really need help.

[el duderino]

hi guys, its been awhile since i posted on here , i need some help.

im building a project for my final year in school and its going to be examined in a couple of weeks, i have done all the work but i cant get my circuit to work.

basically i need to split a current in two so an even amount goes along each wire and both currents work when i turn them on.
i.e. i have one wire with a nine volt, i want to split it into two wires
and get i an even current in both
how can i do this?
please please help!!

e.

[davebungo]

Wire 2 resistors in parallel - say 1K, and put this across the battery.  You'll get the same current in each resistor.  Is this what you meant?  I suspect it is something more complicated as this seems too easy.

[niftydog]

google "constant current source." But somehow it seems a strange request. Can you share any more info about the project?

what dave said would work, provided you maintain the same voltage across both resistors.

[el duderino]

ok well its hard for me to try and explain it but the project requires me to build a marble machine which lifts a charge of ten marbles and deliver them to two novel destinations, on arrival an elctronic indicator must be set off to "indicate" the marbles arrival.
all that is ok and i have it done.
the thing is im on the last section of making it which is the circuit.
what im doing is trying to split one wire fcoming from a nine volt battery in two so i can supply even current along two wires.

[toneman]

this is not a stompbox?  is it?
some say i have marbles in my head, but never in my stompbox.

[el duderino]

no this isnt a stompbox! but i have made stompboxes before and the people on this forum really helped alot, so i figured why not post here

[davebungo]

You have repeated your original question which is too vague.  If you want help then try to be more specific.  Just saying you want to split a wire into two wires with an equal current down each doesn't make any sense without further information.

[el duderino]

ok as i said its hard to explain so i did this real quick drawing.... sorry about how bad it is


http://i5.photobucket.com/albums/y165/humlogic/CIRCUIT.bmp

ok so the problem is where the red circle is, i need this section to split so i can supply even power to the motor and the LED, i need both to work when i flick the toggle switch  and i cant put everything in the one line as i need the motor to work constantly.........

i really apreciate the help.
e.

[R.G.]

OK, I'm not really sure about why the currents have to be equal, but I take your word for it.

There are a couple of ways to do this. One is with transistor current sources, another is with current mirrors. If you want to electronically determine how big the equal currents are, a current mirror is the way to go.

Imagine this: two identical NPN transistors. Their emitters are tied together, their bases are tied together. One of them has its collector tied to the two bases. The emitters are tied to the most negative voltage for the pair. The two bases and collector connection is tied to one load, the controlling load. The other collector is tied to the controlled load.

We usually don't think of transistors this way, but you can consider them to be driven by a base voltage, so that you think of a transconductance, a current in the collector as a function of the base voltage. It's highly nonlinear, but it is a one-to-one function.

The controlling load pulls a current through the diode-connected transistor, the one with its collector connected to its base. That forces the base of that transistor to an appropriate voltage that makes the collector current be correct. The other base is tied in parallel to that same voltage, so it sets its collector current to the same value. The two collector currents are equal, within the limits of how well the transistors are matched and the error terms of base current use. Perfectly identical transistors are useful, but don't exist.

To get around some of the nonlinearities, you can use resistors in series with the emitters before they are connected together. Even small resistors, under 100 ohms, can make things much more predictable and reduce the need for perfectly matched transistors.

You don't say how big the currents are, so I suspect that it's many milliamps to a couple of amps since motors are involved. You'll need power transistors, which work fine as current mirrors, but you may need darlington devices or making up your own darlingtons with a couple of extra small signal devices like 2N3904's.

They may get hot, so you might need heat sinks.

Questions?

[ibanezts808]

Questions?

:shock: I haven't even built my first pedal yet.  I hope you have a lot of time r.g. lol j/k, i'll be at your point one day... i hope.

[R.G.]

Here's a quick schemo for reference. I think this is what the circuit was to do. One switch turns on DC to the whole mess, the second switch turns on the LED. The motor runs at whatever current it runs at and the LED is forced to have the same current when it runs.

Note that if you get much over 20ma, the LED probably burns up. The resistor in series with the LED isn't really necessary, but it keeps the current source transistor from getting so hot.

[R.G.]

ooops. I actually  have to put the picture in, don't I?

http://www.geofex.com/FX_images/motor-current-mirror.gif

[triskadecaepyon]

This project sounds outrageously similar to the "marble sorter project", which used a photocell and fisher technic parts and programming to separate 2 different colors of marbles.  I did this last year as a junior in high school.  Is this by chance a Project Lead The Way (PLTW) project?  I have the schematics and everything for it....