Dew strip

[wookie1965]

R = 12²/3 (That’s 12 volts squared divided by 3 watts.)
R = 48 ohms (Meaning we need a length of nichrome wire with resistance of 48 ohms if your objective/corrector is 8 inch and your power supply is 12V)

Using this formula the length of wire I will need a  180 cm piece to aid connections as well does that sound correct please. 

[JamesF]

The resistance calculation looks ok to me.  You can buy nichrome wire in different resistances, so I found it easiest to work out what length I'd want and then find which resistance made the calculation come out with a suitably close length.  For example, an 8" OTA has a circumference of about 60cm.  To have both connections at the same end you'd want about 120cm of nichrome, to make a loop in the heater.  So for 48 ohms total you'd want wire with a resistance somewhere in the region of 40 ohms per metre.  Is 3W enough power though?  For some reason the figure I had in my head was much more than that.

I am reminded of this, which may help out:  https://www.blackwaterskies.co.uk/2013/05/making-your-own-nichrome-dew-heater-bands/

Particularly the spreadsheet linked at the end that does lots of the calculation for you.

I stopped making my own when I found W&W Astro though because they were sufficiently cheap that the faff of buying the parts and making them didn't seem worthwhile.  Unfortunately they appear to be out of stock of heaters for 8" OTAs at the moment.

James

[wookie1965]

When I read that it didn't sound right it was in S&T but I think I would double that especially to get it just over ambient temperature. 

I bought this off the Internet I was told this was right by the company. 

[Seelive]

My dew heaters generate around 0.25W/cm so for an 8" scope I would expect be looking for at least 15W capability. I run mine using a PWM controller which is normally set to around 50% but there have been nights when it's been necessary to turn it up to 100% (for a given supply voltage you can always reduce the power output but you can't increase it).

I believe that that Nichrome wire has a resistance of about 35ohm/m.  A single turn around an 8" scope will be about 0.72m long so the wire resistance would be about 25ohm which at 12V will give a power of under 6W that might just be about suitable for most nights. A return loop would give you under 3W power which to me seems to be too low. Since you have a 10m spool of wire you could use several individuual loops in parallel to get the required wattage (5 loops at 1.44m each = 7.2m total giving a restance of 10ohm which would provide 14.4W) but you would need to ensure that each individual loop is isolated from all the others.

[Carbon Brush]

Nichrome resistnce does, as indicated earlier depend the wire gauge and the profile. Generally there is 12V available at the scope with a PWM controller, which gives flexibility on heating.
If you want (for example) a flat dew strip, you could take a toaster or iron apart and see what is in there. My toaster uses flat wire but I like my toast so won't be using it as a dew strip donor anytime soon.

The BIG disadvantage of nichrome is soldering. You can't solder using conventional tools and materials. Connections have to be crimped which is potentially expensive.
Crimp tools to produce reliable joints are expensive and crimp housings often bulky. Or terminal blocks that are bulky.

An alternative approach is to buy discrete resistors. Through hole variety.
Here is an example.  https://www.rapidonline.com/subminiature

I'm not saying they are the cheapest, or that Rapid is the place to buy. Or that Rapid have the range of useful values.
But Vishay are a known reliable brand vs buying unknown brand from 'under the railway arches'.

As a rough guide, each resistor, with some lead left on is going to account for 2-3cm of heater length.
So if you have for example a refractor with 25cm circumference OTA, that means using about 10 or so resistors. You can tweak this by using different lead lengths.

Lets assume 10 resistors for now. Each one being 4.7ohms. Connecting in series gives you 47ohms. The 3W heater initially proposed.
You can get a similar heating  effect using 14 or 15 resistors with shorter leads, each 3.3ohms.

Obviously this approach can be used to build a heater array for any scope from a 50mm finder up to a big mirror.

Resistor leads are very easy to solder using any old iron (🤣please laugh) and ordinary solder.
If you aren't used to soldering, you might find 60/40 tin/lead easier than the 'new' lead free stuff.

Buy some heat tolerant sleeving. Ordinary PVC is not a good idea. For how warm these are going to get, heatshrink won't degrade in long term use.
By heating the sleeve (possibly off the scope) before use, you can form it to the desired bend.

Using this approach you can make custom heaters with power to suit the application and small tidy cables.
No chunky 'one size fits all' heaters with thick leads.
Why not put a resistor array in the bottom of a newt tube using a few drops of silicone adhesive? It can be clear of mirror screws, etc.
A much neater solution than external heater tape, and less likely to get disturbed in the dark.

HTH,

David.

