Nichrome dew heater questions

[ballaratdragons]

I bought some Nichrome wire today at Jaycar in Ballarat to hopefully make dew heaters for my ED80 and 100mm guidescope.

I have read about 15 websites today and read the posts in here about how to make it work and I am now more confused than before I started reading.

Some sites say just connect the Nichrome wire to a 12v power supply, others say I have to use a Pulse Wave Generator (whatever that is). Otheres say do not use a Potentiometer, others say use one.
I am very confused.

I'll tell you what I DO have so one of you might be able to help me:

I have an Apple PC power supply (13.5v, 1.0Amp, 15VA) and 4 metres of RES80 Nichrome wire.

What I want to do is heat both the ED80 and the 100mm Guidescope.

I won't be going back into Ballarat for quite some time so I am hoping I can build my unit without having to go buy more bits.

I know I have to cut the wire to length, but do I really need all the electronic gizmos to make it work?
Any help would be appreciated :)

P.S. I also bought Cloth Gaffer tape to house the wire in, and some sticky-backed Velcro tape to mount it.

[rogerg]

My heater is simply the Nichrome wire trimmed to the right length attached directly to a 16v AC power supply. Works like a dream. I don't have it infront of me now, but I'm pretty darn sure there's not even a resistor in-line, let alone a matter to anti-matter converter or something! (what'd you say, Pulse Wave Generator?? - sounds like a warp engine). I would like a variable resistor inline to tune it's heat.

[rogerg]

Oh, and what "the right length" is, I don't know sorry - it comes down to how hot you want it - the longer the length for the same input voltage & amperage, the colder it is. So you will need someone to pop up and explain how long it should be. My cousin worked it out for me, he's knowledgeable in such things. If you don't get help, I can ask him. I think it's a pretty simple case of dividing the resistance of the wire in to the output of the 16v power pack, or something like that...

[ballaratdragons]

I can work out the length, that's not the problem. Laurie (RAJAH) mentioned how in one of his old posts.

The problem was 'do I need all the do-hickeys' or can it be directly wired to the power supply, but you have answered that. You do it, so it must work :thumbsup:

Oh, I looked up what a Pulse Wave Generator is. It works better than a potentiometer. A pot sucks some of the power making it like a resister, whereas a PWG doesn't. It turns the power on and off according to how you set it. :)

Now we both know :lol:

I'll give the power supply a try and see how I go, thanks Roger :thumbsup:

[montewilson]

My FSQ (tube 106mm) uses about 18 ohms and my FS-60c (tube 60mm) uses 28 ohms. You must insulate the outer surface to ensure you don't lose too much heat outwards. I don't bother with PWM stuff. I know it makes great sense, but I have got by without it so far - 8 years now. A modest, constant temp is perfect.

If you are interested to know more PM me and I will give you the details on how to do it. Seriously, it is really cheap and very efective.

[montewilson]

I should add - those values are for 12 volts.

[ballaratdragons]

Thanks Monte. I will be mounting the heater strips inside the Dew Shields in front of the Objective lenses.

There is plenty of room between the Dew shield and the light path on both scopes :thumbsup:

I will be placing the Nichrome in between layers of Cloth Gaffer tape, and attaching the whole assembly with sticky-backed Velcro tape :)

P.S. All that Ohms talk means nothing to me. As soon as I saw all the Mathematical stuff on the sites I closed them :lol:

I'll work out the length by feeling how warm the wire gets using Lauries method with a crocodile clip :thumbsup:

[Bassnut]

I agree with Monte, as long as the temp is a degree or so (more, a bit) above ambient, dont bother with a controller (on my experience).

[ballaratdragons]

Hmmmm, I just tested the length.

At 1 metre is was very hot. Almost too hot to touch. At 1300mm it was only just slightly warm to touch. Big difference for such a small change of only 300mm in length!!!!

How warm should it feel to the touch in your hands? Only barely noticable, or slightly warm?

[ballaratdragons]

In that case, about -1 degree should do for here :lol:

[ballaratdragons]

and also, if I connect 2 heater straps (1 for each scope) should they be in series or parallel.

I'm thinking that 2 running off the 1 power supply means I should only need shorter Nichrome wire length in each one :shrug:

[Bassnut]

Oi sunshine, was that on the scope or free air, sheesh, that makes ALL the difference ;-). Whilst on the ota, its should feel a bit warmer, not scorchingly hot!.

