Subject: Re: LEDs in Series with LM334 Current Source
From: Mike H
[email protected]
Date: 4/30/2004 4:04 PM Central Standard Time
Message-id: <
[email protected]>
[email protected] (CFoley1064) wrote in
In general, the concern is what happens when the voltage regulator dies.
The existing circuit, that failed, relies on resistors to limit current
flow. In my case something catastrophic happened that damaged the PCB.
But more commonly, the LED's in the original circuit simply die to to
excess current.
I haven't received any LED's yet to verify brightness. My goal was to run
them below their rated current as what I have read thus far leads me to
believe that their rated current is generally a maximum and to be avoided.
This application uses the LED's with a reflector and a lenses that
disburses the light in a wider range than a LED normally would. It is
this reflection of light that is visible in the marker lens. Since there
is a right and a left, and I only need to replace a right one. The goal
would be to adjust the current so that the right side would match the left
side.
The stock circuit (on the left side) was developed in 1993, so I'm
guessing the stock design is not all that "bright" in comparison to the
LED's available today. But I may be wrong.
Regarding the circuit you proposed.
If've done some further research and am wondering if all of the components
are necessary. Here is a second circuit diagram I drew where the LED's
are placed into an array, and then the current into that Array is
controled by a single device.
http://www.frontiernet.net/~miketoni/circut2.jpg
Would that work?
Could that be translated to what you proposed?
I.E. reducing the number of components in the circuit.
And can you clarify the calculations for me in your diagram. Where did
the value for .7 come from for the LED?
Thank you for your assistance.
Hi again, Mike. The second circuit shows several strings of three LEDs in
parallel, driven by a current source. That won't work too well, because minor
changes in the Vf of the LEDs will mean that the current will get hogged up by
the strings of LEDs that have the lowest combined forward voltage. That means
major differences in brightness between LED strings. If you match forward
voltages, it might work pretty well, but as a paractical circuit, no. (I guess
the business with having two LEDs in one string is a typo -- that will limit
forward voltage to 6.4V, and only those 2 LEDs will light. Not to mention that
the LM334 has no chance of pumping enough current for all those LED strings.).
Let's take a look at the original "HP" circuit for best and worst cases (view
in fixed font or M$ Notepad):
+12V from regulator ~ ~ ~ GND
___ ~ ~ ~
o---|___|--->|---->|---->|------o
Let's assume you've got a series resistor of 200 ohms +/- 5%, an LED forward
voltage between 3.0V and 3.4V with a 3.2V nominal, and a regulator voltage that
varies between 11.7V (low battery, motor off) and 14.5V (high regulator, motor
on).
Nominal LED current: (13.8V - (3 * 3.2V)) / 200 ohms = 21 mA
High LED Current: (14.5V - (3 * 3.0V)) / 190 ohms = 28 mA
Low LED Current: (11.7V - (3 * 3.4V)) / 210 ohms = 7.14 mA
As you can see, high case to low case is almost a 4:1 ratio. Assuming the
motor is on, you can assume a minimum of 13.5V, which would give a low case of
Low LED Current: (13.5V - (3 * 3.4V))/210 ohms = 15.7 mA
Which gives you less than 2:1 when the motor is on. If you can get better
control over the LED Vf (after all, HP has been making LEDs since the 70s --
they know what they're doing), that means a better ratio.
Now let's assume your voltage regulator has gone south. In that case, your car
voltage may go as high as 16V or so. When that happens, you're killing your
battery, and bad things would happen to incandescent lights too because of the
higher power. In that event your nominal current would be:
Nominal LED current = (16V - (3 * 3.2v)) / 200 ohms = 32 mA
That is 50% over nominal, which isn't a problem if you're at 20 mA. If you're
at or over 40 mA, it might be fatal, depending on the LED.
That gets us back to the current source. The idea is a good one -- keep the
same current going through the LEDs despite variations in voltage. Here's a
segment of the circuit:
12V
.-o-.
| |
| |
D1 V .-.
- | |33 Ohm
| | |
| '-'
D2 V |
- | Ve
| |<
Vb o-|
| |\
.-. |Vc
1K| | |
| | V ~
'-' - ~
| |
| V ~
| - ~
| |
| V ~
| - ~
| |
'-o-'
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
You're probably familiar with the assumption that a forward-biased diode will
have about 0.7V across it. Two diodes in series, then, will have about 1.4V
across them. So, Vb is 1.4V less than the 12V. By the same token, the forward
biased PNP transistor will act so there is 0.7V from the emitter to the base.
That will mean that the transistor will tend to act so that Ve is 0.7V less
than the 12V. Your job is to choose the emitter resistor so that 0.7V across
it will give you the current you need. That would be (0.7V)/0.02A = 35 ohms,
or 33 ohms to the nearest standard value.
Of course this is a first cut approximation. Collector current is emitter
current plus base current (which should be less than 1/100th of emitter current
with a stiff base voltage source). More importantly, Vf of the diode and Vbe
of the PNP transistor is somewhat dependent on junction temperature. But under
normal circumstances in an automotive environment, Vbe will only drift by about
0.2V, and more importantly, the Vf of the diode will tend to track Vbe if
they're the same temp. Without going through the math, I can tell you that the
current source will give far better results than the series resistor alone,
like the example above. Also, a jellybean SOT-23 PNP transistor and a 1/8W SMT
resistor will cost a lot less than an LM334. The voltage source of the two
diodes and the 1/4W resistor is used by all the diode strings.
The main issue here is the voltage burden of the current source. However, even
if you assume 3.4V per diode, the current source can drive a 10.2V burden as
long as the voltage source is above 11.7V or so. Below that, the transistor
will begin to saturate, and current will go down. That should be acceptable --
people are used to having lights dim a little when the car is starting. The
main reason automotive manufacturers didn't use current sources is cost and
reduced reliability due to circuit complexity. Also, they specify LED Vf, and
generally will not accept delivery of LEDs with excessive Vf (which can be
controlled, to an extent, in manufacture).
Good luck
Chris