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Heated chair

moelleux

A while back, i was bored of my computer chair because the cushions were flat. I had a friend with a damaged Volvo that he had for parts, so we made a new desktop chair with the leather seat! Only had to remove the airbag and the rails underneath+make some welds. Now the best part is, the heating pads are still there and i was looking forward to hook it up to AC.
I already have pretty much all the parts i need(had the 166n12 and dc-dc and i wanted to post it here to be sure there was no major flaw in my simple design
image

so, there are 2 heating pads, the back is 0.8 ohm and the seat is 1.4 ohm. I was wondering what was best, in parallel with high amperage(more or less 5v/9A) or in series (more or less 10v4.5A). At the same wattage, would more amps produce more heat?
Thanks

  • 0

    Hi William,

     

    You have a marginal plan here If you put the Back 0.8 ohms and the seat 1.4 ohms in series you will have 2.2 ohms total If you put this resistance across the 12.6 volt transformer you will have 5.72 Amps of current The specifications on the  166N12166N12 transformer is 12.6 volts 4 Amps As you can see the transformer will be overloaded by 43 For long term safe operation it will probably be best to have a transformer that can comfortable supply 8 Amps This will allow it to power the heaters without over heating itself If you plan to control the heat by turning it on and off with a switch you probably do not need to use the diodes or the filter cap The AC from the transformer will work just fine Whether you use the AC directly or make it DC you should not use a rheostat to control the heat A rheostat that can control 72 Watts would be expensive and might give off more waste heat than would be desirable depending on its setting If you want to control the power to the heater elements it will be better to go back to converting the AC to DC and using a circuit with a Pulse Width Modulated output This way only the power desired gets sent to the heater elements with very little waste heat to complicate things If you decide to hook the elements up in parallel instead of series the power demand goes way up to about 25 Amps and the wattage output of the elements would likely be a fire hazard at 315 Watts

     

    John

  • 0 in reply to jw0752

    Would the dc-dc step down converter be a efficient solution? if i lower the voltage to around 10v, the amperage in serie would be around 4.5A (45w).

  • 0 in reply to moelleux

    Hi William,

         In order to use the DC to DC converter you will have to convert the AC to DC. Go back to your original schematic. Even the 5A diodes should be ok. In each conversion step some power will be lost.  You are correct that putting less voltage across the elements will mean less power demand on the transformer and system. That is what the DC to DC converter will allow you to do. This of course also means that there will be less power to heat the elements too. One final caution, the primary of your transformer connects to 120 VAC which is dangerous. Unless you have experience working with mains voltage it might be a good idea to get some help.

     

         I decided to add a little to this post. If you start with the voltage on the converter turned low and slowly bring it up I believe that somewhere around 8 volts your transformer will begin to over heat and the voltage will go no higher because of the offset. It will be a good idea to monitor the transformer temperature and since there is a 4 to 7 volt offset between the input voltage to the converter and the output you will not be able to turn the converter up any higher. Offset means that if you have 12 volts in the most it will put out is 8 volts. This will be an interesting experiment but it may not have enough power to produce the heat you want in the seat.

     

         Sorry I have had to edit this post a couple times as I studied the converter and your application more thoroughly.

     

    John

  • 0 in reply to jw0752

    Hi John
    i thought about it a little, would the most effective solution be to remove the dc-dc converter, keep the rectifier to power a PWM circuit with a 555 chip. Then the chair could be powered by 12v AC through a mosfet directly from the source? Of course i would need to be careful not to override the transformer current output.

     

    sorry for the delay and thanks

  • 0 in reply to moelleux

    Hi William,

    The DC to DC converters are really very efficient. It is doubtful that we could design any type of PWM switch using a 555 and a  MOSFET that would be anywhere near as efficient. You can get DC to DC converters on ebay for less than $10.00 that are adjustable and hence would allow you to adjust your power out to the chair. The converters usually have a small trimmer to adjust the voltage out but I have substituted a regular potentiometer on several occasions for convenience. Your weak link is still the transformer which may not be able to give you the power you expect but hey its worth a try.

    John