Voltage drop

[Psychedelic Theatre]

Hi there,

 

I'm part way through making a very basic (crude) cue light system for a local Am-Dram group. Basically, they gave me £30 and said do what you can.

 

I have a load of old ABS boxes for outstations, I have a stock of LEDs.

 

Now I'm looking at the power supply. Because there will only be three or four outstations, I am looking at running each 'channel' from a PP3 battery, so there will be a battery running each channel.

 

So here are my questions;

 

Will a PP3 battery provide enough current to light the LEDs after running 30-50m down the cable?

 

And, following on from that, what kind of cable would be best? I'm currently looking at some form of ribbon arrangement, is this suitable?

 

 

Many thanks,

 

Tim

[Renny]

Wouldn't you be better to use a 12v birdie transformer and branch the power out from it? Then put the switches after that? Even then you would get voltage drop. However if you only need 9v for the LED then the drop may be acceptable. If it to be partly permanant I would use as heavy a cable as you can manage, perhaps twin and earth.

[Psychedelic Theatre]

Yep, I've just realised it would be much easier to use a transformer!!

 

I've ordered a 12V transformer.

 

This isn't going to be permanent, the shows are in a church hall so everything apart from the lights are taken out after show week.

[Roderick]

As for cable, CAT-5 is pretty cheap. Gives you 8 cores and will withstand regular 'rolling out - rolling up' much better than any ribbon cable.

And if you use RJ-45 connectors (with boots) it will be easy to extend cables and make the system more flexible.

[Malcolm Gordon]

For a number of years I have been using a 9V PP3 system which I constructed from an unpublished article intended for Sound on Sound but given to me by the author. This uses two PP3s in a push-pull arrangement to drive alternate red or green LEDs, with their 'forward' directions in opposition, using two wires only. It uses ordinary 3XLR mic cable. Each channel has four Hi-Brite LEDs in Clip-lite lenses, two at the master and two at the outstation, one pair being in operation at any time, and the PP3 has no difficulty in driving this 30m down the cable, which for me is usually from the prompt position to behind doors in the set for entry cueing. On one occasion, when the set had moving pieces that made the cable difficult to string across the stage, I made the connections up the SL mic tie-lines to the control room and back down to SR, without loss of brightness.

 

In fact, the system has four channels switched to allow for group cueing and one PP3 drives all four, i.e. 8 LEDs, without noticeable loss of brightness. However, the extra complication meant that the switches were quite expensive.

 

The LED current is so low, the cable voltage drop is insignificant compared with the dropper resistor, which in my case is 220ohm. If you change from 9V to 12V, remember to change the LED dropper resistor to suit.

[musht]

Seconded for Cat5,plenty capable at lowest cost. Voltage drop really wont be a problem.

 

Remember their diodes need a rectifier as well.In fact see what you have in wall warts lying around......

 

Typical modern True green, not yellow green, LED about 3.2V at 20mA red about a volt less, probably want to run them at more like 5mA or they will be too bright, at 12V start around 1K5 and adjust to taste.

 

http://led.linear1.org/1led.wiz

[Ekij]

Typical LED is (about) 2V and 10 to 20mA

With a 9V pp3 you have to drop 7V anyway so loosing a little to the resistance of the cable isn't an issue.

Given the tiny current you could probably run a single LED off a pp3 with a mile (literally) of bell wire.

 

However since the OP has already decided not to use a pp3 and use a 12V transformer instead should this topic be closed?

[gyro_gearloose]

You can LEDs off far higher voltages than 2V. It doesn't really matter what voltage you use as long as you limit the current. The current limit will depend on the LEDs you are using, but 20mA seems to be the limit on most cheap LEDs I've seen.

 

The easiest way of limiting the current is, as people have suggested, is to use a resistor in series with each LED. The thing to bear in mind about this is that is is not the most efficient way of doing it, particularly if the difference between supply voltage and LED forward voltage is high, and and this may be an issue if you ever want to use batteries as a power source (and I know that the OP has said that he's going to use a transformer, but other people may find this info useful). A more efficient way of powering LEDs is to use a simple voltage regulator like the one here. The voltage regulator is better than a simple resistor because it doesn't matter too much what the input voltage is so voltage drop along a long and/or poor quality cable won't affect the LEDs light output.

 

If you want to use a resistor, then you need to know this equation :

 

R = (VS-VL)/I

 

where

 

R = Value of current limiting resistor in Ohms

VS = power supply voltage

VL = LED voltage

I = LED current in Amps, so 20mA = 0.02.

 

So if we have a 12V power supply, and our LED needs 3.3Volts and 20mA, then we get (12-3.3)/0.02=435Ohms. As long as we use a resistor of AT LEAST 435Ohms (...and it doesn't matter if it is higher, the current will just be slightly lower and the LED slightly dimmer...) our LED will be driven with 20mA.

[Johnno]

Just for reference, PP3s are the most expensive solution to battery powering equipment. Avoid!

[Psychedelic Theatre]

Thanks everyone, plenty of information to suck on there, especially your idea Malcolm.

 

Thanks for the equation, literally ten minutes ago I looked through my GCSE Electronics notes and found it!!

 

I'm sorted with everything now, so if mods want to close the thread they can do

[lightsource]

My knowledge is a little rusty here, so please be gentle if I'm wrong

 

As far as I'm aware, the manufacture of any mains operated equipment comes under the CE marking system,

 

Also, let's say one of the transformer was faulty, overheated and burned the venue down. There are insurance and H&S issues there.

 

I'll let others reply to this post as my knowledge is sketchy.

 

But, a thermal fuse on the PSU is probably a good idea.

[Ike]

CE marking only applies to products that you "place on the market" and doesn't include things built for in-house use (usually).

[d_korman]

CE marking only applies to products that you "place on the market" and doesn't include things built for in-house use (usually).

 

And in any case, the CE mark does not denote any independent testing, it is just a statement by the manufacturer that the product complies with the relevant standards. Whether is does or not is another matter!

[musht]

Hence use of wall wart, plug top transformers, by lots of manufacturers, the line voltage bit is somonelse`s approval problem and easy to supply appropriate trafo to a worldwide market.

 

Means usually have several unused ones lying around, anything above 6V DC/100mA would suit the OPs requirement.