PTC relay compressor, the simplest of design

7/26/2016

PTC - Compressor starter - Positive Temperature Co-efficient
Hey, simplicity works but it has its limits
The most cheapest compressor starting device, and can I say it is not a relay , more a resistive switch.
For very small class capillary system refrigerator, usually a bar fridge or cubic fridge enter the PTC.

Now these are not high tech, and they need to have decent intervals in time before starts so they can cool down.
Also the system must equalise pressure between the high and low sides to allow for a low compression start.
The PTC starter is similar to the potential relay not by design but where the start winding is engaged immediately from first power up. . At start up the PTC is cool and has no opposition to current flow by a preset low resistance which is in series with the compressor start winding.

The compressor starts by resistive start and induction run, as the current flows through the PTC , it heats up increasing its internal resistance until it will not allow more current to flow , disengaging the start winding. The start winding is isolated and will not engage further as a trickle of current to the start winding will maintain heat and high resistance during the operation of the compressor.

When the unit stops the PTC MUST!!! cool down to allow current to flow again on next start-up. If it trys to start to early the internal overload will activate in the compressor.
So it really is not designed for frequent starts and requires very low starting torque.
Don't expect too much , price and capacity dictates control, yet I do like how simple these are.

The PTC is small, no bigger than a 10c (Australian) coin and as about 4mm thick.
Note also the compressor terminals are inverted to this design as per the Danfoss types. I have marked them.
These starter types are typically only used on low capacity, low torque compressor where off cycle pressure equalisation occurs , as you see in capillary systems

Now does anyone see a Star Trek logo on the PTC relay disc in the pictures above? , or am I just seeing things :D

29 Comments

joe head

9/9/2018 10:10:44 am

Hi. I think you have mislabeled the start and run winding in the third picture. When ohming compressor windings there are the three values. The lowest value is the run or main winding. The next higher value is the start winding. The highest value is the sum of the run and start winding together.

Admin

9/9/2018 11:23:48 am

I agree with your analogy but the readings are as shown , Nothing has been represented differently.

Dirk

3/7/2019 11:38:34 pm

Good Day,
Thank you for your great article, on PTC.
I have cleaned and put the broken pieces of the PTC back in the holder, as an emergency fix until I could order the part.
For years I could not figure out what that disk did and have asked people, no answer.
. One just happened to fail this week, and from my previous experience the fix worked.
Thank you very much, my mind can rest now!
Dirk

admin

4/25/2019 01:00:47 pm

Hi Dirk, thanks for your feedback and kind words

Well done for fixing one up, to be honest they are not very servicable in nature.

A PTC disk, plastic housing and metal bus links

Its no kitchen sink in design and was made to be simple in functionality
Pleased you could make sense of it and get it working

Dana Perry

4/11/2021 01:59:19 pm

is this the dirk i no how you doin

Richard Dixon

4/24/2019 11:29:41 am

You're right! That is a Star Trek Logo. I keep telling people that this piece of ceramic must be Alien Technology. One piece of ceramic acting as a capacitor, relay and starter all with no moving parts. It must be alien tech from what it costs, one of these little jewels for a Whirlpool fridge is $60.

admin

4/25/2019 12:53:14 pm

Hi Richard, thanks for posting in

$60 for the simplest of starters? not bad profit here.These PTC types are limited in capacity they can handle and are the basic starter for an RSIR or CSIR compressor.

Being temperature operation based they dont reset immediately like a current or potential relay type and need time to cool. An equalisation refrigeration system such as capillary and cyclic control is all they are really good for..

Alien tech? , I like that coment

JEAN RUTHEMANN

8/1/2019 02:11:33 am

HI and thanks for explanations :) , my fridge is a samsung and my ptc start relay DA35-00135A has one more part : a m8jz47 BI−DIRECTIONAL TRIODE THYRISTOR SILICON PLANAR TYPE and gave me a 1200 ohms reading and is out of stock a few weeks to get it and the transistor is availlable in a different contry far far away ...
How bad would it be to bypass the transistor and only use the ptc until i get my hands on a new part ? thanks

admin

8/2/2019 05:36:44 pm

Hi Jean.

