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Urgent help needed with secondary dew heater.


Daz Type-R
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All.

Due to holidays/sickness/family life/work etc, I only have 11 days to sort my secondary mirror dew heater out before EMS1.

I need to do a spot of DIY as I am unable to buy a proper one due to the size of the mirror.

Can somebody/anybody confirm if the below will work / be sutible for my needs and not blow either myself or my Kendrick dew controller up in the process?

1. 1 x 7 watt, 47 ohm wire wound ceramic resistor.

2. A sutible length of 2 core flex (all ready got)

3. A phono plug.

4. Some means of sticking the resistor to the back of the mirror - no idea what to use???????

Once all the above is wired up (God knows how but it cant be that difficult - right???) would you bother with any insulation around the resistor? I know somebody (Martyn) who has this set-up and it seems to work perfectly with no issues around the resistor giving off too much heat.

If anybody can help - it would be very much appreciated.

Regards,

Daz.

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I'm waiting for my resistor to arrive, but I am using or going to be using a 220ohm 5w aluminium clad resistor, stuck to the back of the mirror with a spot of silicone. Steve (Ibbo) has this on his and it works a treat. I'm just wiring mine up direct to the battery though, so it's either on or off.

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So I take it then it doesn't matter how little or how much juice you power up to it? If you are running direct from the battery, will that send the full force/watts/ what ever it's called of the battery up to the resistor? Will it not get too hot?

Sorry about the vaugness, I am a non sparky - :(

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The watt rating of the resistor doesn't indicate how hot it will get etc. Bassically the higher the ohms the lower the watts it will use / produce. A 200 ohm resistor at 12v will use / make 0.5 watts. And will be drawing approx 0.04 amps, so very little. a 20 ohm will use and produce approx 10 x that at 12v. The only important thing is that you don't exceed the watt rating of the resistor. I've ordered a 10watt only because that was what I could find at the right price and around the right resistance, also because that's what steve has :lol:

Left connected to 12v it will use 0.04 amps for as long as left on, but will only generate IRO 0.5 watts of heat. That should be nice to keep the mirror from dewing up, but not enough to make it hot. It certainly should not burn out at that voltage. I'm using an aliminuim clad one only because they are nice and small and have a very flat base which will allow the most surface contact to pass as much heat through.

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Ok, think I get that - so if I was to ask you nicely for a link as to where you got that resistor from, then wire it up to my Kendrick - I should be happy as larry (who ever he is)?

Also, I will be running it all off a 38A/H Baz battery via a Kendrick dew controller, so as long as the output of the dew controller does not exceed 5 watts, will that be okay?

Are you going to cover it in some sort of insulation?

And what sort of silicon will you be using to stick it to the back of the mirror?

Sorry all the questions but I am only going to have chance to try this once before EMS.

Edited by Daz type-r
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I'm interested in the ideas too please, for Bob. I've ordered some dew it yourself kit's for Waynes C8, but have drawn a blank for Bob's small secondary. :-)

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http://www.ebay.co.uk/itm/250924485700?var=550056839566&ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

I chose 220 ohm, 10watt. The Kendrick as I understand it will be acting like a pulse generator. All that means is it's turning the voltage on and off, it doesn't actually alter the voltage as such, just alters the amount of time it is live. It won't be sending watts to anything as such either. Without getting too complicated, I would say you should be fine yes. I'm just connecting mine directly to the battery because it's not going to get hot enough to need turning down, and I am too tight to buy a controller.

Reef Bob the Dob, not sure if there would be enough space for this type of thing on the mirror? the resistor is 19mm x 11mm x 10 mm thick so if you can fit that on then it's a suitable idea.

Re covering..... not bothering. Re sticking it, whatever silicone I have at the time I guess, or I have some very good double sided tape I might try instead.

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Guest Rooster

47 ohm sounds a little small in value to me...

Using 12v to power it, it will be using 0.25 Amps; Therefore dissipating 3 Watts.

The maths you need are:

Current in Amps ( i )

Resistance in Ohms ( r )

Volts. ( v )

Watts ( w )

i=v/r. (volts divided by resistance)

w=vi. (volts multiplied by current)

I'm experienced in electronics... But a noob in astronomy.From what I've seen the "dew controllers" use simple PWM (pulse width modulation) this acts like a switch, turning the voltage on/off in variable ratios... If this sounds Greek, imagine turning a lamp on and off quickly... On average the lamp would appear dimmer than usual... (exactly how a lamp dimmer works)

Just bought some PWM controllers off ebay to dim some led lighting strips.. About £5....I bet one would suit this purpose a treat !

Not sure how all the html tags got added - I posted from "tapatalk".... hopefully its easier to read now !

Edited by Rooster
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Hi Darren,

Where it states the wattage on a resistor, that is the max power they are capable of handling. It's isn't an indication of the heat they generate.

My controller failed once, and put full load onto the Telrad resistor, which is rated at 3W. It melted the plastic and I had to butcher the cover to get it open. It will generate a significant amount of heat if not controlled, and it risks causing irreparable damage to the mirror.

The battery doesn't care, if it has a load on it, it will want to discharge, by what in effect is a controlled short. The heat generated is simply the resistor getting rid of the energy it doesn't want to allow through.

