help on diode selection - Page 1
Forum section: electronics / beginners

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 27, 2012, 11:27:32 pm
content: I need help on which diode will suit this circuit best. There needs to be a diode for every key on a keyboard that runs off a USB cable (5v). I've narrowed it down to these two diodes:MMSD41481N4148W-7-FI'm leaning towards the 1N4148 because it's slightly cheaper and has better reverse leakage specs, but has a forward voltage of 1.25v. The MMSD4148 has a higher current leakage, but has a 1.0v forward voltage, and costs more. Would the difference in forward voltage matter much for this USB-powered keyboard?

author: Shuggsy
author_rank: Regular Contributor
timestamp: November 28, 2012, 12:10:31 am
content: If you look over the datasheets, note that the forward voltages for each of those diodes are specified at different forward currents. The MMSD4148 has a 1.0V forward voltage for a forward current (I_f) of 10mA. The 1N4148W has a forward voltage of 1.25V but at an I_f of 150mA. Deeper in the 1N4148W datasheet they list a forward voltage of 0.855V for an I_f of 10mA, so the 1N4148W actually has a lower forward voltage drop. Maybe the MMSD4148 actually has a lower forward voltage drop at 150mA, but since Fairchild didn't see fit to provide any other I_f specs or curve for its forward voltage compared to I_f, I couldn't tell you.As for whether or not the voltage drops will matter, are you building this keyboard yourself or are you repairing an existing one? I haven't cracked one open to see if it has diodes on every key... I would assume all a commercial keyboard would do if it has diodes on every key is to give some indication for which key is pressed. Maybe a bunch of pullup resistors on buffer inputs where a keypress connects the cathode of the diode to ground and takes the input of the buffers low. Something like that... See the circuit below for what I mean.My guess is that the 1N4148W should work just fine.If you're building the keyboard yourself, maybe show us the schematic and we could give you a better answer?

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 28, 2012, 04:10:37 am
content: I'm building this keyboard for myself. Others that have built this keyboard used MMSD4148 and it works fine, but based on the specs, I'm just wondering if I can use the 1N4148W as an alternative. At 10mA, the 1N4148W actually has a lower voltage like you mentioned.All mechanical keyboards have diodes for every key so that no other keys activate when you press on one key. So for this scenario, is 0.85v at 10mA better than 1.0v at 10mA? Many other mechanical keyboards actually use these through-hole1N4148diodes, which is near identical to MMSD4148. However, the through-hole 1N4148 part number is almost the same as the SMD 1N4148W-7-F part number.

author: Shuggsy
author_rank: Regular Contributor
timestamp: November 28, 2012, 06:02:23 am
content: What's your schematic look like? If you're just using them to stop other keys from being pressed, I think you can use either. Won't know for sure without looking at your circuit.

author: WhiteFireDragon
author_rank: Regular Contributor
timestamp: November 28, 2012, 06:31:06 am
content: Thanks for the detailed posts and quick picture, Shuggsy! I ended up going with the SMD 1N4148W-7-F based on what you said since it has lower forward voltage, capacitance, and reverse current leakage.I actually don't know the schematics or matrix lol. All I have to do is solder the SMD diode on the PCB where solder pad is for each switch. And sorry for the typo last post, I meant to say I'm building the keyboard myself, notfor*myself. It's not repairing an existing one.

author: Shuggsy
author_rank: Regular Contributor
timestamp: November 28, 2012, 06:32:25 am
content: Cool.Good luck! Let us know now it goes.

author: SeanB
author_rank: Super Contributor
timestamp: November 28, 2012, 04:14:24 pm
content: Most modern keyboard controllers ( at least since 1990) will handle n key rollover quite well, and do not need diodes between the keys.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 28, 2012, 11:02:41 pm
content: I just opened up my corsair k90 (recently started playiing up) and it has a diode on each key,