I dont have the specifications on that device but unfortunately advising bypasses on manufacturer equipment parts with other parts on this page is a no-no for me. You have quoted what seems to be a TRIAC component and normally these are switching some heavy load AC (current) function

You dont want to go down the road of installing non manufacturer parts unless they themselves sanction a retrofit component. Since the introduction of interface controlling pcbs in refrigerators , everything has changed.

Lola

9/11/2019 04:46:48 am

Great page - done obviously by someone who gets their hands on the equipment!

I have a very ancient freezer that failed unexpectedly and after a LOT of poking around, determined that it has this PTC starter device and that the disk inside was broken into several pieces. I'm prepared to give a replacement a go (cheap online) but when I do the resistance check on the compressor pins, I don't get a high resistance (it appears to be < 10 ohms). Is the ration you show a typical one for most compressors? Or is there a wider range?

Admin

9/11/2019 06:37:41 pm

Hi Lola

Hard one to guess, here, Compressors are inductive motors and need lower ohms to create inductive forces to drive the motor. Without knowing the specificatons of your unit, I cant say what it should be.

Domestic compressors have multiple designs depending on capacity which changes their design type. A PTC relay is on the smaller class and would typically be an RSIR motor or resistance start, induction run. Typically the start winding has a higher reading the run winding.

A PTC starter is in series with the start winding and adds to the total resistance when measured , it open circuits as it heats up and trickle flows to keep it open during operation.

In order to find a replacement , you need to have one suited with the compressor. In the absence of one, many opt for solid state relays which service a range of compressor capacity and some have a built in overload function.

Lola

9/17/2019 09:19:35 am

Thank you. As it turns out, it worked. I found something that looked both physically and otherwise to be very close to the original for $7 - worth a try. Installed and still no go until I realized that there were a few corroded wires. Repaired those and presto: freezer is working well. I plan to run it for a week to be sure, then clean it and use it.

Andy Earnshaw

11/26/2019 10:40:49 pm

Hello & thanks for the article. I'm in a similar position of having a PTC disc still in one piece, but its cold resistance is 1.2kΩ & so I've no idea what it should be other than obviously quite low?
Anyway my more conventionally pinned compressor with common pin at the top has the start winding that connects top pin to LH pin measuring 13.6Ω & run winding that connects top pin to RH (neutral) pin measuring 14.0Ω. so quite evenly matched assuming that they are OK of course. The appliance is an upright under worktop freezer, so not a full height job.

Your PTC is shown as 25Ω, is that what it measured when cold?

Many thanks

Andy

Admin

11/27/2019 07:27:28 pm

Hi Andy, thanks for your message

First answer,, yes this PTC is around 25 ohms cold (room ambient) and when measured will add that resistance to the start winding. The PTC as you know works by heat from current flow increasing temperature and resistance to open circuit the start winding. Your compressor seems unusual with windings close to each other, normallly the start has a higher resistance but this all depends on the manufacturer data which I dont have.

That resistance you have sounds very high and of your getting compressor stall on start , chances are the start is not engaging.
The issue is I dont know your specifications but I would check it with the supplier. If that relay is no longer avail as a spare part you can use a solid state ,model if one is in the capacity range of the unit

Matt

6/10/2020 12:27:54 am

Hi.

Wounder if you can help me with a faulty PTC i have. My part number is QP2-15L. Cannot fond any with the last letter “L” does this matter?

Thanks

Matt

Angus Jones

5/21/2021 06:28:32 pm

I have the same question whats the difference between a QPT 15G and 15L

Glen Bulman

11/2/2021 08:57:45 pm

Hi Matt, did you ever find out an answer to your question? (Obviosuly not on this website) I have the same issue. Thanks, Glen

admin

11/5/2021 09:58:32 am

Hi Matt, sorry about the horrid delay but I have not been able to access my site for a while. Unfortunately , I do not have the technical specifications for your relay or even know about the part number nomenclature. PTC relays can should be access from the manufacturer or agent for specific models unless there is an approved multi range type from refrigeration wholesalers. These days the generic types either wont fit or, are not suitable for a customer model type. If the refrigerator is running on hydrocarbons , then in the interests of intrinsic safety , only genuine parts should be used.

