Dew Zapper and PWM DC Motor Controller

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hamburger
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Dew Zapper and PWM DC Motor Controller

Post by hamburger » Mon Jan 04, 2010 4:22 pm

I am making a dew zapper for a telescope using a motor controller (12v in, 12v out 15A). I figure I need maybe 40 watts (drawing 2 amps) at each of the heating bands (I plan on having 4 heating bands, each will wrap a couple times around a eyepiece and finder scope, etc).

I am using the leftovers of a roof deicing system and when I connect a length of about a foot to the controller it smokes and melts the insulation off.

What I'm confused about is that the deicing wire dosen't give me a reading on my dmm, it jumps from 0 to inf, so I can't figure out what length of wire I need to get 40 watts (ideally it wouldn't be more than a foot of wire but like I said it melts the insulation if its less than that even with the motor controller only turned a quarter of the way up)

Is there something I could put in parallel with the wire to drop the amps, through the four bands, to around 2amps. And am I right thinking that the amps will add together. In other words with each band drawing 2 amps the motor controller will be outputting 8 amps.

Thanks for any help

dyarker
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Re: Dew Zapper and PWM DC Motor Controller

Post by dyarker » Mon Jan 04, 2010 10:17 pm

Is there something I could put in parallel with the wire to drop the amps, through the four bands, to around 2amps. And am I right thinking that the amps will add together. In other words with each band drawing 2 amps the motor controller will be outputting 8 amps.
Anything in parallel with the (heater) wire will increase amps, not drop the amps.

I suggest you put the four sections in series.

Cheers,
Dale Y

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jwax
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Re: Dew Zapper and PWM DC Motor Controller

Post by jwax » Tue Jan 05, 2010 6:29 am

More information about your "deicing wire" would help! Original length and power rating, or getting a good read of its resistance per foot is needed to design a hookup that will do what you need. Where did the "40 watts per band" come from?
Welcome to the "NUTS" forum! :mrgreen:

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Re: Dew Zapper and PWM DC Motor Controller

Post by haklesup » Tue Jan 05, 2010 11:00 am

By any means lower the voltage on your controller and you won't have so many amps to worry about. Alternatively, if that controller is PWM, decrease the duty cycle to a very low setting like 5% and work your way up until you have the right amount of heating.

To measure the very low resistance you probably have, you should use a kelvin 4 wire setup. Assuming your DMM is not 4 wire capable the easiest thing to do is put an amp (or less) through it and measure the voltage drop using your DMM.

Bypassing the heater strip with a parallel resistor to divide and thus lower the current in the coil will just result in a very hot and probably expensive resistor on a heatsink hanging out in the air waiting to burn you.

You might be better off with another variable AC or DC supply or a proper temperature controller (PID with PWM capability). If you insist on using the 12V source, then more series resistance is a better choice. Putting the segments in series will more likely result in 2A through all of them.

Yes, Amps add for parallel circuits by decreasing R as seen by the load. The only way that the bands will use less current is if you added resistance such that the supply went into current limit and reduced its voltage. As long as the supply remains at 12V, the current through the band will remain the same regardless of what you put in parallel with it.

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Re: Dew Zapper and PWM DC Motor Controller

Post by hamburger » Tue Jan 05, 2010 4:17 pm

Thanks for the replies, I'm a new N&V subscriber and figured I'd try these forums.

The wire is from an old installation that plugged into a outdoor outlet. I got about 50ft and don't have any information about the specs or power rating. Just know it was used to prevent ice dams on roofs. The 40 watts is taken from the specs of this dew zapper http://www.amazon.com/Orion-Zapper-4-Ch ... B0016Q670E

Sounds like I'm going to do a series setup because the controller is going to be run from a 12v field battery powering other items. Now I'm going to need a few power resistors (maybe 10w?) to max out the controller...

Going to play around with it later. It was 0 degrees F a few days ago and didn't have any problems with frost so I'm not in as much of a hurry I thought I was.

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Bob Scott
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Re: Dew Zapper and PWM DC Motor Controller

Post by Bob Scott » Tue Jan 05, 2010 4:54 pm

hamburger wrote:Now I'm going to need a few power resistors (maybe 10w?) to max out the controller...
If you are going to use power resistors, you don't have to fool around with that resistance wire for gutters. The resistors will be your heaters.
-=VA7KOR=- My solar system includes Pluto.

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Re: Dew Zapper and PWM DC Motor Controller

Post by haklesup » Tue Jan 05, 2010 5:27 pm

The product you linked uses a Pulse Width Modulator (PWM) as the main control. In case your not familiar with this technology, it basically turns that 12V on and off very fast. It modulates the on time of the 12V from off to 1% to 100% of the time (that being the Pulse width of the 12V supply).

It sounds like you are trying to replicate the action using DC voltage on for 100% of the time.

