Electrical Everybody wants to know how to disable auto shutoff on a heating pad

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I am working on used device works on low not the three higher levels. I drew out circuit so I could see how it works.
The ground goes to three of the four wires to the pad fourth wire is ac positive which goes thru some thing like scr comes in on a1 and goes out on a2 and controlled by gate what I am wondering is if you use jumper wire from a1 to a2 this will bypass the IC and associated parts then positive goes to pad and switch sends grounds to different wire on pad disabling the 2 hour auto off feature what I don'r know is if the ic could control how hot or how much current passes thru? Also the a1 to a2 must pass current or it would not work in low. Some say four wired units send signal back to controller but all four wires on mine are connected with different ohm's between wires.
 

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rude man

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Did you read the instructions for the pad? I've never seen a pad for which auto-off can't be bypassed.

I would not mess with the circuit. Tear the whole thing out & replace with a new 3-way mechanical switch (or 4-way incl. OFF). Auto-off is a stupid idea anyway.
 
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device works on low not the three higher levels
Two higher levels (medium and high).

It looks like Lo-Med-Hi selection is done with a slider mechanism pressing against conductive pads on the back side of the PC board (labeled 'SW1'). See how HT2, HT3, and HT4 all have traces going to the switch pads. It could be the reason medium and high aren't working is the slider isn't in good electrical contact with the board.

Sunbeam PCB Switch Side.jpg


Went looking for a board schematic, and found this article directly applicable to your question.

'Modding a Sunbeam Heat Pad'
http://jimlund.org/blog/?p=352

Can't make it out in your photo, but it is probably the same CD4060, 14 stage ripple counter chip, and its sole purpose appears to be switching TRIAC 'T1' to provide the auto-off time delay.
 
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I noticed they have a capacitor and resistor on series with the ac input and low wire this must be to let a little current pass to ic so it can turn on triac I do not know this for sure without testing a triac, but a triac without signal applied to gate must be in an open state. I will try to shim up circuit board to make switch make better contact marks on foil are more visible on low.
 

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I noticed they have a capacitor and resistor on series with the ac input and low wire this must be to let a little current pass to ic so it can turn on triac I do not know this for sure without testing a triac, but a triac without signal applied to gate must be in an open state. I will try to shim up circuit board to make switch make better contact marks on foil are more visible on low.
With no gate signal the TRIAC will be effectively off (but even when off, there's always a small amount of leakage current through it, a few milliamps at most).

What are the heater resistances?
HT1 to HT2?
HT3 to HT4?
Is there measurable resistance between HT1 to HT3? HT1 to HT4?

What is the heating pad's model number?

I was looking at one of the cheaper Sunbeams with what appears to be the same power controller, but they don't break down the wattages for each heat setting, specifying only a maximum power of 50 watts. My suspicion is HT1 and HT2 is a discrete heating element, and HT3 and HT4 is another. They may be different wattage ratings from one another, and when set to 'lo' the lower wattage element is selected, 'medium' selects the higher wattage element, and 'hi' selects both.

However, diode 'D2' is in series with the HT3 line, which means it will operate at one-half current (only 1/2 wave AC will flow) so it could be, and is perhaps more likely, that the heater elements are rated identically, and this is how they implemented the 'lo' heat setting.
 
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I noticed they have a capacitor and resistor on series with the ac input and low wire this must be to let a little current pass to ic so it can turn on triac
Probably the other way around. It looks like a resistor/capacitor (RC) snubber across the TRIAC, and would serve a dual purpose of reducing electrical noise caused by TRIAC switching, and also reduce the possibility of the TRIAC turning on without a gate signal applied. The latter can occur if the 120V supply the heating pad is plugged into has other electrically noisy equipment producing spikes that go higher than the TRIAC's "dv/dt" rating.

ST Microelectronics has a good app note on the subject.
https://www.st.com/content/ccc/resource/technical/document/application_note/38/88/44/b8/2c/26/44/b8/CD00004096.pdf/files/CD00004096.pdf/jcr:content/translations/en.CD00004096.pdf
 
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Not quite what you asked, but I fitted 'notch select' time switches in line with our two auto-off 'pet-pads', thus resetting them after a quarter hour down-time. As they were both under our kittens' mega-bed, it meant at least one was usually on...
 
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Mine says EA SLA104 and it looks like it almost draws 50 watts in the lowest setting which is enough for what I want anyway.
 