Again apologies for the delay

cletus noronha

7/27/2020 10:24:18 pm

liked your explanation on the working of the PTC. However if I can't get the exact ohm PTC for my mini Westinghouse fridge, can you suggest an alternative design.

What is the trickle value current through the start winding when the compressor is running in order for the PTC to maintain a high value resistance .

Admin

8/2/2020 04:21:25 pm

There will be solid state relay or solid state relay/overload combinations from refrigeration wholesalers. These basically take the place of most small to medium domestic compressor starters. They will need to your compressor capacity size to see if it can be matched then it requires installtion by qualified personel.

Thanks for your comments

kitt

10/26/2020 08:33:52 pm

Wow very nice info. I need help on my samsung ref. I am caught which part could be the problem. Run capacitor, PTC relay, OLP and compressor.. It is RSCR type of motor meaning PTC will help start it then once started, the Run capacitor will take over and connect with the start winding. My questions are the OLP will trigger and turn off the compressor and I measured some 8 amps. This happens around 7 seconds from startup of motor. My run cap is new 5uF at 450V AC and my PTC relay is new with 22ohms ambient reading. the old PTC relay is 33 ohms. What could be the problem. My guess is it could be the PTC which could be defective. I tried to heat the PTC using hair dryer heater but the resistance goes lower. how can this be? I thought it should go higher? Cud it be the compressor d problem? Thanks for your help.

admin

11/5/2021 10:19:10 am

Hi Kitt

PTC relays are in series with the start winding and as they use positive temperature co-efficient operation the NTC operation you described should not be. These tyoes of relays on really exist on the lower power end of the lower torque motors in the refrigeration market and as such the compressors need time to equalise pressure through the cappillary Hopefully your relay has been the issue

Jim Ruedebusch

4/27/2021 07:30:37 pm

Have 50 year old Sears refrigerator—- need compressor start relay. Know ohms for start and run windings on compressor. I can find a relay for this old compressor but they require purchase of10 units at $250. How to design and select relay??

Jim

Admin

11/5/2021 10:27:06 am

Thats an old fridge Jim, sizing is either based on horsepower or refrigeration kW. They make combination solid state relays for even the older hot wire relay starts on classic fridges. You do need to know the capacity of your fridge and that might be hard if there is no info on it.

Sears is a brand Im not familiar with but if those people quoting you parts can tell you that then check with a refrigeration wholsaler to see if one is compatible , regards Dennis

Malx

5/26/2021 03:59:14 am

Brilliant, thanks for explaining this, refrigeration seems a bit like a black art, particularly now electronics are involved. The prices of these electronics seem to be stunningly expensive. Fortunately the PCRs seem reasonably cheap. I will run the tests you suggested, I have measured the current consumption on switching on, it starts at 60W and falls quite quickly, Down to 25 W. But the freezer has cooled Down once after failure to a sufficiently low temp -22. Deg C. But testing after that it seems to stick at 25w, but only cool to about 0 deg.

Admin

11/5/2021 10:29:35 am

Hi

Yes , the interface pcb is now in control of most fridge designs taking the place for temperature, defrost, fan and compressor switching functions. Its a good thing too as most wont start up the compressor straight from a sudden power loss and that saves compressors.

Penito17 link

7/29/2021 03:12:18 am

The PTC shown on the picture above can it be installed without overload relay

admin

11/5/2021 10:30:56 am

Hi

Yes , the one shown has an internal compressor overload and others use external. Most PTC are only starters not overload combinations.

Penito17 link

7/29/2021 03:16:51 am

The PTC shown is installed without overload, my is that will the compressor run normally when supply is given?

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PTC relay compressor, the simplest of design

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