Aftermarket Heated Seat Pads and OEM Switches.

I think you are over complicating this. We tested on the bench with a meter connected to the live and neutral that would feed the heating pad. With the thermistor connected to the switch and heated with a hair dryer it simply turned the heating circuit off. Then once the thermistor cooled it turned the heating circuit on again. There is no control, its simply on and off.
The resistance of the pad makes no difference as its not in the same circuit as the thermistor, you could connect a lamp instead of the pad and it would still work.

Ian
 
This is oversimplified. The pads used will dictate what NTC value you need to use, just like it will dictate whether it's series or parallel connection for seat base / seat back. The resistor is important as it will allow some form of current to flow even when the thermistor is open. The whole system is a simple PWM and I'm assuming that the roller reads the current that is being returned to it via the thermistor / resistor and then switches as appropriate.

Bret, you are misunderstanding how this works. The switch is controlled by the resistance of the thermistor, nothing to do with current. The thermistor changes resistance as it changes temperature.

Ian
 
As temperature increases, thermistor resistance goes down. If the resistance of the thermistor controls the heat, how does this work? It is not a simple on-off.

Attached a photo of the innards of a 4B0 963 563 switch.
 

Attachments

  • PSX_20201110_212045.jpg
    PSX_20201110_212045.jpg
    489.6 KB · Views: 187
I have no idea how the inners of the switch works. When I say it’s a simple on and off, I mean it either feeds 12v to the heating pad or 0v, it does not regulate in between, therefore the power to the heated pad is either on or off.
The resistance from the thermistor circuit must control something within the switch to turn the heated pad circuit on or off.
The power for the heated pad does not go through the thermistor, the thermistor circuit is just a few mA.

Ian
 
Would be Interesting to observe with an oscilloscope with which frequently the switch alternates the pad feed between 12V and 0V. It could be a slow switching like having a period time of 10seconds, but it could also be a pwm modulation of say 10Hz. A pity I dont have an oscilloscop, otherwise I would go out and check in one of my cars.
 
It's a PWM. The amount of time on vs off is dictated by the control circuit. The feedback loop for how much difference there is between on and off is provided by the thermistor and that dictates what the FET switches. That would be the big transistor on the back of the switch, the side you can't see. I looked at this for a couple of minutes earlier and it's a more complex version of what I was expecting.

Essentially, over a period of time. say 6 seconds - at level 1, 5 seconds would be off and 1 on, when the temperature that the circuit is set to achieve is reached. For level 6, it might be 6 seconds on, for level 3, 3 on, 3 off. This "temperature to be reached" is probably of the order of 40C if the newer Octavia VCDS threads are anything to go by (and they probably are). So the control board in the switch is turning the pad on or off relative to the thermistor resistance and the dialed-in temperature. This is achieved by comparing the currents pushed back via the thermistor to the "should be" and then the pad is either turned off or on to suit. The setting of the heat level really is a 5k pot, attached with a gear to the dial.
I was expecting just a transistor but a FET makes more sense. It's a BUK7513, which can switch up to 75A. Sounds familiar - the FETs in the CCCU are quite significantly oversized.

Just getting the pad to react is a good start. But the thermistor value should work with the controls in the OEM switch, and knowing what the approximate currents are that are used for control would be a good start. The implication of the work from Thomas (A2TDI) is that the current goes up as the temperature does and that feedback current is important to stop things getting too hot. The calcs from earlier imply <10mA allow current to flow - I'd be looking to emulate this with any replacement mat.

I don't have proof of this, it's conjecture based on logic and some electronics understanding.

@Joga: I suspect it's a really slow frequency, of the order of 0,1Hz - what's the point in going faster? Still, there's an ST Microelectronics chip on there, too, a 90M something that I can't identify, so it's not just an analogue feedback loop - this is the predecessor to the current fully digital systems.

- Bret
 
Well over my head now. I was just posting in case it helped someone. I know it works and I know it will work with any heating pad.

Thanks

Ian
I’m with you Ian,

Went straight over my head too. I’ll be in touch for the simplified answer when I build up enough courage to remove the seat coverings.

Thank you for the information and offer to help sir.

Kind regards,

Tom
 
@bretti_kivi Higher frequency could be beneficial if the thermal mass in the pads is low, i.e. the temperature would fluctuate so much that it could be sense by the body. But just speculating. Apologies to Ian if messing up the thread, best of intentions, I do find it quite an interesting how this works in detail.
 
Well over my head now. I was just posting in case it helped someone. I know it works and I know it will work with any heating pad.

Thanks

Ian
What you don't know and can't say for sure is how hot it will get with any specific heating pad and how effective the settings on the dial will be.

Either of those could be really interesting in terms of 60C and / or a choice between hot, boiling and vindaloo settings.

If you don't get the feedback loop and can't implement a good one, please leave the OEM switches out of it to avoid hazards.

- Bret
 
I actually have heated seats and wheel in the latest Octavia, my daily driver. The wheel is awesome, but it takes *forever* to warm. The seats are superfast - even on low, I turn them off again after a minute or two. But the wheel? That's lovely and stays on "full blast" for several minutes.

I would want to do this in the A2. It's normally linked to the driver's seat heating. I don't currently have heated seats. Alcantara means even at -30 I haven't really missed them. I'm up for doing it correctly which is why I think it's going to end up being carbon mats manipulated with an Arduino with specific temperature feedback from an 18DS20. Some testing will give me temperature ranges to go for and then can set those to presets. ESP8266 would give phone tweaking, too. I will end up with an interior thermometer to monitor the Termini and report that back to a server in the house...

- Bret
 
Back
Top