I am an electrical novice and have a Therm-O-Disc 12S20 H24V (original image) relay that I'm attempting to understand.
The only meaningful documentation I've been able to find is this, 12S, 14S, 15S Series Time Delay Relays and Sequencers. Page 3 of the PDF (recorded as page 7 in the footer) of the documentation shows a table which indicates on and off timings in seconds. Could someone explain what this means?
Also, how would I determine what the control voltage is to activate the relay?
You did your homework! Thank you for getting this datasheet out.
This is not a 'normal' relay. It is of a time delay type.
I believe your relay will perform an action after a certain time delay. Internally the device seems to compensate for different operating temperatures so that you get consistent delays. You can determine the pinout by the part number, 12S20 H24V on page 9
The control voltage in your case is the 'standard' (as per datasheet) 24VAC. They have data for 120VAC, 240VAC and 277VAC as well for those different models.
The time delay is specified somewhere in the part number. You really need to contact the manufacturer because this datasheet is somewhat vague. The only hint I saw was
A variety of standard timings are available for general time delay applications. You appear to have a relay that delays between 22 and 55 seconds to turn on and between 15 and 45 seconds to turn off. All of the information appears to be marked on the device.
For more details please contact the manufacturer. Their information
The pairs of timing ranges represent different combinations of heater and bi-metal disc. You would have to order accordingly. Love the cautionary statement that warns "at end-of-life of the bi-metal disc, the contacts may remain open or closed".
I realize the first posting was well over a year ago from my post, but I thought I would share what I found with my 12s20 Therm-O-Disc. My gas heater would act like it was going to cycle, you could hear the ignition, gas valve and subsequent lighting of the gas. This would continue for maybe a minute or so. During that time you could tell the unit was getting hot, but the fan would not start. I imagine a thermistor or other safety feature shut the gas valve off when it was found to be getting too hot.
After trouble shooting and finding that bypassing the Therm-o-disc worked to run the fan, I commenced to tear into it and this is what I found....hopefully this helps someone save a few bucks down the road.
In the picture you'll see that I carefully drilled out the rivets which held the tabs onto the top. The cover between the tabs can be pryed off carefully with a small sharp instrument. Underneath you'll find this. It basically is a mechanical relay with silver contacts. In my picture you see that the lower relay has a lot of oxidation on it. The upper relay had some as well, but I had already cleaned it before I remembered to take a picture. Anyways I cleaned both relays and with a 1000 grit sandpaper I lightly buffed both surfaces. I reassembled the unit using very small screws, reinstalled and nothing happened....
On my second round I disassembled, took that little white pin out (seen to the right of the lower contact), cleaned that with alcohol and re installed. Still nothing....
On my third round I bent the copper down at the tab and straightened it at contact point. I threw an OHM meter on it and it showed normally open. With a very small amount of pressure over the white pin - the circuit closed. I re-assembled and voila! It worked. On my furnace, power to the timer is supplied at the same time the ignition takes place. I didn't time the relay but I would guess that it's around 20-30 seconds or so.
I suspect the resistance built up from the oxidation causes the copper to heat up and possibly change shape ever-so-slightly. Mine wasn't even close to working before I bent the copper.
PS the copper relay is up-side-down in the picture. Flip it over before assembly.
FYI these relays are used to stage a fan on when used on a gas fired heater, but also used to turn a fan on when used with an electric furnace as well. Variable fan timing is why the different timing designs per manufacturer requirements. The amount of timing to "off" has a span because of simple thermal mass, which varies depending on how "hot" the relay is when the control voltage is removed from the lower terminals, it takes varying amounts of time to "cool off" the relay and reset to normal contact position.
After 8 years my thermal switch 12s20 was intermittent at best. I would switch the boiler system off and on rapidly several times and it would work for several times then I would have to "switch" again. I took it out to examine and get the numbers so I could order a replacement. I accidentally dropped it and part of the bake-o-lite case broke off. I said bad words thinking I would be without heat until a new one arrived. Then I looked inside and saw the pitted contacts, extremely pitted! A tiny file and a little super glue and the heat was on. That cure lasted about 5 years. Now I will replace it. I found it for $20 with free shipping on Ebay.
The PDF you mentioned above describes the same thermal delay switch with two or three sets of contacts to operate other devices independently. If the price were right you might be able to have two spare sets of contacts already installed and waiting for the first one to become pitted. Also, the right capacitor in the right place MIGHT minimize the pitting.
