Heating pad needs help.
KAZ (Dunlap) model 7788-tt
This pad stopped working. I took the control cover open but didn’t see any problems, so I opened the pad itself. I found a hair sized insulated wire that had broken on the low-level setting of the circuit. In the pad, it is connected to a short piece of resistive wire that makes a loop around one of the thermal switches and then connects to the other lead that delivers the High and Medium setting.
Trying to get a good grasp of how this works, I studied it and have drawn up a circuit diagram of the electrical section but not the electronics that run the timer and apparently also has something to do with the low setting.
I could find no info on this pad nor any of its electrical components using every method of searching I know of. I did find others also looking for info, but they also did not find it.
I made a lot of resistive measurements and then put everything back together and tested it for voltage and power. Miraculously it started to semi work again, but I had made no changes in the controller.
On High, it measures 47 watts, medium 23 watts and low 45 watts. This low setting is not what I expected. After further checking, there is a connection from the low lead to the timing circuit. This apparently effects the power on this lead, probably by shutting down the Q1 transistor partially, causing the Neutral connection to break the loop of the circuit by not bieng turned on all the time. This is my best guess. But it just isn’t doing it and the low setting is almost the same as the high setting.
The medium setting goes through a diode cutting one side of the sine wave out and giving only ½ power.
The high setting is directly connects the AC with the heating pad, no other connections in the circuit.
All the settings use Q1 to supply the natural to the circuit.
This Q1 transistor stops the pad from working if power isn’t applied to the timing circuit. (Testing with jumpers proved this).
So my question is, How does this work? Are the thermal switches just for safety? They look like thermal cut out switches in motors. Their markings didn’t show up on the searches.
I want to make this work again, even if I just cut out the low setting and use the other two. I would like to bypass the timing altogether since I need this to run for much longer than 2 hours.
Is there a resistor that can be added to the low setting to take it down to 10 or 15 watts and still be safe and not over heat the controller or pad? Maybe a half wave signal through a diode and a resistor?
If I can find a way, I’ll attach the drawing.
Thanks for your thoughts, there appreciated.
Tuneup
KAZ (Dunlap) model 7788-tt
This pad stopped working. I took the control cover open but didn’t see any problems, so I opened the pad itself. I found a hair sized insulated wire that had broken on the low-level setting of the circuit. In the pad, it is connected to a short piece of resistive wire that makes a loop around one of the thermal switches and then connects to the other lead that delivers the High and Medium setting.
Trying to get a good grasp of how this works, I studied it and have drawn up a circuit diagram of the electrical section but not the electronics that run the timer and apparently also has something to do with the low setting.
I could find no info on this pad nor any of its electrical components using every method of searching I know of. I did find others also looking for info, but they also did not find it.
I made a lot of resistive measurements and then put everything back together and tested it for voltage and power. Miraculously it started to semi work again, but I had made no changes in the controller.
On High, it measures 47 watts, medium 23 watts and low 45 watts. This low setting is not what I expected. After further checking, there is a connection from the low lead to the timing circuit. This apparently effects the power on this lead, probably by shutting down the Q1 transistor partially, causing the Neutral connection to break the loop of the circuit by not bieng turned on all the time. This is my best guess. But it just isn’t doing it and the low setting is almost the same as the high setting.
The medium setting goes through a diode cutting one side of the sine wave out and giving only ½ power.
The high setting is directly connects the AC with the heating pad, no other connections in the circuit.
All the settings use Q1 to supply the natural to the circuit.
This Q1 transistor stops the pad from working if power isn’t applied to the timing circuit. (Testing with jumpers proved this).
So my question is, How does this work? Are the thermal switches just for safety? They look like thermal cut out switches in motors. Their markings didn’t show up on the searches.
I want to make this work again, even if I just cut out the low setting and use the other two. I would like to bypass the timing altogether since I need this to run for much longer than 2 hours.
Is there a resistor that can be added to the low setting to take it down to 10 or 15 watts and still be safe and not over heat the controller or pad? Maybe a half wave signal through a diode and a resistor?
If I can find a way, I’ll attach the drawing.
Thanks for your thoughts, there appreciated.
Tuneup
