# Transistor H-bridge for bicolor LED?

Discussion in 'Electronic Basics' started by Impmon, Sep 5, 2005.

1. ### ImpmonGuest

I have a pile of 25 bi-color LED and I wanted to drive them all. I
was thinking of using the H-bridge to control the direction of current
soI can have red and green (yellow with high frequency switching).

I read something on the internet that H-bridge may not be able to work
with high current device(s) and the 25 LEDs would be drawing about
750mA total. Will this work or will I need to go with MOSFET instead?
Never used MOSFET before and I have no idea what I need. Or maybe
solid state DPDT relay (regular relay will be too noisy) if they can
handle the power and cheap as transistor.

2. ### ehsjrGuest

Regarding the bipolars: they don't need to provide 750 mA, and,
in fact, should NOT do so. You would be using the LEDs in a
poor design if they did. What you should do is make strings of
several LEDS in series, and place the strings in parallel, driven
by the transistors. You design the strings based on the voltage
the LEDs need, and the power source you will use. For example,
say your voltage source is 12 volts, and your LEDs need 1.8 volts
each. If you put 5 of those 1.8 volt LEDS in series, the string
would need 9 volts (5*1.8). I would limit the current to 20 mA
instead of the 30 mA your post implies. That requires a resistor
computed this way:
(Vsupply - Vleds)/current That's 150 ohms (3/.02)
So the circuit for each series string would look like:
---[LED]---[LED]---[LED]---[LED]---[LED]---[R]---

Each string uses 20 mA, and there are 5 in parallel,
so the total current needed would be 100 mA. You
should use the above design concept for either bipolar
transistors or mosfets.

You definitely do NOT want to put all 25 LEDs in parallel.
You DO want to limit the current through each LED to some
maximum value (I used 20 mA), and the simplest way to do
that is with series resistance.

Bipolars will work just fine, if you do it right, but you
could go with the mosfets - not because you have to, but
because it's a chance to learn to use them. They don't cost
all that much. Here's a url:
http://www.armory.com/~rstevew/Public/Motors/H-Bridges/Blanchard/h-bridge.htm

(watch the line wrap). Remove the motor from the schematic. You
do not need diodes D1,2,3 and 4.

Ed

3. ### Jasen BettsGuest

If possible connect them in series
that's not real high, and if they're connected in series, or in series groups
it'll be significantly less.

Bye.
Jasen

4. ### ImpmonGuest

In series I'd need more volts than my circuit is designed for and I
really don't want to have 50v or more running among the mostly TTL
chips. One bare wire in the wrong place and the whole circuit could
blow up.

I'm going to give MOSFET a try. Since I wanted to make a rope light
style, making them shorter with multiple branches wouldn't work well
as suggested by a different poster. Too many wires running around in
the end.

5. ### Randy DayGuest

Run the LED's in series off a transformer; have
two back-to-back SCR's with opto-isolators to
trigger them. When one SCR is triggered, the
LEDs turn red; when the other SCR is triggered,
they turn green. When *both* SCRs are triggered,
they turn yellow.

I didn't include the resistors needed on the SCR
gates - I just wanted to pass along the concept...

|TTL in|
.------.
| Opto |
| 1 |
'------'
| /
->|-
|-------| |---------|
| -|<- |
| / | |
| .-------. |
| | Opto | - LED 1
| | 2 | ^
----- `-------' |
AC in |TTL in |
------ .
| .
|
| .
| .
|
| .
| .
|
| |
| - LED n
| ^
| |
| .-.
| | | R
| | |
| '-'
|--------------------- |
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

6. ### ehsjrGuest

It answers the above. You use some leds in series with a
resistor, to make a string. You put the strings in parallel.
For example, say you have 2(red) to 2.4(green) volt bicolor leds.
You could string 4 in series and the total voltage drop
would be 9.6 volts, so you could run them from a 12 volt
supply, with a series resistor of 120 ohms. Six of those
strings in parallel would draw 120 mA.

That said, if you still intend to parallel the leds
and drive them with a mosfet or bipolar, be sure to
provide current limiting to each led. If your Vcc is
5 volts, you can still use a bipolar - something like
a TIP120 darlington will provide over an amp and the
V-C drop will be around 2 volts, leaving three volts
for the LEDs.

Ed