PTC Thermistor for doorbell? - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 02, 2020, 04:56:36 pm
content: So I've been burned (literally) twice so far with our doorbell. It's old school analog ding-dong (which I want), so 24VAC transformer, and pushing the doorbell will briefly energize the coil to engage the mechanism.Unfortunately this means if the doorbell switch gets stuck (it did), the coil turns into a heated and things get melty.I'd like to put a PTC thermistor in line and wanted to confirm that should work as intended. Basically allowing for occasional bursts of current, but should something like this happen again, the thermistor will heat up and then jack up the resistance.Or, should I just stick with a slow-blow fuse?A typical ring is what, maybe half a second? From what I can gather before, we had a delivery, and it took about an hour to say "Hey, do you smell that?" and then another 30 minutes to realize what it was. I mean at that point the thing was cooked. Reacting between 5-30 seconds would probably be ideal.I can measure the standby current, since one of the doorbells is lit, therefore conducting a small amount of electricity, and can measure the "peak" current once my replacement dinger arrives

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2020, 05:24:40 pm
content: You could try it with the PTC resistor stuck to the coil, that way it'll turn off, when the coil overheats.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 02, 2020, 08:56:14 pm
content: Are you sure the doorbel is not supposed to run on 12Vac?I don't think sticking the PTC to the doorbell coil is a good idea.PTC's were often used in the old fashioned TV's where a PTC was placed in series with the degauss coil, and the PTC used to have some heat insulation around it.More thermal isolation for the PTC means higher temperature for the PTC after it heats up, which results in higher resistance, and lower current.You also do not want the PTC heating the coils of the doorbell.You will need to read some datasheets and/or experimentation to get good results, but the idea is plausible enough to give it a try.

author: Gyro
author_rank: Super Contributor
timestamp: October 02, 2020, 09:37:32 pm
content: 24V does sound high - I'm sure the ones over here are 12V with a tap to allow 4, 8, or 12V.Agreed, a small insulated PTC would probably do the job - thermally attaching it to the (cold) coil will heatsink it for a long time until the winding heats up too, causing it to draw excessive current for much longer.Attacking the problem at source should probably be the priority, the button shouldn't be sticking in the first place. Maybe it is mechanically worn and just needs replacing, or maybe the contacts are sticking/welding due to the excessive load (if 24V actually is too high). An RC snubber across the switch contacts would probably reduce contact wear.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 04, 2020, 11:58:30 pm
content: 24VAC is the voltage used in Canada for doorbells.I would prefer putting it on the transformer output which is near the electrical panel rather then inside the doorbell. My thoughts were that the heat-rise of continous current would cause the PTC temp to rise, and not by actually attaching it to the doorbell coil.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 05, 2020, 12:06:24 am
content: Yeah a PTC should do what you want. Definitely insulate it, you don't want to wait until the coil gets hot to trip.

author: floobydust
author_rank: Super Contributor
timestamp: October 05, 2020, 12:13:12 am
content: Doorbells in North America are16VAC Class IIwhich means energy limited. They will work at 24VAC but super loud.The transformer is special and has thermal+overcurrent fuse built inside. This is to protect the transformer and low voltage bell wire and solenoid from fire.I'm assuming a proper Class II transformer is being used?

author: IanB
author_rank: Super Contributor
timestamp: October 05, 2020, 12:22:01 am
content: My doorbell here in the USA was installed with an 11 V transformer and it works fine, it's plenty loud. The bell pusher has got stuck once or twice and all that happens is the coil makes a buzzing noise from the 60 Hz. I don't know how hot it gets when that happens, but it has not got hot enough to make a smell or damage the doorbell.I suggest you check your transformer and see if you can drop the voltage before trying more complicated things?

author: floobydust
author_rank: Super Contributor
timestamp: October 05, 2020, 01:12:39 am
content: I've only ever seen 16VAC but a quick search:"In the US, only about 20% of households built before 1995 supply sufficient voltage for the Nest Hello. The power requirement is 16V-24V.""A doorbell or buzzer works on 10 volts, and chimes work on 16 volts."It seems 24VAC is preferred because HVAC uses that.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 05, 2020, 01:35:42 am
content: Sorry about that, you are correct, it is indeed 16VAC. I've also been doing a lot of HVAC latelyRegardless, the issue remains.

