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CJK
professor emeritus
Reged: 12/05/12
Loc: Scenic SE TN
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Re: Where to get flexible 2 core power cable?
[Re: csrlice12]
#5628394 - 01/17/13 04:13 PM
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I also ordered some bulk microphone cable -- that should be here tomorrow also. Maybe one of them will be satisfactory.
-- Chris
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csrlice12
Post Laureate
Reged: 05/22/12
Loc: Denver, CO
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Re: Where to get flexible 2 core power cable?
[Re: CJK]
#5628432 - 01/17/13 04:33 PM
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Microphone cable might not hold the amperage though. It might get hot.
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CJK
professor emeritus
Reged: 12/05/12
Loc: Scenic SE TN
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Re: Where to get flexible 2 core power cable?
[Re: csrlice12]
#5628477 - 01/17/13 05:04 PM
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It's #21/2, so it should do okay for short runs, considering the low current draw of the equipment I need to build power cords for. I'm more worried about the voltage drop, actually.
-- Chris
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johndgaul
member
Reged: 05/19/11
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Re: Where to get flexible 2 core power cable?
[Re: CJK]
#5628534 - 01/17/13 05:37 PM
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I still haven't got around to ordering some cable. I did try McMaster-Carr who wrote back saying they wouldn't ship outside of CONUS.
Having thought about this some more, I think my original 10 amp criterion might be overkill to use all round, including for short runs to individual items like heater bands. Heavier gauge would be ideal for the main longer run to the battery or mains power supply to a dew heater controller or distribution box, from which lighter gauge could be used.
I reckon that mic cable would be fine for short runs, especially if making use of the shielding as one conductor and combining the signal cables as the other conductor.
I must actually try this and take some voltage measurements 
John
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CJK
professor emeritus
Reged: 12/05/12
Loc: Scenic SE TN
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Re: Where to get flexible 2 core power cable?
[Re: johndgaul]
#5632887 - 01/20/13 10:15 AM Attachment (3 downloads)
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So here's the final outcome of my quest:
This cable works PERFECTLY.
I ended up doing as John suggested and using the signal cables as the tip conductor and the shielding as the ring conductor, and everything worked beautifully. Resistance for the pair of signal cables (21 AWG each) tied together was 0.3 ohms over a 10' cable, while resistance for the shielding was 0.2 ohms.
I used the same Switchcraft 5 mm power connectors used by Astro-Physics and SBIG -- I had to enlarge the opening in the back of the plastic housing to 1/4" with a step drill bit in order for the rubber insulation to fit.
The cable is very flexible and the rubber insulation should tolerate the cold much better than the PVC on the power cables I'm replacing.
I love it when a plan comes together!
-- Chris
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johndgaul
member
Reged: 05/19/11
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Re: Where to get flexible 2 core power cable?
[Re: CJK]
#5633763 - 01/20/13 06:48 PM
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Chris, that looks great! Thanks for posting - and Anderson Powerpoles too - I like your style
Say 0.3 + 0.2 = ~0.5 ohm on a 10ft run, so might be ~0.2 ohm on a 4 ft run from dew heater controller to dew heater strip, assuming negligible resistance at the electrical contacts.
As an example, a Kendrick Astro 5" optic "Premium" dew heater is specified to draw 1.2A. Note that the Kendrick Firefly strips draw half the current of the "Premium" stips (and perhaps other strips like the Dew Nots?). I would assume a dew heater controller uses pulse width modulation to adjust the output, so 12V or 0V is put across the ouput, giving 1.2A or 0A. Voltage drop across a 0.2 ohm cable would be V = IR = 1.2A * 0.2 ohm = ~ 0.2V. Another way of looking at it would be 0.2V / 12 V = ~2% power loss in the cable (P = VI, and I is the same in cable and heater strip). This isn't much, and might just keep the cable flexible Fine!
from Kendrick Astro, specs for the "Premium" strips:
Item No Heater WattageAmperageHeater DimensioHeater Dimensions
Metric* Imperial*
2003Telrad Heater 6 0.4 50 x 90 mm 2" x 3.5"
2003-RDRed Dot Finder Heater 2.5 0.2 Press fit Press fit
2004.965" Eyepiece Heater 2.5 0.2 16 x 75 mm 1/2" x 3"
20051.25" Eyepiece Heater 3 0.2 16 x 125 mm 1/2" x 5"
20062" Eyepiece Heater 4 0.3 16 x 175 mm 1/2" x 7"
2006-L DEW-Minator Large Eyepiece Heater 10 0.8 75 x 205 mm 3" x 8"
20073" Optic Heater 10 0.8 25 x 280 mm 1" x 11"
20084" Optic Heater 11 0.9 25 x 370 mm 1" x 14.5"
20095" Optic Heater 15 1.2 25 x 460 mm 1" x 18"
20106" Optic Heater 17.5 1.4 25 x 560 mm 1" x 22"
20117/8" Optic Heater 20 1.6 25 x 660 mm 1" x 26"
20129/10" Optic Heater 25 2 25 x 890 mm 1" x 35"
201311" Optic Heater 28 2.3 25 x 965 mm 1" x 38"
201412" Optic Heater 33 2.6 25 x 1115 mm 1" x 44"
201514/16" Optic Heater 52 4.1 25 x 1270 mm 1" x 50"
2005-B1Bino-viewer Heaters for eyepieces up to 1.5 in 5 0.4 16 x 125 mm 1/2" x 5"
2005-B2Bino-viewer Heaters for eyepieces over 1.5 inc 8 0.6 16 x 175 mm 1/2" x 7"
2016Secondary Mirror heater for Newtonian secondar 2.5 0.2 44 x 66 mm elli1.75" x 2.6" ellipse
2017Secondary Mirror heater for Newtonian secondar 4 0.3 50 x 90 mm 2.5" x 3.625"ellipse
2018Secondary Mirror heater for Newtonian secondar 9 0.7 50 x 90 mm 3.75"x5.50"ellipse
2019Electronic Hand Control Pad Heater 10 0.8 57mm x 165mm 2.25" x 6.5"
2020Eyepiece Box Heater 10 0.8 57mm x 165mm 2.25" x 6.5"
2022Rigel Quik Finder Heater 2.5 0.2 16 x 75 mm .5" x 3"
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