Electronics - advice on circuits and LEDs

BullBoyShoes · 30 Aug 2012 at 12:45

Just FYI - electronics.stackexchange have a good community going for questions like this, heres a similar one:

http://electronics.stackexchange.com/questions/38946/20-3-2v-3-8v-leds-in-a-line-on-a-12v-circuit

dbov22 · 30 Aug 2012 at 15:28

Well, just been into Maplins to get some matrix board and resistors - the resistors were a fortune and one of the assistants asked if I wanted some help - the entire staff did not know that there was a difference between series and parallel and anyway...long story short they didn't have a clue.

One thing they were all CERTAIN of (hmm) was that the transformer I originally used is a AC transformer and NOT and DC. I measured the voltage coming from the transformer and it was 13.64....apparently this is a give away sign it's AC and also the fact it has the following symbol on it:

____
- - -

back to square one dudes

BullBoyShoes · 30 Aug 2012 at 15:35

That is the symbol for DC.

http://en.wikipedia.org/wiki/File:Direct_current_symbol.svg

You must have set your multimeter to read DC volts to get a reading of 13.64v.
It will be that high as its unloaded, draw some current and it will be closer to 12v.

dbov22 · 30 Aug 2012 at 15:40

Yeah, I googled it and I knew I was right but when 7 "electronics assistant experts" in Maplin tell you you're wrong you believe it!

I put my multimeter on the number 20 in the top left corner V section. No idea where it's meant to go.

Tealc · 30 Aug 2012 at 17:02

That's the right section for 12v DC.

I never pay more than £1 for 100 basic resistors. The only time I would is if it were for a precision job such as audio, then I might go up to 10p each.

The giveaway for AC is that it won't read the voltage until you put the meter in AC mode and the little sine wave symbol comes up.

That shop just employ people with no clues about the stuff they sell.

dbov22 · 30 Aug 2012 at 18:37

I think I'm starting to get somewhere now. Tealc can you help with this one please, no matter where I place the multimeter in the circuit it always comes up around 3.4V on average, this is with a 3x 1.5v cells together as well as my 12v transformer.

I don't understand why it would do this.

Also, am I correct in thinking that if I keep the LEDs in parallel (I know this isn't the best way to do it) I'm saying If... Then my 40 LEDs x 3.7v (each LED) x 0.02 (amps per led) = 2.96watts. My power supply is rated 5w so this should be ok?

But if wired in series this would obviously be different and closer to overloading the transformer?

I have also just been to buy a cheap LED torch, it has nine white LEDs. There are no resistors anywhere and it is powered by 3x 1.5v batteries. So this is overpowering the LEDs by at the most 1.1V? So at the worst this will just reduce the life of the torch?

Tealc · 30 Aug 2012 at 18:57

3.4v sound suspiciously like the forward voltage drop of your white LEDs.

I don't know much about current limited or controlled power supplies (haven't studied them in any way) but suspect that it always shows the voltage drop of the lowest LED in your configuration.

2.96 Watts would be correct if each LED only draws 20mA. Why not measure it?

Put the multimeter in series with the circuit.

Put the meter on the 10ADC/10A setting and plug the red lead into the 10A hole, leaving the black lead in the COM hole.

box · 30 Aug 2012 at 19:05

dbov22 said:
Well, just been into Maplins to get some matrix board and resistors - the resistors were a fortune and one of the assistants asked if I wanted some help - the entire staff did not know that there was a difference between series and parallel and anyway...long story short they didn't have a clue.

One thing they were all CERTAIN of (hmm) was that the transformer I originally used is a AC transformer and NOT and DC. I measured the voltage coming from the transformer and it was 13.64....apparently this is a give away sign it's AC and also the fact it has the following symbol on it:

____
- - -

back to square one dudes

Maplin are a joke a lot of the time. If the prices aren't bad enough, a lot of the younger staff there seem to be rude and arrogant in my experience. Probably better to buy your components online if you can.