PWM frequency for led dimmer

[Johan]

Hello,

I'm making a PWM dimmer for leds with a ST7 microcontroller. The
lowest PWM frequency this µ-chip can generate is 1000 Hz. I believe
this is quite high for a led-pwm dimmer (long cables and many leds -->
high currents).

Has anybody some experience with this?

Has the PWM frequency on the warming up of the led?

thanks!

 

[Ban]

Hello,

I'm making a PWM dimmer for leds with a ST7 microcontroller. The
lowest PWM frequency this µ-chip can generate is 1000 Hz. I believe
this is quite high for a led-pwm dimmer (long cables and many leds -->
high currents).

Has anybody some experience with this?

Has the PWM frequency on the warming up of the led?

thanks!

Johan,
no need to warm up LEDs, they do not have a filement.
1000 Hz is ok, unless you hear some whining noise. (then try 30kHz)
If you operate your ST7 with 5V, try this circuit: (view\font\fixed)

The outputs are current sources, which deliver 20mA each. (I= 0.7V/33R). So
you do not need series resistors. If ou have a higher/lower DC-voltage for
the LEDs, you can chain more of them in one string. just remember, the added
LED voltages should be not higher than U_supply - 1V.
The transistors are very cheap. If you need more strings ou can parallel
more drivers(upto 20). The port-pin is active high, only 0.1mA drive
current.
It might be useful to have an optocoupler inserted too.
+---------+---------+-------o +13.8V
| | |
+5Vdigital V
o-------+ -
| | upto 3wht/bl LEDs
| V
port-pin |/ - upto 5 yel/gn LEDs
o-----|2N3904 |
|> V upto 6 red LEDs
| -
.-. | per string
270R| | V
| | -
'-' | | |
| |/ |/ |/
+--------------------------|2N3904
| |> |> |>
1N4148 V | | |
- .-. .-. .-.
| | | | | | |
1N4148 V | |33R | | | |
- '-' '-' '-'
| | | |
o------GND-------+---------+---------+
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

ciao Ban

 

[Kevin McMurtrie]

Hello,

I'm making a PWM dimmer for leds with a ST7 microcontroller. The
lowest PWM frequency this µ-chip can generate is 1000 Hz. I believe
this is quite high for a led-pwm dimmer (long cables and many leds -->
high currents).

Has anybody some experience with this?

Has the PWM frequency on the warming up of the led?

thanks!

You either need to limit the peak current and/or you need to eliminate
the pulses with an LC filter. That's the case regardless of the
frequency. Both would be best. DC will travel long cables better and
current limiting will keep your LEDs from exploding if your
microcontroller should crash in the "on" state.

Unless you need a digital interface, there are many easier ways to pulse
width modulate something using analog electronics. A 555 chip, a pair
of op-amps, a PWM chip, an LED illumination chip, etc.