1n4148s, so I used a couple of 1n4006s. With their heavy gauge leads for high current carrying, the 1n4006 diodes are hard to plug into a breadboard. Then I remembered the bag of tl431s I have, and that just like the zener diodes they are meant to replace, they can be used forward biased. The tl431 block diagram above shows the diode from the anode to cathode.
One other difference I noticed with the 1n4006 diodes is their lower voltage drop. As I discussed in my 5c lithium ion battery charger post, diodes don't have a fixed voltage drop, but have a voltage drop that increases with increased current. At 1mA, 1n4148 diodes have voltage drop of 0.6 - 0.65V, while 1n4006 diodes have a voltage drop around 0.5V. None of the datasheets for the 1n4148 or 1n400x had detailed specs for low current. The first couple of 1n400x datasheets I found didn't even show current below 100mA. However Jack Smith has a great write-up on the forward voltage of the 1n400x diodes including curves going down to 10uA. I measured the forward voltage of the 1n4006 diodes using my multimeter (which uses a current of about 500uA) in the .502V to .515V range. Jack's tests don't include the 4006, though my measurements would suggest that it falls in between the curves of the 4005 and the 4007.
Next I measured the voltage drop across the tl431 anode-cathode diode:
.542, .546, .540, .543, .539, .546, .541V
I also measured reference-cathode diode. It isn't a discrete diode like one between the anode and cathode, it is the diode formed by the base-collector of the reference input transistor:
.729, .730, .728, .728, .727, .728, .730V
I also measured the base-emitter voltage of some old 2N2222a NPN transistors:
.601, .608, .612, .608V
and I measured the base-collector voltage too:
.605, .613, .616, .614V
Another common but often forgotten diode is the clamping or protection diodes on MCU input pins. Here's a diagram from an Atmel AVR datasheet:
Although the datasheet and schematic suggest there is no difference between the protection diodes, there is a noticeable difference in the voltage drop. The measurements for the protection diodes going to Vcc on an ATtiny13a were as follows:
.631, .631, .633, .629, .628V
and from ground:
.560, .561, .538, .537, .538V
The measurements from an ATtiny85 going to Vcc and from ground were as follows:
.634, .612, .613, .633, .634V
.554, .555, .555, .551, .553V
The voltage drop across the ground protection diodes is close to the 1N4006, which would suggest a current handling ability in the hundreds of mA. Although circuits that take advantage of the ground protection diodes are extremely rare, the Vcc protection diodes allow for simple level shifting. An AVR running at 3.3V can receive 5V signal simply by using a current limiting series resistor of around 1K Ohm on the input.