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Help with electronics please.

michaelmorris

By michaelmorris
in DIY Astronomer

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michaelmorris

michaelmorris

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After finishing my 50mm finder-to-guidescope conversion I feel empowered and omnipotent. So I've decided (rather rashly) to have a go at building a lightbox for taking flats. I want a very small and lightweight system, but cannot afford an EL panel.

I've bought some mega cheap flexible strips of LED lights on Fleabay.

2 White 48 LED Flexible Strip Waterproof Car Light Bulb - eBay (item 290461500821 end time Aug-11-10 04:09:22 PDT)

So here is the plan. Coil the LEDs in a spiral shape, glueing them onto a piece of white foam board. In front of this I put a sheet of white perspex and a white artists canvas to diffuse the light. The box will be made of white foam board, possibly strengthened in the corners/edges with some thin aluminium angle strips. The lights are 12v so I figured I could power them from one of the continuous output phono sockets on my dew heater controller. Or so I thought....

A friend has said that he thinks there is a risk I could fry the LEDs by either putting too much current through them or by getting the polarity wrong. How do I avoid this?

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martin_h

martin_h

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Looking at the ad on ebay it looks as if they are ready to go into a car, so I would imagine they have inbuilt resistors or are wired in paralell for 12V. As for polarity what colour are the flying leads attached to it?

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barkis

barkis

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Are you certain you will get an even illumination using these strings of LED's Michael?. I suppose it's a question of try it and see really.

-----------------------------

I bought some blue ones from Maplin, and I placed them out of sight, and above my garage up and over door.

Looks quite nice having the door bathed in a gentle blue light at night. It's powered by a Syquest drive 12volt Transformer on a timer.

The drive packed in, but I kept the transformer for a job I knew would crop up one day.:)

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michaelmorris

michaelmorris

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As for polarity what colour are the flying leads attached to it?

The top lead (white) appears to be wired come in at the top and the copper coloured lead at the bottom. Other than that I'm afraid I just can't tell where they go from there. :)

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michaelmorris

michaelmorris

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Are you certain you will get an even illumination using these strings of LED's Michael?. I suppose it's a question of try it and see really.

-----------------------------

I bought some blue ones from Maplin, and I placed them out of sight, and above my garage up and over door.

Looks quite nice having the door bathed in a gentle blue light at night. It's powered by a Syquest drive 12volt Transformer on a timer.

The drive packed in, but I kept the transformer for a job I knew would crop up one day.:)

Does these mean it would be okay to wire them up to the continuous output of my dew heater controller?

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martin_h

martin_h

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Try connecting them to 1 AA battery, if you get a glimmer of light you can get the polarity from that. if no glimmer reverse the connection. If still no glimmer try 2 AA batteries in series and repeat, but dont use any more than 2..Failing that do you know anyone with a test meter that can test them for you?

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Beyond_Vision

Beyond_Vision

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The ad says they are rated for 12V so should be fine.

They are either in parallel with series resistors or connected in series with no resitors. Although with a 12V supply and 48 leds this would only mean 0.25v across each LED if they were connected in series. I would have expected each one to have something like around a 1 to 2 V forward voltage across each.

I suspect that based on the number of LEDS in the strip they may well be in parallel with a series current limiting resistors. If you have a multimeter with a diode test function it will show you the polarity if they are connected in parallel. If in series the meter will not register conduction in either direction as its test voltage will be too low to turn on the LEDs

Reverse biasing will probably do it no harm if the diodes are in series but could possibly if they are wired in parallel. A typical LED will have a reverse breakdown voltage of around 5V.

Regards

Kevin

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michaelmorris

michaelmorris

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Try connecting them to 1 AA battery, if you get a glimmer of light you can get the polarity from that. if no glimmer reverse the connection. If still no glimmer try 2 AA batteries in series and repeat, but dont use any more than 2..Failing that do you know anyone with a test meter that can test them for you?

Hi Martin

I tried the battery method with no result. I don't think I know anyone with a test meter, I'll ask around. I've e-mailed the seller but to be honest I'm not hopeful he/she will know.

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yesyes

yesyes

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If the LED rope comes with bare wires (no plug), then the seller must tell you what the voltage, amperage and polarity is, otherwise the item will be useless to anyone (unless they have additional equipment to test it).

Seeing that they are 48 LEDs I'd assume they are arranged in groups of 6 in series and 8 of these groups in parallel.

I haven't done much with LEDs in the last few years but remember that they can take (ans survive) quite a beating when the polarity is wrong. The 5 Volt mentioned earlier sounds about right for something that would be on a data sheet. The real value is probably higher but you wouldn't want to try that out.. ;-)

Since some of the LEDs will be arranged in series, you can add this voltage. Assume 6 LEDs in series per group, that would make the "break" voltage about 30V, far higher than the 12V your would supply.

I think it's safe to try it out on a 12V power supply. If they don't light up, try reversing polarity. (If they still don't light up, I might have been wrong... ;-) )

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michaelmorris

michaelmorris

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The answer form the seller was "they have a positive polarity and a negative polarity". Not much help there then! Anyway, I connected them to a 12v power supply and bingo, they work first time.

I'll wire them up at the weekend. I presume I wire the two strips in parallel, is this correct?

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michaelmorris

michaelmorris

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they will no doubt be center positive.

You are correct.

Thanks everyone for their help with this. I've wired up one of the strips to an old phono lead and Robert's your father's brother - it works. Now, to build a light box.

To the bat cave Robin ........

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