Another crazy DMX project

[cimo]

Carnival comes closer and i can t just finish my older projects without starting a new one: I am going to dress a girl with around 2000-2500 Watts of light power.

Discarding the idea of using 12 volts cause of the extreme amount of amperes that would be involved i have to step back to a "normal" 220 volts system; ie: a car battery with a 12 to 220 volts stepup transformer.

Then a laptop, a gm5, a core, a dout, and a distribution box with a bunch of these

http://www.reichelt.de/?;ACTION=3;LA=4;GROUP=A45;GROUPID=2999;ARTICLE=21349;START=0;SORT=artnr;OFFSET=16;SID=25UQ1hMawQARkAADNghYcea08697fb0e5051c2c30584076abf772

Now about the lights, different otions:

-normal bulbs at 220 volts connected in parallel

-halogen lights at 220 volts

-halogen lights at 24 volts wired in series in group of 8

Anybody with similar experiences, any good advice?

Simone

[bugfight]

reconsider strapping a homebrew 220v system on a person, unless you want them to fry.  even if you get lucky there, the heat may kill...

why not leds?

the battery will need to supply the necessary power regardless of final voltage, so your current out of the battery will be high anywayz.  unless you are driving very long lines, the transformer efficiency will actually increase your power requirements.

car batteries are not designed to handle repeated charge/discharge cycles.  you'll want a marine battery instead. 

lead acid batteries give off hydrogen when they charge, and can explode if an internal short happens.  this is especially nasty because of the sulfuric acid.

dc thru a transformer will not work, you need an ac inverter or likesuchas for that.

[cimo]

good points but i forgot to mention some details:

reconsider strapping a homebrew 220v system on a person, unless you want them to fry.  even if you get lucky there, the heat may kill...

why not leds?

-I am not going to strap stuff on her body, she s going to carry a 200-300 kilos wheeled "stuff" and the system is going to be inbuit there, and i can also isolate the parts that she is touching form the rest of the frame

the battery will need to supply the necessary power regardless of final voltage, so your current out of the battery will be high anywayz.  unless you are driving very long lines, the transformer efficiency will actually increase your power requirements.

-The idea is to use a step up transformer so the only "thick cabled" connection would be 20-30 cms

car batteries are not designed to handle repeated charge/discharge cycles.  you'll want a marine battery instead. 

-The whole thing will last only 210 seconds and only once

lead acid batteries give off hydrogen when they charge, and can explode if an internal short happens.  this is especially nasty because of the sulfuric acid.

-Would a sealed plastic box be of any help?

dc thru a transformer will not work, you need an ac inverter or likesuchas for that.

-Yep, that s what i meant, i just used the wrong word

[bugfight]

hehe some important details there...

a box will help for sure, but you'll want a hole to let the hydrogen out.  this only happens while charging so you could charge it and then install it... i'd still use a box though, i've only seen two car batteries explode this way, but that was enough to scare me.  since it can happen, it's good to take reasonable precaution...

re: thick cables, it seems you'll want those for 220v anyways...

i have an led flashlight that throws a beam several blocks, brighter than any of my regular ones...

maybe consider getting some of those in bulk and replace the switches with relays or likesuchas...

[cimo]

re: thick cables, it seems you'll want those for 220v anyways...

but that would be just for the 12 volts path then i could use 1mm cable formt he inverter to the ditribuitor and then just 0.75mm.

i have an led flashlight that throws a beam several blocks, brighter than any of my regular ones...

maybe consider getting some of those in bulk and replace the switches with relays or likesuchas...

ok, more details: we are desperately short on money, and time for that it matters.I am going to use those Triacs with, in case, some optoisolators, btw any advice for optoisolators that can be found at Reichelt or RS Components?

[MRE]

-The idea is to use a step up transformer so the only "thick cabled" connection would be 20-30 cms

Umm.. not sure I get you here.

Steeping up the voltage will allow you to get away with thin cable, only if you plan on stepping it back down to be used.

at what voltage is your load operating at, compared to what voltage you would be stepping up to?

