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Author Topic: Which thermostat works with 24V DC?  (Read 5003 times)

« on: September 04, 2006, 06:42:25 PM »

Is there a termostat that works with 24V DC or do I need a relay or something with another type of thermostat?

Brian Elfert
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« Reply #1 on: September 04, 2006, 06:49:51 PM »

Most house type thermostats are 24 volts a.c., Most also use a small low current relay for the contacts.

A common, a N.O and a N.C. connection.

Depending on what you are hooking up will make a difference of whether you need an external relay to handle currents higher
that about 50 m.a. ( the contact ratings are usually on the package. )

The electronic ones that work on their own batteries are more universal since you don't have to actually supply power
to the display.

I got some at HD a while back that worked fine operating some 12 volt relays to turn on and off an a/c compressor and fans.
( the relays were 24 v.a.c. used in air handlers and worked fine on 12 volts d.c. for the coils. )

Never take a knife to a gunfight!

« Reply #2 on: September 04, 2006, 06:54:40 PM »

Would I need to convert 24V DC to 24V AC for a 24V AC thermostat?

I will be operating three blower fans from the thermostat for now.  Eventually I will have a seperate thermostat for each blower.  I have a bunch of 24V relays that are good for 30 or 40 amps I can use.

Brian Elfert
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1995 Prevost H3-41, series 60, B500 Allison

« Reply #3 on: September 04, 2006, 06:57:57 PM »

Yes, most aer made for 24 volts to start with.  The real issue is to make sure that you don't use one with a mercury switch in it.  They have to be dead level, and motion is even worse.  They'd be turning on and off every time you came to a stop. Or if you parked slightly out of level they would not be calibrated to the temp you ask it for.  Having said that I"m not sure other than Dick Wright, where to get the RV types, probably any big  RV accessory store would work.

Jim Stewart
El Cajon, Ca.  (San Diego area)

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Clarke Echols
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« Reply #4 on: September 04, 2006, 07:11:49 PM »

At low currents, thermostat switches don't much care whether it's AC or DC.  Use a 24-volt relay
to carry the bigger currents so you have only the coil current through the thermostat.  A small
capacitor across the points of the relay can help cut down on arcing across the contact points;
something on the order of 0.1 microfarad, but isn't critical.  If you use a capacitor get one that
is rated for at least 50 volts; 100 volts is better.  When the magnetic field in the relay coil collapses
at the time the points open, it can create a substantial voltage for a very short time.

Mercury switches don't work well in a moving environment as previously stated.

Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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Nick & Michelle Badame

« Reply #5 on: September 05, 2006, 03:53:49 AM »


You can use a line voltage thormostat, they are used for baseboard electric heaters and are capeable of handeling

25+ amps ac. Which would be able to handle dc fan motors of that size.[ No Mercury, Bimetal operation!]

They come in different combinations, either Close on the rise or Open on the rise. I am assumeing you want close on the rise of tempature.

You can find them in Mcmaster Carr or Grainger.


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Commercial Refrigeration- Ice machines- Heating & Air/ Atlantic Custom Coach Inc.
Master Mason- Cannon Lodge #104

« Reply #6 on: September 05, 2006, 04:52:45 AM »

I am unsure of the exact application that you're working on there.
I swapped out our Custom Coaches thermostats -- which were line-voltage type.
They had (7) total in the coach, (3) for AC's and (4) for heaters.
In an effort to tie both AC and heat into one thermostat, I found one at Graingers that seemed to work.
The TA-155 from Peco Mfg.  I bought a bunch and do have more.  Refer to http://www.busconversions.com/bbs/index.php?topic=1850.0

Not sure if they could help with your needs.
Or try...

Graingers - http://www.grainger.com/Grainger/wwg/productIndex.shtml
or Johnstone - http://www.johnstonesupply.com/scripts/cgiip.exe/WService=wsbroker1/login.htm?store=Corp&browse=yes&category=

Good luck,
Jerry H.
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