Re: current regulator -- explanation?

On Dec 6, 2:56 am, Rex <r...@xxxxxxxxxxxxx> wrote:
I found my way to this page today:http://www.gizmology.net/LEDs.htm
looking for information about light output.

As I was reading, I found a circuit for regulating the current through
LEDs. It has two transistors with one having the base-collector shorted.
The circuit is about 2/3 down the page and has the caption,

"LEDs driven by a simple current regulator".

I don't see exactly how this senses the current through the LEDs and
regulates it. Does it work? If so, can someone explain or give a link to
how the circuit works.

Posted here because maybe this is basic stuff I have missed or am just
too dumb to see.

Assuming it does work (or even if not), is this a good implementation
(function/parts)? Is there a better way?

Hi, Rex. Current mirrors are a good choice if you're making and IC,
because you can match the transistors well, and temperature tracking
between transistors is easy.

If you want an easy current source that has pretty good compliance
over a fairly wide range, you could do worse than this (view in fixed
font or M$ Notepad):

|
| VCC
| +
| |
| .-.
| | |
| | |
| '-'
| |
| |
| Vb |<
| <------| PNP
| |\
| |
| |
| |
| |
| ^
| Isource
|
| |
| ===
| GND
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

By controlling Vb, you can achieve pretty good control of current over
a fairly high voltage range with only one transistor and one resistor.

The emitter voltage will be one diode drop higher than the base
voltage. Let's say you wanted a current source of 20mA, and you had a
power supply that varied from 8 to 16VDC. You might do something like
this (view in fixed font or M$ Notepad):
|
| VCC VCC
| + +
| | |
| V .-.
| - | |36 ohm
| |1N914X2 | |
| | '-'
| V |
| - |
| | |<
| o-------| 2N3906
| | |\
| | |
| | |
| | |LED
| |3.3K |
| .-. V ~
| | | - ~
| | | |
| '-' |
| | |
| === ===
| GND GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

By adding the two diodes and the 3.3K resistor, you're ensuring that
Vb will be two diode drops below Vcc. That means Ve will be one diode
drop below Vcc. A 36 ohm resistor is chosen using Ohms Law:

0.7V / 0.02A = 35 ohms

This will not work perfectly, but it is an acceptable current source
for an LED.

Cheers
Chris
.

Relevant Pages

  • Re: Astable Multivibrator help!
    ... In order to guarantee oscillation, you need to add a diode logic OR function which provides base drive only if one OR the other of the transistors is OFF, like shown below, a lock-up cannot occur because there will be insufficient base drive. ... Oddly, with the two diodes, it takes the circuit about 20 seconds to start up. ... the circuit will fire the LEDs up immediately and start the alternation in one or two seconds. ... Even at 9V, which I assume means an alkaline battery that will spend most of its time in the 7.2-8.4V range, you have the caps charging to ...
    (sci.electronics.design)
  • Re: High Current LED flasher
    ... I doubt that I will be able to find the germanium transistors so I am ... to simulate this circuit but I cannot get it to work. ... try putting a 10 ohm resistor in series with the LED. ... may try two or more LEDs in series to obtain the same effect producing more ...
    (sci.electronics.basics)
  • Re: Flashing LEDs - 9V
    ... LEDs - the sort you get in cycle lights. ... choosing the right transistors of course. ... That's not to say you can apply 9V to a circuit designed to be used at 5V. ...
    (sci.electronics.basics)
  • Re: Simple circuit, stupid problem
    ... >>actually pluck the capacitors right out of the circuit and both LEDs ... >>Does anyone have any suggestions as to what stupid mistake I'm making ... But that circuit has a stable non-oscillating state: both transistors ...
    (sci.electronics.basics)
  • Re: 12 LED resistance circuit help
    ... be more humble and merely post "Circuit designer"? ... >Perhaps "dissipate more power" would have been more appropriate than ... "Since there are two LEDs in series, one may hog more current than the ... >> dissipation spec of an LED with Vf max in series with an LED with Vf ...
    (sci.electronics.basics)