Watts per inch?

[GyCx]

Hi all.

How many Watts are drawn per inch of dew heater? I'm trying to find an in-line dimmer unit for my non adjustable dew heater controller, any help much appreciated.

[IanL]

Read this for the low down on the calculations:

http://www.blackwaterskies.co.uk/2013/05/making-your-own-nichrome-dew-heater.html

To find an appropriate dimmer you need to know the voltage (Volts) and the maximum current (Amps).

1. Measure the resistance of a heater band (in Ohms) using a multi-meter.

2. Measure the voltage of your power supply (in Volts, obv.) again using a multi-meter.

3. The current drawn is Amps = Volts / Ohms

So if you have a 15 Ohm Band and a 12 Volt power supply then you would draw 0.8 Amps.  (12 / 15 = 0.8).

4. You can calculate the power per Inch of band by multiplying the voltage by the current to find the total power (in Watts) and then dividing the power output by the length of the band.

So our example band puts out 9.6 Watts (12 x 0.8 = 9.6) and if the band was 12 inches long that would be 0.8 Watts per inch, which is about the right ballpark (0.3 Watts per cm for us metric types).

You don't actually need to know the power output though, as I said you need to know the voltage and the current for your purposes.

5. If you are running multiple heater bands through one supply, then you just do the same calculation for each and sum the currents to find the total amps drawn.

Not sure what you mean by a 'non adjustable dew heater controller' though?  The point of a dew heater controller is to adjust the power output of the heater bands, otherwise you'd just wire them directly to the power supply (with a fuse between).  So what function is it performing, or is it converting from one voltage to another for example?

The LED dimmers in the article below which are used by many DIYers on here can handle 12 to 24 volts and 8 Amps current, which should be plenty for most purposes.  Usually you just use one per heater band to provide adjustment for each.  Details here:

http://www.blackwaterskies.co.uk/2013/05/a-cheap-multi-channel-dew-heater.html

[GyCx]

Sorry I should of said it was a 4 way splitter as sold by modern astronomy. I was hoping just to put one, in line dimmer to adjust the power going in  because at the moment its full or nothing. Many thanks for the info i'll go through it and see what I want to do is possible.

[IanL]

I've had a look and you could do one of:

- Put one of the LED dimmers between the lighter socket plug and the splitter.  The splitter is rated for 6A and the dimmers for 8A so assuming you aren't exceeding the load of the splitter it would work. That would only give you control of all four outputs at one setting though, and the dimmers are not exactly high quality item so might be better run at a lower current to avoid premature failure.

- Put an LED dimmer between each of the outputs and the dimmer bands to give control over each channel and lower the current in each dimmer.  That would require a fair few additional phono sockets and plugs though, and some kind of housing maybe.

- Might just be easier to make a new controller as per the second article linked above and sell the existing splitter!

[faulksy]

I bought my heater controllers from maplins, pulse width modulation controlers, I tink they were around £15 each part no 919d2p

[GyCx]

Dew heater controller project now complete, Thanks Proto star.

[faulksy]

similar to mine, but i put leds on the outputs as well so you can adjust them by brightness to determine how much power is going to your heaters.

[GyCx]

I did think about LED's but measuring with a volt meter I know that just past half way round with the control knob gives half power so quite easy to know where im at with power setting.

Whole build cost less than £20.00 so happy with that.

(Thanks IanL **proto star in previous post)