The only problem I can see with double sided tape is the heat will soften the adhesive and it may fall off.

I have some silicone rated at 300 degrees C, and is black, I'll pop round Wednesday night if you are in. PM me and let me know Darren.

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Bob's mirror has literally a few mm on one edge showing, the rest is covered in the mirror holder. :wacko:

Our dew it yourself heater strip kits have come, that's going to fun - wish us luck. I wonder if a couple of the little resister things that have come with it can be made to fix the little secondary mirror? If so, I can tap off of our 2011 model 4 way controller, and do some cable knitting lol.

How effective is a hair drier? Is that too noisy - even at a star party? Does it blow to much dust and things into the tube?

I think for now he will have to look down with his cover on. ;)

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Baz, I was going to mention that about the double sided tape. That silicon you mentioned sounds good, pleased it's not glow in the dark lol.

Good luck Darren.

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I've wired it all up, plugged it in and left it on to see how warm it gets. So far coming up on 30 mins now and it is warm but not getting hot. That's straight across the battery with no control. I'm guessing it might be around 30 degrees. Bare in mind when on the mirror it will dissipate the temperature so will be considerably cooler.

I am confident it will work well. I have had to chop off both mounting lugs though as one stopped the resistor from sitting far enough towards the stalk, and the other was just starting to show from behind the mirror. Other than that, so far so good.

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I've wired it all up, plugged it in and left it on to see how warm it gets. So far coming up on 30 mins now and it is warm but not getting hot. That's straight across the battery with no control. I'm guessing it might be around 30 degrees. Bare in mind when on the mirror it will dissipate the temperature so will be considerably cooler.

I am confident it will work well. I have had to chop off both mounting lugs though as one stopped the resistor from sitting far enough towards the stalk, and the other was just starting to show from behind the mirror. Other than that, so far so good.

Excellent news, it sounds like you have is sorted. My mirror only has a tiny bit of the back showing, so I'm going to a struggle I think.

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The resistor does show a tiny bit but opinion is that it will be fine. I've now siliconed it into place and taped the wire edge on to the top of one of the spider vanes. Seems good so far. I would not think it a suitable method for anything smaller than a 200p

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Guest Ely Ellis

Darren,

One thing to mention if your using a single 47 ohm resistor is that if you use any other dew heaters from the same power supply they should be the same resistance.

It seems that most comercial dew strips I have seen have a resistance around 27 ohm, so if you have just 1 or up to 4 on a controller, they will all take the same amount of current from the controller and therefore around the same level of heat will be produced from each strip.

If you have a comercial dew strip connected and then connect your 47 ohm resistor, then the 47 ohm resistor will draw less current for the same voltage, thus you would get less heat from it. If you turn up your controller to get it hotter, then any other comercial strip would also get hotter, which could be bad news as it will already be hotter than your 47 ohm resistor.

In this respect there are a few options:

1. If using 47 ohm resistor, and investing in a comercial dew strip later, you would probably require seperate controllers.

2. If you use a 27 ohm resistor via a controller, then if you add a commercial dew strip later, you can use the same controller. (Check the resistance of the commercial dew stip first of course).

3. You could do what Mike has done and use a 220 ohm resistor which you can connect directly to your 12 volt supply without having to use a controller, then if you buy a commercial dew strip and controller, they will be kept separate.

4. A site Mike pointed me to suggests using 330 ohm resistors, when connected to a 12 volt supply that will give around 0.45 watts disipation. Need more heat, just add more resistors (in parrallel).

This latter option is what I will be going for. The beauty of this idea is that you can add as many resistors as you like to get the correct heat to the lense / mirror yet you don't have to worry about the voltage, although a controller will give you that added control. I have ordered a pack of 50 330 ohm 1/2 watt resistors, that is £1.24 including delivery.

http://www.ebay.co.u...=item4ab59a974c

With this pack I can then affored to make up a dew strip with as many resistors as I want, the estimate being around 22 to go around the 120mm lens on my refractor.

I could comfortably add more resistors to spread the heat out more evenly and use a controller (one of those LED light controllers), to keep the heat down. If any failure of the controller was to occur like in Martyn's case, I should not get excessive heat as each resistor at full load would dissipate only 0.5 watts max.

My original plan (taken from the internet of course), was to use 15 x 1.8 ohm resistors in series, but this required higher power rating of the resistors and although only using half the amount, would cost 3 times more to produce. Actually considering I would only use half of the 50 x 330 ohm resistors, it would cost around 6 times more to produce, however, it would allow me to use comercial dew strips on the same controller at 27 ohms later on!

Ok, rambled on enough, probably confused you by now.... I hope this all makes sense.

Cheers

Martin

Edited by Ely Ellis
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Hi, makes sense, but the back of the secondary mirror on the 200 is tiny, I can only fit one resistor, there just is not the room for any more.

I am running it from a Kendrick controller, so I can vary the output, also, I am no longer going for that resistor, found another one, though not sure of the ohm.

Thanks for all the input though. :)

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Had a very good test with the dew heater secondary resistor mod. Worked brilliantly and has caused no ill effects on my images from what I can tell. Very happy.

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