[wookie1965]

2 hours ago, Carbon Brush said:

Nichrome resistnce does, as indicated earlier depend the wire gauge and the profile. Generally there is 12V available at the scope with a PWM controller, which gives flexibility on heating.
If you want (for example) a flat dew strip, you could take a toaster or iron apart and see what is in there. My toaster uses flat wire but I like my toast so won't be using it as a dew strip donor anytime soon.

The BIG disadvantage of nichrome is soldering. You can't solder using conventional tools and materials. Connections have to be crimped which is potentially expensive.
Crimp tools to produce reliable joints are expensive and crimp housings often bulky. Or terminal blocks that are bulky.

An alternative approach is to buy discrete resistors. Through hole variety.
Here is an example.  https://www.rapidonline.com/subminiature

I'm not saying they are the cheapest, or that Rapid is the place to buy. Or that Rapid have the range of useful values.
But Vishay are a known reliable brand vs buying unknown brand from 'under the railway arches'.

As a rough guide, each resistor, with some lead left on is going to account for 2-3cm of heater length.
So if you have for example a refractor with 25cm circumference OTA, that means using about 10 or so resistors. You can tweak this by using different lead lengths.

Lets assume 10 resistors for now. Each one being 4.7ohms. Connecting in series gives you 47ohms. The 3W heater initially proposed.
You can get a similar heating  effect using 14 or 15 resistors with shorter leads, each 3.3ohms.

Obviously this approach can be used to build a heater array for any scope from a 50mm finder up to a big mirror.

Resistor leads are very easy to solder using any old iron (🤣please laugh) and ordinary solder.
If you aren't used to soldering, you might find 60/40 tin/lead easier than the 'new' lead free stuff.

Buy some heat tolerant sleeving. Ordinary PVC is not a good idea. For how warm these are going to get, heatshrink won't degrade in long term use.
By heating the sleeve (possibly off the scope) before use, you can form it to the desired bend.

Using this approach you can make custom heaters with power to suit the application and small tidy cables.
No chunky 'one size fits all' heaters with thick leads.
Why not put a resistor array in the bottom of a newt tube using a few drops of silicone adhesive? It can be clear of mirror screws, etc.
A much neater solution than external heater tape, and less likely to get disturbed in the dark.

HTH,

David.

Hi David,

                   It is for my Frac the Meade127 I was just trying get my calculations right I have made a ladder dew strip out of resistors 330ohm and have plenty left which I could use in series have you a link to anywhere that shows you how to build this I have heat shrink but not in one length I bought a pack of mixed ones so not sure if trying to join these together would work. 

Thanks 

Paul

[Seelive]

1 hour ago, wookie1965 said:

Hi David,

                   It is for my Frac the Meade127 I was just trying get my calculations right I have made a ladder dew strip out of resistors 330ohm and have plenty left which I could use in series have you a link to anywhere that shows you how to build this I have heat shrink but not in one length I bought a pack of mixed ones so not sure if trying to join these together would work. 

Thanks 

Paul

If you intend to use 330ohm resistors they will need to be used in parallel rather than in series. There was a post on here very recently regarding a home made dew heater strip. The build details of my own heaters are here

https://www.eastmidlandsstargazers.org.uk/topic/16935-diy-dew-heaters-and-controller/

[Carbon Brush]

@Seelive thanks. You beat me to the reply.

Using 330ohms means parallel connection. Two wires to each resistor with the resistors as the rungs of a ladder and the wires the uprights.
Low value resistors in series (like a chain) allows easier construction.
Each 330 ohm resistor on 12V will contribute 0.43W, should you go for this option.
Given that resistors are low cost, I would go for low value resistors in series and ease the construction.

HTH, David.

[wookie1965]

3 hours ago, Carbon Brush said:

@Seelive thanks. You beat me to the reply.

Using 330ohms means parallel connection. Two wires to each resistor with the resistors as the rungs of a ladder and the wires the uprights.
Low value resistors in series (like a chain) allows easier construction.
Each 330 ohm resistor on 12V will contribute 0.43W, should you go for this option.
Given that resistors are low cost, I would go for low value resistors in series and ease the construction.

HTH, David.

So in series I would need much lower ohm resistors as per your previous reply 4.7ohm is that correct I could make two then and run them both off the dimmer switch I have. Would these resistors suffice. 

Edited January 26, 2021 by wookie1965

[wookie1965]

Thank you everyone I really appreciate your help I will take pictures of my builds when I get new resistors. I may as well make both at the same time. 

[Carbon Brush]

4.7 OHM.
NOT 4.7K OHM.
The K denotes 1000 multliplier.

4.7K ohm = 4700 ohms. Not suitable.

[wookie1965]

Do you know I clicked on 4.7ohm and this picture turned up unless they have different resistors for sale I will make sure I buy the correct ones. 

