check out the specs and let you know.

scott