75 Volt DC LED light globe

Submitted: Sunday, Jun 27, 2010 at 18:36
ThreadID: 79682 Views:5326 Replies:2 FollowUps:11
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Hi All,

I randomly got my hands on a 75 volt DC LED light globe, it looks fairly good and was thinking I could use it in addition / instead of a 12 volt fluoro.

I currently have an 80watt solar panel attached via steca regulator to a 12 volt 100 amphour AGM battery, and was wondering if anybody had any ideas how to power it?

Appreciate any ideas.

Regards

Glen
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Reply By: Ianw - Sunday, Jun 27, 2010 at 18:44

Sunday, Jun 27, 2010 at 18:44
Where are you going to get a 12v to 75v inverter? I haven't seen any around !

Ian
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Follow Up By: Member - Christopher P (NSW) - Sunday, Jun 27, 2010 at 18:48

Sunday, Jun 27, 2010 at 18:48
google

inverter from 12v
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Follow Up By: Member - Christopher P (NSW) - Sunday, Jun 27, 2010 at 18:50

Sunday, Jun 27, 2010 at 18:50
75 light
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Follow Up By: Ianw - Sunday, Jun 27, 2010 at 19:02

Sunday, Jun 27, 2010 at 19:02
Yep, plenty of 75v lights, but no inverters ! Appear to be for trains!

Ian
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Follow Up By: Ianw - Sunday, Jun 27, 2010 at 19:03

Sunday, Jun 27, 2010 at 19:03
First ref was for a 75 Watt inverter. Not Volt.





Ian
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Follow Up By: Member - Christopher P (NSW) - Sunday, Jun 27, 2010 at 20:04

Sunday, Jun 27, 2010 at 20:04
i can ask a mate that works with trains what you can do. or you can use it to make a 12v led light...

Instructables, hack a day.
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Reply By: Geoff (Newcastle, NSW) - Sunday, Jun 27, 2010 at 21:40

Sunday, Jun 27, 2010 at 21:40
Glen,
Really the only difference between a 75V LED and a 12V LED is the resistance of the current limiting resistor.

Is the light sealed?

Can you get access to the current limiting resistor?

If so, find the value of the installed resistor, divide the value by 6.25, buy one of the calculated size and install it.

Do not up your 12V DC to 75V DC, Medium DC voltages are considerably more dangerous than the equivalent value in AC.

Your LED is out of a locomotive, the people trained to work on DC voltages of that level are vastly different to a home handy man!

Geoff,
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Follow Up By: pop2jocem - Sunday, Jun 27, 2010 at 22:56

Sunday, Jun 27, 2010 at 22:56
"Medium DC voltages are considerably more dangerous than equivalent in AC"

Amen to that, the older mining dump trucks that ran a 1200 HP engine with a DC generator hooked up would produce a most startling noise if the current decided to track to earth through a faulty insulation. You would not want to be between where the current was coming from and where it wanted to go. Mind you at that llevel of power either AC or DC would cause grief.

Cheers Pop
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Follow Up By: Ianw - Sunday, Jun 27, 2010 at 23:23

Sunday, Jun 27, 2010 at 23:23
Depending on the wattage of the light. it may have 20 LEDs in series. Would work on 75v but certainly would not work on 12v.

Ian
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Follow Up By: Geoff (Newcastle, NSW) - Sunday, Jun 27, 2010 at 23:40

Sunday, Jun 27, 2010 at 23:40
Ian,
100 LED's in series without a current limiting resistor wouldn't have sufficient internal resistance to run successfully on 75V DC.

A diode has an internal drop of 0.6 Volts,

100 x 0.6 = 60Volts = Smoke at 75 Volts.

20 x 0.6 Volts = 12 Volts = Marginal, probably work.

As I said above, divide the value of the fitted current limiting resistor by 6.25 and change it to that value.

The light is unlikely to use a series network of 2, 10 or 200 LED's. A single LED failure will render the whole light useless.

Geoff,
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Follow Up By: postieboy - Monday, Jun 28, 2010 at 16:18

Monday, Jun 28, 2010 at 16:18
Thanks heaps for the advice. The light seems pretty sealed but will try and pop the light open tonight and have a look. I imagine the current limiting resistor would either be directly on the negative or positive side of the input, and would be physically large to handle a bit of power dissipation?

Thanks also for the heads up about the dangers of 75 volts, I wasnt too keen to up the voltage anyway.

Cheers,

Glen
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Follow Up By: Geoff (Newcastle, NSW) - Monday, Jun 28, 2010 at 17:40

Monday, Jun 28, 2010 at 17:40
Hello Glen,
Yep, the current limiting resistor will be either directly connected to the positive or negative terminals of the light.

Don't be too concerned if there is more than one resistor. Without seeing the unit I'm guessing it has multiple individual LED's and these are connected in parallel for the reasons I mentioned in my last post. (Individual LED failure doesn't render the whole light inoperable)

Just repeat the resistor change for each and every resistor in the light.

Geoff,
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Follow Up By: Ianw - Monday, Jun 28, 2010 at 20:02

Monday, Jun 28, 2010 at 20:02
Geoff, I guess we had better go back to the theory books eh? I will agree that some diodes have a nominal voltage drop of .6v i.e those rectifiers made of silicon.
Others (made of germanium) have a .2 volt drop. However, there is not an LED made that has anything near those !!! Most LEDs have (depending on colour , and therefore construction) a voltage drop between 3v to even 4.5 volts each. So 100 LEDS would require at least 300volts b4 they even glow. Try it yourself; connect say, 5 LEDs in series and put across 12v. They will not light up. That is why I said 20 LEDs in series. If they are red LEDs they will require 72v to even light up. Actually, white LEDS are mostly made from blue LEDS which require even more volts to work. You can run a red led on a 3v lithium battery with no resistor. It will not blow up !!!

75v DC is not life threatening in most circumstances. I would have no hesitation in grabbing hold of a 75vDC supply because ordinary skin resistance is too high to allow lethal current to flow. Old radios used to run on 90v batteries, and never got a shock from them! 6 car batteries in series ( 75.6v) are capable of melting steel etc, but they certainly won't kill you if you grab hold of it. The resistance of the human skin is just too high.

Ian

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