[wookie1965]

[Seelive]

Are you intending to use just 10 4.7ohm resistors around an 8" scope? That would space them about 70mm apart so any heating will be very localised rather than being evently spread out around the scope, and at just 0.3Watt per resistor I don't believe that would be enough to raise the temperature sufficiently to act as a dew repellent under most circumstances. 

[wulfrun]

Have you considered Constantin wire instead of Nichrome? It can be soldered. I think Kanthal might be solderable too but maybe not as readily. Kanthal is commonly used in E-cig heaters so it's cheap.

[SMF]

 

Personally I used 330ohm resistors in parallel spaced 25mm. Work a treat wrapped in Velcro.

RS are far cheaper and you’re guaranteed good products.

https://uk.rs-online.com/web/p/through-hole-fixed-resistors/2141888

https://smile.amazon.co.uk/gp/product/B00186Q7XY/ref=ppx_yo_dt_b_asin_title_o08_s01?ie=UTF8&psc=1
 

With these as controllers:

https://smile.amazon.co.uk/gp/product/B07RHMMM9W/ref=ppx_yo_dt_b_asin_title_o08_s01?ie=UTF8&psc=1

Edited January 26, 2021 by SMF

[SMF]

One other thing to make up whilst your at the dew strips is a hand warmer or two. A god send the last few nights. 

[wookie1965]

1 hour ago, Seelive said:

Are you intending to use just 10 4.7ohm resistors around an 8" scope? That would space them about 70mm apart so any heating will be very localised rather than being evently spread out around the scope, and at just 0.3Watt per resistor I don't believe that would be enough to raise the temperature sufficiently to act as a dew repellent under most circumstances. 

No I want it for a 5" Refractor. 

1 hour ago, wulfrun said:

Have you considered Constantin wire instead of Nichrome? It can be soldered. I think Kanthal might be solderable too but maybe not as readily. Kanthal is commonly used in E-cig heaters so it's cheap.

No I have never heard of that I will look into thanks. 

33 minutes ago, SMF said:

 

Personally I used 330ohm resistors in parallel spaced 25mm. Work a treat wrapped in Velcro.

RS are far cheaper and you’re guaranteed good products.

https://uk.rs-online.com/web/p/through-hole-fixed-resistors/2141888

https://smile.amazon.co.uk/gp/product/B00186Q7XY/ref=ppx_yo_dt_b_asin_title_o08_s01?ie=UTF8&psc=1
 

With these as controllers:

https://smile.amazon.co.uk/gp/product/B07RHMMM9W/ref=ppx_yo_dt_b_asin_title_o08_s01?ie=UTF8&psc=1

I already have 330ohm resistors which I am going to make a dew heater with in a ladder configuration. 

[wookie1965]

31 minutes ago, SMF said:

One other thing to make up whilst your at the dew strips is a hand warmer or two. A god send the last few nights. 

That is a good idea I do wear gloves and they seem to be warm enough at the moment. 

[wulfrun]

1 hour ago, wookie1965 said:

No I have never heard of that I will look into thanks.

Apologies I mis-typed it. It's constantan not constantin.

[7170]

Soldering nichrome wire to the power supply copper cable is easy. Just clean it, then twist together the nichrome and copper wire strands into a tight bunch (or ball shape), and then solder (so it is all covered).

The solder will hold onto the copper fine and should fully encapsulate the nichrome wire, resulting in a very strong joint.
 

Another option is to tie the nichrome and copper into a knot, and again solder, so it fixes onto the copper and fully encapsulates the nichrome wire at the joint.

Both ways are fine, the secret is encapsulation by solder rather than normal methods. A higher wattage soldering iron with large tip helps to keep the solder all molten when you are encapsulating.

Edited January 27, 2021 by 7170

[wookie1965]

10 hours ago, wulfrun said:

Apologies I mis-typed it. It's constantan not constantin.

I wondered why I was getting listings for razor wire 😆

[wookie1965]

9 hours ago, 7170 said:

Soldering nichrome wire to the power supply copper cable is easy. Just clean it, then twist together the nichrome and copper wire strands into a tight bunch (or ball shape), and then solder (so it is all covered).

The solder will hold onto the copper fine and should fully encapsulate the nichrome wire, resulting in a very strong joint.
 

Another option is to tie the nichrome and copper into a knot, and again solder, so it fixes onto the copper and fully encapsulates the nichrome wire at the joint.

Both ways are fine, the secret is encapsulation by solder rather than normal methods. A higher wattage soldering iron with large tip helps to keep the solder all molten when you are encapsulating.

Yes I have seen someone make a hook with the nichrome and hook the normal copper cable around it then soldering very similar to what you are describing thanks.

[wookie1965]

Update made two dew strips one in series using 8. 4.7ohm resistors and one in parallel with 11 330ohm resistors both getting warm but not overly warm.