Probably a silly question (protection diode)

[jimbob]

I'm working on a project that requires a 1n757 9.1v 0.5w zener as a BS 170 protection diode but I only have 1N5818's (Schottky Rectifier 30V 1.0 amp)

I assume this would work just fine? Or is there something I'm missing?

[aron]

I don't know. I would think not....

If it's like this:

http://www.muzique.com/schem/mosfet.htm

[jimbob]

Im cloning/making a pooper scooper duper 2 in 1

[kurtlives]

 

I used a 1N4148 and most schems I have seen use them. Wored for me.

[jimbob]

Would that work as a protection diode against 9.1v?

[kurtlives]

If its oriented right I imagine it would.

The SHO uses two 1N4148s to form an input clamp (kinda looks like a voltage divider). Are we talking about the resistor going from the B+ to the gate?

[jimbob]

Its d3 the one that goes between the 10m and the 5kl pot/Source

[PerroGrande]

The purpose of the Zener diode is to protect the gate-channel junction insulation which, on a MOSFET is only about 20 Volts "thick".  This is why MOSFET-based devices are considered to be static sensitive.

The idea of the Zener is that it will prevent the gate from becoming more than X volts more positive than the channel.  If you increase the value of the Zener's voltage (12V, for example), the circuit will still work and your MOSFET will still be protected.  Decreasing the value will work, too, as long as positive-going signals don't exceed the Zener's rated voltage -- in which a case they'll be clipped.  The 9V zener selection offers a nice compromise with lots of room for safety.

The circuit will work just fine without the diode at all.  You'll just have your MOSFET at greater risk.

Using a regular diode like a 4148 or a Schottky will serve a different purpose (clamping/clipping) and won't really protect your MOSFET.  Diodes of the "standard" variety don't exhibit quite the same electrical characteristics as a Zener and therefor don't function the same way.  Even the way a zener is oriented in the circuit is different than a regular diode -- they're very different beasts.

So if you're building something like a MOSFET boost, you'd be better off with no diode than the wrong diode.  If you're looking to create clamping or clipping action, then yes, go with the standard diodes.  However, since you mentioned both the BS-170 and a 9.1v zener, I'm assuming you're building a MOSFET boost. 

[jimbob]

Building a SHO clone and about 3 other types/clones ect.. A booster collection of sorts. That actually makes good sense- Thanks for the explanation.