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I'm pretty sure all transistors as "switches" need a resistor, or will burn something(?) sooner or later, buy a h-bridge in a chip, like the sn754410ne.
You would not need these 1k resistors with MOSFETs,
The 10k resistors are just pull-ups / pull-downs to control the sensitivity of the phototransistors by requiring a certain current to be sourced before the base voltage increases goes above a threshold,
Quote from: futmacl on July 25, 2010, 07:30:57 PMYou would not need these 1k resistors with MOSFETs,Not 1k, but you need something in the vicinity of 10 Ohm to 100 Ohm, if a well mannered circuit is the goal.
The 10k base-emitter resistors is there solely to keep the base in check when the transistor in the opto coupler is closed - not to control the sensitivity of the opto.
I read that link, and it said that the 1K resistor was limiting the current coming OUT of the base... Why would current come out of the base? I thought you applied current TO the base!!
Hm thats interesting... But i still dont understand why the resistor limits current... I thought it just created a voltage drop which would limit voltage not current... The only drop in current i can see is a total circuit current drop due to the total circuit resistance going up...