author: james_s
author_rank: Super Contributor
timestamp: October 05, 2020, 03:43:15 am
content: I think a PTC is a sound idea, I would start by measuring the current that the coil draws and then pick a PTC that will not limit the current too much. They're cheap enough that you could buy a few to try out, I think you'll want something that is fairly small physically and relatively low resistance at room temperature. There are also those polyfuse things that you could try.

author: Gyro
author_rank: Super Contributor
timestamp: October 05, 2020, 09:20:40 am
content: You can probably rate it quite low. A normal ring won't give the PTC significant time to warm up and lots of cooling off time. Ideally set it to frustrate persistent button pushers.

author: Circlotron
author_rank: Super Contributor
timestamp: October 05, 2020, 11:50:31 am
content: Maybe even some kind of light globe in series with the bell? The cold resistance of the filament should be low enough to ring the bell, and if the button is held it will light somewhat and reduce the current. Same deal as a PTC resistor but maybe you have a car globe on hand that would do the job.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 05, 2020, 12:22:43 pm
content: The Idea of adding a PTC is charmingly simple.A more complicated idea could be an oscillator with a startup delay, so your bell starts ding-donging after a minute or so if the button gets stuck, or shuts of the bell and lights a LED instead.

author: Berni
author_rank: Super Contributor
timestamp: October 05, 2020, 12:23:58 pm
content: Yep a PTC Polyfuse is the right thing for the job.They all have pretty slow reaction times and you can fine tune the reaction time by choosing one rated for a higher or lower current. Basically the DC resistance at room temp makes it get warm as current flows then at some point its temperature characteristic gets to a sharp rise in resistance and this puts it into a very fast thermal runaway until pretty much all of the voltage is across the PTC, so very little current is required to keep it hot in the high resistance tripped state. Once power is removed for a while it cools down and gets reset into the normal low resistance state.Going far above its rated current will trip it in < 1 second, going just on the edge might take it 30 seconds to trip.Tho you might also want to wire a bright LED in parallel with the PTC so the doorbell starts glowing in the tripped state to tell you something is wrong.

author: Zero999
author_rank: Super Contributor
timestamp: October 05, 2020, 12:37:19 pm
content: One big downside with PTC resistors is limited life. After ach time it trips, it becomes more sensitive and eventually reachs a point, when it trips during normal operation, rendering it useless. I've experianced this before and is why I suggested fixing it to the coil, since it will ensure it doesn't trip unless the coil is actually overheating.What about a timer relay to dissonnect it, after a few seconds? Get an off the shelf unit, or roll your own from a rectifier, capacitor & regulator, 555 timer/comparator/MCU & a normal relay. Normally closed, with delayed on will give a longer life, since the contacts will only ever open, when the button is pressed for long enough.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 05, 2020, 12:50:15 pm
content: Polyfuses are PPTC's (Plastic PTC's), and these have some limitations.I don't think the old fashioned PTC's have this limitation. They used to be used for televisions where they switched the degaussing coil each time the TV started, with a decaying AC signal as the PTC heated up. And they did this each day for years on end.They are also used in big numbers in refrigerator motors. They switch off the start coil of the motor as the PTC heats up, so it goes to a cycle each time the motor is started.In addition to this, the circuit is only supposed to trip when the button gets stuck, so it will not see many cycles.

author: Berni
author_rank: Super Contributor
timestamp: October 05, 2020, 01:01:55 pm
content: True Polyfuses have limited cycles, but this thing will only trip if the button is stuck for too long, it won't get hot from just normal door bell rings.The other keep it stupid simple way of fixing it is a small bimetal thermal cutoff switch stuck to the doorbell coil (Don't use the big round can types, they would take too long to heat up). Get one that is set to trip at a pretty low temperature of say 50°C so by the time the windings get above 100°C the switch will have tripped. One benefit of this is that once it cools off it resets again suddenly and rings the bell as a result, so you get a audible warning every so often that something is wrong.

