OT - Heater wire changes if covered?

[Secpcb]

This is something I figured out that electrically doesn't make sense to me.


An electric blanket with a single heating wire looped through it and a single thermal sensor wire also, I'm guessing a 30 foot long RTD basically.


I am comfortable with the temperature control set at 2-1/2 under normal conditions just to break the chill off the bed. But when my dog jumps on the bed I have to turn it up to 6 or 8 to be the same.


When he lays on the blanket the area under him gets very hot, I'm guessing over 120 degrees and the rest of the blanket gets cold.



The only thing using physics I can think of is the wire being compressed by him changes resistance and uses most of the voltage so the rest of the blanket gets little voltage. Being a single wire this has to create a voltage divider under him.


But not being an electrical engineer I'm still puzzled by this happening. The area under him would have to greatly reduce resistance from being compressed, not stretched to make thinner like a strain link works. And he's not a 600 pound tiger, just a shepard.

 

[Steve Bailey]

Electric blankets aren't supposed to have anything on top of them. The dog is preventing the area he's on from radiating heat away, trapping the heat beneath him and raising he temperature of that area. The blanket's temperature sensor detects the higher temperature and adjusts the control accordingly.

 

[g.mccormick]

Electric blankets aren't supposed to have anything on top of them. The dog is preventing the area he's on from radiating heat away, trapping the heat beneath him and raising he temperature of that area. The blanket's temperature sensor detects the higher temperature and adjusts the control accordingly.

Moral of the story....

Dogs don't belong on your bed.

 

[Steve Bailey]

Dogs don't belong on your bed.

The IUCC (International Union of Canine Companions) respectfully disagrees.

Add a couple more dogs and the OP won't need an electric blanket.

 

[drbitboy]

Electric blankets aren't supposed to have anything on top of them. The dog is preventing the area he's on from radiating heat away, trapping the heat beneath him and raising he temperature of that area. The blanket's temperature sensor detects the higher temperature and adjusts the control accordingly.

Nice.

This is not electrical engineering, btw, or at least not all of it.

It is roughly, but not exactly, analogous to whole-house a thermostat being in a better-insulated room of the house, or on an upper floor or directly over a furnace vent. Since the thermostat controls the furnace to keep the air near the thermostat at a temperature ("thermo-") that is near-constant ("-stat"), and the thermostat measures warmer air than the rest of the house, the air in the rest of the house is cooler than the air around the thermostat. There is probably a better analogy.

In the blanket, the design assumption of the control system measurement is that the 30-foot long RTD is at a more-or-less constant temperature throughout its length, but in actuality it is measuring the equivalent of many resistors in series at various temperatures. So when it is controlling to a "temperature" setting it is actually controlling the total resistance of the 30-foot RTD by controlling the RTDs temperature. So if the warm section of RTD under the dog has a higher resistance because it is at a higher temperature than the rest (because the dog provides extra insulation), which raises the total resistance, then the controller must drop the current (and heat) input to the whole system to drop the temperature along the whole RTD wire (though still warmer under the dog), which drops the total resistance to match the setpoint.

A possibly better analogy is pumping water into a tank to maintain its level, while there are a leaks near the bottom of the tank. The level is analogous to mean RTD temperature (total resistance) measurement, and the pump flow rate is analogous to the current (heat) provided to the heating element. If we plug some of the leaks in one area of the tank (analogous to the dog reducing heat loss from one section of the blanket), then there will be less flow at the level setpoint, so the level will rise, the pump controller will detect the increasing level (error) and reduce the flow rate to match the lower leakage rate, and the level will eventually return to setpoint at a lower flow rate. The lower pump flow rate is analogous to less current through the heating wire, so the heat balance over you makes you colder. N.B. this analogy does not reproduce the hot spot under the dog.

Here is an interesting question: if the dog jumps on the blanket and dial is moved from 2-1/2 to 6 or 8 to restore a comfortable equilibrium over you, then is the system it using more current or less current?

 

[danw]

Traditionally heat control on a consumer electric blanket has been a bimetallic strip that opens and closes which conducts or interrupts the current flow through the heating element. Cheap and reliable. The control dial controls the on/off time ratio. And I suspect UL requires an over temp, high limit shutoff.

