Ring my bell

July 15, 2012 11:46 PM

Where might I find a replacement (low-profile) doorbell transformer that is similar to this? It is about the size of a Matchbox or Hot Wheels toy car and has FRONT, REAR and TRANS connectors. The replacement transformer I bought looks like this, which is about the size of a tennis ball and has white, black and green wires, but no FRONT, REAR and TRANS connectors. Big chain hardware stores shrug their shoulders, lighting stores only carry the big TF I linked to, so where might someone go to find something like this, to fix an older doorbell?

It has to be a low-profile TF, because of the tiny, low-profile space where the previous TF was installed. Google isn't much help. Thanks for pointers if you've had to fix this sort of thing before.

posted by Blazecock Pileon to home & garden (5 answers total)

Are you sure that's a transformer and not some sort of ... terminal block, maybe? I'm thinking that the transformer might be at the other end off the wire that was connected to the terminals labeled "trans."
posted by jon1270 at 1:59 AM on July 16, 2012

Your twitter picture is not the doorbell system transformer. It's the chime solenoid.

Re your twitter pic: there should be a pair of plungers that fit into the tubes in the top. Those plungers move when power from the transformer (usually located in either the furnace room or next to/on the electrical panel) is applied to either the Front and Trans or Rear and Trans set of terminals when someone presses the door bell button. IE: you need both a chime solenoid and a transformer like the second picture for the system to work.

Pictures of hardware stores doorbell solenoid used to make custom door chime (you can see the plungers). Another chime; the terminals from left to right would be labelled Rear, Trans, Front. Wiring Diagram.

No suggestion on where to get a replacement chime solenoid. Yours looks really old so you might be forced to open a bunch of chimes at the hardware store until you find one the right size or that can be made to work. If you don't care about rebuilding the existing chime then pretty well any chime unit would work. Having said that how did you determine your unit is bad? They are incredibly simple devices with power only applied intermittently (IE very low duty cycle). It's a lot more likely the transformer has failed (it's on all the time) or breaker supplying power to the transformer has tripped. Also if your doorbell has never worked it is possible the plungers are missing.
posted by Mitheral at 4:36 AM on July 16, 2012 [1 favorite]

To back up Mitheral, you can see it here, about 1/3 of the way down the page. It is the model NuTone K28 Imperial.
posted by Climber at 5:02 AM on July 16, 2012

Having said that how did you determine your unit is bad? They are incredibly simple devices with power only applied intermittently (IE very low duty cycle). It's a lot more likely the transformer has failed (it's on all the time) or breaker supplying power to the transformer has tripped.

AC solenoids are different from DC solenoids in that the inrush current is high when activated and then goes down as the plunger is drawn into the solenoid which increases the inductance and thereby reduces the current. If the plunger ever sticks in the open position while the circuit is closed, the solenoid current remains high and can burn out the coil. That is why the solenoid has a warning "do not oil." Oil attracts dirt which may eventually jam the plunger and cause the solenoid to burn up. This is one of the limitations unique to AC solenoids.

It is easy to check the solenoid with an ohm meter by measuring between the transformer to the front and rear terminals. In both cases it should be a small resistance, not infinite.

You can check the transformer with a meter to see if it is outputting something close to either 12 VAC or 24 VAC.

If the transformer is the problem, it is easy to replace with a similar one having the same voltage output. The transformer is usually located in a nearby closet or in the basement because it requires a 120 VAC source.

If the solenoid is the problem, you probably need to see if you can get an exact replacement from the manufacturer because it needs to meet the exact mechanical and electrical specifications for size and stroke length to impact the tone bars correctly. If the stroke length is wrong, then the plunger may not fully extend into the solenoid which means the AC current could be excessive as explained in the first paragraph, causing it to fail.
posted by JackFlash at 9:50 AM on July 16, 2012

Holy crap — $500 for a doorbell. This midcentury fad is getting way out of hand. Thanks for pointing me in the right direction, folks!
posted by Blazecock Pileon at 2:32 PM on July 16, 2012