15V zener for AMZ Mosfet Booster?

[knutolai]

I wanna build the AMZ Mosfet Booster and I don't have any 9v zeners at the moment. Would a 15v zener do the job or would it have to be equal to the supply voltage? I'm a little unsure how this protection circuit works.

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

[Eddododo]

I don't quite understand the application but I'm pretty sure that you would want the forward voltage to meet the supply voltage . For what it's worth most of the MOSFET I have used have zener protection even the ones I thought did not

[PRR]

Most MOSFETs will tolerate 20V.

Signal at this point is rarely even 5V.

From 5V to 20V should work OK.

[PRR]

> understand the application

The Gate of a MOSFET is nearly un-connected to the rest of the world. It is thin metal on a nano-thin layer of glass (SiO). It is vulnerable to Static Charge. Even in not-dry air, if you shuffle across the carpet and build just 500V of static on your body, then touch a MOSFET gate leg on a grounded surface, ZAP the MOSFET is an expensive odd-value resistor.

Other semiconductor break-downs are more "bulk" actions. When you take a BJT Base or JFET Gate way negative (with a limiting resistor), breakdown happens throughout a significant volume of material. A 7V 10ma breakdown in half the volume of a 300mW device does no real harm.

The MOSFET gate breakdown is nano-thin and can't hardly hardly any energy.

The intrinsic breakdown is often above 50V, but thinner glass is better gain so it won't be much higher. And the oxide process can have defects (dust, edge-effects) that may break-down at lower voltage. The pretty-darn-sure safe value is in the data, but often around 20V.

Audio MOSFET applications often have some part to limit the Gate-Channel voltage. A 9V Zener Source-Gate will keep the Gate in the range +9V/-0.6V, which is safe. The real goal is to be well below 20V. 15V is safe enough.

[knutolai]

Thanks PRR! Now I don't need to order another package. Would be hard to find out if 15V was sufficient without potentially ruining my MOSFET. 

[duck_arse]

someone, somewhere, advocates a normal red led in place of the zener.

[PRR]

> red led in place of the zener

All the same if the MOSFET turn-on is under 1.5V.

In that we find LEDs everywhere, and Zeners are special-purchase, it is worth a try.

[R.G.]

Also note that the base-emitter junction of a normal NPN transistor is a zener of something between 5.2 and 8V. That's a handy thing to remember.

[knutolai]

Also note that the base-emitter junction of a normal NPN transistor is a zener of something between 5.2 and 8V. That's a handy thing to remember.

Good tip! So this could potentially be used with the Source left unconnected?

[PRR]

> used with the Source left unconnected?

I think if you don't connect a MOSFET's Source some place, it isn't a MOSFET.

Perhaps a diagram?

[knutolai]

Ye thats what I meant. sorry, typo on my part. Meant the collector of the NPN. I'm getting the terms mixed

[Bill Mountain]

> used with the Source left unconnected?

I think if you don't connect a MOSFET's Source some place, it isn't a MOSFET.

Perhaps a diagram?

This will be in an over-hyped boutique pedal in about 3 weeks.

[R.G.]

This will be in an over-hyped boutique pedal in about 3 weeks.

Yep.

If you REALLY want to mess them up, hook a 1M from the open collector to B+ and talk about hyper cross modulation of the intermediate frequencies and the regenerative effect of the NPN putting back the signals normally lost through the source of the MOSFET.

[knutolai]

This will be in an over-hyped boutique pedal in about 3 weeks.

[petey twofinger]

srry to hijack .

i found a layout for that one that did not have the zener diode . built it out of ignorance , the boost worked for fine a few months  , then it blew the mosfett .

live and learn . well , screw up , then some kind soul here helped me get it goin again .

i have become fond of the smally npn boost since then .

[duck_arse]

hmmm, that transistor is starting to make the circuit look something like a bi-directional boost, but with just a few parts missing. obviously, it would need an extra input socket on the base/source connection, but otherwise, maybe someone could try it?

I love thread jacking, but it would be nice if un-loaded images in these pages (I use "only show cached images" as my page default) had some sort of placemarker visible, like html used to. I thought prr's question mark indicated he wanted someone to provide a circuit, not that he had drawn the jellybeans hisself. at least the drawing I did matches his.

[PRR]

> nice if un-loaded images in these pages (I use "only show cached images" as my page default) had some sort of placemarker

Your browser should do that. It may be a setting.

After all the labor to hack-up a gorilla jack cartoon, I'm too tired to type "Dere's an Image here!" for people who don't load images.

[knutolai]

Edit: Off topic, moved to separate thread