In control terms: variable PWM open loop temperature control, with integrated current overload protection, but the marketing department doesn't want that printed on the box.

I seriously doubt that consumer electric blankets use an RTD. WAAAAAYYYYYY TOO expensive. Even a thermistor, positive or negative coefficient, require a sensor input, logic running on DC supply and a relay to switch the heating current. China is cheap but can uE beat out the bimetallic switch? For the incredibly competitive consumer market?

 

[alan_505]

The electric blanket I use has separate heating for upper body and lower body to 9 levels of heat and up to 12hrs on time with an LCD screen.

 

[drbitboy]

The only thing using physics I can think of is the wire being compressed by him changes resistance and uses most of the voltage so the rest of the blanket gets little voltage. Being a single wire this has to create a voltage divider under him.

Assuming a single heating wire, it would be current not voltage, and Power = Current² x Resistance; with a single wire the current would be the same throughout that wire, so if the dog changed the resistance of a section of wire* it would be to increase it.

If the dog on top causes that section of the blanket to be near 120°F and that is considered dangerous, then a high-limit shutoff could override the PWM and shut things down until it cooled off, but if that were the case then increasing the dial setting could not cause any form of recovery.

I think the thermal explanation is the most likely, and an RTD or equivalent measurement is needed to understand the rest.

* Physical compression of/force on the wire changing its specific resistance seems unlikely; making the wire warmer via increased insulation around it could increase its resistance, but I have my doubts that the effect would be enough to be significant.

 

[Secpcb]

Thanks guys.



Steve,
Makes sense from that perspective


G.mccormick,
You never had a dog I take it. The band Three Dog Night took their name from an Eskimo saying for a night was so cold they brought in 3 sled dogs to sleep with them - hence the worst nights were a 3 Dog Night



Danw,
This blanket has 4 wires, 2 power 14AWG and 2 look like 22AWG that are one long thermal sensor that when measured only has about 5 volts to it. The blanket maintains about the same internal temperature no matter the temperature of the room that would effect a thermal switch in the controller



Alan 505,
I'm cheap. Mine's only got 1 zone and a potentiometer dial



Drbitboy,
Monitoring the output to the blanket the voltage is controlled between 12 to 90 volts depending on the heat needed. Turning the dial up to max suddenly makes the voltage jump to 90

 

[padees]

Three Dog Night is from Cali.
The reference is they got so ****ed up, it took 3 days to recover from the partying. Hence: 3 Dog Night. I understand the other meanings.


Nazareth: Hair of the Dog...


You wake up the next morning after a bit too much, time to taper it off for (3) Days.


Alice has a song or two about this.

 

[drbitboy]

Drbitboy,
Monitoring the output to the blanket the voltage is controlled ...

I understand the controller will control voltage, my post was about the phrase "... the wire [...] uses most of the voltage ..."

If that were the case then resistance of the dog-affected section would have gone up, so the total resistance would be up, so the current - through any and all cross sections of heater wire - would go down, which is what would actually be causing the loss of heat in the non-dog-affected sections.

That said, Power = Voltage² ÷ Resistance. I suppose that works too; it just sounds awkward.

 

[Secpcb]

Three Dog Night is from Cali.
The reference is they got so ****ed up, it took 3 days to recover from the partying. Hence: 3 Dog Night. I understand the other meanings.

Wikipedia:

The commentary included in the CD set Celebrate: The Three Dog Night Story, 1965–1975 states that vocalist Danny Hutton's girlfriend, actress June Fairchild (best known as the "Ajax Lady" from the Cheech and Chong movie Up In Smoke) suggested the name after reading a magazine article about Aboriginal Australians, in which it was explained that on cold nights they would customarily sleep while embracing a dingo, a native species of wild dog. On colder nights they would sleep with two dogs and, if the night were freezing, it was a "three dog night".[5]

Plus I remember it from an interview the band did but he said on the show it was Eskimo sled dogs, maybe he had little too much on the way to the studio.

 

[ASF]

But when my dog jumps on the bed I have to turn it up to 6 or 8 to be the same.

You know the rule about the dog tax, right? Pay up!