Club 80-90 Forums

I found out you can buy "positive output" dimmers, which is what I would need http://www.abeltronics.co.uk/products/l ... tage/dim12" onclick="window.open(this.href);return false;
But at around £45 all in I'm going to see how I get on without first. Spent yesterday rewiring so I could have two switches Including one you can reach from the bed, and a separate feed for some existing non-led reading lights. Hopefully might get the actual LEDs hooked up today.

It's worth noting that the "official" dimmers do it by using square wave pulses of ever reducing length, but constant voltage, to reduce the light output. However, introducing a resistor into the circuit to reduce the voltage fed to the LEDs will reduce the output noticeably. The only issue is they will go dimmer and dimmer as the voltage is reduced, but at a certain reduction, they will simply go completely off. I would bet though, especially as resistor are only pennies, that it is worth a try. I would if I did not already have a dimmer on mine.

If you would like me to work out what resistor would be needed to reduce the brightness by a reasonable amount, you will need to let me know the current flowing when they are working normally.

A simple "change over" switch with a central "off" position (commonly available) is all that would be needed to switch from "full" to "somewhat dimmer".

Thanks ghost123uk, I will do that when its all hooked up. In that case is there such a thing as a variable resistor to vary dimming ? What sort if dimmer do you have and does it work on positive or negative ? There are many cheap remote dimmers on eBay. I asked various sellers which output they work on but none so far either know the answer or understand the question.

http://pages.ebay.com/link/?nav=item.vi ... 58&alt=web" onclick="window.open(this.href);return false;

Something like this would do the job, just need to work out which resistance range would be best to go from off to fully on using it.

I know what you mean e when asking a question and they don't know what you mean

My "dimmer" is wired directly to the strips, with no common earths, so I don't have your problem. Actually my "dimmer" is remote controlled and also can change the colour of the strip to anything I desire, as well as dimming and fading between colours automatically

There is a type of variable resistor that may be suitable, but it is not your standard "carbon track" design as that would not be suitable because of the current involved. Once "we" know the current involved I will be able to give you a link to a suitable (likely resistive wire wound) potentiometer that will work, but note, at some point during the reduction of brightness, the LEDs will just go off. It will be something similar to this -> http://www.ebay.co.uk/itm/25W-30-OHM-Hi ... BNBh230aUw" onclick="window.open(this.href);return false;

This all sounds great. I'll let you know when I've done it. My existing wiring and layout lends itself to having 2, 3 or 4 strips with them being either in series or parallel to each other or a combination. Will the length of each strip and the wiring layout have any effect on each strips brightness or brightness of each strip relative to each other, and would a potentiometer dim the same amount and turn each off at the same time regardless of how I wire them, or would I need a particular configuration to acheive equal brightness and dimming and concurrent dimming to off state ? Thanks again for all the help !

Regardless of dimmers or not, they cannot be wired in series.

I presumed that as you can cut the led strip with scissors you would be able to join the bits you cut with 2 wires and it would be no different electrically speaking. Maybe that counts as in parallel rather than series. So if its all done in parallel will the length of strip i cut affect relative brightness or dimming characteristics per led or strip ? Ideally each led should be equally bright and dim/ go off at the same rate/time. I know one way to find out but it involves chopping up my led strip. Sorry to be such a dimwit on electronics !

What you are describing is wiring in parallel

For any resistor / dimmer to work the same on more than one strip, each strip will need to have the same amount of LEDs.

Surely in each strip all the leds are in parallel, so if the strips are connected to each other in parallel then a resistor connected in the common feed will affect each led equally?

Oh yes, you are of course correct

Hi Ghost123uk
I now have my strip lights all wired up. Quite bright, but bearable. Still, a dimmer might not go amiss.
I have measured the current, which is 1.06 Amps. Do you have any suggestions on what spec of variable resistor I might need ?
Also, with it dimmed, will it still draw the same 1.06 Amps or so, or does dimming it reduce the power draw ?
Many thanks

Just to let you know I'm not ignoring this, I will get the figure for the resistor when I have had chance to do a little experiment with regard to the voltage that is optimum for getting the correct level of "dimness" before it drops low enough to cause the LEDs to extinguish.

Ha, bet you thought I had forgotten this

I just needed to do some tests "in the real world" because doing it using maths wouldn't work with the way these LED strips work.

So, after eventually finding enough bits of LED strips to make up enough to get to the 1 Amp figure requested I did some tests, with interesting results.

Because of the way LEDs work, they do not dim 50% when 50% of the voltage is applied.
My tests showed that from full brightness to almost out completely was from 12.6 Volts (obviously) to 5 Volts.
However, In my tests, the current drops off very rapidly as the voltage is decreased. It was very non linear (as LEDs are "semi conductors"). This means one cannot use a simple (wirewound, ~15W) potentiometer. E D I T = reading back over my results below, if I had a 15W 500Ω pot, I would have tried it Looking again at the results, i think it might be OK.

Also, as the voltage (and therefore the current) drops below about 8 Volts, the LEDs stop being white, the red LEDs in the strip obviously require less voltage to keep going, so as you go below 8 Volts the LEDs become redder and redder. This might be considered a quite nice feature.

By the way, I am assuming here we are talking about the tri-colour versions of the strips?

So, what is the answer. Well, one cannot use one of the £6.70 DC adjustable voltage regulators available on ebay because they drop 3 Volts, meaning that on "full" with 12.6 Volts going in, they would only chuck out 8.6 Volts = no good. What one could do is arrange a multi position switch, just like the one that controls the three speeds on your heater blower. Feed it with 12 Volts, then, on the three output legs, solder on resistors. Or, you could use a rotary switch with more than three terminals, like these :-

http://www.ebay.co.uk/itm/Rotary-Switch ... 1Jf61Oa1OQ" onclick="window.open(this.href);return false;

http://www.ebay.co.uk/itm/2P6W-2-Pole-6 ... SwKtlWrRel" onclick="window.open(this.href);return false;

http://www.ebay.co.uk/itm/Rotary-Switch ... SwY45URh1i" onclick="window.open(this.href);return false;

And get more control.

Resistor values =
470‎Ω = very dim red @ less than 1mA !!
220‎Ω = dull red @ 1mA
100‎Ω = bright red @ 4mA
33‎Ω = almost pure white @ .15A (a 1Watt resistor would be needed here = pennies still)
15‎Ω = not quite full brightness @ .25A (a 2Watt resistor would be needed here = pennies still)
And of course one connection to give the full 12.6 Volts from the battery. (1 whole Amp for my strip totalling 48 SMD LEDs)

Or, you could just have a three position ordinary switch = off / full power / via a 220‎Ω resistor for a medium red output. This would be cheap and easy option (about £2 would cover it).

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Slight aside = I bet Ralph and some other electronics nerds will be a bit surprised at those very low current figures the LEDs draw whilst still emitting useful amounts of light. Turn um down lads and save your leisure batteries power for the Propex

Thanks so much for going to all that effort. The main problem I see is that I would have no idea whatsoever how I would wire the 6 way switch, the incoming/outgoing wire (s?) and all the resistors together. Would I want a shorting or a non-shorting rotary switch ? You list a 1watt and a 2watt resistor. What would the others be ? 1/4w, 1/2W ?
I'm a bit bewildered by the many different types of resistor on Ebay, would any type do (any material) ? Is a 33R resistor the same as a 33 Ohm resistor. Bit out of my depth here !
I already tried three cheapo dimmers from ebay, but they all turned out to be negative output ones.