Hi. I have a PWM that i'd like to isolate from the business end of the dc motor. i bought some cheap 4n25 optocouplers for starts and I can't figure out how to get them to work for my situation.

I don't know how to explain this well so let me give you a scenario.

Lets say my optocoupler has a PNP photo-transistor on the output side. To drive my gate driver circuit, I would connect the collector pin to Vcc and the emitter pin to the gate driver input to source current. Correct?

Well the problem I have is that my optocoupler has an npn phototransistor on the output side and npn transistors sink current, right? Meaning I can't use it to pulse current to my gate driver because the load has to go between the collector and Vcc?

I'm not sure if that made any sense but here's a graphical representation of what I mean:

I know I could buy the pnp version to solve my problem but it there any way I could make this work for what I have?

bare in mind I'm really new to electronics so it could just be that I have no idea what I'm talking about, so go easy on me

Thanks

Originally Posted by ericgibbs

hi,
Look at the image for a typical connection.

so I can connect the "pulse out" to my gate driver? I'm guess this basically inverts my PWM signal then, right?

I tried hooking i up like this and it didn't work

something tells me it can't sink enough current to drive my 2n2222 and 2n2907 transistors for my gate drive. i guess i just need to order a better optocoupler or better yet one that's made to directly drive a gate?

Originally Posted by MrAl

Hi,

Eric, that opto coupler works a little better with some base resistance to ground.
Just thought i would mention that...

5ky, what speed are you after here? 20kHz, higher?

I really don't have a set frequency I'm after to be honest--I'm just building random projects for fun and to learn from. right now, to make this easier to work with, I'm running below 1 kHz if I did my math right.

Hi again,

Eric, i meant a base resistor to ground.

Originally Posted by ericgibbs

hi,
If you need a pullup, add a PNP transistor across the 10K, with a 1K series resistor to the base.OK.?

EDIT:

Look at this image.

i'll give that a shot, thanks =)

you need to swap the bipolars in the gate drive circuit. You need the PNP up top and the NPN down low. With the way they are connected, neither will ever turn on.

5ky,

This is more along the lines of what you originally posted: just an opto + transistor buffer to drive the gate, providing current amplification for both charge & discharge. The resistor pulls down the voltage on the bases when the opto is inactive.

Hi,

Oh i see now he wants to drive a mosfet...that is a different story.
To drive the mosfet properly you need a faster rise signal than an
opto can provide, unless the mosfet can tolerate a bit of extra heating.

The typical way this is done is to drive a logic gate with the opto, such
as a schmitt trigger inverter, then use the output to drive the mosfet
driver.
Another even better way is to get a dedicated mosfet driver chip. These
not only can provide the proper high current gate drive, but can drive
the gate high and low with fast rise and fall times...both a very good idea.
Many mosfet driver IC's already have a schmitt trigger built in too.

Originally Posted by MrAl

Another even better way is to get a dedicated mosfet driver chip.

This then, would be the cream of the crop then for what I'm wanting, yea? Optical gate driver.

http://www.fairchildsemi.com/ds/FO/FOD3120.pdf