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DIY 4 channel dew heater controller and heater strap!


abhoriel
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Yet another DIY dew heater controller made using the Chinese 12v LED dimmers..! I had lots of help from my wife on this one, she hand-sewed the dew strap, helped me interior design and assemble the box and had to affix a red bow at the end!

This one has 4 channels which is kind of overkill.. but I ended up with enough bits to do it and realised that the box was large enough so why not? Internally it has a fuse holder with four 5amp blade fuses (one for each channel). There are also LEDs which get brighter proportionally to the 'heat' you want. It was quite hard to assemble it all in the box because I connected everything with relatively thick 18AWG wire. Also, I managed to destroy an LED dimmer at one point in a most unfortunate accident! I was testing the circuit and the bottom of the dimmer PCB touched a metal screw hole on the corner of the box in such a way that it shorted out a capacitor and blew the circuit with a nice blue flash. The outer layer is waterproof nylon fabric.

I can provide a parts list and cost if anyone wants. But this is something where DIY really does save some money, and its not too complicated. I'm not very good at DIY or electronics, but at least I remember the basic equations like ohms law from physics at school.

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Edited by abhoriel
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Nice build. What gauge nichrome wire did you use, and what was your ohm's calculation for the band shown? 5 amp fuses for each channel are a bit big for this IMO. I used 1x 5 amp master fuse for the whole of my box, and 1 amp for each double heater channel, and even that is likely to be too big and may need changing. I used 0.5 amp fuses for my adjustable voltage channels to run my DSLR's. 

 

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8 hours ago, Daz69 said:

Nice build. What gauge nichrome wire did you use, and what was your ohm's calculation for the band shown? 5 amp fuses for each channel are a bit big for this IMO. I used 1x 5 amp master fuse for the whole of my box, and 1 amp for each double heater channel, and even that is likely to be too big and may need changing. I used 0.5 amp fuses for my adjustable voltage channels to run my DSLR's. 

 

 
 
 

Yeah I agree on the fuses! 5 amps is too high. If 5 amps flowed through an 80ED sized dew heater, it would get very hot indeed. 5 amps are just the smallest blade fuses that I have around. I would rather use 1 or 2amp fuses. I think that 1amp fuses protecting a double channel might be a little on the small size, but it depends how you use it! If you plugged in two astrozap 4 inch telescope dew straps into one double channel and had it at maximum power (which I think is a somewhat reasonable usage scenario), they would draw 1.22amps. 

 I thought I'd written about the wire that I used, but I must have accidentally deleted it..

I'm using nichrome wire with a diameter of 0.35mm. I measured its resistance at 10ohms/meter. I use two 40cm loops of nichrome wire (for a total length of 160cm) within the dew straps, which results in a total resistance of 16 ohms. The current with a 12v supply is therefore 0.75amps. So there is a maximum of 9 watts of power (22.5watts per meter) This is quite warm, but that's a maximum temperature.

Edited by abhoriel
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To correctly rate the fuse take the maximum current being drawn on the circuit, add 10% and then use the next readily available rated fuse above that.  It isn't really an exact science which is why I say next readily available fuse, because as noted above it is actually there to protect the circuit and further damage to equipment, not necessarily directly protect the equipment, but you can rate it too high i.e. a 10A fuse on a circuit with 6A rated cable isn't going to work as the cable will melt before the fuse blows

Edited by RayD
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I'd rather the fuse blow too soon than get to the point where the nichrome wire is burning through the strap and damaging the scope/eye piece. My understanding was to keep the heater band just above the air temp around it to stop the dew, not to heat it up so it's much hotter? A lot of people keep their E/Ps in their pocket to keep warm, so I based my theory on that, and from what I've read on here about dew bands and temps.

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On 22/02/2017 at 11:23, Daz69 said:

I'd rather the fuse blow too soon than get to the point where the nichrome wire is burning through the strap and damaging the scope/eye piece. My understanding was to keep the heater band just above the air temp around it to stop the dew, not to heat it up so it's much hotter? A lot of people keep their E/Ps in their pocket to keep warm, so I based my theory on that, and from what I've read on here about dew bands and temps.

Sorry for the slow reply. Yeah, with a refractor, you only want to heat the front element to above the dew point. It shouldn't really feel warm/hot, because that'll probably be high above the dew point. I've read that if you it gets too hot, you can cause air currents which will affect your images. 

As for what the temperature of the dew strap itself should be, I guess it depends. Depends on the ambient temperature, dew point, how well its conducting heat to the dew shield, how well the dew shield conducts/radiates heat onto the front element. You really want it hot enough to stop dew, but no hotter than that..

The way I've made my straps, its unlikely that the nichome will short out in such a way that the heat output doubles or quadruples (the nichrome loops have been sewed onto the fabric, so it cant really move). Probably the most likely thing is that a solder joint or wire breaks loose and touches another wire either within the strap, the plug or the controller box which will result in lots of current flowing, so even a 5amp fuse would be fine. But a 2-3 amp fuse may be a little safer.

Choosing a 1amp fuse for a two plug channel might make using both plugs simultaneously unreliable. It also limits you to using dew straps for small scopes.

Its really a balance between safety, allowing the device to be useful for larger straps and reliability. A fuse blowing in the middle of an imaging session might go unnoticed for some time. Where you draw this line is up to you.

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7 minutes ago, abhoriel said:

Its really a balance between safety, allowing the device to be useful for larger straps and reliability. A fuse blowing in the middle of an imaging session might go unnoticed for some time. Where you draw this line is up to you.

Absolutely. I started at 1amp as I'm more likely to use only 2 straps at once, one through each controller. If they blow then I'll go to 2amp. If I get to using both channels on each controller, then it may have to go up, but I don't see it getting to 5amp. I could be wrong and if so I'll stand to be corrected. I'll only be using short straps around the EP and secondary boss. If it gets to the point where I need a heated seat to sit on, I'll take up knitting by the fire :D 

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2 minutes ago, Daz69 said:

Absolutely. I started at 1amp as I'm more likely to use only 2 straps at once, one through each controller. If they blow then I'll go to 2amp. If I get to using both channels on each controller, then it may have to go up, but I don't see it getting to 5amp. I could be wrong and if so I'll stand to be corrected. I'll only be using short straps around the EP and secondary boss. If it gets to the point where I need a heated seat to sit on, I'll take up knitting by the fire :D 

https://www.astrozap.com/scripts/prodView.asp?idproduct=232

This is the largest strap astrozap have, and it only draws 2.15amps.. I'm unlikely to ever have a 16 inch scope, but you never know!!

 

The irony of my situation is that I purchased a fused 12v cigarette lighter plug for the entire dew heater controller, which comes with a fuse. I checked the fuse and its only 3amps anyway...!

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Nice build, looks similar to my one I built, I added little rocker switches to each channel so I could isolated the unused ones. I used some of the outputs for LEDs around the scope like on the clutches so I can easily locate them in the dark and being dimable, also one output runs the cooling fan for the primary mirror also being able to control the speed.

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18 minutes ago, SW130p said:

Nice build, looks similar to my one I built, I added little rocker switches to each channel so I could isolated the unused ones. I used some of the outputs for LEDs around the scope like on the clutches so I can easily locate them in the dark and being dimable, also one output runs the cooling fan for the primary mirror also being able to control the speed.

Cool, sounds good. I didn't bother with rocker switches, but considered it. I was trying to keep costs to a minimum. I was considering getting some LED strips for this purpose actually! For controlling the speed of a motor, assume you are adjusting the voltage rather than using a pulse-width modulation method.

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16 minutes ago, abhoriel said:

............ assume you are adjusting the voltage rather than using a pulse-width modulation method.

Not quite. You're slowing the voltage ripple so the motor spins slower. Pulse width is exactly that. The closer the pulses, the brighter, hotter, or quicker something happens. Turn the dial down to lessen the width and things get dimmer, warmer, and slower. 

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1 minute ago, Daz69 said:

Not quite. You're slowing the voltage ripple so the motor spins slower. Pulse width is exactly that. The closer the pulses, the brighter, hotter, or quicker something happens. Turn the dial down to lessen the width and things get dimmer, warmer, and slower. 

The fan is connected to the PWM output, works fine.

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Just now, SW130p said:

The fan is connected to the PWM output, works fine.

The fan will work fine, as described in my reply above. You are slowing the pulse width, and so the motor will spin slower. It doesn't adjust the voltage as voltage regulator would do by taking away excess power in the form of heat. 

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2 minutes ago, Daz69 said:

Not quite. You're slowing the voltage ripple so the motor spins slower. Pulse width is exactly that. The closer the pulses, the brighter, hotter, or quicker something happens. Turn the dial down to lessen the width and things get dimmer, warmer, and slower. 

Yeah I was wondering if the motor would be designed to cope with the current being turned on and off all the time. But thinking about it, I suppose it doesn't matter. I guess the current is turned on and off constantly in a motor anyway when the brushes contact and decontact.

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Yes, that's right. However PC fans are brushless. It still won't cause any problems for the motor, except that the rotor just doesn't spin as quickly. PC fans will probably run even if your voltage input was as low as 3v, and will still work fine, just slower. A "PWM" is the flashier, more cleaner way of making something work slower/faster, cooler/hotter or dimmer/brighter by slowing down the DC ripple.

I use a PC fan in a home made magnetic stirrer for making vape E-liquid at home, and that runs at 5.5v via an LM317T chip, a 1k ohm potentiometer and a 330 ohm resistor and various smoothing capacitors. If I used a 120 ohm resistor, I'd get around 11v. However because the mixing requires a long slow mix to start, if I had 11v running through, things would get hot and the carbon track potentiometer will eventually burn out. I could've used a PWM for this, but had the parts available to build my own. 

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