author: Seekonk
author_rank: Super Contributor
timestamp: October 05, 2020, 01:28:09 pm
content: It is an ideal use. As said before a little foam insulation will give faster response. Any cooling really slows them down. I've replaced numerous fuses in my car with PTC when a fuse blows with a rating of less than half for momentary loads. Never had one fail. A fraction of the circuit's current is all that is needed because of time element. Added bonus, those who keep ringing your doorbell will soon get turned off.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 05, 2020, 07:23:13 pm
content: Thanks all! Once I measure the standby and "active" current, I'll try to pick values.I just need to ensure the Trip Current is above the in-use current while the coil is energized, or else it would immediately trigger, right? Or does trip current still need to be held for some period of time specified in the trip time?

author: james_s
author_rank: Super Contributor
timestamp: October 05, 2020, 08:00:21 pm
content: The standby current will be zero, or if you have an illuminated button it will be whatever insignificant current the little light bulb draws. In normal use the time constant for a doorbell will be <1 second so you can get by with quite a small PTC, something rated to continuously carry maybe 1/4-1/2 of the current the bell draws when engaged ought to do. You should have a good deal of wiggle room here as this is a very non-critical application. A "trip" time of anywhere from a few seconds to 30 minutes ought to be fine.Careful with the advice of wrapping the PTC in foam, they can get quite hot and some foams may break down or even ignite. I suspect it will be fine just hanging out inside the chime housing unless it is in a very drafty location.

author: Berni
author_rank: Super Contributor
timestamp: October 05, 2020, 08:22:50 pm
content: The trip times of PTCs are really slow, even slower than regular fuses (Even those will happily conduct 150% the rated current for a few seconds in most cases)Have a look at the graph here. The lowest rated partnumber at line A is rated for a holding current of 0.9A and a trip current of 1.8A. Yet it will still conduct 2A for 40 seconds before tripping. You need to put about 8A of current trough the thing to trip it in under one second, that is almost an order of magnitude above the rated holding current.So just mesure the current the doorbell coil consumes and then look up this graph for various PTCs until you find one that has the desired trip time at that current. Tho id say 10 seconds is plenty enugh since the doorbell switch is only supposed to be briefly tapped anyway. If someone wants to really bang the bell they can proabobly still brifly hammer the switch multiple times a second for a good half minute before the PTC would trip.

author: Seekonk
author_rank: Super Contributor
timestamp: October 06, 2020, 12:54:03 pm
content: I actually experimented making a stabilized thermal oven with a standard PTC fuse and a resistor in foam. They reach about 100C and stay there. If I remember it only took 1/4W to maintain the temperature of the PTC.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 07, 2020, 05:41:54 pm
content: Okay so idle current with the lit doorbell is 80mA, the current when ringing is 2A, which is nice and high actually, should generate some nice heat if it gets stuck. I'll look for something accordingly.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 07, 2020, 05:56:47 pm
content: I found this which trips at 1.5A after 6 secondshttps://www.digikey.ca/en/products/detail/littelfuse-inc/RXEF075/5015922Would it be suitable?

author: Gyro
author_rank: Super Contributor
timestamp: October 07, 2020, 06:03:22 pm
content: Wow, potentially up to 48W then, no wonder things got smelly!Nearly 2W on the lamp too.That's quite a switching load for the button contacts.

author: james_s
author_rank: Super Contributor
timestamp: October 07, 2020, 08:24:21 pm
content: Quote from: joeyjoejoe on October 07, 2020, 05:56:47 pmThat's a polyfuse. I'd be looking at a PTC thermistor, not a polyfuse. The latter degrade fairly quickly after a few trip cycles.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 08, 2020, 12:22:46 am
content: Hmm, Digikey only has PTC Resettable fuses... or sensor thermistors. What should I be looking for?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2020, 03:53:51 pm
content: The "PTC-resettable-Fuses" category seems to do the trick. There are many more devices there then the PPTC's, such as these:https://www.vishay.com/docs/29087/ptccl265vseries.pdf

