Zener diode question

[armdnrdy]

I have a question about how the zener diode is being used in the power input section of a Boss VB-2.

A RD11FB3 (rated at 10.87-11.43) is connected between +V and ground.

There is no current limiting resistor in the zener circuit.

In this configuration, does the zener "pass" the incoming voltage until the incoming voltage is over it's rated voltage, then the zener sinks the additional voltage?

Like a very crude overvoltage protection circuit?

[GibsonGM]

To my understanding, a zener set up this way won't do anything as long as the voltage is BELOW its reverse breakdown voltage.  It will remain off.    In this case, where it will turn on is, as you say, 10.87 to 11.43V.    If the input voltage exceeds this reverse breakdown voltage, somewhere in that range the zener will turn on and short to ground.
 
This will shunt the power supply, and hopefully protect components downstream. It would work much better with a fuse, to act like a 'crowbar'.    Whether or not it kills the zener depends on its rating, but I suspect it would given the low resistance it will have.  After that, well, doesn't it kill the rest of the stuff??

D1 - looks to be for polarity protection on the other side.

[armdnrdy]

Thanks for the reply Mike.

That's kind of what I figured.

Low voltage BBD IC circuits generally regulate a rail down for the BBD circuit.
This circuit has a separate rail provided by Q7, but it is proportional to the input voltage.
So...without the zener, if you plug in a 15 volt supply ....you would fry the BBD.

[R.G.]

And it's a polarity protector to the extent that it can conduct the amount of current the wrong polarity supply feeds it.

Like all shunt diode protectors, it will burn up and quit protecting if the applied power is AC, not reversed DC.

[GibsonGM]

Thanks for the reply Mike.

That's kind of what I figured.

Low voltage BBD IC circuits generally regulate a rail down for the BBD circuit.
This circuit has a separate rail provided by Q7, but it is proportional to the input voltage.
So...without the zener, if you plug in a 15 volt supply ....you would fry the BBD.

It's interesting, and not how I'd want to do it (I'd probably use a zener AND transistor if I really wanted surety - or just a diode in series if i could take the drop).    I'm glad my take on it was right,  ha ha.  Zeners are a place I really don't go to, just once in a blue moon for simple regulation.

[thelonious]

Sorry to revive an old thread - but I haven't found this answer elsewhere, and it was part of the original question:

There is no current limiting resistor in the zener circuit.

A lot of simple zener shunt protection schemes I see are like this- there's no current limiting resistor. Would it be just as effective at protection but help keep the zener alive if a small current limiting resistor were added to this scheme? Is it not done here because it would drop the supply voltage a bit?

[PRR]

> there's no current limiting resistor.

There's always some resistance. In the cord, in the wall-wart, in wall power lines, in the electric company generator.

That Zener may clip small short transients from the factory wall-wart. Say the 115V power line spikes to 180V for a split second (A/C motor kick-off?), we try to force 14V across the 11V Zener. Say the wal-wart and cord has 15 ohms winding and wire resistance. 14V-11V= 3V forced across 15 Ohms, 0.2 Amps current, 2.2 Watts of heat in the Zener. If the kick-up is short, the Zener survives; meanwhile the rest of the pedal never sees more than 11V-12V and it survives.

Replace the factory wart with one with lower resistance, or higher voltage and low resistance, the Zener fries. IMHO, this is a poor protection; however the 11V is significantly above the nominal 9V so it would very rarely come into play (unless you swap the factory wall-wart with a beefier unit).

[duck_arse]

so if you had series schottky, followed by R//C line filter, then the shunt zener? no ac, no line noise, no overvolts, no burnings?

[slacker]

Depends, if the input Voltage is high enough for long enough then you could burn out the schottky, the resistor or the zener. The best you can do is decide what level of over voltage you want to protect against and use components that can stand that, above that level one or more of them might fry. You could arrange it so the schottky or resistor will die first which stands more chance of protecting the rest of the circuit.

[amptramp]

There is a specialized zener called the Transzorb which is used to clip spikes in the incoming voltage.  I used them wherever I could and on one occasion, it saved our bacon.  There was a piece of equipment designed to go in an aircraft and had to survive MIL-STD-704 voltage transients and I also added transzorbs to an interface at a cable input.  One day (it was a Friday at quitting time), we decided to see how well it worked from the standard aircraft 400 Hz supply, so we connected up our trusty Elgar 400 Hz supply, switched it on and heard a loud bang.  We pulled the fuse and it was vapourized.  We had one of the technicians check the circuit and found that the 5 VDC logic supply used by both the unit and the test set had been connected to the external 117 VAC line.  This was the first prototype that had to undergo testing and it was made with parts that were difficult and time-consuming to manufacture before we set up the production line, so major problems at this point would set our schedule back by an unacceptable amount.  We repaired the test set (it blew 6 out of 83 IC's) then checked the unit under test and found it had survived with no failures!  The 3-foot length of the cable was enough to keep the voltage within survivable limits at the unit under test, but the length of cable was able to let the voltages in the test set get out of hand to the extent that six devices were fried.  We were able to repair the test set within a week and did not lose too much schedule.

[duck_arse]

well, I was only thinking as far as constraining a 9V circuit with a 12V zener. if bozo with the plug in hand wants to try a wart over 18V or so, bad luck bozo.

[amptramp]

It may be possible to use a lamp in series with the input so that if the voltage is significantly higher than optimum, the lamp lights up and drops the voltage.  There was  Knight - Kit amplifier that used lamps as current regulators for the output transistors.  The lamp had negligible resistance until the overload was enough to cause it to glow, then the resistance was high enough to protect the outputs.  You can do the same (within limits) for a power input.