Very low power heater: how many Watts?

[orion.osiris]

Hi guys,

I've got this 3hp inverter unit bolted to the wall of my garage. It's about the size of a rugby ball and contains a fair amount of electronics that do esn't get on well with condensation - and I do get a lot of condensation in there, most obvious to spot on cold, metal surfaces. Anyway, this device n eeds to be protected from it, so I've thought about mounting a couple of ch unky resistors bolted to a heat sink under it to provide a continuous gentl e plume of warm rising air which I shall leave on 24/7. The component values I've come up with will provide 25W of heat from the ma ins supply to keep the condensation at bay. This 25W is just a figure I've pulled out of my a$^e, though. A pal of mine reckons it's on the high side, so I thought I'd run it past you and get your thoughts. I don't really want to box the inverter in if I can possibly help it since I need access to the control panel on the front, so a bit of heat from belo w seems the best solution. What say y'all?

thanks

[John Williamson]

Why re-invent the wheel?

Or use a 60 watt bulb fed through a diode in a damp-proof housing.

[The Natural Philosopher]

I'd say a 5W light bulb is more than adequate.

That should give a 10C rise and render the condensation unlikely,.. I have to say that condensation inside garages is seldom an issue except on windows and occasionally steel rooves. By night its generally colder outside and the air coming in has already dropped its load somewhere else.

But you don't say whether this is in fact inside or not.

[Harry Bloomfield]

all you are looking to do, is raise the temperature of the unit, a couple or so degrees above everything else. In a previous life, I used to install control cabinets almost large enough to walk into and they were installed on water authority sites. You cannot imagine a more humid environement. All they had in the base, was a 10w gold resistor, sometimes contoleed by a room thermostat.

[Bob Minchin]

Use two bulbs in series. These will last much much longer. the total power will half of 1 bulb rating eg 2 x 15watt fridge bulbs (cheap in ALDI from time to time) will give 7.5 watts which will be more than enough. Mount under the inverter in an improvised chimney/tube the same size as the cooling air intake of the inverter fan.

I have a similar system in a tool cabinet and everything stays rust free.

[Brian Gaff]

Couple of light bulbs in series?

Brian

[The Other Mike]

Take it partly to bits and spray it with this

[orion.osiris]

Many useful and interesting replies here for which many thanks, guys.

I'm obviously going to cut back the power I was planning to use, but I think I'll stick with resistors which dissipate all the energy as heat, rather than bulbs, which waste a fair bit as light (which I don't want in this case).

I'll work out some values and mount two power resistors (bolt-on types) to a nice piece of heat-sinking I've got set aside for just such a purpose, then ensure the resulting heat is guided to the right area via some improvised flueing.

Perhaps I might even also spray the insides of the inverter with the 'varnish' type stuff one of the last respondents suggested, just for good measure! Belt 'n' braces indeed!!

[Tim Watts]

On Thursday 25 July 2013 19:35 [e-mail] wrote in uk.d-i-y:

You can get self limiting resistive heaters too - which are instrinsically safe (cannot catch fire if buried in crap). I was thinking such a thing would be a good idea in a tool cabinet that may get full of sawdust and spiderwebs. Farnell do them.

This may not be an issue for your application.

[orion.osiris]

Mind you, paint the bulbs black or very dark red and then (if my physics is right) ALL the energy will be dissipated as heat..?

[Vir Campestris]

Resistance varies a _lot_ with temp so it won't be half. And incidentally as they'll barely be glowing you'll get damn all light.

and that I can well believe. I suspect this is a little OTT for the OP, I'd box it with a removable box that also had a wallwart in it. That's all the heat you'd need.

Andy

[Dave Liquorice]

I'd go for the temporary box as well. Boxing in will reduce the energy input required greatly and help ensure that the whole unit is warmed. With the unit open Sods Law would have the bit most sensitive to damp staying cold...

Wall wart? Well an iron one not a modern SMPS one, though a resistor on it's output to produce a couple of watts of heat would safer than having mains across a resistor.

[Bill Wright]

stick with resistors which dissipate all the energy as heat, rather than bulbs, which

waste a fair bit as light (which I don't want in this case).

ALL the energy will be dissipated as heat..?

80% of it is dissipated as heat anyway. That's why tungsten bulbs are inefficient.

Bill

[John Williamson]

Or just put them in a box with a black interior and a small crack. All the light gets absorbed and turned into heat, and the crack lets you check it's all working without opening the box.

[PeterC]

Use 2 bulbs in series - not too bright, gentle warmth and last forever.

[PeterC]

Sorry - posted before reading!