Surge / overvoltage input protection?

Hi. I've read some articles about surge input protection; I think it was regarding switches. Basically, they had a diode's anode connected to the input terminal and the cathode connected to the voltage source. It looks like this:

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My question is that most diode datasheets that I've seen shows the minimum forward voltage drop as somewhere around 700mV. Another article that I read (I think this is about opamps this time), mentioned that the input is capable of handling a maximum signal amplitude that is VCC

  • 0.3V.

In this situation, does that mean that I would need a diode with a lower forward voltage drop?

Also, I seem to be confused as to how this type of diode protection scheme works. Using the picture link, if the signal is higher than the VCC voltage by an amount greater than the diode forward voltage, then the diode conducts. So that would mean that the input is now at VCC voltage levels? If so, then wouldn't the diode "open" up again since the anode is now at the same voltage level as VCC?

Thanks!

Reply to
MRW
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Often a second diode is connected between the input and the ground rail to do the same thing for negative spikes.

Usually, the 300 mV limit is an operational one (possible malfunctions if exceeded), and there is a higher absolute maximum (possible damage if exceeded) spec.

A lower drop diode, such as a Schottky or germanium, is sometimes used, but these also have higher leakage current, so may not by useful for very low current signals.

But in many cases, the 300 mV limit can be exceeded a bit, without damage, if the input current is limited. This might be accomplished by using an ordinary silicon diode (that may drop a volt or so while it is passing a surge current) and adding a current limiting resistor between the diode and the chip input. Sometimes it is possible to choose such a resistor that will safely limit the surge current from a source 1 volt higher then Vcc but below enough to not interfere with normal signals.

Right. And if the surge source impedance is considerably higher than the impedance of the Vcc source, most of the rest of the surge voltage is dropped across the surge source impedance. However, if the Vcc source impedance is high, the surge may just jack up Vcc to damaging levels. For this reason, some designs use a zener diode to ground, that breaks down and conducts the surge current to ground when the voltage gets close to Vcc. I have seem 4.8 volt zeners used with 5 volt Vcc rails, this way, especially for logic signals that don't have to go all the way to Vcc to work. For negative spikes, the zener acts like a normal diode in forward conduction. The weakness of the zener approach is that it lets the input go all the way to zener break down voltage, even if the chip being protected is unpowered. In that case, the chip ends up seeing an input that is a lot more than a volt higher than its Vcc.

No, Vcc plus the diode forward drop voltage.

Reply to
John Popelish

And why do you think your equipment's inputs needs protecting from these 'surges' ?

Graham

Reply to
Eeyore

Thanks, John!

Reply to
MRW

Hi Graham. Oh, I was just curious. I figured it'd come handy sometime in the future when I encounter something like this.

Reply to
MRW

Powerline 'surges' (let's call them transients shall we?) if that's what you meant are very unlikely to be an issue for equipment inputs (or outputs).

The biggest problems for your ins and outs are likely to be electrostatic discharges and RF pickup.

Graham

Reply to
Eeyore

Gotcha. I was actually thinking more of the CMOS switch application notes that I've seen. They had were depicting a sine wave input that was much greater than VCC. One solution they mention was using diodes. I've probably seen that configuration before in some textbook, but I guess I paid attention to them the second time around. Thanks!

Reply to
MRW

There's a couple of things worth noting here.

Firstly it's not a good idea to use CMOS switches with voltages larger than their power supplies !

Secondly, mamy ICs have an internal 'parasitic' diode structure to the supply rails on input nodes. Combined with a modest value current limiting resistor these may be able to handle at least transient excursions beyond the power supply voltage.

If you regularly expect excessive voltage inputs then it's a good idea to fit additioanl diodes to deal with it. Do bear in mind that the input has to be current limited for these to be effective.

It's good to see you're aware of this issue.

Graham

Reply to
Eeyore

Dear friends

Surges may come due to many reasons. they can be as high as hundred of volts. Suppose the connection is made to wires going outside the equipment. these wires might be long. the longer the worst is the case. when the switch opens (the worst condition is opening and not closing), due to inductance of the wire, there will be a spike good enough to damage the cmos ics. surges can also be due to noisy envoirnment.

please correct me if i am wrong.

the internal protective diodes of the cmos are not very great. if you expect surges / spikes you should always put external diodes. but a series resistor before the diode is also a must. without this series resistance a spike of >50 Volts would mean the whole current passes through the diode with negliglible source resistance. this can damage the diode itself.

with regards

manoj

Reply to
Manoj

You're quite correct about that possibility.

Correct.

Exactly.

Graham

Reply to
Eeyore

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