Resistance Wire Cut and Splice - Too Much Current Now

[clubgitmo]

Hey Guys - Long time reader, first time poster. I am attempting to develop a prototype of a new product. Part of this product will involve a good length of resitive wire that will generate heat. To accomplish this I cannibalized an electric blanket for the resistive wire and power controller. The problem is, I cut and quick spliced the wire and now I am getting an error from the power controller that possibly could be related to too much current flowing.

I assumed the resistance from the wire is related to length. eg, cutting and splicing the wire would not affect total resistance. Apparently I'm wrong as the power controller doesn't like it.

So the question is, where did I go wrong and how can I fix it? I thought about adding a resistor to the loop but I have no idea what the total resistance should be. So, my next thought is a simple adjustable power controller. However, I am short on time and I need to buy most materials locally. Is this an item radio shack could help me with? Any other advice or things I am missing?

 

[vk6kro]

You haven't said what you mean by "cut and splice".

However, it is very difficult to make a reliable connection to resistance wire and this is possibly the cause of your problem. An apparently good looking connection can be high resistance or even open circuit.

Some resistance wire needs silver soldering to make a good connection.

If you don't already have a digital multimeter, you should get one. Adequate meters cost under $20.
With this, you can quickly check if the wire is really presenting the resistance you expect.

 

[clubgitmo]

Thanks for the reply.

You haven't said what you mean by "cut and splice".

Simple automotive quick splice. Although that doesn't seem to be the issue. The resistor wire provides ~24 ohms of resistance which sounds about right... Would pull ~5amps at 110v. Which brings me back to the question of how to power this for testing. Is there a relatively simple way to build a variable voltage A/C power supply? COTS products seem quite expensive.

 

[markmal]

Thanks for the reply.
Simple automotive quick splice. Although that doesn't seem to be the issue. The resistor wire provides ~24 ohms of resistance which sounds about right... Would pull ~5amps at 110v. Which brings me back to the question of how to power this for testing. Is there a relatively simple way to build a variable voltage A/C power supply? COTS products seem quite expensive.

Instead of reducing voltage you can simply increase length (and resistance) of the wire. (you'll need another blanket for this :)

 

[vk6kro]

If you are using the controller from an old electric blanket, do you know it isn't faulty?

Testing the resistance wire should'nt be necessary if you already know it has a resistance of 24 ohms.

Older electric blankets had a slow switching system where full power was applied for some time then it was removed for another time. The ratio of these times determined how hot the bed got.
You could substitute a 110 volt lamp for the resistance wire to check the controller.
If it was faulty, you may be able to purchase a lamp dimmer as an alternative.

If you have doubts about doing any of this safely, you should seek local advice. 110 volts from the mains or anywhere else can kill you, and that wire will dissipate 500 watts, which could start a fire.
So you need to be careful.

 

[vk6kro]

Electric blankets typically dissipate about 60 watts maximum, so if you are using all the wire from such a blanket, it might have been operated at about 40 volts. Power = 40 volts * 40 volts / 24 ohms = 66 watts.

 

[clubgitmo]

If it was faulty, you may be able to purchase a lamp dimmer as an alternative.

Great idea. Much appreciated. Dimmer switch does exactly what I need.