PTC heater mistery

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ArtemRu
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Posted by ArtemRu: Tue. Apr 9, 2019 - 12:45 PM
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I found one more thing that i cannot understand.

 

I have a ceramic heater. Presumably  it is based on PTC.

As we know, PTC increases resistance when temperature rises. I have read many datasheet on PTC, app notes and some theoretical and experimental articles.

They do not state anything special about PTC. Just that that R does drop rise right away. It actually goes down up to Curie  temperature and the rise very rapidly.

 

The drop in R from datasheet is about 3-5 times from 25C to 130C

Alright. Thta's clear.

 

Now, i disassemble a simple PTC heater. It is a 500W heater. It have two ceramic modules in paralle.

In cold state R is about 360 Ohm.

Then i heat it and watch power consumption on wattmeter. It goes up to almost 700W and then drops below 100W.

Good, looks like PTC behavior.

 

Then i measure the resistance and i see 40 Ohm!

 

Here is video (it is in russian, but there is no point in listening to me rating, just watch the numbers on instruments)

 

https://www.youtube.com/watch?v=...

 

So, temperature is crazy high, but resistance dropped almost 10 times.

But i am lost here. If resistance is 40 ohm per section, then current should be around 5A per section, 10A total, 2 KW total.

But wattmeter shows less than 100 W. HOW?????

 

Then i did an experiment. I used another heater fan to heat  the heating element and measured resistance while heating it.

The resistance dropped to 60 Ohm. Well, maybe i cannot reach the breaking point, but when it was selfheating it should have reached it.

 

I am totally lost.

 

Some people report and interesting finding about NTC resistors. If they a heated at the edge the resistance actually goes up, instead of going down.

Maybe the same effect is present here. Maybe power gradient is important.

But i did not find anything about this in anything i have read about PTC and NTC.

 

Any idea?

 

 

 

 

turn it on without fan and protection cicuit an

 

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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Learning and Information, General Electronics
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ki0bk
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Posted by ki0bk: Tue. Apr 9, 2019 - 01:30 PM
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From earlier discussions, I'll assume your using AC to power your heater element(s).

So although the DC resistance may go down, I would take from the evidence the AC reactance goes up with temperature....  

I once worked on a "high speed" pizza oven project, but did not worry about such properties such as this at the time, as the customer just wanted to get the pizza baked fast, without burning it!

 

Jim

 

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ArtemRu
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Posted by ArtemRu: Tue. Apr 9, 2019 - 01:47 PM
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Yes, as you can see from the video i plug it into an outlet. 220v.

 

Well, your comment implies that PTC has different properties for AC and DC current. However this

https://pdfs.semanticscholar.org...

does not confirm such idea and many other datasheets never mention different  behavior for ac and dc.

 

 

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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wagnerlip
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Posted by wagnerlip: Tue. Apr 9, 2019 - 02:54 PM
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ArtemRu, you do realize about hot metals generate milivolts of electricity on their extremities, right?

You are measuring the hot heater resistance with a regular multi-meter in Ohms range, right?

You do realize that the multi-meter for low ohms range use to measure milivolts as Vdrop over the resistance, right?

Think.  You never reverted the ohmsmeter probes polarity over the measurement... 

 

Instead to measure Ohms, apply any power supply DC voltage you may have on the bench, lets say 20Vdc to the "hot" heater and measure current.

If, the heater is really 40Ω you will have half Ampere of current.   Ohms multi-meter range  is to be used in a very controlled and steady bench environment, always remembering the principles of their workings, and considering the environment will not interfere with them.

 

Another issue is regarding the heater thermostat, the ramp down power on the power meter is not just a natural decay with seconds (average) of a power consumed that is actually open?   The way you are doing the tests (as shown in the video) is not very conclusive and practical.   You need to have a voltmeter showing the voltage over the heater all the time, alligator clips would make the measurement more satisfying, your drop of probes make me mad, not willing to keep watching.

 

I have one "good brand" multi-meter that I can NOT use it to measure the DC voltage of any switching power supply primary capacitor, around +380Vdc, this meter shows a fluctuating 700Vdc or more over that 450V capacitor... the reason is the electromagnetic interference generated by the high frequency oscillator of the power supply, makes the meter crazy.

 

Now, when you say PTC, you are saying ALL the heaters resistance materials, a simple tungsten incandescente lamp filament is to be considered a PTC.

 

Wagner Lipnharski
Orlando Florida USA

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avrcandies
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Posted by avrcandies: Tue. Apr 9, 2019 - 03:06 PM
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Don't measure the resistance...measure the current...you know the line voltage, so you can calculate the resistance while it is operating.  That way, you get the true reading.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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ArtemRu
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Posted by ArtemRu: Tue. Apr 9, 2019 - 03:37 PM (Reply to #4)
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As usual, you comments are highly valuable. :)

 

So, i really did not think that the ceramic ptc generate voltage when heated. My bad.