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ht1 to ht2 = 290 ohms
ht1 to ht4 = 305 ohms
ht2 to ht4 = 15 ohms
ht3 to ht4 = 25 ohms

seems normal
 

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The unit is actually drawing over 40 watts in any of the four positions and I never owned it when it was working so I do not know how hot would be in high as when switch is in low this would draw the minimum amount not the maximum amount of current. One would think proper functioning unit would be 10,20,30,40 watts as it is no change in amount of current draw when changing heat positions.
 
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The unit is actually drawing over 40 watts in any of the four positions and I never owned it when it was working so I do not know how hot would be in high as when switch is in low this would draw the minimum amount not the maximum amount of current. One would think proper functioning unit would be 10,20,30,40 watts as it is no change in amount of current draw when changing heat positions.
Does this SLA104 controller have three, or four heat settings?

Current demand ought to change in relation to switch position. If it doesn't, then switch 'SW1' and/or heater elements aren't functioning correctly.

Resistances across the heater connections are throwing me.
HT1/HT2 = 290Ω. HT1/HT4 = 305Ω. Each of these produce close to 50W if 120V is placed across them.

HT3/HT4 = 25Ω and HT2/HT4 = 15Ω. These points cannot be connected across 120V. Doing so would generate 576 and 960 watts, respectively. Normally I'd isolate the heater pad from the circuit board by disconnecting the wires, and testing the heater pad alone, but in this case it look like the ring terminals are riveted and soldered to the PCB, and this would be impractical.

Other observations, trying to wrap my head around this thing.
'AC1' is connected to the rest of the circuit through a particularly narrow PCB trace labeled 'F1', which I'm guessing is how they fused this device.

'AC1' appears to be connected to the junction of 'C5' and 'D4'. These components are part of a low current AC/DC converter used to supply the IC timer and provide gate voltage.

TRIAC 'T1' connects 'AC1' to 'HT1' when it is gated on.

'AC2' is connected to the two longer pads in switch 'SW1'.

From right to left on 'SW1'.
Position 4 - Both pads are connected to 'HT4'.
Position 3 - One pad isn't connected. The other pad is tied to 'HT3', and the AC/DC converter circuit.
Position 2 - One pad is also tied to 'HT3' and the AC/DC converter. The other pad goes to the anode of diode 'D2'.
Position 1 - One pad isn't connected. The other pad is connected to 'HT2', and the cathode of diode 'D2'.

Can't say without seeing what the crossbars look like in the slider mechanism, but it seems to me the controller will be off unless the 'dual pad' connected to 'HT3' and the other side of the AC/DC converter circuit is connected to 'AC2'. No DC supply; nothing to gate on TRIAC 'T1'.

In position 2, 'HT2' is connected to 'AC2' through diode 'D2'.
In position 1, 'HT2' is connected to 'AC2' directly.
 
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If where highest current from controller to pad goes is shorted to all other positions this seems only way to get the same current draw in all positions. You know slider bar is working as it draws same amount current in all positions and gate must be closing triac as it works in all positions so this only leaves pad. HT4 to HT1 is highest ohms at 305 when switch is in low seems right. HT2 to HT3 is 11 ohms and HT2 to HT4 is 15 ohms. Switch sends it to HT4 and HT4 to HT1 is 305 ohms highest resistance makes lowest heat seems right, Switch sends it to HT2 and HT2 to HT1 is 290 ohms this seems almost the same between high and low this don't seem right. HT1 to HT3 is 300 ohms this makes only 5 to 10 ohms resistance drop over three settings is this right? Maybe unit is working right 5 ohm resistance drop for each setting is change that you would not even notice though,
 

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In position 2, 'HT2' is connected to 'AC2' through diode 'D2'.
In position 1, 'HT2' is connected to 'AC2' directly.
Position 1 = Highest
Position 2 = High seems diode reduce voltage to give less heat
IC and Triac get power thru resistor connected to HT3

I drew it out and this is the way it is connected and the ohm readings.
Does not seem to be any thing wrong unless I can't see it.
 

