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Mozhoven
professor emeritus
Reged: 05/26/05
Posts: 727
Loc: Middletown, CT
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Hello all,
A while back I purchased DC to Pulse Width Modulator kit from Hobbytron.com based on the advice of Tim (Tberdon)to make my own Dew Controller.
I finally got it assembled and working, but I have a few questinos about the calibration/voltage settings.
When I hook it up to a multimeter, I can asjust the voltage from 0 - 13.30volts with the 1k Pot I installed in the box. The problem is it goes from 0-12v withing the first few degrees of the knob. So, if the knob where labled with numbers (1 being the lowest voltage), it will max out at 2, and not change all the way to 10.
I've tried adjusting the min/max voltage settings on the board, but it's pretty much the same deal. If I set the max votage at 6v, it'll max out at the same place.
Now, I'm not all that concerned about being able to control the voltage right now, I just need it working. So, for those of you who don't know about electronics, but do know about temperature settings, perhaps you can chime in here.
When I set the controller on 13 volts and wrap a heater strip around a thermometer, it gets up to 120F. When I put it on 5 Volts, it hovers around 100F. I'm not sure how this will translate when hooked up to an EP (like an Ethos). Do I leave the voltage at the max assuming it will take a while to heat the Ethos, or is that overkill?
Any advice will be much appreciated.
BTW:Here is the link to the thread regarding the construction of this kit:
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=atm&Number=1737499&fpart=1&PHPSESSID=
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Jeremy Davis
Orion XT12i - 12" f/4.9 "Alfred E. Newtonian"
Wyorock Custom Crayford
13mm Ethos The "Freshmaker"
12x60 Oberwerk Binos
www.jdavisgallery.com
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walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2312
Loc: Doylestown, PA
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It sounds like either the circuit or the pot are non-linear on the PW adjustment. If its the pot then a different pot will help. If its the circuit then you'll just have to live with it.
Getting to 120°F seems to be a bit on the too warm side but then again on a dewy summer night you do need to go above ambient. Having the heater on an EP that has most of its surface exposed to the air will probably reduce this temperature.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1345
Loc: Hockessin, De
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What setting are you using on your meter to measure the voltage ? Pulse width modulation is always putting out the same voltage, but varies the duty cycle, ie the time the voltage is full on vs full off. If your using the DC mode on your voltmeter, the meter has a time constant and once the "on time" of the pulse is over about 30%, the meter will read the same voltage. You need an oscilliscope to see the wave form.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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tatarjj
scholastic sledgehammer
Reged: 04/20/04
Posts: 900
Loc: Auburn, AL
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Yea, you really need an o-scope to characterize what your PWM signal is really doing. Voltage tells you very little, unless it is time averaged over at least a period or two... who knows what the heck your multimeter is doing.
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John T.
Auburn, AL
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
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GlennLeDrew
sage
Reged: 06/18/08
Posts: 464
Loc: Ottawa, Ontario, Canada
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A heating element that is not heat-sunk will get rather hot. You should always have them in good contact with something into which that heat can go. Said object will not warm up to near the same degree, especially if it's radiating/convecting some of that into the cool night air/sky.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Achernar
Post Laureate
Reged: 02/25/06
Posts: 3572
Loc: Alabama, USA
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Make sure you put a fuse between the battery and everything else. It need be rated to handle the load you'll be placing upon your controller, but not so high circuits can be damaged. A bad RCA jack could short the whole works out and you really wouldn't want 200 amps of current surging through your controller because it will destroy it and possibly damage your telescope. Shorting a gel cell battery can also blow it apart or otherwise damage it as well. It happened to me once and the fuse blew as it was supposed to, preventing any damage. I made my own controller, which functions very much like a single phase load center or breaker panel found in your house, with linear variable resistors to control the amount of heat that each dew heater receives. It also supplies power to a cooling fan and my digital setting circles.
Taras
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10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
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Mozhoven
professor emeritus
Reged: 05/26/05
Posts: 727
Loc: Middletown, CT
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Taras,
Do you have plans (or pictures at least) of your controller. I would like to see that.
As for my controller, I will see if I can track down and O-scope. I tried it out last night and it heated the Ethos and my secondary quite well. They both felt rather warm, so I will definitely need to adjust the output.
"If" I were to change pots from the 1k I currently have, what should I look at?
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Jeremy Davis
Orion XT12i - 12" f/4.9 "Alfred E. Newtonian"
Wyorock Custom Crayford
13mm Ethos The "Freshmaker"
12x60 Oberwerk Binos
www.jdavisgallery.com
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tatarjj
scholastic sledgehammer
Reged: 04/20/04
Posts: 900
Loc: Auburn, AL
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Quote:
"If" I were to change pots from the 1k I currently have, what should I look at?
I donno, show us a circuit diagram and I might be able to give you some suggestions.
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John T.
Auburn, AL
25" f/4.2 Dob
18" Obsession #701
4" Stellar Vue Achromat
8X56 Binos
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Mozhoven
professor emeritus
Reged: 05/26/05
Posts: 727
Loc: Middletown, CT
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I'll have to hunt around for the print out, but maybe this will suffice for now?