[Bassnut]

What... eek. In parrallel !. The length for each one is irrelavent to how many you have, the supply should be able to supply the total draw.

[ballaratdragons]

Just laying the wire along my kitchen bench, not outside.

Looks like I'll have to run them on seperate lines from the power supply as I can't work out a way to run the wires in Parallel.

[Bassnut]

Nah, your not that thick hehe. Run a pair of wires to the scope, then connect all the heaters to the supply pair, easy. No need for seperate supply wires. Cant work it out?, whaaa, plus and minus to the heaters, each connected to these wires, what dont you understand about parrallel?.

[Bassnut]

Oh, OK, you have 2 RCA connectors I guess, just connect 2 RCA sockets together at the end of the supply pair.

[ballaratdragons]

Fred, I have no RCA connectors. I'll just be twisting wires together and soldering.

Here is a quick diagram to show you what I mean about the wiring:

[Bassnut]

Good grief, thats pretty ;-). Use the middle pic and just connect the 2 blue loops together at the same point as the top loop. The current draw for my 12" with a kendrick was about 3A and less than 2A for the ED80, so with fairly modest speaker cable, it would easily carry the 5 amp total. There is really no need to run seperate supplies for each.

[ballaratdragons]

Sorry Fred but you have lost me. Connect the 2 blue loops together at the same point as the top loop?

Can you draw some sort of wiring diagram so I can understand what you mean please.

[wasyoungonce]

I don't think you can solder nichrome wire. Have to crimp join it.

[ballaratdragons]

Yep, or use a terminal block, or twist wires around it and solder the wire. :thumbsup:

[RAJAH235]

Hi Ken................................ ...........th,
The third diagram on right will work just fine.
Make up a heater to be a little 'warmer' than you think, then duplicate it.
No problemo.
You've just "completed a parallel circuit". :D L.
ps. After finishing your heater, try wrapping it around your arm to test for temp.
You'll soon know if it's way too hot. hahahahahaha..
pps. I'd forgotten all about my 'alligator/crocodile clip' trick/post, errr, way of getting the correct temp.
ppps. A PWM (Pulse Width Modulation), unit will let you control the temp more accurately. Cost about $25 odd?? Not sure, as I make my own for lot less, as you know.
Would you like one? Only cost ya $40....:D

[ballaratdragons]

Hiya Laurie, long time no see! I've been waiting for you to pop up in Skype!!!

Thanks for the info mate. I have some pots I can try. Hopefully they won't be the Carbon core ones. They are from a busted monitor.

I am attempting to make a single control box for 4 items all run through pots. ED80 heater, Guidescope Heater, Toucam fan, and a spare for whatever.

Hopefully the Power supply will drive them all. :)

[Bassnut]

Sorry, as Laurie says, the middle and right diagrams are both the same, parellel, and will do the same thing, only the right one uses more wire ;-). Use either.

Youll find the heater gets much hotter off the tube, without the tube to soak up the heat.

[gary]

Danger, Will Robinson!

Ken, I would recommend you post here your final wiring topology including the
individual lengths of the two segments of nichrome before using the system
in the field and we can double-check it for you.

The math is very important from a safety view point. For example, there is
concern you could exceed the rating of the power supply, the very worse case
scenario in the type of environment you are operating being lethal, so please
run it by us first. ;)

We'd hate to see you suddenly stop posting.

You absolutely must also run an inline fuse. We can advise on final value once
you advise on lengths and power supply.

Best Regards

Gary Kopff B.E. (Elec. Eng.)

[ballaratdragons]

Thanks Fred.

Bit hard to test length on the OTA, but I'll try work something out :thumbsup:

[ballaratdragons]

Thanks Gary. Once I know lengths etc I'll let you check it :)

[gary]

Thanks Ken,

You're too valuable a forum contributor. :thumbsup:

Besides, it might be one of us that gets zapped when we come down to look
through your scope. ;)

Best Regards

Gary

[ballaratdragons]

Now there's an idea :lol:

I am confused about the Fuses. I looked in my shed and I can only find one fuse and fuse holder. It is 250v 6amp.

How many fuses would I need, what ratings, and where would they go. On each heater line? or just for the power supply?