But i did reverse the polarity actually (not in video) and it is the same value.

So, i don't think it generates anything at all when heated. Or something funny is going on.

 

I happen to have a lab power supply :)  I pushed 14V to it and got myself 0.1A. Which gives us 140 Ohm. It was hot anyway.

Then i measured the the resistance with multimeter again, having the lab power supply wires connected (but supply is cut off) and now one way i got 20 ohm and reversed i got 140 Ohm!

Bigo! But if i disconect wires i read 200 Ohm any direction. This world is broken, next, please :)

 

Here is video ( i even tried to speak INGRISH )

https://www.youtube.com/watch?v=...

 

Video i my second measurement and this time measured R did not match U/I. But well, the number are too low, maybe it is just errors.

But question is: why measuring works when lab power supply connected? (i repeat, lab power supply output is turned off when i measure with DMM).

 

I did not get it about measuring voltage over the heater. The voltage over the heater is always 220V. It is directly connected to 220V outlet.

Don't worry about probes drop. I use them only when everything is disconnected. I have a small son and hope to see his children :)

 

As for filament as PTC. Well, i refer to PTC as described in wikipedia (sorry to mention it in  a good society): BaTiO3 infused ceramic.

 

Another issue is regarding the heater thermostat, the ramp down power on the power meter is not just a natural decay with seconds (average) of a power consumed that is actually open? 

 

What thermostat do you mean? There is no thermostat as a separate device here. And not, it is not natural decay. If i open the cicruit the power meter goes to zero under 1 second.

 

Artem Kuchin.
Electronics hobbyist. Born in 1976.

Last Edited: Tue. Apr 9, 2019 - 04:24 PM
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avrcandies
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Posted by avrcandies: Tue. Apr 9, 2019 - 04:17 PM
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 The voltage over the heater is always 220V. 

If you want the highest accuracy, measure the voltage right at the heater...any wiring could drop the voltage slightly.  However, this is probably less than 1%.  If you measure the current, then you know R=V/I (and you know I).

 

You might try putting the ptc in an oven & let it soak for an hour & measure the temperature....then you can have fun measuring any generated voltage.   Note the any self-generated voltage will  completely screw up any ohm meter readings.

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. Apr 9, 2019 - 04:21 PM
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wagnerlip
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Posted by wagnerlip: Tue. Apr 9, 2019 - 04:27 PM
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ArtemRu wrote:
I did not get it about measuring voltage over the heater. The voltage over the heater is always 220V. It is directly connected to 220V outlet.

 

I believe I saw a thermostat along with the thermal fuse, in series with the heating element, isn't it?

Measuring voltage right over the heating element is just to make sure the thermostat is not open and you think it still flowing current...

Wagner Lipnharski
Orlando Florida USA

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ArtemRu
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Posted by ArtemRu: Tue. Apr 9, 2019 - 07:55 PM (Reply to #8)
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There is no thermostat. Why should it be? PTC (ceramic one) is self regulating and never overheats by itself under normal condition. The little pcb is just DOUBLE protection.  There is a bimetal thermostat on that pcb and thermal fuse. However, i monitored the thermostat and it never opens if fan is blowing on the heater. Maybe it will when the air is hot. But still it is just a precaution, not a regulating thermostat. Anyway, that pcb was far far away when i  was doing my tests.

Artem Kuchin.
Electronics hobbyist. Born in 1976.

Last Edited: Tue. Apr 9, 2019 - 07:57 PM
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Who-me
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Posted by Who-me: Tue. Apr 9, 2019 - 08:23 PM
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ArtemRu wrote:

There is no thermostat. Why should it be? PTC (ceramic one) is self regulating and never overheats by itself under normal condition. The little pcb is just DOUBLE protection.  There is a bimetal thermostat on that pcb and thermal fuse. However, i monitored the thermostat and it never opens if fan is blowing on the heater. Maybe it will when the air is hot. But still it is just a precaution, not a regulating thermostat. Anyway, that pcb was far far away when i  was doing my tests.

 

In testing this sort of heater, it seems it is working as expected ?

You are better to put an AC Current meter in series with the mains, and measure that (which may already be how you are determining watts )

Measuring DC resistance does not really matter to how this works, as it is not designed to run on DC.

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meslomp
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Posted by meslomp: Wed. Apr 10, 2019 - 05:24 AM
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Put an AC volt meter over the Ceramic element and an AC current meter in series with the ceramic element.

