Heater Element (Electric Blanket)

[nicosuave]

I am trying to make a simple heating element like that of an electric blanket.

Please let me know if I have the concept right with this schematic

 

[kchriste]

It's OK, except for the fact that the potentiometer will be as much a heater as the heating element when set for half power. Use a TRIAC control instead, or a multi tap element. Plus 50K is too high of a value. It's value depends on the element resistance and how low in power you'd want to go heat wise.

 

[dknguyen]

Shouldn't you have an isolation transformer to isolate yourself from the mains and step down the voltage to a safer level (48V or lower)? I wouldn't want to wrap myself in a high voltage wire and then go to sleep.

 

[nicosuave]

It's OK, except for the fact that the potentiometer will be as much a heater as the heating element when set for half power. Use a TRIAC control instead, or a multi tap element. Plus 50K is too high of a value. It's value depends on the element resistance and how low in power you'd want to go heat wise.

I believe your right about the potentiometer becoming as much of a heater as the heating element/wire, when I connect the circuit the potentiometer blows out (can no longer measure resistance across potentiometer).

When I measure resistance across heating element I get 2.4Ω. I am not familiar with TRIAC (sorry my experience is limited). What does a TRIAC do exactly??

How would you wire this circuit with the TRIAC??

 

[audioguru]

If you measure 2.4 ohms for the heater then the leads of your multimeter are probably at least 0.4 ohms. Then the max power in the 2 ohm heater is 7200W which will cook you and burn down your home. The power will drop a little as the heater gets hot and starts to glow red hot.

Actually, a 15A breaker will disconnect when the power is more than 1800W.

A triac is the controlling electronic part in a light dimmer which can power a small heater but yours takes way too much power.

 

[nicosuave]

If you measure 2.4 ohms for the heater then the leads of your multimeter are probably at least 0.4 ohms. Then the max power in the 2 ohm heater is 7200W which will cook you and burn down your home. The power will drop a little as the heater gets hot and starts to glow red hot.

Actually, a 15A breaker will disconnect when the power is more than 1800W.

A triac is the controlling electronic part in a light dimmer which can power a small heater but yours takes way too much power.

I obviously need to supply less volts and amps. Because the wire I'm using can NOT take 1800W. I need to be working with 10 - 40W.

Can someone make me a schematic of how to use the TRIAC to vary the power so that I can hone in on the correct amount of power to deliver to the heating element??

Thank you for all you help.

 

[kchriste]

Where did you get the heating element from?
As AudioGuru has pointed out, a 2Ω element would mean it would dissipate 7200 watts with 120VAC input to it. For 10 watts you'd need a heating element that measures 1440Ω and for 40W a heating element that measures 360Ω. So find a better suited element, such as a 360Ω one, and use a regular light dimmer to control it.

 

[MikeMl]

Shouldn't you have an isolation transformer to isolate yourself from the mains and step down the voltage to a safer level (48V or lower)? I wouldn't want to wrap myself in a high voltage wire and then go to sleep.

That is exactly what you do if you have ever slept under an electric blanket or heating pad. There is no isolation tranny; the heating element is connected to the 120V line through a bi-metal thermostat. It is ok unless you pee the bed

 

[Hero999]

No offence intended but going by the schematic you first posted you probably don't have enough experience to build an electric blanket.

I wouldn't recommend building an electric blanket; buy one it's much safer and if it catches fire of electrocutes someone at least you can blame the manufacturer. If you want to use a pot as a temperature control, buy a plug-in lamp dimmer and use it with your nice new electric blanket.

 

[Nigel Goodwin]

That is exactly what you do if you have ever slept under an electric blanket or heating pad. There is no isolation tranny; the heating element is connected to the 120V line through a bi-metal thermostat. It is ok unless you pee the bed

240V in the UK

I've always thought them EXTREMELY dangerous devices.

Years back we used to send them back to the manufacturers for servicing for our customers, but it got to the stage that every single one was said to be beyond repair, and a new one offered at a discount price. I always presumed they had stopped the servicing facilities, and just binned the old blankets.

 

[Hero999]

If you pee the bed, it shouldn't be a problem, you'll receive a short shock before the RCD (GFCI in US) kicks in and saves you.

I suspect at most electric blankets are waterproofed to some extent anyway.

 

[Nigel Goodwin]

If you pee the bed, it shouldn't be a problem, you'll receive a short shock before the RCD (GFCI in US) kicks in and saves you.

I suspect at most electric blankets are waterproofed to some extent anyway.

You're assuming the house has an RCD, many don't - there's no requirement for older properties to have one added (although a rewire would onviously include one).

Personally, RCD or not, I'd rather not sleep with 240V over or under me

 

[Andrew Leigh]

My advice,

don't do it.

Nearly lost a son, daughter in law and two grandbuddies through an electric blanket. They lost everything, house, contents EVERYTHING. My son was severly burned, spent three weeks in hospital, they lost their two dogs in the blaze.

Cheers
Andrew

 

[dknguyen]

I'm not even sure about why anyone uses electric blankets indoors. It get freakin' cold here and two blankets always does the trick. Most of the time it's actually too hot.

 

[Hero999]

My advice,

don't do it.

Agreed.

Nearly lost a son, daughter in law and two grandbuddies through an electric blanket. They lost everything, house, contents EVERYTHING. My son was severly burned, spent three weeks in hospital, they lost their two dogs in the blaze.

That's really didn't the insurance pay up?

My mum uses an electric blanked, maybe I should tell her not to.

I have an electric foot warmer but it has a timer on the maximum hear setting to stop this sort of thing from happening.

 

[Andrew Leigh]

Hi

They were going through a real tough patch financially and had cancelled their household contents insurance. The structure was repaired as it is mandatory to pay for the house structure for as long as the house is bonded to the bank.

Cheers
Andrew

 

[Hero999]

They were going through a real tough patch financially and had cancelled their household contents insurance. The structure was repaired as it is mandatory to pay for the house structure for as long as the house is bonded to the bank.

That's really bad. It's just typical Murphy's law that your house burns down when you can't afford the insurance.

Well you probably know that you should have downsized or took a lone, if you couldn't afford the insurance but hindsight is a great thing.

 

[bailey45]

Couple of notes.

The actual resistance of the element is likly 2-3 times as high when the element gets to its operating temperature.
A simple way to control this type of heater element would be to use a light switch dimmer.
In your original post you said 'Like and electric blanket'. I agree with all the other comments don't make a home made electric blank. The commercial version is an accident waiting to happen.

 

[Hero999]

The actual resistance of the element is likly 2-3 times as high when the element gets to its operating temperature.

That's true at high temperatures such as electric fire not at low temperatures such as an electric blanket. Unless you want it to cause burns that maximum temperature should be under 40°C and the difference between the resistance of nichrome wire at 20°C (a cool room) and 40°C (a warm bed, just above body temperature) is negligible.

 

[audioguru]

I cancelled my life insurance 2 months before my heart attack. I have savings, my kids are grown up and the insurance kept getting more and more expensive.

I am still alive so I beat Murphy's Law.

The smoke caused by an electric blanket keeps you warm.