Dew Heater Advice

[faulksy]

Hi again Mike & Alan,

Found this, http://www.dewbuster.com/heaters-330ohm-resistors.html

I'll have a good read.

Thanks everyone for all the help!

there spot on richard, you can get them on ebay about 30 pence each, i spaced them about every 1" but connect them in parallel

[Rustysplit]

Follow that guide Richard and you can't go wrong. I used the 330 ohm .5w resistors as recommended in that link. For my secondary iirc, I used 5 or 6 in a ladder arrangement.There are spaced about 8mm apart. I could just fit this between the silicone blobs holding the mirror. I also squeezed some soft black foam around the edge of the stalk to keep the heat on the mirror as much as possible. It only wants to be soft and lightly pushed in there though.

[Tommohawk]

Just a thought - depending on your time/cash balance, the DIY route may not be the most effective, especially given the risk of incidental damage to the mirror.

TBH if I could find a heater pad that would fit direct to the secondary I would definitely go for that. DIY controllers I reckon does make sense. (see the other thread I referred to earlier #17)

Heating the stalk sounds easy and I'd like to go that way, but I just dont see how you can get enough heat to conduct along the stalk to the mirror especially if its wooden and especially if the mirror is separated on a pad or dobs of adhesive sealant. I'm not saying its impossible - clearly others have done it - but only that the amount of heat required must cause turbulence. Sound like I'm on my own though!

Maybe a heated collar made of conductive material, with an elliptical flange at 45 degrees which buts up to the mirror, all covered in insulation....?

Follow that guide Richard and you can't go wrong. I used the 330 ohm .5w resistors as recommended in that link. For my secondary iirc, I used 5 or 6 in a ladder arrangement.There are spaced about 8mm apart. I could just fit this between the silicone blobs holding the mirror. I also squeezed some soft black foam around the edge of the stalk to keep the heat on the mirror as much as possible. It only wants to be soft and lightly pushed in there though.

Trying to figure out this.. do you mean you wrapped the ladder around the stalk, or applied it direct to the secondary?

[Scooot]

Just a thought - depending on your time/cash balance, the DIY route may not be the most effective, especially given the risk of incidental damage to the mirror.

TBH if I could find a heater pad that would fit direct to the secondary I would definitely go for that. DIY controllers I reckon does make sense. (see the other thread I referred to earlier #17)

Heating the stalk sounds easy and I'd like to go that way, but I just dont see how you can get enough heat to conduct along the stalk to the mirror especially if its wooden and especially if the mirror is separated on a pad or dobs of adhesive sealant. I'm not saying its impossible - clearly others have done it - but only that the amount of heat required must cause turbulence. Sound like I'm on my own though!

Maybe a heated collar made of conductive material, with an elliptical flange at 45 degrees which buts up to the mirror, all covered in insulation....?

Trying to figure out this.. do you mean you wrapped the ladder around the stalk, or applied it direct to the secondary?

Thanks Tommo,

I understood Alan's post as fitting the ladder between the mirror and the stalk, as I would have to do.

Yes I would much rather buy something to slide between the mirror and stalk if there was something available that fitted, It needs to be quite narrow to fit between the glue blobs, a ladder configuration could fit. I would have thought the mirror would be fairly safe either way as I'd be working behind it and not removing it.

[Sporadic Dobstronomer]

Thanks Tommo,

I understood Alan's post as fitting the ladder between the mirror and the stalk, as I would have to do.

Yes I would much rather buy something to slide between the mirror and stalk if there was something available that fitted, It needs to be quite narrow to fit between the glue blobs, a ladder configuration could fit. I would have thought the mirror would be fairly safe either way as I'd be working behind it and not removing it.

You might try sliding things in there and then measuring the ones that fit to see how much space you really have.  It is possible to buy resistors that are quite small but you will need about 2mm clearance even for 1/8W size resistors.

You might be able to go thinner using surface mount resistors and cunning use of bits of wire so you solder them on their ends rather than their undersides.  Doable, but VERY fiddly... An 0805 size resistor is under 0.7mm thick  but only 2mm long .

If you use mechanically small resistors, you will need a fair number of them because they will individually have low power ratings.  A benefit is doing that inherently distributes the heat.

[Tommohawk]

You might try sliding things in there and then measuring the ones that fit to see how much space you really have.  It is possible to buy resistors that are quite small but you will need about 2mm clearance even for 1/8W size resistors.

You might be able to go thinner using surface mount resistors and cunning use of bits of wire so you solder them on their ends rather than their undersides.  Doable, but VERY fiddly... An 0805 size resistor is under 0.7mm thick  but only 2mm long .

If you use mechanically small resistors, you will need a fair number of them because they will individually have low power ratings.  A benefit is doing that inherently distributes the heat.

Agreed. I think Richard and I have different problems.

On my scope the pad holding the mirror is only about 2mm max thick, so no room to slide anything between. On the other hand the mirror is somewhat larger than the stalk with space - just - to place a heater in contact with the mirror.

Richards mirror is only marginally larger than the stalk dia, but has a more significant gap between mirror and stalk due to a thicker pad/blob attachment.

I also now a slightly scratched secondary, and want to source another (52mm or slightly larger) which seems to be a problem. But I'll start another thread for that.

[Scooot]

We've had incessant rain here today so a perfect for astronomical pottering. I went over to maplin and bought some 0.5w 330ohm resistors and recruited my neighbour's help to put this together as he has a soldering iron.

I can just about slide it behind the mirror from the top between the 2 bobs of glue. I wont be able to if I attach it to duct tape or anything else so does this matter?

I plan on getting this controller for it unless anyone points out a problem with it?

http://www.amazon.co.uk/Dimming-Controller-Lights-Ribbon-Volt/dp/B003L4KKF2/ref=pd_sim_sbs_light_2?ie=UTF8&refRID=1X2DXJYB8G02ZKEMEER9

We tested it and it did get quite hot but it was working properly. So it's 3 watts on full power but I can turn it down with the controller. If I left it on full power for too long is there any danger I'd crack the mirror, generally, what setting should I have it on?

[Sporadic Dobstronomer]

We've had incessant rain here today so a perfect for astronomical pottering. I went over to maplin and bought some 0.5w 330ohm resistors and recruited my neighbour's help to put this together as he has a soldering iron.

image.jpg

I can just about slide it behind the mirror from the top between the 2 bobs of glue. I wont be able to if I attach it to duct tape or anything else so does this matter?

I plan on getting this controller for it unless anyone points out a problem with it?

http://www.amazon.co.uk/Dimming-Controller-Lights-Ribbon-Volt/dp/B003L4KKF2/ref=pd_sim_sbs_light_2?ie=UTF8&refRID=1X2DXJYB8G02ZKEMEER9

We tested it and it did get quite hot but it was working properly. So it's 3 watts on full power but I can turn it down with the controller. If I left it on full power for too long is there any danger I'd crack the mirror, generally, what setting should I have it on?

I'm not sure I like your use of the words "just about slide it".  That says to me "mechanical strain which might flex the mirror or cause the glue to fail".  (I have no direct experience of this application but I am an engineer with conservative tendencies.)

You might consider having another go with 12x 680R 1/4W resistors instead (after checking their mechanical size).

That would also distribute the heat more evenly and might make room for the duct tape or whatever to further distribute the heat.

I can't see any obvious problem with that controller.  From my extensive research (a quick look at Kendrick's site), you will need 100% power for defrosting and about 10% for keeping the dew off thereafter.

[Scooot]

I'm not sure I like your use of the words "just about slide it". That says to me "mechanical strain which might flex the mirror or cause the glue to fail". (I have no direct experience of this application but I am an engineer with conservative tendencies.)

You might consider having another go with 12x 680R 1/4W resistors instead (after checking their mechanical size).

That would also distribute the heat more evenly and might make room for the duct tape or whatever to further distribute the heat.

I can't see any obvious problem with that controller. From my extensive research (a quick look at Kendrick's site), you will need 100% power for defrosting and about 10% for keeping the dew off thereafter.

Thanks,

There isn't any strain, it's more to do with it not being perfectly straight/flat and bits of solder catching. I have flattened it a bit and now wrapped a piece of duct tape around the back and sides and it slides in more easily although it is a tight fit. I am confident it won't loosen the mirror.

[Rustysplit]

Nice one Richard. Just the job. So long as you are not having to wedge it in it will be fine. So much cheaper than the ready made ones!!

[Daniel-K]

We've had incessant rain here today so a perfect for astronomical pottering. I went over to maplin and bought some 0.5w 330ohm resistors and recruited my neighbour's help to put this together as he has a soldering iron.

image.jpg

I can just about slide it behind the mirror from the top between the 2 bobs of glue. I wont be able to if I attach it to duct tape or anything else so does this matter?

I plan on getting this controller for it unless anyone points out a problem with it?

http://www.amazon.co.uk/Dimming-Controller-Lights-Ribbon-Volt/dp/B003L4KKF2/ref=pd_sim_sbs_light_2?ie=UTF8&refRID=1X2DXJYB8G02ZKEMEER9

We tested it and it did get quite hot but it was working properly. So it's 3 watts on full power but I can turn it down with the controller. If I left it on full power for too long is there any danger I'd crack the mirror, generally, what setting should I have it on?

those controllers are not that great i think mike aka faulksy had one it started smelling of burning plastic.drop him a pm he will help you out

[Tommohawk]

hi Richard - couple of thoughts.

Not sure what youre planning to use for power, but I've been using a 12V power tank to run my HEQ5, and it also has lowwer voltage outputs - 6V and 3V.

