lowering wattage

[jr#8]

I'm looking for some assistance on making a controller for a dew heater.
Where do I start.
I have a 12v dc power supply and I want to be able to heat a nicrome wire with the ability to control the heat output.
So lets see if I can explain what I have allready tried.
12v dc power supply to a inline fuse to a rheostat to the nicrome wire.
Results fried rheostat.
Next I reduced the voltage to 6vdc through 2- 2.2ohm resistors.
Results not enough volts to heat the wire.
Is there a way that I can reduce the wattage but keep the volts to 12v or be able to heat the wire without frying components?

 

[Styx]

PWM the voltage onto the wire

 

[jr#8]

Styx whats PWM ?

 

[JimB]

What is the resistance of the nichrome wire?

JimB

 

[bonxer]

Pulse Width Modulation.

Instead of just connecting the 12v 100% of the time, you only have it on part of the time. You give it a waveform that turns on and off many times per second. You would use a knob to control the actual width of the pulse. So for a 10% duty cycle, it would be off 90% of the time, then on 10% of the time. For a 60% duty cycle, say it's on 60 milliseconds, off 40 milliseconds, on 60ms, off 40ms, repeat.

 

[jr#8]

The resistance of the wire is 2.9.

 

[jr#8]

Would a 555 chip work for the PWM and where can I find what prongs to what MS to use?

 

[Styx]

yes a 555 is fine

 

[JimB]

How accurate does the control have to be?

For a simple approach, why not use a transformer with a multi-tapped secondary and select the voltage with a switch?

for a 2.9 ohm wire
12v = 50 watts
10v = 34 watts
8v = 22 watts

etc

W = V x V / R

JimB

 

[FRIED]

Could also use a transistor in linear mode as long as the transistor and heatsink can be part of the heater.

 

[jr#8]

If useing a 555. Do I need a PCB and I would be looking to get a 50% duty cycle. What other parts would I need for this? How should I be looking at the 555 to know which pin is which.

 

[gerty]

How much heat do you need? We have used 10 watt resistors, as a heating element, to heat a camera housing lens for outdoor use. The resistor is about 3/8" square by 2" long and gives pretty good heat transfer to the lens. The resistance you need will depend on how much heat you need, and for about 75 cents each you can afford to experiment for the correct value. Keep in mind that the lower the resistance , the higher the current and heat.. The resistor was mounted low on the lens, below the view of the camera , and was held in place by a bent wire clip that was screwed down, almost like a pipe clamp..
Hope this helps...

 

[jr#8]

The wire is for a telescope which has a dia. of 10" and a cir. of 39". It dosen't have to get hot to the touch only warm enough to keep the wire 2 deg. above the ambient air temp. I found this link on a automatic dew heater.
I think i'll have to use this. Thankx every one for all your help. and I'm sure I'll be back on this.
**broken link removed**

 

[Styx]

it might be worth putting two NTC down - one in free air and one on what you are trying to keep warm.

pasing these two into a comparator (taking into account this 2C difference needed, easily done with voltage DIv). This will then give a HI output IF it is too cool or LO if too warm.

This signal can then be directly fed into a MOSFET'S gate to engage the RESISTOR.

using a 555 will work but it is open-loop. This way is a simple closed-loop BANG-BANG controller that will give you better temerature control

 

[jr#8]

Whats NTC down mean

 

[Styx]

NTC is "Negative Temperature Co-efficient" Resistor

Soz for the "down" more of a slang, as in to place.

Thus put an NTC down near a point that would represnt your local air temp

Then one near (not on the resistor) what you are wanting to warm.


These two NTC's basically will form part of two separate voltage-dividors.


Asumming the "Ambient" NTC will stay the same value (it wont in practice but that doesn't matter atm for explanation). This "Ambient" NTC will act as a reference to the COMPARATOR

As the temp cools at the other NTC, its resistance will change. With this NTC part of a voltage dividor at the other COMPARATOR pin, Thus the voltage at that COMPARATOR pin will change.

This change wll eventually flip the COMPARATOR, the output of which can then drive a MOSFET. Once the temp starts to rise, the NTC will warm up and thus the voltage at the COMPARATOR will change, thus changing the COMPARATOR state (and thus turning the FET off)

a very simple and faily decent temperature controller