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That depends on the used voltage...
for example: V= 12,5 ; needed max. power 20 Watts Current needed: W : V =  1. 6 A
Ohms Law: R = U/I R= ohms; U = Volts;  I = Amperes. So R = 12,5 : 1,6 = ± 7,8 Ohms
You will need 330 : 7,8 =  42 x 330 Ohm resistors in parallel 

This is not critical, because you are able to dimm the power. 20 Watts will be enough to get rid of the dew

 

 

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I'd normally work it the other way round - how many resistors spaced approx 1 cm gap do you need for a certain length..  Looking at the 'dewNot' table below, suggests 8W, 0.66A at 12vdc

Power (P) of a resistor "ladder" is calculated the following way:  P = ( Supply voltage ^ 2 ) / ( resistor value / number of resistors ) (ref: http://www.deepskywatch.com/Articles/newtonian-dew-heater.html)

8= 144 / (330 / X)

X = ~18

but .. they are 1/4 w resistors and using that calculation you would be putting nearly 1/2 W through them... a bit hot!

so.. it looks like you would need 660 ohm resistors or rely on the PWM drive to limit the max power/heat (EDIT: this would half the power to 4W, maybe not enough to keep things clear)

or use  18 330 ohm 1/2 W resistors...

 

PS:  connect them all up to the cable, and try it out before adding the covering, you can feel how hot they get

image.thumb.png.7f5ef43aa7eb70bfbcb20d4e0d1417f8.png

Edited by mikeyj1
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look closely at the dimmer control. There is a decimal point before the 8 so I think this is a 800 ma unit not 8 amps.

How about     -50-110°C W1209 Digital Thermostat Sensor Temperature Control Switch Module UK from Ebay

Edited by Tomatobro
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I was going to make one with a ladder of resistors like this (half way down the page)

http://www.deepskywatch.com/Articles/newtonian-dew-heater.html

This page is the one I was going to use just need to know how many resistors please.

http://www.dewbuster.com/heaters-330ohm-resistors.html

Edited by wookie1965
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You don't want to run the resistors near their maximum rating as they will be too hot to touch in free air and could cause localized hot spots around the scope.

To dissipate 0.2W per resistor (using 0.25W resistors) rearranging Ohm's law R = V^2/W = 12* 12 / 0.2 = 720 ohms. 750 ohms is the closest preferred value which would dissipate 0.19 Watts. If you have a stock of 330 ohm resistors you want to use up, you could put 2 in series for each 'rung' of the ladder where each rung would dissipate 0.22W (that is 0.11W per resistor).

If using 750 ohm resistors for each rung to dissipate 8W in total (from the above chart) you would need 8/0.19 total resistors = 42

If using 2 x 330 ohms in series for each rung, you would need 8/0.22 rungs = 36 rungs. So 72 resistors in total.

A 5" scope has a circumference of around 16" which is 40cm. So the 750 ohm resistors would then be spaced 40/42 cm apart or 0.95cm apart. :smile:

Using 2 x 330 ohm rungs each rung would be spaced 1.1cm apart.

Alan

 

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2 hours ago, Tomatobro said:

look closely at the dimmer control. There is a decimal point before the 8 so I think this is a 800 ma unit not 8 amps.

That's just a full stop to separate the values. Poorly written I admit as it could be confusing. They are max 8A rated. :smile:

Alan

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3 hours ago, symmetal said:

You don't want to run the resistors near their maximum rating as they will be too hot to touch in free air and could cause localized hot spots around the scope.

To dissipate 0.2W per resistor (using 0.25W resistors) rearranging Ohm's law R = V^2/W = 12* 12 / 0.2 = 720 ohms. 750 ohms is the closest preferred value which would dissipate 0.19 Watts. If you have a stock of 330 ohm resistors you want to use up, you could put 2 in series for each 'rung' of the ladder where each rung would dissipate 0.22W (that is 0.11W per resistor).

If using 750 ohm resistors for each rung to dissipate 8W in total (from the above chart) you would need 8/0.19 total resistors = 42

If using 2 x 330 ohms in series for each rung, you would need 8/0.22 rungs = 36 rungs. So 72 resistors in total.

A 5" scope has a circumference of around 16" which is 40cm. So the 750 ohm resistors would then be spaced 40/42 cm apart or 0.95cm apart. :smile:

Using 2 x 330 ohm rungs each rung would be spaced 1.1cm apart.

Alan

 

Mine measures 48cm around the objective on its dew shield. 

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23 minutes ago, wookie1965 said:

Mine measures 48cm around the objective on its dew shield. 

I just took the 5" as the diameter and didn't add on the extra bits. :smile:

Therefore using the 750 ohm resistors means spacing them at 1.14cm or using the 2x330 ohm means spacing at 1.3cm. In reality you could just space the rungs at 1cm and add a few more resistor rungs to get the 48cm length. This means overall it will be around 9W with the 750 ohms and around 10W using the 2x330 ohms. Whatever method takes your fancy. :D

Alan

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