Two nights back I used my home made wire nichrome heater (same wattage as yours) and simply waited about 1 hr for the scope to cool down and then plugged it into the 3V output (actually meter shows more like 5V) and left it. This should have produced only about 1/2 watt, but it was enought to keep it perfectly clear for a couple of hours. So using the tank means I dont really need a controller, just a well charged tank.

I plan to heat the primary and the guide - maybe 6W total , so altogether may draw about 0.5amps. The HEQ5 uses 1-2 amps, so it might be possible to run everything from the battery for around 5 hours. That said, I have PSU and controller on the go to avoid dependence on the battery.

If you dont have a tank, you could just wire a number batteries in series, say 4x 3V cells, and just connect to the appropriate voltage point giving 3V, 6V 9V and 12V options.

I also suggest you keep your heater ladder insulated from the scope body if the mount is driven, because otherwise there is a risk of voltage conflict on the rig.

If you use one of the vanes as a conductor as some folk have done or if you mount an unisolated power socket on the metal scope body this could be a big issue, and something may get hurt!

Most particularly be aware that some PWM controllers have common anodes, ie the +ve on the input side is the same as the +ve on the output - hence the potential problem with a tip +ve mount. This isnt an issue all the while that neither side of the heater supply contacts the scope/mount.

Have fun!

[Scooot]

Thanks Tommo, I use a 12v , one if these http://www.hardwarexpress.co.uk/np7-12l-yuasa-12v-7ah-sealed-lead-acid-battery-1333-p.asp?gclid=CIzY4_jomMICFWL4wgodYQYABg

to power my nexus push to so I'll just attach it to this. I'd forgotten I actually got 0.6w resistors so in total it's 3.6w which I don't think I need so I might cut one or 2 off. I think I will use a controller to be on the safe side. Maybe this one that mike showed me.

http://www.maplin.co.uk/p/kemo-10a-pwm-power-controller-n80ql

The mount isn't driven but there's nothing between the mirror and stalk that could conduct anything to the scope body so I should be ok with that, but thank you anyway.

[Sporadic Dobstronomer]

Thanks Tommo, I use a 12v , one if these http://www.hardwarexpress.co.uk/np7-12l-yuasa-12v-7ah-sealed-lead-acid-battery-1333-p.asp?gclid=CIzY4_jomMICFWL4wgodYQYABg

to power my nexus push to so I'll just attach it to this. I'd forgotten I actually got 0.6w resistors so in total it's 3.6w which I don't think I need so I might cut one or 2 off. I think I will use a controller to be on the safe side. Maybe this one that mike showed me.

http://www.maplin.co.uk/p/kemo-10a-pwm-power-controller-n80ql

The mount isn't driven but there's nothing between the mirror and stalk that could conduct anything to the scope body so I should be ok with that, but thank you anyway.

The power in each resistor is Volts² / Ohms so at 12V each 330R resistor will produce 0.44W.  The relevence of the power rating is that you must not exceed that power or the resistors will burn out.

[Scooot]

The power in each resistor is Volts² / Ohms so at 12V each 330R resistor will produce 0.44W.  The relevence of the power rating is that you must not exceed that power or the resistors will burn out.

I just went by this, (near end of first paragraph). http://www.dewbuster.com/heaters-330ohm-resistors.html

If wired in parallel it says you just add the wattage of each resistor?

[Sporadic Dobstronomer]

I just went by this, (near end of first paragraph). http://www.dewbuster.com/heaters-330ohm-resistors.html

If wired in parallel it says you just add the wattage of each resistor?

If the resistors are in parallel (as yours are) then each one separately generates its power V²/R.

So if you have 6x 330R resistors at 12V you will get 12x12 / 330 x 6= 2.6W from the six taken together.  At 13.8V you will get 0.58W (ie 0.6W near enough) from each 330R resistor so that might be why 330R is the recommended value.

The heat power generated in each resistor depends only on the voltage and the resistance, not on the power rating.  Whether it will or won't catch fire does depend on the power rating!

[Scooot]

If the resistors are in parallel (as yours are) then each one separately generates its power V²/R.

So if you have 6x 330R resistors at 12V you will get 12x12 / 330 x 6= 2.6W from the six taken together.  At 13.8V you will get 0.58W (ie 0.6W near enough) from each 330R resistor so that might be why 330R is the recommended value.

The heat power generated in each resistor depends only on the voltage and the resistance, not on the power rating.  Whether it will or won't catch fire does depend on the power rating!

Ah ok, I didn't delve into it too much and just took it at face value. Thanks for explaining it, So my 6 are 2.6w which should be fine, I'll leave them as they are

[Scooot]

I've finally finished and tried my home made secondary dew heater. I was out for 2.5 hours last night and it worked a treat.

I've taped the cables along the top of the two spider veins to the right in the pic below. I bought a hitec Astro controller in the end, together with an Astrozap eyepiece dew strap. So I've attached a phone jack to the other end of the heater wire to plug into the controller.

Thanks everyone for your advice and assistance.