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Dew controler based LED dimmers

astrovirus

By astrovirus
in DIY Astronomer

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malc-c

malc-c

Posted
Posted

Thanks guys, my language. :icon_salut:

LOL - please note that this is for illustration purposes only... ( I couldn't remember the connections on the dimmer... positive - ground - ground - positive, or ground - positive - positive - ground !)

Just had a look at the image above and I got it right :) !!! ---- just the input /output ass about face !! :(

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Gina

Gina

Posted
Posted

If your using a metal box you might need to use insulating bushes on the phono sockets...

Yes, you will need to - it's the negative (Gnd) side that's switched, so both connections to the dew heater(s) are "hot".
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Freff

Freff

Posted
Posted

If your using a metal box you might need to use insulating bushes on the phono sockets...

Thanks, I wouldn't have known that. I'm using a plastic box I have spare. Bit OTT but better than buying something else.

post-7131-0-72867000-1348244824_thumb.jp

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malc-c

malc-c

Posted
Posted

This is a crude schematic of how I wired my dimmers / control box up.(drawn in Tony's language :) )

The 1st phono on the right is the 12v input, moving right to left, I have 8.5v out for the canon DSLR, two 12v outlets for mount and accessories, then the two outlets for each dimmer. Note that the outputs from the dimmers are separate and do not share a common ground.

Please note that if using a regulator to supply a camera you check the pin outs - whilst the middle pin is normally GND the input and outputs can be reversed -

post-10726-0-57117400-1348305619_thumb.j

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Freff

Freff

Posted
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Malcolm thanks for that. I spent an hour last night trying to figure out what sockets were for what on your picture.

I could sussed the ones for the dimmer outlets, but got confused with the others. This schematic explains it all.

As they say "A pictures worth a thousand words". Thanks.

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Freff

Freff

Posted
Posted

One LED and one resistor of something like 2.2K in series across the output of the controller. Make sure the LED is connected the right way round or it won't light up. Anode, longest wire is positive. The brightness of the LED will vary with the output power :)

Gina, I have hundreds of 330ohm 1/2w resisters, can I use these instead of the 2.2k you mention.

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malc-c

malc-c

Posted
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Gina, I have hundreds of 330ohm 1/2w resisters, can I use these instead of the 2.2k you mention.

Depends on the LED you are using. Before the advent of 5v and 12v LEDs, most common LEDs were 2v operation and had a forward current draw of 20mA for max brightness. Therefore you had to base the series resistor based on a 2v voltage drop from the supply voltage and 20mA if you wanted a bright LED. Using Ohms law the minimum series resistor for a 12v supply is 500 ohms. 330ohms may work, but it allows the LED to draw more current, and thus may not last long. I find that 1K series resistor with a 13.8v supply is a decent compromise.

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Gina

Gina

Posted
Posted

You could connect two 330 ohm resistors in series to give 660 ohms - not parallel. I suggested 2.2K to give a bit dimmer light for an observatory - you don't want lights to be too bright.

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Freff

Freff

Posted
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That's what I want Gina, dimmer LED's. I bought several from Maplins today and specifically asked for the dimmer type.

I also bought the 2.2k resisters as you suggested, so I have a few options but will probably use them.

This is quite a learning curve for me, but getting there. :confused: Thanks.

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Bizibilder

Bizibilder

Posted
Posted

Could someone confirm if this is a source of supply of voltage regulators for camera output of 8v (suitable for most Canons)?:

http://cpc.farnell.com/stmicroelectronics/l7808acv/v-reg-1a-8-0v-7808-to-220-3/dp/SC08347?in_merch=Products

Heat sink is here:

http://cpc.farnell.com/aavid-thermalloy/436966/to220-to218-with-solder-pins-heat/dp/SC00670

Handling charge seems to be £5.95

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Epicyclus Dave

Epicyclus Dave

Posted
Posted

Hi all :) , I got my other dimmers delivered today to finish off my controller .

I was testing the output of these PWM controllers again with my oscilliscope and found that just like some analog synth PWMs the width is varied from about 20% to 80% of the pot turn, this means the max temp. is reached at 3/4 of the turn on the 1K control pot , past this point to full clockwise on the pot , the temp starts to drop again.

....So ..... to stretch the max temp to corespond with 100% pot travel i soldered a 680R (680 ohm) in series with the red wire and the max lug on the pot .

I tried a few values and 680R gave good results for me.

LEDs will dim just fine with these controllers but resistive heaters seem to exhibit temp drop at max pot setting , wiring the resistor onto the pot & red lead seems to fix this.

Have fun .

Dave .

:) :)

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Stargazer33

Stargazer33

Posted
Posted

I thought there was something funny going on when I turned the pot to max!

Thanks for that Dave, I will give it a try. No hope of getting the scope out at the moment due to the torrential rain & strong winds here in the south. So I might as well occupy myself with something else!

Bryan

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Epicyclus Dave

Epicyclus Dave

Posted
Posted

:) good news / results , my new batch of dimmers have a green pcb (first batch had a red pcb) , i think it's an identical circuit but slightly different component layout, the resistor pulse width mod works with these too , i tried a 5W diy band i have just constructed for my old threaded M42 metal camera lenses , low setting = room temp 18c ./ Half way = 33c . / Max setting = 49c .

Should give enough variation for damp nights to -17c rural hilltops :D .

Dave.

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Epicyclus Dave

Epicyclus Dave

Posted
Posted

Hi again.

That's the control box finished :)

post-16688-0-27717100-1348601250_thumb.j

The red pcbs came with a red / black / yellow wire that was so thin it snapped just looking at it :D , for those pcbs i re-wired the pots back to the 3 pin header on the pcb , removed the header pvc socket housing to leave the 3 pins sticking up and soldered direct , one of the pots had a bad carbon track / lug weld , changed it for an alpha pot (1K) i had lying around tho i need to chop the splined shaft down a bit to match the others (dremil job) , chuffed it's all working , i coupled the panel red leds to the output side of the circuit, they dim / glow as per heat setting.

post-16688-0-61428200-1348601267_thumb.j

Sound as a pound :) .

Dave.

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Stargazer33

Stargazer33

Posted
Posted

My final version of the dew band controller. Looks now next to Dave's! :embarrassed:

This final version has an LED (top right corner) that dims with the output, an on/off switch just above the downlead and I have incorporated Dave's resistor mod to the pot (as above).

All I need now is for Noah to get his ark out of our garden so that I can set my scope up and give it a go! :clouds2:

post-21511-0-64216200-1348663700_thumb.j

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