Mini Dew: Lens Specific Dew Heater [Prototype]

[AstroMadd]

THE MINI DEW PROJECT:

Being as I don't own a proper scope (yet) I will be doing Astro Photography with either a CMOS/CCD imager or a DSLR and Canon L series lenses ... hence why this project is called Mini Dew ...

On the cold nights we have an added task to keep the dew at bay so I decided to build a specific Dew Heater for both my Canon 24-70mm (67mm / 3" dia) and 70-200mm (77mm / 3.5" dia) lenses ... all I done was measure the length needed for the wider diameter lens (the 24-70mm) which ended up being 27cm (10.5") long ... the overlap on the 70-200mm will not cause a problem, as this is wrapped around on to the existing Velcro strip, to tidy it up.

The Research:

After doing a lot of research of various sites and the different variations of dew heaters out there, I decided on the resistor network system, as opposed to Nichrome (NiCr) wire ... the decision was down to ease of the build and soldering resistors to tinned copper wire seemed easiest, for me ... one issue you can get with NiCr wire is burnt-spots, this is where the wire can have weak points or short-outs if not insulated well ... with resistors soldered in the parallel setup they never pose a shorting issue and would rarely burn out, unless abused with voltages higher than specified ... there's a few links at the base of this post.

The Build:

The Mini Dew Project ended up being based on Ron Keating's excellent 'Homemade Heater Strips' documentation, from his site on Dewbuster.com ... on there he shows examples of different resistor networks and setups as well as the calculations needed to create the ideal resistor network for your scope.

The Resistor Network:

The easiest way to determine how many resistors are needed, is to measure the circumference of your lens (or scope) to get the actual length of the strap (which I call the 'heat zone') this is the area of the scope needed to be heated ... then add a few inches for the Velcro 'catch' ... then draw out the resistor positions, I found metric squared paper perfect for this and put vertical lines at 15mm (5/8") intervals ... it's up to you whether you want to add or takeaway the final resistor, if it appears to be over or under the measured length ... the width of the resistor network, once all soldered together, cut & cleaned, is approx 12mm with the resistors that I used ... then finally I soldered the power cable to the ends, this was a short length of very flexible 5mm silicon cable, with some sleeving added.

My (max) lens circumference, for the Canon 24-70mm L Lens, came to 27cm (10.5") ... so @ 15mm intervals that equated to bang on 19 resistors ... the resistors used were 330ohms, 1/2Watt, metal film ... the resistors will be generating their full wattage, when the controller is at full power ... with the 70-200mm lens there is a little bit of overlap, due to being a smaller diameter lens (67mm), this is attached onto the existing Velcro strip.

The Calculations:

• Voltage (supply) = 12v DC @1 Amp
• Resistance = 19 x 330 Ohm (wired in parallel)
• Overall parallel resistance = 17.37 Ohms
• Current = V / R = 12v / 17.37Ohms = 0.690 Amps (when @max)
• Wattage = V * I = 12v * 0.690A = 8.28 Watts (when @max)

Those numbers fall in-line with Ken's calculations ... the resistor network does get damn hot (to the touch) on full power, as it should ... but when it's in the Dew Strap it's comfortably hot ... so heat is getting through.

The Controller [PWM]:

For the controller I used a simple PWM motor speed controller from China, along with a bulk of other electronics I bought for Arduino projects ... this controller is based on the very simple 555 Timer PWM circuit ... PWM is the best way to go for controlling Dew Heaters, as they are pretty efficient, especially for your battery packs ... I could've actually built the circuit smaller than the PCB version from China (using Veroboard) but not for less, as the whole unit cost only $1.50  (at that time) ... I did have to cut & trim the PCB, so it fit snugly in the small enclosure ... and some of the on-board circuitry was not needed, i.e. the 5v regulator stage ... I also de-soldered the connectors to make space.

 

 

The Dew Strap:

I've seen on various sites where duck tap and self sticking draft excluder have been used ... but duck tape being common on a lot of designs though ... I understand this is to get maximum heat exchange but this is an experiment and I prefer a more reliable solution, as in stitching the materials together ... cue Mother-in-Law .

The Mini Dew strap is 40mm wide and consists of 5 layers ... the bottom layer (lens/scope contact) is denim, which is then followed by the resistor network, followed by folded cotton ribbon (2 layers), followed by the Velcro strip (the fluffy side) ... I asked M-in-L to create a pocket (a glove, so to speak), which will be approx 20mm wide, so the resistor network can be feed down inside of it ... this can then be removed, mainly for testing but also for tweaking values if need be ... being as the resistor network is roughly 12mm wide it's a perfect fit for the pocket.

In order to get the dew strap and the (glove) pocket just right I used 20mm (width) Velcro ... firstly a full length piece of Velcro was sewn on to the folded cotton ribbon material ... yes this Velcro length could be a lot shorter in reality but being a prototype I wanted to try other small lenses like a 50mm ... so this length can be set to taste.

 

Next, a small length of the opposite side of the Velcro (the hook), is sewn on to the denim, at the pocket entrance, this piece is only 70mm long.

With the denim layer lined up and underneath the other layers, a stitch is sewn down both sides of the Velcro (not on it), which in turn creates a 20mm (approx) glove pocket ... it's gets a tighter fit when you take into account the thickness of the resistors ... but it's free enough to feed the network to the end of the strip.

As a final finish the whole lot is sewn together, around the outsides but allowing the pocket hole exposed, to feed the resistor network in ... I must say this turned out better than expected ... especially for a prototype.

If this was a final design I would tightly sew some stitching around the power cable entrance to anchor that a bit more and I would also add a bit of stitching at the far end by the last resistor, which would then anchor the whole network in place and prevent it from being pulled out of the (glove) pocket.

After Thoughts:

As an after thought (while typing this up) I realized there is a second (glove) pocket, which was created with the folded cotton ribbon ... this could be an opportunity to slide an insulator in there of some kind ... and would help with any escaping heat issues ... obviously the next version will have a bit more padding on the outer section.

Version 2.0 [Revisions]:

1. Number one priority is to swap out the potentiometer with a switched version, as this would be far more practical to have a power switch built-in.
2. Add an LED Power Output indicator ... I was going to use just a standard LED and rely on its brightness proportional to power output but the brightness indication is not really practical, so I've opted for a digital read-out device which will give a better rendition of the output  ... basically it's a voltmeter and yep, a few are on order from China @ $1.00 ... the device is very small and 'should' fit into the existing enclosure, if not then the next size up will be fine.
3. Thicker outer insulation for the Dew Strap itself.

Testing:

I have yet to test this out in the field but I'm confident it will work ... the controller works well and varies the heat pretty good ... I shall put this on test for a good hour to make sure no burn marks occur, or a visit from the fire brigade happens.

 

 

When I get the other bits, as per the revision, I'll update this post ... but first I gotta go test this puppy out with my home-made Barn Door Tracker.

LINKS:

Ron Keating - Homemade Heater Strips [Resistors]:
http://www.dewbuster.com/heaters-330ohm-resistors.html

Ron Keating - Building Nichrome Wire Dew Heaters:
http://www.dewbuster.com/heaters-nichrome.html

Mark Kaye - NiCr Wire Dew Heater:
http://www3.sympatico.ca/mark.kaye/dewheat.htm

NiCr Wire:
https://en.wikipedia.org/wiki/Nichrome

[knobby]

Looking good !

[michael8554]

11 hours ago, AstroMadd said:

one issue you can get with NiCr wire is burnt-spots, this is where the wire can have weak points or short-outs if not insulated well

Before I read this I would have said it's so much faster and easier with NiCr wire.

Michael

[Gina]

I used Nichrome wire in the past but now use resistors.  I too had problems with hot-spots and burning.  Also, with connecting to Nichrome wire, won't solder, has to be crimped or use screw terminals. 

[AstroMadd]

On 12/7/2017 at 13:25, knobby said:

Looking good !

Thanks knobby ...

17 hours ago, michael8554 said:

Before I read this I would have said it's so much faster and easier with NiCr wire.

Michael

Yes you'd think so Michael ... only from the information I've read from some sites my decision swapped to using resistors ...

9 hours ago, Gina said:

I used Nichrome wire in the past but now use resistors.  I too had problems with hot-spots and burning.  Also, with connecting to Nichrome wire, won't solder, has to be crimped or use screw terminals. 

Yes Gina crimping/screwing is the way for most tough (non solder-able) metals ...

[IanL]

Suggest you read comment #2 on my article about building nichrome bands here:

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

I have not built any bands for lenses, but the chap who posted the comment did. He reported that 0.3 W/cm is way too high for a lens and will cause the grease in the diaphragm mechanism to become mobile and end up damaging the lens badly.  Appreciate you will be dialling the power down somewhat but it is a risk you should be aware of.

Basically if the band is warm (or hot!) to the touch then it is far too warm for a lens or small eyepiece. It might be OK for a large SCT where you're going to need a reasonable amount of heat due to the large thermal mass of the scope, but a band for a smaller device should be cold to the touch.

Regarding nichrome:

- My post shows how you can solder a joint securely with a bit of ingenuity - the issue is that the solder won't wet the nichrome at all, so you need to knot the nichrome and the copper connecting wire together using a couple of loops, then solder the copper to itself trapping the nichrome. Crimping also works, but nichrome tends to be very thin and easily slips out of a crimp so knotting it to the copper is a good idea regardless.

- I've not personally had any problems with shorting, burning or hot spots in my DIY nichrome bands which are getting on for five years old. I can't see why a resistor ladder would be any less prone to shorting out to be honest? The resistor leads are fairly fragile (unlike nichrome) and a short is equally likely in my view. Either way, make sure you use an appropriately rated fuse for safety.

[Gina]

Ah yes, I've used knotting of Nichrome and copper wires followed by soldering the copper - I'd forgotten about that.

[BigRD]

Your bands look great, you've put my duct tape and insulation ones to shame.

I've just put together a couple using the parallel resistor method. Initially one for my guide scope (135mm Carl Ziess lens) to wrap around the built in shield then I made another to fit around my camera Tamron lens hood at 67mm diameter. The controller is made up of 2 x switched PWM controllers from Amazon They're only £1.89 each so was worth it to try out. I tested in anger last night and all went very well 3 hours of imaging in -5C my 130PDS was white with frost but my guide scope was clear. Chuffed :-)

Band 1 = 16 x 330ohm @ 12Vdc, 0.58A max = 6.9W

band 2 = 20 x 330ohm @ 12Vdc, 0.72A max = 8.7W

[AstroMadd]

3 hours ago, IanL said:

Suggest you read comment #2 on my article about building nichrome bands here:

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

I have not built any bands for lenses, but the chap who posted the comment did. He reported that 0.3 W/cm is way too high for a lens and will cause the grease in the diaphragm mechanism to become mobile and end up damaging the lens badly.  Appreciate you will be dialling the power down somewhat but it is a risk you should be aware of.

Yes I did read (and bookmark) your article Ian and with all due respect that information is irrelevant from my project's point of view ... but it does show issues using nichrome, more so with the chap's comment ... with resistors the W/cm and heat transfer will be different to NiCr and I am fully aware of the heat Dew Straps can put out and I would be very cautious when heating my expensive lenses ... hence why mine is a 'prototype' and I am testing it fully ... I shall be adding a resistor network, to the potentiometer, to calibrate the correct required temp needed for my lens, when set at max output, therefore not exceeding the rated setting that 'could' damage the lenses ... I would suggest all 'home-made' dew heaters have a power control of some sort and not directly connected to 12v supplies ... unfortunately I cannot edit my original post to clarify a few things ...

6 hours ago, IanL said:

Regarding nichrome: - My post shows how you can solder a joint securely with a bit of ingenuity - the issue is that the solder won't wet the nichrome at all, so you need to knot the nichrome and the copper connecting wire together using a couple of loops, then solder the copper to itself trapping the nichrome. Crimping also works, but nichrome tends to be very thin and easily slips out of a crimp so knotting it to the copper is a good idea regardless.

I did add a link at the end of my thread that shows how Mark twists & weaves the nichrome into the copper wire, then solders the whole lot together and anchors the NiCr in place ... so that method is also effective ...

6 hours ago, IanL said:

- I've not personally had any problems with shorting, burning or hot spots in my DIY nichrome bands which are getting on for five years old. I can't see why a resistor ladder would be any less prone to shorting out to be honest? The resistor leads are fairly fragile (unlike nichrome) and a short is equally likely in my view. Either way, make sure you use an appropriately rated fuse for safety.

Looking at my images, you can see how the resistor ladder is formed, it would be pretty hard to get a short ... even if a resistor breaks ... my resistor leads are soldered at their minimum, their wire is wrapped round the main feed wire ... the only flexing that occurs is by the 22SWG tinned copper wire the resistors are soldered to but to be honest the strap will always be in a curved state, so there won't be much flexing going on ...

I may add a fuse to the actual enclosure, if space permits ...

53 minutes ago, BigRD said:

Your bands look great, you've put my duct tape and insulation ones to shame.

I've just put together a couple using the parallel resistor method. Initially one for my guide scope (135mm Carl Ziess lens) to wrap around the built in shield then I made another to fit around my camera Tamron lens hood at 67mm diameter. The controller is made up of 2 x switched PWM controllers from Amazon They're only £1.89 each so was worth it to try out. I tested in anger last night and all went very well 3 hours of imaging in -5C my 130PDS was white with frost but my guide scope was clear. Chuffed :-)

Band 1 = 16 x 330ohm @ 12Vdc, 0.58A max = 6.9W

band 2 = 20 x 330ohm @ 12Vdc, 0.72A max = 8.7W

Excellent BigRD ... and yes those PWM controllers, on Amazon, are very similar to the one I used, just laid out differently, the circuit is the same apart from the output driver stage ... bargain ...

Martin

[Stub Mandrel]

Very neat. I've 3D printed an enclosure for an ebay 12V PWM controller but not made a dew band yet, I may  shamelessly copy your design.

At the moment my guidscope and now my 58mm prime lens are being kept clear using disposable heat pads, I just ordered another 40  (20 packs of two) working out at about 60p a night!

[BigRD]

I just came across another low cost alternative to the parallel resistor dew heaters or at least the basis for making some very easily :-) Bigclivedotcom worth for the parts alone.

 

Roy

[AstroMadd]

53 minutes ago, BigRD said:

I just came across another low cost alternative to the parallel resistor dew heaters or at least the basis for making some very easily :-) Bigclivedotcom worth for the parts alone.

Roy

How funny is that ... I'm subscribed to Clive's channel and saw this, this morning ... I have seen other heated gloves/pads videos, a few months back and 'was' thinking Dew Strap maybe? ... but after watching Clive's 'teardown' video I thought hmm, maybe not, due to the build quality of the actual pads ... would they really work? ... I suppose for the price they'd probably be worth a punt ...

[Stub Mandrel]

Thanks for that, led me to USB heater elements for gloves hoes etc. for £1.66 a pair, ordered two pairs!

[Uplooker]

great write up, very comprehensive. This will no end of hepl to others, thanks for posting.

[BigRD]

12 hours ago, AstroMadd said:

How funny is that ... I'm subscribed to Clive's channel and saw this, this morning ... I have seen other heated gloves/pads videos, a few months back and 'was' thinking Dew Strap maybe? ... but after watching Clive's 'teardown' video I thought hmm, maybe not, due to the build quality of the actual pads ... would they really work? ... I suppose for the price they'd probably be worth a punt ...

Haha I do enjoy Clive's videos. Possibly not the best quality resistive pad, I wondered if cutting the fingers off and using as a heated woolly sleeve over the guide scope  Mine has an extending shield so unlikely to damage anything and as Clive says it's worth for the cable and plugs alone.

[AstroMadd]

I've traced out the PCB and drew up a schematic ... this circuit diagram is the heart of the Mini Dew Project, i.e. the controller I used ... it's based on the classic NE555 Timer PWM circuit, that produces an output Duty Cycle waveform ... I've also created a Veroboard layout as well, for those who feel brave ... they have both been updated to v.1.10 [14th Dec 2017] ...

NOTE:
The 5volt regulation stage (78L05 - U2) IS used in my design ... I did say above in the OP that it was not but after tracing the PCB I found it actually powers the 555 timer circuit separately ... U2 could be removed, so the PWM circuit gets powered by the 12v supply ... but I have not tested the outcome of this ...
 

Schematic v.1.10:

 

Veroboard Layout v1.10:

 

Martin

 

[Stub Mandrel]

Looks good, but a couple of thoughts. You should not need D4 with a totally resistive load, also the NE555 will run off a 12V supply so you can save yourself a few components if you want.

[AstroMadd]

21 hours ago, Stub Mandrel said:

Looks good, but a couple of thoughts. You should not need D4 with a totally resistive load, also the NE555 will run off a 12V supply so you can save yourself a few components if you want.

Yes exactly that Neil ... I have updated the schematics above ... I just wanted to trace the actual PCB out, so others can easily follow ... by all means MODs can be made to this project, to make it more efficient ... and yes D4 is only needed for inductive loads (hence why the controller was a motor speed device initially) ... ... it's only because I bought the controller ready made for $1.50 and couldn't build that for less, if not bought from China ... but if I was to build this I would definitely MOD it ... ...

When I traced the board out I did wonder why they didn't power the 555, with the 12v DC supply but added the 5volt regulator in-place ... are they being over cautious with the Gate input (of the MOSFET)? ...

Looking at the PCB again, I see there is a location for an R4 resistor, which basically goes across the input and the output of the 78L05 regulator (U2) ... so one 'could' potentially put a short in its place and therefore bypass the 5v regulator stage altogether and have the whole unit powered by the 12v supply ... obviously U2 could then be removed, or not used at all in the MOD ...

Martin

[Stub Mandrel]

4 minutes ago, AstroMadd said:

are they being over cautious with the Gate input (of the MOSFET)? ...

They've already got a zener on it... belt, braces and a bit of string!

[AstroMadd]

21 hours ago, Stub Mandrel said:

They've already got a zener on it... belt, braces and a bit of string!

Well to be honest I'm guessing that's a Zener (due to the packaging) ... it could also be a TVS Diode (Transient-voltage-suppression diode) for ESD, rather than clamping the gate voltage ... dunno ... I'm starting to confuse myself now ... but mainly the information supplied within this thread is for people who want to tinker with this project ...

Martin

[BigRD]

Thanks for drawing this up AstroMadd, with the exception of the pot I'm sure I have enough odds 'n ends to put this together just for the fun of it. Though probably still cheaper to buy from China