Modding a Sunbeam heating pad

I bought a Sunbeam heating pad. It has low/med/high settings, and turns off after two hours.
heating pad

The back of the controller says ‘Sunbeam SLA103′.

I wanted to eliminate the shutoff, I’m using it to heat my fermentation bucket.

I found good information in this instructable, Disabling Auto-Off on a Sunbeam Health at Home heating pad.

I opened the controller up, and the circuit board was similar, but some of the components had moved, and the microcontroller was different.

circuit board

Microcontroller is a NXP Semiconductors HEF4060BF. I figured this is similar to the CD4060, and found this datasheet, 74HC4060D Philips/NXP.

microcontroller
closeup

Looking at the circuit board, there is a capacitor (C3, 480nF) between pins 12 and 16 which makes sense if pin 12 is the master reset. Pin 3, Q13, is connected to the triac (T1). Pins 9 and 19 have a capacitor (C2) and resistor (R7), and this likely makes up the RC timing circuit. So this chip is a 4060.

As you can see on the circuit board, I removed C3. I replaced it with a 10K ohm resistor, and now the heating pad stays on indefinitely.

8 Responses

  1. Peter says:

    Thanks for the info!

    I’ve got the Microchip IC (PIC16F506) more like the one in the instruct-able. I was able to disable the timer by disconnecting pin 2, but I noticed that there was a resistor from pin 2 to pin 3. I was wondering if there is a way to change a few resistor values to modify the length of time that the auto shut-off feature works. My model is a 3 hour timer, but I would like to make it a 10 hour (or something like this) as opposed to just disabling the timer. Do you have any ideas for that? Here’s the datasheet.

    http://www.mouser.com/ProductDetail/Microchip-Technology/PIC16F506-I-P/?qs=WKvCbX%2Fl3UzgnY%252bYIwr6dg%3D%3D

  2. jiml says:

    Well Peter, if your heating pad has a PIC16F506 in place of the counter IC, it is just luck that pin 2 worked! The basic circuit seems to be a triac that sends power to the heating pad. The triac is on until the counter/timer circuit sends a signal to shut off.

    The PIC16F506 has its clock input (CLK) on pin 2, so lifting pin 2 keeps the PIC from running. The circuit on my heating pad has the clock feeding the counter made from an RC oscillator–a resistor + capacitor. Changing the value of either of these components will change the clock frequency and thus the time the timer goes off.

    Replacing the resistor or capacitor with a higher resistance/capacitance component will lower the frequency of the clock and make it take longer to shut off.

    • Peter2 says:

      I have the same controller as Peter and instead of lifting PIN 2, I removed the resistor (10K) between pin 2 and 3, just because it was easier than lifting the pin. I know almost nothing about PIC and only a little about circuitry, but it would seem that pin 2 and 3 can be configured as standard IO ports, while Pin 3 can be configured as clock out. Based on that and the fact that the PIC seems to be running despite having the pin 2 disconnected – I can change heating modes LEDs with a push of a button and the LEDs are PIC driven, I am guessing that for some reason the PIC is programmed to output clock on PIN 3 and then read it on PIN 2 to use that as source for a counter (maybe?). But as I said these are only my conjectures based on my limited knowledge, or lack there of. So take it with a grain of salt. Bottom line is: the blanket heats with no time limit if the resistor between pin 2 and 3 is removed.

  3. Julie says:

    Thanks admin. I have the same board and controller chip you have. I’m going to use your 10Kohm resistor fix, but when I went to purchase it, they asked what wattage? I bought both 1/4 W and 1/2 W but will try with the 1/4 first as they didn’t have surface mount components, so this is going to be a squeeze to get it in. I hope 1/4 W will work. The half watt is a bigger resistor. He told me if the wattage is too low it will smoke.

    • admin says:

      The 1/4 watt is fine. This is a timing circuit, very low current (likely 5-10mA, 5V, so 0.025-0.05 watts), so any resistor would work.

  4. Marc Epstein says:

    Hi! I just read your article on modding your elec. heating pad:

    http://jimlund.org/blog/?p=352

    Peter on there described his circuit.. which also is the one I have now.
    Could you please tell me how to disable my PIC16F506 chip? Should I physically disconnect the pin from the board? Wouldn’t it be easier to just cut the runner from Pin 2 to the surface mount resistor connecting pin 2 to pin 3? [The resistor normally connects between Pin 2 and Pin 3 of the chip]..?
    Any help would be so appreciated! Thank you… marc

  5. Cat G says:

    Can anyone help me figure out to disable the faulty auto shut off on mine? I know nothing about circuits or rewiring. But My sunbeam heating pad fairly new, will not tun on. It seems to be stuck with auto shut off light flashing. used to be able to unplug and hold the on switch while u plugged it back in, and it would trick it into working. But that does not work anymore. Is there anything i can remove so the damn thing will turn back on? I swear they make these fail like this so you are forced to keep buying new. the product number i believe is
    CA T94E E12107..I wish i could post a picture.

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