Reverse polarity protection MOSFET questions

[KORGULL]

In reference to R.G.'s Advanced Polarity Protection setup using a BS-250 MOSFET:

1) Is there a way that I can test it so I know it is set up right and will function correctly?
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2) What is most likely to happen if the MOSFET is defective or gets damaged?
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3) What goes on if you change the value of the 1M gate resistor?
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4) Is a MOSFET's voltage drop measured by hooking the meter up across source and drain?
In a circuit using a 9V supply and measuring a current draw of 5mA at the battery, I get 41mV across the BS-250's source and drain. Does that sound right/typical?

R.G. wrote (in the article):

Note that you probably should use a gate protection zener. For the P-channels, hook anode to gate, cathode to source. For N-channels, hook anode to source, cathode to gate.

Should I use a 1W zener? Why a zener? Why is this necessary?

Sorry for so many (probably dumb) questions at once. Any extra info at all is appreciated.

[davebungo]

1) Is there a way that I can test it so I know it is set up right and will function correctly?

If you are worried about trying it with your circuit in case it damages it, try replacing your circuit with a resistor (say 500 ohms 1/4 watt) and then try reversing the supply.  Simply measure the voltage across your resistor and that across the MOSFET.  You should have no current flowing through the resistor (i.e. as good as 0V).

2) What is most likely to happen if the MOSFET is defective or gets damaged?

Consider the cases. a) open circuit (you won't get any supply to your effect), b) short circuit (your effect will simply work as normal, c) somewhere inbetween (not sure how likely this is), the MOSFET may drop your supply voltage down somewhat.  I think the most probable mode of failure is damage to the gate insulation due to static discharge.  Perhaps someone else can chip in and explain how this would affect the device.  I suspect that it may end up in a lowish impedance state since the gate can no longer maintain a VGS enough to bias the device off.

3) What goes on if you change the value of the 1M gate resistor?

Not much, since the gate is insulated from the drain and source and there is no (steady state) current flow.  It is usually wise to have some resistance in the path to the gate to limit transient currents, which can flow through the gate-source and gate-drain capacitance, to a safe level.

4) Is a MOSFET's voltage drop measured by hooking the meter up across source and drain?
In a circuit using a 9V supply and measuring a current draw of 5mA at the battery, I get 41mV across the BS-250's source and drain. Does that sound right/typical?

Yes and 41mV at 5mA = 8 ohms so doesn't sound bad at all.

R.G. wrote (in the article):

Note that you probably should use a gate protection zener. For the P-channels, hook anode to gate, cathode to source. For N-channels, hook anode to source, cathode to gate.

Should I use a 1W zener? Why a zener? Why is this necessary?

The Zener will start to conduct at the rated voltage (nominally) and will therefore clamp the gate-source junction to a safe level.  An ordinary diode will not go into reverse breakdown until it reaches the rated reverse breakdown voltage which can be typically > 100V (depending on the device).  The gate-source junction will probably have broken down long before this.  The BS250P which is specified by R.G. can withstand only +/- 20V absolute maximum.

Sorry for so many (probably dumb) questions at once. Any extra info at all is appreciated.

No dumb questions detected.

[R.G.]

Good answers, Dave.

Just one point of clarification.

2) What is most likely to happen if the MOSFET is defective or gets damaged?

Consider the cases. a) open circuit (you won't get any supply to your effect), b) short circuit (your effect will simply work as normal, c) somewhere inbetween (not sure how likely this is), the MOSFET may drop your supply voltage down somewhat.  I think the most probable mode of failure is damage to the gate insulation due to static discharge.  Perhaps someone else can chip in and explain how this would affect the device.  I suspect that it may end up in a lowish impedance state since the gate can no longer maintain a VGS enough to bias the device off.

Actually, while bipolars tend to fail shorted, MOSFETs tend to fail open. For the MOSFETs we can normally get, they are enhancement mode devices - meaning that they are normally off, unless you do something to turn them on, unlike JFETs, which are depletion mode devices and have to have something done to their gate to turn them off.