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 08, 2020, 04:31:45 pm
content: I'm just thrown off by james_s comment that I picked the wrong thing... that was in the PTC Resettable fuses section. Is there a "type" i need to filter on or something? Or do I need to just read the literature for all the series?

author: ArthurDent
author_rank: Super Contributor
timestamp: October 08, 2020, 05:45:45 pm
content: If the problem is a sticky doorbell switch, instead of adding complexity to such a simple circuit, why not just replace the faulty switch?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2020, 05:55:23 pm
content: The different types of PTC's seem to be intermixed in the same category.The Plastic / or "Polymer" versions often start with 2 P's, as in: "PPTC".https://en.wikipedia.org/wiki/Thermistor#PTChttps://en.wikipedia.org/wiki/Resettable_fuse------------ 8<--------------- 8<--------------- 8<--------------- 8<--------------- 8<---Why would anyone ever get the idea of replacing a faulty switch?The Idea alone is just ludicrous!!!

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 08, 2020, 06:02:14 pm
content: I did replace the switch. The goal is to prevent melting another doorbell. This switch was replaced less than 1 year ago by myself.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 08, 2020, 06:02:56 pm
content: Okay, so I want to find these PPTC's as these would be the desirable kind?Quote from: Doctorandus_P on October 08, 2020, 05:55:23 pm

author: james_s
author_rank: Super Contributor
timestamp: October 08, 2020, 07:13:55 pm
content: Quote from: ArthurDent on October 08, 2020, 05:45:45 pmI think adding a protection device is still a good idea. Devices have fuses and branch circuits have breakers not as a workaround for faults, but to prevent a fault from turning into something more serious. The next time the button fails and gets stuck there's no reason to burn out the doorbell and/or power transformer too.

author: floobydust
author_rank: Super Contributor
timestamp: October 08, 2020, 08:07:15 pm
content: OP a car tail light like 'ol #1157 will also work, if you want to do this on the cheap.

author: Gyro
author_rank: Super Contributor
timestamp: October 08, 2020, 08:28:52 pm
content: Quote from: joeyjoejoe on October 08, 2020, 04:31:45 pmThe Vishay ones that Doctorandus_P referenced are rated for 5000 trip cycles - I think that ought to see you out.

author: floobydust
author_rank: Super Contributor
timestamp: October 08, 2020, 08:40:33 pm
content: PTC fuses trip at lower currents after their initial trip. If I'm designing something with one, I will force them to trip a few times which gives a new hold/trip current value lower than a brand new part. I believe reflow also alters the crystalline structure so "surprise" they behave weaker in production.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 08, 2020, 10:30:29 pm
content: Okay, I'm gonna grab a PTCCL13H321HBE‎ which should trip in about 3 seconds of continous 2A current and call it a day!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 09, 2020, 12:01:19 am
content: I was not clear in this, buy you do **NOT** want the PPTC variant, these have the lowest number of cycles.When in doubt, just check the datasheets.

author: james_s
author_rank: Super Contributor
timestamp: October 09, 2020, 12:33:30 am
content: This is a very non-critical application though, lots of wiggle room, so don't stress over it too much. The very low duty cycle in normal use is a near ideal circumstance for this sort of protection scheme. If you find it eventually starts making the doorbell sound weak you can always try a different part. Nobody is likely to die if the doorbell stops working.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 09, 2020, 12:40:05 am
content: Vishay is tested to 5000 trips. I think I'm good.

author: Berni
author_rank: Super Contributor
timestamp: October 09, 2020, 07:48:43 pm
content: And the typical lithium phone battery is only rated to 1000 cycles (or even less for crappy ones) yet its still used everywhere.If anything id say this is a better use for PPTC polyfuses than using them as an actual fuse. They suck as those as they lose quite a bit of voltage across them, they get hot and their response characteristic is kinda all over the place, especially if your device is expected to work over a wide temperature range. You would never want one of these to protect anything sensitive or critical, but they are the cheapest self resetting way to keep wires from melting and catching fire in case of a short circuit. But this slow lazy response is the perfect thing for this doorbell.Sure you could solve it a lot more elegantly with a timing circuit and a relay and so on, but how much would that solution cost. Not only the BOM cost but the cost of time to figure it out, draw a design, order all the parts, put it together, test it etc... Time is money after all.