Now you have the voltage over and current through the element all you need to do now is divide and concur...... ehhh then you have the actual resistance of the element.

 

Perhaps you can even use a scope with a series resistor of about 1R to measure current and voltage and then let the scope do the math.

That could give you the resistance change over time.

have to be carefull then as you are working with 220V mains on the scope input and have to tripple check what point to take as ground/common

 

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avrcandies
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Posted by avrcandies: Wed. Apr 10, 2019 - 06:55 AM
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have to be carefull then as you are working with 220V mains on the scope input and have to tripple check what point to take as ground/common

The scope is prob not floating....so  you must ground your probe to the neutral...maybe add 100ohms in the gnd lead for safety (prevent the scope from going poof).  That would impact viewing any really high speed waves (MHz).

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Paulvdh
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Posted by Paulvdh: Wed. Apr 10, 2019 - 09:42 AM
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I'm getting the idea that either your DMM is broken or you're using it in the wrong way.

For example, there is no hope of measuring the resistance of your heater accurately as long as it is also connected to a power supply, even if that power supply is turned off.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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ArtemRu
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Posted by ArtemRu: Wed. Apr 10, 2019 - 12:54 PM
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Ohh, new comments are fantastic :)

Alrighty, thank you people, you are great!

 

New day, new brain.

I thought once again about what wagnerlip  said and  did a couple of tests.

 

A little PTC test:

https://www.youtube.com/watch?v=...

(read video description)

 

The only strange thing is that when heating the R measurement always drops either way. One way is slower, another way is faster.

Strange. But i think it is more related to internal workings of this DMM.

 

So, no way anyone cam measure resistance of PTC which is heating, cooling.

 

Cooling after being connected to from 220V:

 

https://www.youtube.com/watch?v=...

(read video description)

 

 

 

Artem Kuchin.
Electronics hobbyist. Born in 1976.

Last Edited: Wed. Apr 10, 2019 - 12:55 PM
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wagnerlip
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Posted by wagnerlip: Wed. Apr 10, 2019 - 05:47 PM
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ArtemRu wrote:

Ohh, new comments are fantastic :)

Alrighty, thank you people, you are great!

 

New day, new brain.

I thought once again about what wagnerlip  said and  did a couple of tests.

 

A little PTC test:

https://www.youtube.com/watch?v=...

(read video description)

The only strange thing is that when heating the R measurement always drops either way. One way is slower, another way is faster.

Strange. But i think it is more related to internal workings of this DMM.

So, no way anyone cam measure resistance of PTC which is heating, cooling.

Cooling after being connected to from 220V:

https://www.youtube.com/watch?v=...

(read video description)

 

Due to the thermo-electric effect of metals and conductors, it becomes complicated to use DMMs and OhmsMeter to measure low values of resistances.  For a better understanding of PTCs and NTCs the correct way is really measuring actual current with an AmperMeter, or as you already did, a power-meter, but you are not very sure as the power-meter works, if what it is showing is an average of the last seconds, or the real actual split second power information.

 

Another nice way to test it, is like you did before, use your 14Vdc power supply, measure current, then leave wires connected and put the whole ceramic heater inside your sandwich oven, heat it and using a temperature measurement (I would use a thermocouple if my multimeter allows it) and make a table for current vs temperature, then you can calculate resistance from that.

 

By the way, this effects you are experiencing with millivolts, you can learn a lot if you read about how thermocouples works, type K and J are the most popular, they are just two wires made of different metals, welded at the end and immersed into the temperature you want to measure, they develop few millivolts that represent the temperature. There are other tricks in this area, as ice block (ice point reference) to compensate for the temperature differences between what you are measuring and the temperature on the multimeter (the voltage generated over the wire is relative to both sides temperature difference) - today this compensation is done electronically by measuring the ambient temperature and make deductions.  That is in fact a good area of study and work. People make money working with those things. You can find more info also on "Peltier Junction".

Wagner Lipnharski
Orlando Florida USA

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avrcandies
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Posted by avrcandies: Wed. Apr 10, 2019 - 09:51 PM
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Are you actually have some issue or failure, or is everything actually working in a satisfactory manner? If so, what is the failure?

   

For example, my car engine may be running great, but I'm not going to tear it apart to look at the camshaft to see why it is running great.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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ArtemRu
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Posted by ArtemRu: Fri. Apr 19, 2019 - 07:40 PM
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No, no issues and no failue. I just needed to understand how it work to  build my own heater for a small greenbhouse. Iput the heater into a tube and fan air through it. The air flow is higher than from original fan and now i  get 600W from the heater,

 

Artem Kuchin.
Electronics hobbyist. Born in 1976.

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