A punched-through gate on this MOSFET would probably make it fail open. That means that only the body-diode of the MOSFET would still be operating. In this circuit, the body diode actually points in the way you want a protection diode to point, so a damaged MOSFET would likeliest leave you with the MOSFET body diode still doing series protection, and the only result would be that the available voltage to the effect would be lower.

It's funny - I hadn't thought about the fail-safe action here with the body diode still operating. I wonder if it actually works that way. I guess I'll have to go kill one and try it.

[KORGULL]

Thank you both. Great replies all 'round.
Now I also realize exactly what 'depletion mode' and 'enhancement mode' means. I knew basically how JFET's and MOSFET's operate, and I've seen those terms in passing, but never put two and two together ... and another piece of the electronics puzzle falls into place ... only 1,367,954,231 more pieces to go.

So, about the gate protection zener, what one(s) should I use? I haven't ever actually used a zener yet, but I have a variety pack of them ready to go.
I'm guessing I need one with a breakdown voltage that is lower than my supply voltage, so if polarity gets reversed it will be sure to open up and conduct.
The smallest value one I have says 1W. I don't know if I have all this right, but I figure that if I'm running a circuit at 9V/5mA - that only comes to 45mW - too low for my 1W zener to function?
Is the variety pack that I bought basically useless for 9-12V effect building if the lowest value zener is 1W?
I quickly looked on the Mouser site, and the smallest zener I saw was .25W, which would still be too high. I know I'm missing something obvious here. I think I'm getting confused because the ratings are in watts and not simply the breakdown voltage.
*edit*: Is the wattage rating just signifying how much power, in general, it can dissipate when forward biased without burning up, and you need to look up the reverse breakdown voltage (avalanche point (IIRC) on a datasheet?

[R.G.]

So, about the gate protection zener, what one(s) should I use? I haven't ever actually used a zener yet, but I have a variety pack of them ready to go.
I'm guessing I need one with a breakdown voltage that is lower than my supply voltage, so if polarity gets reversed it will be sure to open up and conduct.

Actually, you want a zener that never conducts, except in conditions where the gate would be damaged. The gate is damaged at 20V, so you want one lower than that, and you'd like the actual power supply not to turn it on, so it needs to be more than 9V plus any variation above 9V. A fresh battery is 9.3-9.4V, and some power supplies get to 10V. I personally would use a 12-15V zener. 12V is kind of standard for MOSFET gate zeners in many apps.

The smallest value one I have says 1W. I don't know if I have all this right, but I figure that if I'm running a circuit at 9V/5mA - that only comes to 45mW - too low for my 1W zener to function?

Zeners used to be known as "backwards diodes". This is because they are used under conditions reverse to what normal diodes are. They are intended to be used in reverse breakdown mode, where their cathode is more positive than their anode, and the voltage causes conduction anyway. When that happens, the voltage across them stays the same (relatively) and the current can vary from zero up to burnout. If you hook them up like normal diodes, making anode positive and cathode negative, they conduct at 0.5V - 0.7V just like normal diodes.

In one sense every diode is a zener diode, because there is always a reverse voltage where the diode starts letting through current. It's just very high for most diodes, and not tightly controlled. So two 1N4001 rectifier diodes rated at 1A/50V might actually start 'zenering' at different reverse voltages - the maker just guarantees "more than 50V" not exactly something. For zeners, the maker actually sorts out the ones that are exactly the same voltage and sells you the ones sorted by voltage.

Zener diodes are rated in terms of voltage and power. The voltage is easy - it's just the voltage at which the junction starts conducting in the reverse direction. Power is the power where it burns up - or, I guess, where it just barely doesn't burn up. So if you have a 10V zener and it's rated at 1W, then you can put through it 1W/10V = 100ma of current, because the power there is 10V*0.1A = 1W. The 10V zener is just as happy running at 10ma as 100ma, and it still gives you nominally 10V across it. So zeners can be run at less than their rated power - and should be.

Is the variety pack that I bought basically useless for 9-12V effect building if the lowest value zener is 1W?

No - as long as the voltages are small enough to be the right values, you're fine. You don't want to run them at full power. Power is a "how far can I go" specification, not a "you gotta do this much" specification.

[KORGULL]

Thank you. It's all much clearer to me now.