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If where highest current from controller to pad goes is shorted to all other positions this seems only way to get the same current draw in all positions. You know slider bar is working as it draws same amount current in all positions and gate must be closing triac as it works in all positions so this only leaves pad. HT4 to HT1 is highest ohms at 305 when switch is in low seems right. HT2 to HT3 is 11 ohms and HT2 to HT4 is 15 ohms. Switch sends it to HT4 and HT4 to HT1 is 305 ohms highest resistance makes lowest heat seems right, Switch sends it to HT2 and HT2 to HT1 is 290 ohms this seems almost the same between high and low this don't seem right. HT1 to HT3 is 300 ohms this makes only 5 to 10 ohms resistance drop over three settings is this right? Maybe unit is working right 5 ohm resistance drop for each setting is change that you would not even notice though,
You know slider bar is working as it draws same amount current in all positions
But it shouldn't. AC current must change, else power isn't changing. Power = Volts * Amps.

only 5 to 10 ohms resistance drop over three settings is this right? Maybe unit is working right 5 ohm resistance drop for each setting is change that you would not even notice though,
This is why I wish they hadn't permanently attached heater pad wires to the board. Normally, the thing to do is isolate heater elements from the power source, and measure their resistances separately. Otherwise, it isn't possible to tell whether those lower resistance measurements are due to components on the circuit board , or a problem in the heater pad.

Your resistance readings could suggest (3) identical elements (290, 300, and 305 ohms are within 5% of one another) using HT1 as a common point, but this would make sense only if the crossbar was progressive (that is, only HT4 was turned on when slid all the way to that side, then adding the other heaters in order - HT3, then adding HT2 (through D2, at 1/2 current), then finally HT2 at full power) as it was slid to the other side.

Instead, the pictured crossbar acts as a 2 pole, 4 position switch, and connects either HT4, OR HT3, OR HT2 (through D2), OR HT2 to the 'AC2' return. I don't know what to make of this, but it makes me wonder about the heater elements.

IC and Triac get power thru resistor connected to HT3
I missed it before (and can't follow the circuit path from the photo), but resistor R1 is connected to AC1 at the TRIAC 'A2' lead. Somehow, this must be routed to the AC/DC converter. That large resistor (can't clearly read the silk-screened label, but it looks like 'R9', rated 39 ohms), and many other nearby parts (C8, R12, D1, D4, C7) all appear to be in the AC/DC converter.

DC power for the CD4060 IC chip 'U1' must be between 5 to 15 volts DC. The sole purpose of 'U1' is timing the auto shut-off function. Pin 9 (connected to C2) and pin 8 (connected to the series pair R7 and R6) make up the timebase oscillator. These R and C component values in combination determine overall time range. The oscillator is fed into pin 11 (clock input). Vdd (+ supply) is pin 16, Vss (supply common) is pin 8, reset is pin 12, and the rest of them are outputs. They are using output Q14 (pin 3) to drive the TRIAC gate low at the end of timeout.

ST datasheet - https://www.st.com/resource/en/datasheet/m74hc4060.pdf
While closing open PDF files, noticed the ST datasheet is for a lower voltage part. Although harder to read (looks like it was scanned in from a paper manual) the TI datasheet is more applicable.
http://www.ti.com/lit/ds/symlink/cd4060b.pdf

Does the LED light up when the heating pad is turned on?
 
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Your resistance readings could suggest (3) identical elements (290, 300, and 305 ohms are within 5% of one another) using HT1 as a common point, but this would make sense only if the crossbar was progressive (that is, only HT4 was turned on when slid all the way to that side, then adding the other heaters in order - HT3, then adding HT2 (through D2, at 1/2 current), then finally HT2 at full power) as it was slid to the other side.

Instead, the pictured crossbar acts as a 2 pole, 4 position switch, and connects either HT4, OR HT3, OR HT2 (through D2), OR HT2 to the 'AC2' return. I don't know what to make of this, but it makes me wonder about the heater elements.
When one looks at the readings it looks like all the windings in the pad are always used regardless off which position switch is in and following ohms they are all similar that is why you get same heat and current draw. I had a different pad that had only 3 wires to pad and that was hooked similar although it had two little devices inside between heating wires but I don't know if they where to act as a thermostat when get hot resistance goes up or they where to be used as just a safety switch. I am going to leave it as it as I do not need any thing hotter but would like cooler . Path from ac2 to ht4 is straight thru if one cuts trace on board and inserts common diode of 2 amp rating will this not lower heat and power draw?
 
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If switch position doesn't influence current demand then short circuits between what ought to be discrete elements within the pad must be occurring. I wouldn't recommend using it in any role that places it in contact with your body (if insulation is breaking down between elements, chances are sections of bare conductor already exist, and pose a shock hazard). Also, if heater wire insulation is already breaking down this process will continue, and eventually the pad will short out in a more spectacular fashion (sparks, fire ... that sort of thing).

Path from ac2 to ht4 is straight thru if one cuts trace on board and inserts common diode of 2 amp rating will this not lower heat and power draw?
Yes. Current will flow from AC2 to HT4 during only 1/2 of the AC cycle.
 

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