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Old 28-01-2018, 09:10 AM   #1
Silvernovice
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Default LEDs in series

Can Leds be wired in series?
I am wiring some platform lamps rated at 3 volt. I was intending to use an old laptop transformer that pushes out 18 volt DC. Would wiring 6 lamps inseries be a problem?
Peter
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Old 28-01-2018, 09:18 AM   #2
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Use this calculator to work out the resistor you require for that psu/led combination.

http://led.linear1.org/led.wiz

Or look for something like this to adjust your supply voltage.

https://rover.ebay.com/rover/0/0/0?m...2F332344627723
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Old 28-01-2018, 09:58 AM   #3
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Thans Steve useful tool
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Old 28-01-2018, 10:24 AM   #4
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Quote:
Originally Posted by Silvernovice View Post
Thans Steve useful tool
Peter
Yes, I am.

That calculator is invaluable - Flashbang shared it originally and I have it bookmarked now.

I think it was Tinker that found those little control units. I have a couple on my layout where I wanted a very specific and stable supply and have plans to introduce more as my early wiring efforts are ‘tidied up’.

Another suggestion, if you want to have a variable supply for adjusting lighting or even controlling a turntable these are very good.

https://rover.ebay.com/rover/0/0/0?m...2F282655377856
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Old 28-01-2018, 11:18 AM   #5
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Again many thanks, a veritable font of knowledge
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Old 28-01-2018, 11:28 AM   #6
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and a little late I have just read Flashbang's sticky on LEDs which would have answered my question

My problem now is I have a couple of supposedly 12volt dc output transformers and on measuring with two different multimeters and a 0-15 volt lab meter I get a reading around over 18 volt. So a bit of experimentation with resistors needed!
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Old 28-01-2018, 11:33 AM   #7
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If you are measuring the transformer/PSU open circuit then you will get a higher than rated voltage. Once a load is applied then you should find the voltage drops down to the rated voltage.

Richard
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Old 28-01-2018, 11:34 AM   #8
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Quote:
Originally Posted by Tricky Dicky View Post
If you are measuring the transformer/PSU open circuit then you will get a higher than rated voltage. Once a load is applied then you should find the voltage drops down to the rated voltage.

Richard
Thanks Richard
that would explain it
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Old 28-01-2018, 11:44 AM   #9
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Hi
I would advocate whenever possible using a Regulated DC power supply. These output a constant voltage regardless of loading. A simple 12volt regulated plug in power supply can be purchased for around £6.00! Example EBay link to 12v PSU This one can run up to 190 ish LEDs! and comes with a easy to wire connector.

Your ex laptop power supply will also be regulated, but at a higher output voltage and current and therefore I would split the circuits into several 1.0Amp sub circuits immediately after the PSU. Use 1.0 Amp self resetting circuit breakers to protect each sub circuit. Maplin sell these in the UK for around £1.79 each. Part No AK07H. https://www.maplin.co.uk/p/10a-auto-...-breaker-ak07h
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Old 28-01-2018, 02:12 PM   #10
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Many thanks FB, the idea of several separate circuits makes sence, MAplins here I come.
Peter
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