Using DC you can easily adjust things so that you get a constant power output but that dosen't necessarily translate into constant temperature. Since heat can be cumulative you could end up with temps quite a bit higher than you want. Furthermore operating the heating bands with no load (not attached to anything) and un regulated (DC source) I would expect the temp to run away and get way too hot very fast. Attach that heating band to a cold metal pipe and you might find it operating at a more useful temp but once that pipe gets warm and if there is not sufficient surface area to radiate the heat at the same rate that it is put in, you will experience an uncontrolled rise in temp. You will need a temp regulating circuit to be successful at this project.

Now ideally you have a proper temp controller (just punch temperature controller or PID into ebay and you'll see a bunch). for $40 or so you can get an adequate controller but you may also need an H bridge. This is just a FET switch driven by the controller, it buffers the high current from the supply to the low current of the controller. Before you buy any PID, make sure to get the instruction sheet and verify it has the right kind of hookups, many require 120V but 12V versions are available for example. You may not need a true H bridge (works pos and neg voltage) but some sort of solid state switch to buffer the controller and source may be necessary depending on the features of the temp controller.

A simpler temp controller makes use of thermostats. Similar to those transistor looking things found in any heating appliance (like a clothes dryer), these are bimetal switches that open and close at preset temperatures. For your application you would need a high limit thermostat to turn off the power when its hot enough, these have 10-20 degrees hystereses so they don't turn back on again until its cooled by that amount. Additionally a thermal fuse with a rating a bit above the high limit switch is advisable, these go open perminantly if a certain temp is exceeded and are a last line of defence against failure of the primary control circuit. Just go to Digikey.com and type in thermostats then select the temperatures you need and you'll be presented with a list. Make sure to read and understand the datasheet especially the accuracy of the setpoint and hysterisis characteristics.

Here is a 75F thermostat (sufficient to be above the dew point on all but the muggiest summer nights). Its normally closed which means it opens when 75F is exceeded stopping the heater until it cools to 40F. Other manufacturers might have a tighter on off band.
http://media.digikey.com/pdf/Data%20She ... Series.PDF

These things need to be in intimate contact with the mass you are heating. This form factor might be a better fit between the bands and the telescope cylinder
http://media.digikey.com/pdf/Data%20She ... 20Type.pdf

Think of these as a heat powered relay. Some even have NO and NC contacts (see M3 on datasheet above) and others have the thermal fuse built in (presumably wired in series with the switch)

Below freezing especially well below freezing, you shouldn't have any condensation since the air is already dessicated by the cold (unless your breath gets on the eyepiece for example) your most troublesome times will be when the telescope is cooler than the dew point for some reason.

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Re: Dew Zapper and PWM DC Motor Controller

Post by sghioto » Tue Jan 05, 2010 6:38 pm

hamburger,

I think you are going about this the wrong way. That deicing wire you have is not designed for 12 volt DC operation. Most dew heaters use nichrome wire for the heating element. Below is a link to a homemade dew zapper you might be interested in.
http://www.deep-sky.co.uk/telescopemaking/tm16.htm

Steve G.

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Re: Dew Zapper and PWM DC Motor Controller

Post by dyarker » Tue Jan 05, 2010 9:53 pm

I looked up some deicing cables. They all seem to be 5W per foot. They come in different lengths from 30ft to 250ft, all running from 120VAC; that means the resistance per foot is different depending on the original length.

You said you had about 50ft, implying that it it a cut out section. The warm (running) resistance will be higher than the cold resistance; so the ohm meter wouldn't be much help anyway. You need to know the original length.

FOR EXAMPLE, if original length was 100ft; then warm resistance per foot is:
120V / 100ft = 1.2 VperFt
5WpFt / 1.2VpFt = 4.166A
1.2VpFt / 4.166A = 0.288 Ohms per Ft

4 one foot sections in series would need a supply voltage of:
1.2V * 4 = 4.8V

No wonder the 12V supply was melting things!!!

Cheers,
Dale Y

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Re: Dew Zapper and PWM DC Motor Controller

Post by jimmy101 » Wed Jan 06, 2010 9:51 am

I think this is a case where KISS applies.

You really don't require any type of controller like PWM or PID. It's just a heater after all that doesn't really need precise temperature control. Besides, as soon as you add temp control the complexity of the circuit goes way up.

If the original heater setup was designed for 120VAC and you still have access to 120VAC then you might just try a standard light dimmer.

If you don't have access to 120VAC and are using 12VDC then just make a very simple oscillator. A 555 or perhaps a couple inverters. A bit of fiddling with the 555 and you can control the duty cycle with a potentiometer. A bit more fiddling with a pair of 555's (556) and you can easily control the on time and the on frequency. It's probably possible to do the same with inverters. You'll need a power transistor to couple the timing circuit to the heater wire. A relay would work but I think the constant clicking would probably drive you nuts.

To use just turn it on and put your hand on one of the heater elements. Turn the controller up until the heater feels warmer than the surroundings.

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Re: Dew Zapper and PWM DC Motor Controller

Post by jimmy101 » Thu Jan 07, 2010 7:39 am

Looking at dyarker's numbers, it looks like you could just use a total length of about 10 to 15 feet of the heater wire. If that is too long for what you thought you needed then just double or triple the amount of heater wire at each location. Or, just hang the excess someplace, like on a chair to keep your butt warm!

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