120VAC Heating Pad changes temperature while voltage stays same?

Thread Starter

BarryTron

Joined Nov 18, 2018
76
Hi everyone, for about a year I have become interested in electronics more than usual. I take things apart and try to understand how things work. Couple of days ago I took a heating pad apart and tried to understand how it ticks, to my surprise some of the voltmeter measurements did not make any sense.

The input voltage to the pad is 120VAC, the output to the pad is 170VAC. What is confusing is that when I change the temperature of the pad the output voltage stays at 170VAC. My question is how is it possible that the pad changes temperature without voltage change?

I took a picture of the board, let me know if you need me to measure anything else. Thank you in advance.

pic.jpg
 

crutschow

Joined Mar 14, 2008
24,946
It's likely because you meter is an average responding type that is not measuring the true RMS voltage.
What type of meter is it?
 

wayneh

Joined Sep 9, 2010
16,390
Your meter expects a sine wave and gives a reasonable estimate of the time-average voltage when it sees one. For other wave shapes, it may or may not be accurate. The controller is probably chopping the signal and so the heater spends more or less time “off” or on during each cycle. The meter cannot show you that.
 

hexreader

Joined Apr 16, 2011
404
Maybe the circuitry in the photo gives a constant voltage output, and it is the current that varies. I would go with the earlier replies as more likely though.

Is there any identification mark on T1? If we knew what device T1 was, we might guess at circuit function.

EDIT: That is a really nice third picture, by the way. Impressive. I reckon the 14-pin chip is a microprocessor, but can't work out what kind. Looks like +5V is on pin 4, and common (AC neutral) on pin 11. That should give a clue, but not found a match yet.
 
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Thread Starter

BarryTron

Joined Nov 18, 2018
76
It's likely because you meter is an average responding type that is not measuring the true RMS voltage.
What type of meter is it?
The meters that i am using are Innova 3320 and Uni-T.
I have an old Tektronix 475 oscilloscope that i use for DC projects and low voltage AC. I am not sure if i can use it with 120VAC. Can you recommend a low budget meter that i can use to see the RMS voltage.
 

AlbertHall

Joined Jun 4, 2014
9,831
One way of measuring RMS voltage is to connect it to a resistor and measure the temperature rise of the resistor. So the temperature of the heating pad is giving you some indication of the RMS voltage.

RF power meters used to work this way - I don't know whether they still do.
 

Thread Starter

BarryTron

Joined Nov 18, 2018
76
Maybe the circuitry in the photo gives a constant voltage output, and it is the current that varies. I would go with the earlier replies as more likely though.

Is there any identification mark on T1? If we knew what device T1 was, we might guess at circuit function.

EDIT: That is a really nice third picture, by the way. Impressive. I reckon the 14-pin chip is a microprocessor, but can't work out what kind. Looks like +5V is on pin 4, and common (AC neutral) on pin 11. That should give a clue, but not found a match yet.
Thank you very much Hexreader. I took your advice and measured the current and to my surprise the A=0.370, same as voltage it did not change when switching temperature of the pad.

The T1 is a BT131 / J40487

I am adding another picture with labeled parts to answer some of your questions: I took some measurements across with my voltmeter (reference to picture please). No change in voltage was observed when the temperature of the pad was change:

A-B = 120 VAC

A-C = 117 VAC (in the original post I typed 170VAC, that was obviously wrong)

A-D = 117 VAC

D-B = 0.05 VAC

F-E or E-K = 1.4 VAC

C-B = 3.9 VAC

pic2.jpg
 

crutschow

Joined Mar 14, 2008
24,946
I have an old Tektronix 475 oscilloscope that i use for DC projects and low voltage AC. I am not sure if i can use it with 120VAC.
No, you do not want to connect an oscilloscope to measure line voltages because it can create a short circuit through the oscilloscope safety ground.
Can you recommend a low budget meter that i can use to see the RMS voltage.
Don't know of any particular brand, but look for one that states it measures "true RMS".
 

hexreader

Joined Apr 16, 2011
404
Advising on a circuit connected directly to mains makes me very nervous. I think I had better stop due to safety concerns.

... not that I had much more to add anyway.

I advise that you stop messing with mains circuits, and change to experimenting with low voltage circuits.

If you really enjoy the risk of death, then my last hint is that I would expect voltage B to D to be somewhere around 5 Volts DC. Measuring DC voltages with multi-meter on AC range is bound to give confusing results.
 

MisterBill2

Joined Jan 23, 2018
6,037
My guess is that the controller varies the duty cycle, since I don't see anything that looks like a heat sink. It is also probable that there is a bit more electronics inside the blanket used with this controller.
 

Sensacell

Joined Jun 19, 2012
2,567
The device "T1" is likely a Triac.

The power to the heater pad is most likely controlled by phase angle chopping, or bursts of 1/2 sine cycles.
The current is low, a small triac could easily drive it without generating much heat, it's either conducting fully, or off.

Your meter readings are going to be way off, the complex wave forms of voltage and current cannot be accurately measured with a simple meter.
 

ebeowulf17

Joined Aug 12, 2014
3,274
Maybe the circuitry in the photo gives a constant voltage output, and it is the current that varies.
You can't provide varying current without also varying either the voltage or the load. It's a pretty safe bet that a resistive heating element has a more or less fixed resistance, so, per ohms law, the only way to change current is to also change voltage.
 

Thread Starter

BarryTron

Joined Nov 18, 2018
76
My guess is that the controller varies the duty cycle, since I don't see anything that looks like a heat sink. It is also probable that there is a bit more electronics inside the blanket used with this controller.
If I have to guess I would think the pad is just a resistant wire with some fuse for safety. On the heating pad it’s written: "60Hz 50W 120VAC ONLY". And as you can see that only two wires go from the controller to the pad. This is what makes this very confusing for me. Usually when I take things apart I can understand how things work especially with mechanical components.
 

Thread Starter

BarryTron

Joined Nov 18, 2018
76
The device "T1" is likely a Triac.

The power to the heater pad is most likely controlled by phase angle chopping, or bursts of 1/2 sine cycles.
The current is low, a small triac could easily drive it without generating much heat, it's either conducting fully, or off.

Your meter readings are going to be way off, the complex wave forms of voltage and current cannot be accurately measured with a simple meter.
So how would one go about in order to view what actually is going into the pad. I would really love to have a device that would show me visually what is taking place. Do you know of a device that will let me view the sine cycles?

The T1 is a BT131.
 
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Thread Starter

BarryTron

Joined Nov 18, 2018
76
You can't provide varying current without also varying either the voltage or the load. It's a pretty safe bet that a resistive heating element has a more or less fixed resistance, so, per ohms law, the only way to change current is to also change voltage.
That is why I am posting this, because the meter reading contradicts my expectation and ohms law. I posted the board hopping that someone could provide an explanation to the contradiction. I have used 3 different meters and I get same result.
 

Sensacell

Joined Jun 19, 2012
2,567
You need an oscilloscope and an isolation transformer.
You cannot connect a scope to this mains-powered circuit without an isolation transformer.

The function would be abundantly clear once you looked at the wave forms.
 

Thread Starter

BarryTron

Joined Nov 18, 2018
76
You need an oscilloscope and an isolation transformer.
You cannot connect a scope to this mains-powered circuit without an isolation transformer.

The function would be abundantly clear once you looked at the wave forms.
Thank you Snsacell, i was not sure if i could use my Tektonix 475 for high AC application, I will have to look more into the Tektonix documentation and into the isolation transformer (maybe even build the transformer as a project).
 

MisterBill2

Joined Jan 23, 2018
6,037
You can't provide varying current without also varying either the voltage or the load. It's a pretty safe bet that a resistive heating element has a more or less fixed resistance, so, per ohms law, the only way to change current is to also change voltage.
It is certainly possible to provide varying AVERAGE power by any of a number of methods, including cycle counting switching, waveform chopping, as well as just plain off-on controlling. The heater provides an integration function so that in any minute the power can be from zero to the maximum. The control logic can get a bit complex but the hardware can be quite simple. So it is quite easy to control the heat in a slow responding heater system without varying current or voltage, except by on/off, which woukld not be apparent on many meter types.
 

AlbertHall

Joined Jun 4, 2014
9,831
As a simple average power indicator you could connect a neon lamp in series with a 120k resistor across the heating pad. This will be a lot cheaper than than the 'scope/transformer combo.
 
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