author: james_s
author_rank: Super Contributor
timestamp: October 09, 2020, 07:58:21 pm
content: I'd argue that a PTC or PPTC is a more elegant solution than an electronic circuit. There is elegance in simplicity, it's a bit like the old $$$ fancy NASA space pen vs Russian pencil legend.

author: mech_e
author_rank: Newbie
timestamp: November 05, 2024, 09:53:38 pm
content: Sorry to resurrect an old thread, but I'm in a very similar scenario as the original poster. Two follow-up questions:1) Now that it's been a few years, how has this "upgrade" worked and would you do it differently?2) For clarification, did you affix the PTC to the doorbell chime coil or to the transformer?Any images would be helpful of how you attached the PTC.And for the background story, I've got a decorative (read: expensive) doorbell button that the wife loves. For whatever reason, it has gotten stuck by the typical delivery man pressing it and running off. I think I've fixed the sticking issue, and then my wife pressure washes the house, likely gets the button wet, and shorts it again. So, now after two events, I don't want this to happen yet another time for whatever reason. I like this idea of a simple fuse to protect the equipment from overheating and melting.Thanks - mech_e

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: November 06, 2024, 01:01:23 am
content: I put it with the transformer - no room near the doorbell.Almost 5 years later, doorbell still working. Whether that's because of this device, or the button has never been stuck since, we'll never knowI can't remember how I connected it - probably with a Wago spring connector for simplicity.

author: mech_e
author_rank: Newbie
timestamp: November 06, 2024, 04:23:04 pm
content: joeyjoejoe, thanks for the update and the info! It's far easier in my application to also put it with the transformer, so I'm leaning towards this option too.

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2024, 06:15:07 pm
content: Did you guys use aClass IIdoorbell transformer? As I said earlier, these are energy-limited and have the primary thermal fuse. This change prevents house fires where the transformer or the wiring burned up. I had one old 1950's building do this, cotton covered wiring, cable staple or nail cut it etc. there was a fire.An off the shelf transformer will have no fusing at all, so you must add it onbothprimary and secondary.

author: mech_e
author_rank: Newbie
timestamp: November 06, 2024, 10:01:05 pm
content: My setup is pretty standard using a Heath Zenith 8V/16V/24V transfomer. I have a feeling this has a built-in fuse, which means maybe using the thermistor with the transformer may not do much since it's internal fuse may blow before getting too hot? Maybe the only real option then is to add the thermistor to the doorbell chime solenoid?Or maybe I'm missing the point and you're saying to add two thermistors; one to the primary and then one to the secondary lines going to the doorbell chime from the transformer? I schematic would help me out since I'm pretty green with these types of components.Thanks again!

author: MarkS
author_rank: Unknown
timestamp: November 07, 2024, 12:05:34 am
content: You're trying to fix the symptom and not the problem. Replace the doorbell switch. It should not stick.

author: BradC
author_rank: Super Contributor
timestamp: November 07, 2024, 04:29:48 am
content: And set a counter such that 5 presses in quick succession applies 15kv to the metal surrounding the switch.

author: floobydust
author_rank: Super Contributor
timestamp: November 07, 2024, 05:51:24 am
content: Quote from: mech_e on November 06, 2024, 10:01:05 pmI checked and Heath Zenith (Newhouse china) doorbell transformers are claimed to be "UL Class 2 Certified".SL-125or 16TR, 40TR etc.They have overload protection likely a thermal fuse built in per UL 1310 so no need to add fusing.It's common people upgrade their doorbell transformer, problems where they run out of power with smart doorbells/cams.But for your stuck button, a PTC will work to stop the coil from burning out.