In other words: If you are planning to use the inverter to drive the lamp voltage up to 220, then you still have the associated thick cables required to operate whatever lamps you are intending to use, SAFELY.

Are you planning on stepping up to 220, going somewhere with it, then stepping down to 12 to drive the lamps?

Then you will have an ARSE load of transformers all over the place.  Saving on thick cable and replacing it with heavy transformers, for 5 feet of gain.. doesn't sound very cost effective.

Neither is going to save you on over-all current drain either.

If you are short on money, then its a sure bet that an inverter capable of handling that much wattage is going to be a strain on the budget anyway. Add to it step down transformers (where needed), and you have a very convoluted system.

So, Practically, I think your best solution might be:

Large battery (car, or marine) --> thick cable --> distribution box (triacs) --> thinner cable --> 1 12volt lamp (or string of small series/parallel lamps).

Figure that a 50 watt load at 12 volts still needs about the same size lamp cord as a 50 watt load at 120 volts AC. An automotive headlight would be slightly thicker.

If you stick with Automotive lamps (Headlights, tail lights, interior lights, etc) you cant go wrong. The cabling requirements are pretty obvious, and if not, you can always ask the shop clerk.

If you are using much more than the 4 head lamps, I would use two car batteries in parallel.

In case this isn't already obvious: A large lamp, regardless of its voltage/current format, will require a large cable. There is no way around this. A load is a load is a load. Trying to shortcut this is asking for fire.

2500 watts of lighting is going to take some current, and is going to create some heat. No way around it.

Perhaps you should reconsider your plan.

If LED lighting is too expensive (think of the safety of the poor girl), then perhaps it shouldn't be done.

[cimo]

mmm

let s go back to the 12 volts idea.

I ll have, let  say, 16 outputs with 2x 40W/12v (6.7A)

That makes for 108A, that would be for maximum load.

battery<-->108A<-->distribuition box<-->6.7A<-->"light block"

My only worries are about the PCB that will carry the transistors, since there would be a considerably load of current on the copper layer.

MRE: i was not planning to use step down transformer with the 220v setup, and i don t understand: a 12 volts 50W light requires a thicker cable than a 220 volts 50W one, doesn t it?

Simone

[philetaylor]

My only worries are about the PCB that will carry the transistors, since there would be a considerably load of current on the copper layer.

MRE: i was not planning to use step down transformer with the 220v setup, and i don t understand: a 12 volts 50W light requires a thicker cable than a 220 volts 50W one, doesn t it?

Simone

Hi Simone.

Yes for cable/pcb trace sizing you need to think in amps rather than watts. 50W @ 220V is 227mA whereas 50W @ 12V is 4.17A. On this basis, 1mm2 cable should be fine for the connection to each lamp. (maximum 8.75A @ 12V). You will need a very thick cable to connect to the battery(ies) though as even 10mm2 is only rated at 70A @ 12V!!

Take a look at http://www.desmith.net/NMdS/Electronics/TraceWidth.html for calculating PCB trace widths. You will notice that the voltage doesn't make any difference to the required trace widths other than the higher the voltage the bigger the gap between traces is (to prevent arcing).

[cimo]

Just an uptodate for you to follow up:

Thanks to Reichelt s cheap prices and Lucem advice i came up with the following:

-Each group of light will carry 3x 50W (12v) halogen lights.

-Each group will be connected to a DOUT with 2 bc547 and a final power stage with a 2N 3055, the power transistor will be mounted on a large heatsink and i ll use 2mm² copper wire to connect the transistor to the pcb and to the battery

-I ll solder close to each transistor 3x 4700 uF capacitors

-All cables from the transitors going to the battery will be tied together directly on the battery, avoiding having a single cable to carry the whole current.

... more ideas ?

[MRE]

ahh right right right!

yes.

Funny thing is I had 2 mm cord in my head to begin with.

[cimo]

Here we go with an idea for a single channel, it should drive about 100W at 12v.