[gary]

Hi Ken,

You will only need one fuse. Suggest you go to Jaycar and get something like
Cat No. SZ2015 which is a 3AG In-line fuse holder (set you back all of $1.15)
and buy an appropriate 3AG quick blow glass encapsulated fuse (35 cents or twice that
if you want a spare). Since the power supply you mentioned is rated to only 1A,
a 1A fuse would be a good place to start (Jaycar Cat No. SF2190). However,
what I would do is compute an appropriate value fuse based on the lengths
of nichrome you end up using in parallel and the output voltage of the
power supply. We can assist in that simple calculation.

Solder the fuse holder in-line with the positive lead coming from the positive
output of the power supply. Make sure you insulate the connections.
I would recommend a short segment of heat shrink tubing which you slide over
the wire and out of the way before soldering, then slide over the joint after
soldering. If you have a real industrial hot-air gun, use it to shrink the heat-shrink.
A hair drier usually is not hot enough. If you are very careful, you can also
do it over a stove hot-plate. Most heat-shrink halves its diameter on contraction (so
called 2:1), so buy a diameter appropriate for the wire. Just try sliding it
over the wire in the store.

The fuse will serve several purposes. For example, a back-of-the-envelope
calculation hints that for the two lengths of wire you might be using in
parallel and given the rated voltage of the power supply you mentioned, you
might be exceeding its rating current output, which is something you don't
want to do. Secondly, in the event of an inadvertent short circuit, maybe not
today but at sometime in the future when the system has undergone a certain
amount of wear and tear, you don't want to run the risk of burning yourself
or damaging the scope. The fuse will hopefully limit the current sometime
before such an eventuality.

Readers of this thread who use 12V lead-acid or gel cell batteries should
not drop your guard either. Alwys use appropriate short circuit protection,
such as a fuse, as such power sources can deliver very large amounts of current,
and in the case of some battery technologies, such as car batteries, in very
short amounts of time.

Best Regards

Gary

[ballaratdragons]

Thanks Gary. Very helpful.

Hows this:

1. wiring diagram.
2. pic of actual sockets mounted on scope.

[RAJAH235]

Ken,
That is an excellent diagram. Showing off your artistic skills again huh? j/k.
One question tho? I'm not sure if you said you already had a multimeter, digital or otherwise (insert 'moving coil, needle type' here), to measure the RESISTANCE of the heaters? Most Important.
This value is needed to calculate the CURRENT drawn.
(JIC.....To measure, simply connect the meter leads to each lead of each heater separately. Hopefully both heaters will be about the same reading in OHMS).
Alternatively, a digital multimeter can be bought for around $15. It should be capable of measuring up to 10 AMPS DC. Most Jaycar ones are.
This way, you can measure the current drawn by one or both heaters & the fan.
Most computer fans draw ~200 mA or .2 of an AMP.
Connect the meter in series with the fuse in the +ve line.
If the reading is close to or more than 1 AMP for one heater, then I would go looking for a larger capacity power supply. Say, a 3 to 5 AMP one.
Keep us informed as that way, we'll know you're still kicking.:rofl::D
Regards, Laurie..

[gary]

Hi Ken,

Looks good, but you now need to determine the individual
lengths of the two pieces of nichrome plus let us know what the
rated current draw of the fan is. Most of those 80mm box PC
fans draw about 0.08A at 12V. This will then allow us to determine the
total current draw so that will will then know whether it is higher than
the 1A rating of the power supply. If it is not, you will either need to
increase the length of the nichrome segments or add one or
more current limiting resistors in series with the nichrome wire.

Do you also happen to know whether the power supply is 13.5V DC or AC?
This will make a difference as to whether your fan works, which I assume is
a brushless DC fan.

Best regards

Gary

[ballaratdragons]

Thanks Laurie & Gary, but I don't have a Multi-meter, and I can't buy one either. No money left and I won't be going into Ballarat for quite a while.

I do have one of those regulated power supplies I bought as a spare laptop power supply. It goes up to 3.5 amps, but it's lowest voltage is 15 volts.

The settings on it are:
15/16/18/19/20/22 volts @ 3.5amps
24v @ 3amps

I think 15 volts will be too much. Especially for the 12v fan.



The fan works using the 13.5 volt Apple power supply. It is a 12v 1.5W 50mm DC brushless computer fan.

[gary]

Hi Ken,

You won't need the multimeter and I took into account you probably don't
have one. You had mentioned that you had bought the nichrome wire from
Jaycar and earlier in the day I had checked online and saw they only sold one
type which has a resistance of 13.77 ohms per meter, so we just need to know
the two lengths.

Just stick with your 1A power supply for now, but we need to calculate
the total current draw first, hence the need to measure the lengths.

Best Regards

Gary

[ballaratdragons]

Ahhhh, ok. Thanks Gary.

I will more than likely make each one 1100mm, as 1m was too hot and 1300mm could hardly be felt.

So that's: 2 x 1100mm and 1 x 12v 1.5W 50mm DC brushless computer fan.
All running from an Apple 13.5v, 1.0 amp, 15 VA power Adaptor.

[ballaratdragons]

Actually, I think I'll go halves between the 1m Hot and 1300mm lukewarm, and make both Nichrome strips 1150mm.

If it ends up not warm enough I can always chop out 50mm, but harder to add it on if need be.

[ballaratdragons]

Gary? anyone?

Can anyone work out the current draw for me please? I'm waiting to know whether to construct my set-up, or not.

Any help greatly appreciated :)

2 Nichrome heaters x 1150mm each (resistance of 13.77 ohms per meter), and 1 x 12v 1.5W 50mm DC brushless computer fan.
All running from an Apple 13.5v, 1.0 amp, 15 VA power Adaptor.

[erick]

Hi Ken. I'll give it a go, but I'm operating under the disadvantage of not having read the thread - been busy and out under clear skies Sat night - woopee!

1.15m at 13.77 ohms per metre = 15.8 ohm. Connect 13.5 volts to that resistance and the current flow will be 0.85A. Two of these in parallel will draw 1.7A. Add the fan at about 0.2 amp and you reach 1.9A. Nope, I don't think your supply will be capable. Try one length only on the supply - no fan as well, and see what temperature above ambient you reach? Just to try and get a feel for the performance. If you can use twice the length of wire for each heater then maybe you can run them both plus the fan - your supply will be at its limit and will probably drop a bit in voltage output - unless very well regulated.

[ballaratdragons]

Thanks Eric.

But if I use twice the length of Nichrome, it won't even get slightly warmish. Heat is almost undetectable at 1300mm.

Looks like I'll have to use the 3.5 amp power supply, but then I have the other problem. It's lowest voltage is 15v. :shrug:

[erick]

Actually Ken, I did read the earlier posts a few days ago (or whenever it was). I was very surprised with the big difference between 1 metre and 1.3 metre. Maybe someone else can explain that, but it's odd since it should be a linear relationship?

Still, the 15V supply won't be a major problem, you'll just need to add a suitable dropping resister for your fan. We can work that out - but you sure need a multimeter working on the job! I'm hoping to come up Friday night - it's sort of 50:50 at the moment. I'll bring my meter, some leads, and we'll check things out - if it can wait that long? (Remind me later in the week!)

[ballaratdragons]

Yeah Eric, the heat loss over only 300mm was a wierd one.

As for bringing a multi-meter up on Friday, I'll probably have the whole thing built and running by tomorrow afternoon :lol:

I will put a 'Pot' in the fan line, and re-measure the Nichrome at 15 volts. It will be slightly different to the 13.5 volt supply.

I'll work something out :P

EDIT: I might as well put 'Pots' on all the lines while I'm at it. I have some :thumbsup:

[Doug]

Ken why is that a problem?
I'd respectfully suggest that you should repeat your earlier experiment.
If 1m of wire was piping hot, 1.3m would have to be better than just barely warm. You might have had a poor connection for the second test.

I gather Ballarat is somewhat cooler than cold at night, so the wattage of a heater will need to be somewhat more gutsy that what is needed up here in Sydney. The Dew heater I use on the LX 200 is about 28Watts, (at 12v) and it is sometimes necessary to crank the voltage up to about 18v to keep the dew away. You wont fight dew with a 12v 1Amp supply; 12 Watts is way too lite IMO. (Though an 80 or 100mm scope will need less wattage that a larger 12" scope.) As for Pulse circuits and potentiometers, pass transistors and a few other 'name dropping' devices, I believe that you will find that as long as you keep the wattage rating down around the 18-20 watt mark you should have no problems. Why?
Because (assuming good thermal contact) your scope bodies will act as massive heat sinks and so the thermal gradient across the OTA will be negligible, leaving the lens cell barely heated above ambient temperature.
which is more or less what you need. Dew point is of course what you are fighting, but there are still practical limits to the wattage you apply to a dew heater; a fan to augment the dew heater should not be overlooked as a possibility IMO.
Doug

[ballaratdragons]

Doug, sorry to frustrate you too, but what the hell is wattage? :lol:

I've already been confused about amps and volts, now watts! :eyepop:

The test on the length was carried out and measured several times, so it wasn't a bad contact :thumbsup:

[ballaratdragons]

Too late!

2 Heater strips made and working perfectly :D

I measured with the 15v power supply, cut the Nichrome to length, and encased it all inside cloth tape with Felt as a heat barrier on one side to stop heat from escaping into the outside air.

I decided to wrap the heaters around the outside of the dew-shields. It was too much hassle trying to wire them inside.

Tomorrow night I should have it all wired up and fitted :thumbsup:

I'll let you know how successful (or not) it is.

Thanks everyone for your help :)

[montewilson]

Wattage (Watts) = Voltage (Volts) x Amperage (Amps)

Watts may also be written at E (Energy)

W or E = VA

PS are you heating the scope or the dewshield? It is important to get the heat into the scope body. If not you will have the driest dewshield in the land and a dewed up telescope.

[avandonk]

How dew inhibiting heaters work is quite simple. The infra red radiation from the heated dew shield onto the optic surface should only just counteract the infra red radiation of your lens or corrector plate into the clear night sky which has a temperature of about 3 degrees Kelvin (-270C)!
The loss of heat into the sky is called supercooling. That is why a frost can occur even when the air temperatue is about 2 or 3 degrees.

A modest wattage is all that is needed about 10W for a 100mm and 20W+ for a 200mm etc. So for 12V about one amp for a 100mm and two amp for a 200mm. For Ballarat a bit more might be needed on really cold nights ie -5C.

Bert

[wasyoungonce]

I found different types of nichrome wire..different resistances and gauges at Wiltronics on ebay

http://stores.ebay.com.au/wiltronics-research_Laboratory-Products_Nichrome-Resistance-Wire_W0QQcolZ2QQdirZQ2d1QQfsubZ1189 4915QQftidZ2QQtZkm

They come in rolls but at least you can get the resistance and length configuration that suits your needs.

[ballaratdragons]

The ED80 dew shield sits directly on top of the lens cell. That's where I'll be placing it. The heat will have to seep through the dew sheild, then slowly work its way through the cell, then to the lens.

It may take 15 minutes or so to soak in.

The guidescope is easy as it has no cell. The lens it directly up against the OTA.

[Doug]

Ken what a watt is can be a bit confusing I admit. But apart from the mathematical relationship Monte (I think) gave you, Here is a practical (I hope) explanation.
The Watt is a measure of power. It is the Watt what does all the work.
The Ampere, the Volt of the Ohmic resistance do nothing on their own.
If a certain wattage is needed to do a thing, then that is what must be provided. Within reasonable limits it doesn't matter what the voltage is, it doesn't matter what the current is, it doesn't matter what the Ohmic resistance is.
Those reasonable limits would mean that a very High voltage is not desirable because of safety, insulation/ arc over problems.
In the same way, very High currents are to be avoided because of the risk of overheating the wires used to carry the stuff, and the need for larger cross sectioned conductors of higher conductivity.
Have you heard of 66KV +++ transmission lines? Those Extra High Voltage Transmission lines are feeding a City, even a State. Imagine the power that these lines are called upon to carry.
If that power were to be supplied using 240V, refer to Monte's formulas,,,Current = Power/Voltage, then to supply the megawatts needed by a city, the current is going to be very high and the cross sectional area of the conductors will need to be impracticably large to avoid severe losses. However at 66KV and higher, the same power can be delivered at considerably less current, hence smaller /lighter cables.
Ken if you are interested, you might like to look at this link:
http://www.powercor.com.au/docs/education/sheet_f.pdf

So to come back to the point, power (Watts) is what is going to move your objective temperature from ambient temperature to some value above ambient.
12v at 1 amp and 12 ohms will give 12watts. 240V at 0.05 Amp and 4.8K ohm will also give you 12Watts. 1volt at 12 Amps and 0.083 ohms will give you 12 Watts. So theoretically the same power can be delivered by various combinations of voltage and current, but in practical terms we must choose wisely what we will use. Anyways I've probably prattled on needlessly, it is a big subject to tuck under your belt overnight, however you might gain much understanding by studying the material at the link I provided.

cheers,
Doug

[ballaratdragons]

Thanks Doug. A bit of reading might help :thumbsup: