Zener diode suitability

[roger]

I have a couple 1N4729A 3.6V, 1W zener diodes. Will these work with a 20 MHz clock? I didn't even think about the low power requirement when I bought them.

Thanks,
Roger

[christian]

Probably not. If you don't have the right zeners at hand, you may consider LEDs in series which have the same forward voltage drop as the zener would have.

[roger]

Thanks Christian, I got EasyLogger working with a ATTINY45 and some diodes I salvaged.

[Tilex]

can anyone take a look at the circuit of PowerSwitch? http://www.obdev.at/products/avrusb/powerswitch.html

I tried to change the circuit to run with red LEDs instead of Zeners.
I'm using an atmega8 controller running with 5V @ 16 Mhz (quarz).

Where do I have to place the LEDs to work in that circuit? Do I have to change the resistors from pull-up to pull-down and do I have to edit the code either?

thank you in advance,
bye

[christian]

You replace the zeners with LEDs, no other change is required. However, you must use so many LEDs in series that the (measured) forward voltage is ~ 3.3 V. The LEDs must be in reverse direction compared to the zeners, since zeners have their breakdown voltage in reverse direction.

[Tilex]

finally

I've tested it with two red LEDs in series from DATA+ to GND and two LEDs from DATA- to GND. That works fine.

The other solution was the method showed on the circuit of the USB-AVR home page with two normal diodes in series to reduce the voltage of the whole circuit to ~3.6 V.
http://www.obdev.at/Images/avrusb/circuit-zoomed.gif

My device didn't work earlier, because my hex-file was broken

thank you,
bye.