Transistor sizing

[jnnewton]

Hello,
I have an application where I need to send a pulse train to a servo drive from a microcontroller. The input to the servo drive requires that i construct an open collector output and suggests that I use 12VDC through a 1kohm resistor, which will result in 12mA to the drive. On the return side it does not specify the transistor. So, I need to specify a transistor to switch the return side.

For this application I am able to switch at a max rate of 200k pulses per second, which is the equivalent of 200k "ons" and 200k "offs" per second for the transistor. This is where i am stuck. I am unable to search transistors by speed!!!! Can anyone suggest on? Also, if possible I would like to use a surface mount part and possibly one that is opto-isolated as the drive claims that it is not!

 

[j.p.bill]

A 2N7000 MOSFET in SOT23 will switch at that speed, maybe. Why do you need to run a servo at that frequency? A couple thousand Hz should do. Any optocoupler like the venerable MCT-2 will isolate it (guess you might want an SMT optocoupler, too). Try a 4N25S.

 

[jnnewton]

I dont necessarily need to run at that speed. i will need to run at what it thinks is a 1000 ppr encoder at 5000 rpm which would make:
5000/60*1000 = ~85k. I meant to post that 200k is it's (the drive's) max input rate, sorry. The 4N25S looks to me like it will work, and all in one package (diode and opto isolation) is nice, does anyone see anything that I don't?