(looking good?)

but now that i come to think about it: i could use just a single bc547 and invert the 0 and 1 state of the DOUT.

[cimo]

Hi

since the whole thing is to be mounted in a metal structure i was wondering if i can use the chassis to carry the GND lie, is this a good idea?

All in all that s the way it is built in cars.

Simone

[cimo]

hheeelllllpppp

The first stage works, i get 12 volts at every pulse from a dout but somehow the power stage does t get activated an di think that maybe the first stage can t supply enough current to trigger the power transistor. I measured just 1mA between the 2 stages that is between R3 and Q3 base.

Which resistors should i change to increase the current available at the output of the fisrt stage?

thanks

[philetaylor]

I measured just 1mA between the 2 stages that is between R3 and Q3 base.

That's pretty much what I would expect. Ohms law states that I=V/R so with a 10K base resistor (R3), at 12V you should get about 1.2mA across R3.

I don't know the spec of the 2n3055 but you need a collector current of about 8.3A so assuming it can handle a gain of about 40x you will need to replace R3 with something like 56R (you should then measure about 200mA across R3) but you might want to try some higher values and work down until it works! This also means that you will need to reduce R2 as well, probably about 4-500R should be OK.

**WARNING** It is about 18 years since I did transistor theory so I may have got the above completely wrong :)

Phil

[philetaylor]

Also, assuming you are just using one BC547, I would lose R4/R5 completely. the emitter of the BC547 can just go straight to the base of the 2n3055 (via R3) and shouldn't need connecting to the 12v rail at all.

[cimo]

hi

i am now triggering the 2n3055 but i need to go and check it with a 100W load. I ll be back soon (I hope)

Simone

[philetaylor]

Great.

Another thing I did think, R3 will need to be reasonably big (about 2-3W) as it will have about 200mA across it....

Phil

[cimo]

Well it is working but:

1- it s inverted, when the dout is on the light goes off

2-it s slow to power on and off (???)

3-i can t really reach a perfect no-light and full brightness

The change have been reducing all values of the resistors as suggested by Philtaylor

I d really appreciate any kind of help.

Thanks

[philetaylor]

I would try it without the capacitors, with 3 x 4700uF they will take quite a while to discharge which could be why you are seeing a slow switch-off.

Also did you remove R4/R5 as with them in, there will always be some current flow through the base of the 2n3055 which could be why it never fully switches off.

As to why it is never fully on, I would check the current through the lamp, for fully on it should be about 8A. Any less means that you may need an even smaller base resistor on the 2n3055.

Cheers

Phil

[philetaylor]

Just looking at your circuit, is there any reason why the load is wired between the emitter and ground?

Usually you would place the load between collector and ground (with the emitter directly grounded).

I have found a quite nice website that will help you with calculations etc:

http://www.kpsec.freeuk.com/trancirc.htm

Cheers

Phil

[philetaylor]

Obviously I meant between collector and v+ :)

[cimo]

Just looking at your circuit, is there any reason why the load is wired between the emitter and ground?

Usually you would place the load between collector and ground (with the emitter directly grounded).

i ve changed it and it seems to be more responsive this way

See the attached shematics, i understand why it s inverted but i don t understand why if i reply the same circuit and add in serie as i had it before it is still inverted, am i missing something?

thanks

[cimo]

I ve just changed the logic state of douts in the code so now it s ok as it is i go and try it with 85W now

thanks

[cimo]

So far: with up to 35W lamp per channel it works as a charm with the latest schematic, just a single bc547 and a couple of resistors. Problems come with a 50W lamp and up, it can tswitch it on completely or turn it off completely depending on the resistors value i use.

From what i ve learned these days, i am not supplying enough current to the 2n3055 be junction so the transistor can handle all the current required by the 50W lamp, over 4A.

My plan was to go all the way up to 70W per channel.

Would that be a good idea to use another transistor instead of the bc547? maybe something that can handle more current? Or is it just a matter of using the proper resistor value?

Thanks

[cimo]

This is the schem i am going to try now: