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Powering 12v blanket from 24v source

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holabr

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I use a power wheelchair that uses 2 - 12 volt car batteries in series. The charging port gives me easy access to the 24v with no modifications to the chair. I am a big football fan but have a problem with the cold due to circulation problems. I have found a number of electric blankets intended for use in a car. They typically draw anywhere from 4 to 6 amps. Is it feasible to power the blanket from the wheelchair? I'm assuming that the 24v into the 12v designed blanket would cause it to overheat, so what type of control would I need to vary the voltage and therefore be able to adjust the heat output accordingly? I would want something that would be efficient and not just dump off the excess voltage. Also, is there a way to calculate how long the blanket could be used before the voltage in the batteries drops to a certain level? A second option might be to use 18v rechargable battery packs from my DeWalt cordless drill.
Any ideas/comments are welcome.
 
I take it that the blanket is purely resistive and doesn't contain any electronics.

If so you could use a 555 timer running at 50% duty cycle to switch that blanket on and off at a high frequency so the average voltage is just 12V. You'll need a voltage regulator and driver transistor (a MOSFET is best) because the 555 can't run from 24V or directly drive 6A 24V loads.

Adding a 6A fast blow fuse would be a good idea in case the 555 or driver transistor latches on for any reason.
 
I use a power wheelchair that uses 2 - 12 volt car batteries in series. The charging port gives me easy access to the 24v with no modifications to the chair. I am a big football fan but have a problem with the cold due to circulation problems. I have found a number of electric blankets intended for use in a car. They typically draw anywhere from 4 to 6 amps. Is it feasible to power the blanket from the wheelchair? I'm assuming that the 24v into the 12v designed blanket would cause it to overheat, so what type of control would I need to vary the voltage and therefore be able to adjust the heat output accordingly? I would want something that would be efficient and not just dump off the excess voltage. Also, is there a way to calculate how long the blanket could be used before the voltage in the batteries drops to a certain level? A second option might be to use 18v rechargable battery packs from my DeWalt cordless drill.
Any ideas/comments are welcome.
I worked on an Everest-Jennings. What kind you have?
 
Heating elements are forgiving on how fast you switch them. 1 second on 3 off will work fine.
 
Hi Hero999,

The duty cucle should be 25 %, NOT 50 %.
Do you want to fry the bloke ?

on1aag.

Yes you're right, I made a classic schoolboy error.

P = V²/R which means that if he used 50% duty cycle the power would be double and the blanket will overheat.

To give an RMS voltage of 12V from a 24V squarewave the duty cycle needs to be 25% which would give a power dissipation of 36W.

Of course he could just connect two blankets in series.
 
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Willbe: The chair is an Invacare Storm

Can anyone point me to a schematic and the proper MOSFET to handle the voltage/amperage?

Also, can the cycling be variable so I could increase or decrease the heat output within a range? Maybe something like 10% to 40% duty cycle. Would 40% fry the heating element?

Any guess how long I could run the blanket at 25% before the voltage drops from say 26v to 23v?
 
Anything above 25% duty cycle could fry the heating element.

For variable temperature you need to vary the duty cycle from between 0% and 25%.

The IRL540 would be perfect for this.
 
Ok kids, I got to throw in an easy fix here.
Just wire leads from one battery to a connector for your blanket.
Ya, it will be an uneven drain on the batteries but so what.
The controls for that chair are not 24v (At lease I do not think so, my experience with Invacare is 15 years old)
You might even wire a connector to each battery. Stay toasty for 20 min then switch batteries by moving to the other connector. You could even put in a double pole double throw switch in so you don't have to mess with 2 connectors.
The only issue that I can think of is that if the chair is grounded (easy to check) to the battery pack and I don't think it is (cause the motors reverse)
IF the blanket were to become frayed AND the heating element was unshielded or itself frayed AND the chair was grounded The blanket might get hot IF it came in contact with frame. And it might get hot fast!
So fuse the lead with an in line fuse holder. You could wire tie the blanket circuit wires (fairly small wire) right to the charging wires.

How long will it take to drain your batteries? Your battery is rated in amp/hours. How ever many amp/hours your battery is rated at minus 6 (your number) times the number of hours you have the blanket on. It will be half that drain if you toggle the use of your batteries.

Maybe the best suggestion is to mount a 12v 20 to 30 amp/ hour deep cycle (so you can run it all the way down) battrie some where on the chair. Get a gel type battrie (no spill) It could even be a cloth bag that you hang on the chair. That would give you 4 to 7 hours of heat and no danger of loosing your mobility.

good luck have fun at the game.

2:48 am my time
 
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I don't think that's a good idea, as you say the batteries will discharge unevenly which could even lead to cell reversal. A switch would be an inconvenience and he'd need to be careful to make sure that the time on each battery is even. I think it's much easier to switch it on at 25% duty cycle which would work fine.
 
Hi Holabr,

Can anyone point me to a schematic and the proper
MOSFET to handle the voltage/amperage?

Any guess how long I could run the blanket at 25%
before the voltage drops from say 26v to 23v?

Here's a circuit, don't fry yourself !

About the second question, without knowing the capacity of
your batteries and the power dissipation of your blanket it's
impossible to tell.

on1aag.
 

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You can't, that circuit it designed for the TL071, if you want to use a 555 then you need a totally different circuit.

A different op-amp would work, I haven't simulaited but it looks fairly low frequency so a 741 would probably work. You could also use a comparator like the LM311 or LM393.
 
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