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Jeremy Davis
Orion XT12i - 12" f/4.9 "Alfred E. Newtonian"
Wyorock Custom Crayford
13mm Ethos The "Freshmaker"
12x60 Oberwerk Binos
www.jdavisgallery.com
Edited by Mozhoven (06/25/08 05:35 PM)
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Mozhoven
professor emeritus
Reged: 05/26/05
Posts: 727
Loc: Middletown, CT
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OK, here is the link to the manual. It has a PCB layout and circuit schematic on the last few pages.
http://apogeekits.com/PDF_Files/Manual_K8004.pdf
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Jeremy Davis
Orion XT12i - 12" f/4.9 "Alfred E. Newtonian"
Wyorock Custom Crayford
13mm Ethos The "Freshmaker"
12x60 Oberwerk Binos
www.jdavisgallery.com
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randtek
professor emeritus
Reged: 06/18/05
Posts: 659
Loc: Central Indiana
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I would be very hesitant changing any component values until you are able to put an oscilloscope on the output to see what is really happening in the circuit. Depending on the current draw of the heaters, you could end up designing your own version of the famous "voltage to smoke converter" circuit. 
If you can't get access to an oscilloscope, you could get a 12 volt "grain-o-wheat" type bulb (Used in older analog electronic equipment as panel lights. Available at electronic suppliers.) and hook it up in place of the heating element. The brightness of the bulb can give you an idea what the heater would be doing. This would not be as precise, but might be good enough for experimenting safely.
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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Achernar
Post Laureate
Reged: 02/25/06
Posts: 3572
Loc: Alabama, USA
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If the heater strips were very warm even when wrapped around the finder scope or eyepiece, it might be because you wired the heater in series. That is a mistake because as I found out, the heaters can get very hot. Instead, wire the heater strips in parallel, whose resistance is determined by the value of the resistors divided by their number. I made my heater strips from 330 Ohm, 1/2 watt resistors wired this way, and even when powered by 12-volt power directly, they keep dew away but only make the finder scope feel just a little warmer than it would be without the heater. Even when they're not on my finder, secondary mirror and coma corrector, they only get moderately warm. When I wired resistors in series, they got so hot they were enough to cause burns. I didn't build my controller from any plans, but I simply put two "bus bars" in the case, one for incoming power, the other for a ground. I connected leads to each switch, then from there power went to a variable resistor. At the load side of the switch, I connected a red LED and a 680 Ohm resistor to serve as a power on indicator, with a ground wire back to the ground bar. Power then flows to the RCA jack in the central terminal, and return current flows through the outer terminal which then goes back to the ground bus bar. A hot and ground wire goes from each bus bar to another RCA jack, with a fuse between the hot bus bar and the jack. I made the connections with forked lugs into terminal strips that acted as bus bars. I couldn't find any better pictures of my controller at the moment, but I'll post one as soon as I can find one.
Taras
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10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
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Achernar
Post Laureate
Reged: 02/25/06
Posts: 3572
Loc: Alabama, USA
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Post deleted by Achernar
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10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
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Mozhoven
professor emeritus
Reged: 05/26/05
Posts: 727
Loc: Middletown, CT
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Maybe I exaggerated how hot the strips are getting. They certainly aren't hot enough to burn, but seem to be making the mirror (at least) rather more hot than ambient.
The eyepiece strip I have is retail, while the secondary heater is DIY wired in parallel.
I thought that a light bulb would be a good idea and picked up a red light (not LED) from Radioshack. It certainly is a good indicator of power, but again it will top out very close to 0 on the dial.
I suppose If I could dial in the voltage/pulse to a decent temperature above ambient I should be ok on most nights providing the dew doesn't go crazy. But if I can make the pot work correctly, I'll be much happier.
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Jeremy Davis
Orion XT12i - 12" f/4.9 "Alfred E. Newtonian"
Wyorock Custom Crayford
13mm Ethos The "Freshmaker"
12x60 Oberwerk Binos
www.jdavisgallery.com
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Timo Kuhmonen
member
Reged: 08/16/07
Posts: 49
Loc: Espoo, Finland
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As earlier mentioned, I also would recommend the oscilloscope. Some multimeters today are "true RMS" capable, I still am not 100% sure can they display correct output in this case (the used frequency & waveform). Go on with the scope...
What I saw on the diagram, SG3525 is normally used at SMPS controller (switchmode power supply)
-IC1 has a fixed reference output (vref at pin 16)
-min adjustment: reference voltage is fed to voltage divider chain R4-RV1-R5
-max adjustment: from the slider of RV2 is taken another voltage
-these two mentioned voltages are again fed to another voltage divider R6-R7
-at interconnecting point R6-R7 the voltage is fed to non-inverting input (IC1 pin2). This voltage defines the duty cycle of the PWM output (pins 11 and 14 at IC1)
-instead of one trimmer, other components affect to output duty cycle also (R4, RV1, R5, R6, R7 and RV2)
SG3525 specs:
http://www.st.com/stonline/products/literature/ds/4286.pdf
Hopefully these will help... If someone of you noticed incorrect info here, pls tell me.
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http://timokuhmonen.smugmug.com
http://tiku-astro.blogspot.com
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randtek
professor emeritus
Reged: 06/18/05
Posts: 659
Loc: Central Indiana
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After re-reading your posts, and looking at the schematic of the circuit, and the datasheet for the IC, I think you may have a problem on your circuit board. I'm guessing there is a connection problem associated with pin 1,2, or 9 of the IC. Specifically you would get behavior like this if the connection between pins 1 and 9 is not correct. take a couple of close up photos of the board, one showing the top, one showing the solder side, and post them. I will look at them and see if I see any problems.
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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Achernar
Post Laureate
Reged: 02/25/06
Posts: 3572
Loc: Alabama, USA
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Here's a close up photo of my DIY controller for my dew heaters. It's not a thing of beauty, but it does work quite well.
Taras
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10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
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