Mod Number 2 - Secondary Dew Zapper (Long Post)

[daz]

Spurred on by the success of my mirror cooling mod, I have forged ahead a built myself a simple dew zapper for the secondary mirror.

It's fairly straightfoward (in theory) but does require removing the secondary and soldering.

THE MATHS

I did some research and reckon I needed to generate around 3 watts of heat in order to keep the dew off the mirror. Basic Ohms Law tells me that 45 ohms resistance is required for my 12V DC supply. This will draw approx 0.26 amps, so no big load. In order to produce the desired resistance, I made three sets of resistors, using two resistors in each set. Each resistor is 270 ohms.

Parallel resistance is simple (1/n1) + (1/n2) + (1/n3) ... where 'n' is the resistor value. So two values of 270 ohms gives a total of 135. Three lots of 135 in parallel gives me 45. Bingo!!

The kit consisted of:

Small black plastic project box

4 x 2.5mm DC Sockets

4 x 2.5mm DC right angle leads

3 x red LED's

3 x toggle switches

1 x Cigar adaptor

MAJOR STEPS

-----------------

Drill holes for the LED's, sockets and switches

Wire the box

Build resistor sets

Remove the secondary AND KEEP SAFE

Drill small holes for power leads in tube

Glue resistor sets to secondary

Solder power leads to resistors on secondary

TEST

Replace secondary

Run leads through small holes to switch location

Make up the main power lead

THE PICTURES

OK, a short series of some of the manufacturing process...

The box showing the LED's, switches and 2 of the power sockets. The bottom one is the incoming power. The Finder Cap is give an idea of scale.

The secondary mirror with the resistors glued and wired in place. Notice none of the leads overlap the edge of the mirror!

The small holes for the fly-leads. I put them next to the vane holders so they would'nt show too much

The finished secondary back in place. Good old duct tape holds the wires out of the way (silver will be replaced with black v.soon!)

The finished switch box - well nearly! I forgot I needed resistors for the LED's so I had to disconnect them for now.

The box is then velcroed to the tube at a convenient location!!

The circuit works perfectly, just need to road test to make sure I have the resistor values correct. If not, its relatively straightforward to change them.

Stage 2 is then a heater for the finder, which will plug into the same box, as will Stage 3, the eyepiece heater!

I shall keep you updated

[daz]

And yes, this mod was the cause of the mirror disaster. Being careless and lazy when removing the secondary

[GazOC]

Nice one Daz, I didn't understand a word of the first couple of paragraphs though.....

Did you remove the secondary without tilting the scope downwards.....?

[daz]

Did you remove the secondary without tilting the scope downwards.....?

<sheepish voice> Yes

[GazOC]

Daz!! :clouds2:

[The Warthog]

Stage 2 is then a heater for the finder, which will plug into the same box, as will Stage 3, the eyepiece heater!

I shall keep you updated

Hence, three switches, right? I thought at first you might be making a variable heater by powering each resistor separately.

I understand the theory, but not the practice. What did you make the resistance out of? Are these just resistors like you would use in a radio circuit, and you are relying on them to get hot as the current passes through them. I sort of expected nichrome wire or something like that. So, where do you get the components?

[daz]

Hi Warthog

The resistance was indeed standard resistors which I got from Maplins (similar to Radio Shack). I couldn't find any nichrome wire with a high enough resistance per metre to make using that viable for this one. I am searching again for the finder version.

And yes, the idea is to use the resistors natural property of heat dissipation to zap the dew. Each resistor is a 0.25 W rating, and as I am only generating 3 Watts total, I'm under the limit.

At least, that's the theory!!

[mark RD]

You sould have used EL44 valves, you could keep your hands warm with those as well

[daz]

Yeah but Maplins don't stock them anymore!!

[lunator]

Daz

Just put this thread link on a cloudynights post.

Someone was asking how to make a dew zapper and your design is so straightforward.

It may bring a few more US based guys into the forum.

Cheers

Ian

[daz]

Thanks Ian, most kind

In my last observation post I made the comment that the EP dewed up, the finder dewed up, and even with a light frost on the OTA, the secondary was nice and clear.

[daz]

Might have found a suitable wire as well.....

[radioactive]

Might have found a suitable wire as well.....

a pretty common nichrome wire that astronomers use is that taken out of popup toasters

I do have however a full 20 metres of 0.25mm nichrome I got to make a dewzapper or three

if anyone is really stuck

[The Warthog]

Might have found a suitable wire as well.....

a pretty common nichrome wire that astronomers use is that taken out of popup toasters

I do have however a full 20 metres of 0.25mm nichrome I got to make a dewzapper or three

if anyone is really stuck

How much nichrome wire, combined with what power supply, will keep an eyepiece from fogging up? I'd hate to toast an ep. And where do your wrap the nichrome wire? Would putting it at the top of the focuser tube do the job?

[radioactive]

Might have found a suitable wire as well.....

a pretty common nichrome wire that astronomers use is that taken out of popup toasters

I do have however a full 20 metres of 0.25mm nichrome I got to make a dewzapper or three

if anyone is really stuck

How much nichrome wire, combined with what power supply, will keep an eyepiece from fogging up? I'd hate to toast an ep. And where do your wrap the nichrome wire? Would putting it at the top of the focuser tube do the job?

WH this link may help answer that if not there are plenty of others available

http://www.backyard-astro.com/equipment/accessories/dewheater/dewheater.html

[daz]

That's a nice simple design!

Anyone know where to get circuit boards printed??

I would like to convert mine to the switching version....

WH - Around 3 watts of power should do the job on an EP. The resistance needed depends on the power supply. Different wires have different values, so you need to know this before you can calculate.

[Kaptain Klevtsov]

just to be picky - the guy mentions that the Darlington transistor that he uses can dissipate 65W - that is not how much it wil switch, that's how much it can dissipate.

If the switched on Darlington has a resistance of, say 1/4 ohm (ought to be less than that), then the volt drop at a current of 5.4A (65W/12V) the voltage across the Darlingon will be 1.35V (0.25Ohm x 5.4A). The power loss in the Darlington transistor will therefore be 7.29W (1.35V x 5.4A) which is nowhere near the 65W dissipation rating of the transistor.

This aside, the maximum current of the TIP120 is 5A which would give a maximum power rating at 12V of 60W

In summary, the dissipation of the transistor has a relationship to the power being switched, but they are not the same.

HTH

Captain Chaos

[daz]

CC

OK - if then I added another circuit in parallel to the output, I need to figure this into the power loss for the Darlington to ensure it does not exceed the 65W. Correct?