DC powered thermostat

[defed]

i have a small compartment that i want to keep warm, using a 24v battery bank.  i figured it would be simple to put a mechanical thermostat in there to switch the heat on and off...but it appears that home thermostats use 24vac.  i have found some that work on DC, but they are quite expensive.  there has to be a simple (and cheap) way to open/close the 24vdc power source to the heat using temperature...having a hard time finding it tho.

any help would be greatly appreciated.

[rossw]

i have a small compartment that i want to keep warm, using a 24v battery bank.  i figured it would be simple to put a mechanical thermostat in there to switch the heat on and off...but it appears that home thermostats use 24vac.  i have found some that work on DC, but they are quite expensive.  there has to be a simple (and cheap) way to open/close the 24vdc power source to the heat using temperature...having a hard time finding it tho.

any help would be greatly appreciated.

How close to your setpoint do you want it to hold?
How warm is "warm"?
There are any number of smallish, inexpensive temperature-controller kits that use little more than an opamp, a couple of resistors and a thermister to control either a relay or a power-transistor. If your "precision" is not an issue, a thermister and a couple of transistors would probably keep you within 10 deg of your setpoint.

[defed]

i'd like to keep it at about 50F, and it doesn't have to be rock solid.  40F-60F is ok, would just use a little more power if it overshoots.  i did find a really inexpensive one, but it will only handle 30 watts for DC (but 15a @ 120vac).  also found one that will do exactly what i want but it's rather expensive for what i need it for.  i did see a couple kits like you mention, i will have to look at them closer.

it's a really small space, about 12 cubic ft and insulated, so it won't take alot to make it warm...just need some way to control it.

[wpowokal]

Use it to switch a suitably rated relay.

Allan

[rossw]

it's a really small space, about 12 cubic ft and insulated, so it won't take alot to make it warm...just need some way to control it.

Years ago, I used to make crystal ovens (for high-stability radio gear). They were *REALLY* small - a matter of probably half a cubic inch.
The simplest way we came up with that was "adequate" was a PTC thermister in series with the heater (a bunch of resistors). The PTC was in the oven with the crystal and provided the feedback. As long as the voltage remained stable, the temperature got to an equilibrium and it was all good. Not quite what you want though.

Something like this might be suitable - opens at 60 deg, re-closes at 40, cost under $5
https://secure.vividcluster.crox.net.au/jaycar2005/productView.asp?ID=ST3821&keywords=thermal&form=KEYWORD


These things are available in a wide range of temperatures, and NO as well as NC. The one above is too hot for you (I think you want it to open at 10C), not sure I've seen them that low.

But another avenue you might try - look for an OLD junked refrigerator. They used to have bulb-and-bellows type thermostats. The bulb went in the sensing area, connected via little capillary tube to the bellows, which operated a microswitch. The switch had an adjustable cam to set it's trip-point, and could generally be set from about 0 degrees C to 10 or so - about ideal for your application. The switches in the ones I saw were quite happy to switch loads of 5 amps. If thats not enough for you, it could operate a relay.

Ebay item 270581506819 for example, or ebay 160372777342 (adjustable 32 to 46 deg F). I'm sure you can scrounge an old one for little or nothing. Check your small appliance repair centres near you

[WindriderNM]

You can use the mechanical thermostat connected to a suitable relay. Even though it is ac it should work with the small dc current needed to operate the relay. A capacitor across the contacts will decrease arcing. or go to radio shack and get an op amp, a few resisters, thermosister, small IC board and relay. probably less then $30.  they used to have little booklets with the diagrams for these types of projects. There are no longer any radio shacks near me so I don't know if the booklets are still available.

[defed]

Use it to switch a suitably rated relay.

Allan

i didn't think this would work...isn't the thermostat essentially the relay?  or maybe i'm misunderstanding something...is this what you mean:  if i had the 30w DC thermostat...connected to 24v battery...set point is reached, thermostat closes, sending the 24v to the relay...this activates the relay to pass more power to whatever load?  if that is the case, don't SSR's have a minimum amperage on the input?  not sure how you determine the amp output of the thermostat at 24v.

i always think i understand how things work (relays, thermostats) then the more i think/read about it, i get myself messed up w/ details! 

[ghurd]

If you can find any thermostat in the temp range, pretty easy to make it trigger a simple power circuit with a few parts.
See CmeBrew's Reply #26...
http://fieldlines.com/board/index.php/topic,136388.msg900391.html#msg900391
G-

[defed]

let me know if i have this concept correct....if i have an SSR, w/ input control voltage of 3-32vdc...it is active any time it receives voltage within that range?

my charge controller says it sends a 'signal' to activate a relay...for some reason, i thought this meant something different...but i presume now that it sends a constant voltage to the relay to keep it active...when parameters dictate it to turn the relay off, it stops sending the voltage.

[ghurd]

Basically, yes.

Why over complicate it with a SSR?
Can do it with 2 little resistors and a mosfet.
G-

[defed]

i thought the SSR was simple, once i finally got it thru my head how it works.  i already have some SSR's lying around which is why i figured it would be the easiest way.  i guess the schematic you referenced would be good to practice on, i'm not too good w/ a soldering iron!  isn't there 3 resistors on that (150k, 3k, 1m)?  what's the thing across the pump wires?  not up on all of my electric notations.  and i still have to find a thermostat in the right temp range.

[ghurd]

The extra (3K) resistor was sort of an option we discussed.  It is one of those things that won't do any harm, but may help.

The black thing with the pump is a Schottky diode used as a flyback (or freewheel) diode.  It is required with motors being switched with typical semiconductor circuits.  You probably won't need one with a tiny heater element (light bulb?).

Your system is 24V?  The resistor values need to be different than CmeBrew's 12V set up.

Something cheap and simple like that would be a good circuit to get that soldering iron some experience.
G-

[defed]

yea, 24v, and a bulb or 2 (depending on size) for the heater.  electronics stuff interests me, just never found the time to get into it much.

[BrianSmith]

Defed,

I make a DC powered temperature controller you could use for this.  Its best suited and most cost effective for solar heating type stuff as it supports differential temperature control, but it also has a Heat and Cool mode thremostat you can use for what you are describing.  It has 10A relay outputs and runs off 12V.   The other thing is it has some temperature logging for the do it yourself solar guy who want to measure how there collector / tank are doing.

Its probably a bit more than you want to spend, but no soldering required.....

You can see what its about here:  https://sites.google.com/site/mydtcstore/

Maybe in the future you will want to build your own solar hot water system and you'll be ready.....  

The SSR relay is pretty easy to do if you already have a control output for the temperature control.  It sounds like it is your best bet if you already have one.  

[defed]

i found a cheap thermostat on clearance at the store.  i thought it was a mechanical one (since it had no display) but later i found it needed batteries to operate.  i didn't read the package close enough to notice.  not that it matters, but figured a mechanical one would be more forgiving of my experiments.  it is a millivolt/24v AC heat only thermostat.

anyway, i ran the battery power thru the thermostat, to the relay.  hooked the relay outputs to the battery and load.  initially, i used (1) 6v battery to power the thermostat.  it worked, and drew about .2a thru the thermostat when operating the relay.  then i hooked it to the 24v series of batts.  again, worked fine and drew about .04a.  so, either configuration, it appears to pass about 1 watt thru it.  i don't know how long it will work, but for now, it does.