±500 Volt Protection Circuit

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#1
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May I refer you to the original Supertex Inc, now Microchip, application note "AN-D11 +/- 500 Volt Protection Circuit" which purports to provide an example schematic to protect an Lm324 based input stage. ww1.microchip.com/downloads/en/AppNotes/AN-D11.pdf

 

The following two diagrams are included.

 

 

 

... and the statement...

 

 

Q1. Would I be correct in stating that a high voltage applied between the V2 terminal and Ground would blow the Bejezes out of the circuit?

 

Q2. Would I also be correct in suggesting that the protection circuitry comprising Q1, Q2 and R1 should be duplicated in the V2 line to afford the claimed protection?

 

Q3. I wonder how many "very sensitive and expensive high impedance devices" are no longer .... devil

 

Ross McKenzie ValuSoft Melbourne Australia

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Ross,

 

 

Q1. Would I be correct in stating that a high voltage applied between the V2 terminal and Ground would blow the Bejezes out of the circuit?

 

I would suggest a large negative voltage would achieve the same end. 

 

Q2. Would I also be correct in suggesting that the protection circuitry comprising Q1, Q2 and R1 should be duplicated in the V2 line to afford the claimed protection?

 

+1 (assume the protection circuit works).

 

Q3. I wonder how many "very sensitive and expensive high impedance devices" are no longer .... devil

 

A few more if they implement this.

 

David

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LDN150 datasheet:

http://ww1.microchip.com/downloa...

 

You don't see depletion mode mosfets often...

The circuit is nothing but a bi-directional current source / sink / limiter.

 

Q1 Yep.

Q2. Depends

Q3. "Very sensitive and expensive high impedance devices"  With a LM324 ???

Q3. "Very sensitive and expensive high impedance devices"  usually don't depend on the ESD diodes for non ESD events.

 

Haven't seen the AN...

http://ww1.microchip.com/downloa...

(Oops,corrected. (added "http" so the forum recognises the link :))

 

The circuit is probably ment for battery powered measurement devices in an enclosed box where the 2 probes wires are the only connection to the outside world.

The circuit is probably ment to show of the LND150 Fet.

 

So if you want an "high" input impedance ( > 10M ohm) then what's wrong with some good old fashioned resistors?

Resistors also don't have a turn on/off time of 100ns nor a rise time of 450ns. I'll bet there are lot's of transients out there which are shorter than that.

 

But it's fun to see a depletion mode fet again.

Triggers memories of Philips which made a fet once which could feed the antenna signal from a video recorder (= machine with big spools of fancy thin plastic) to the TV if the Video was turned off (or even unplugged from the power outlet).

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Fri. Jan 13, 2017 - 01:31 PM
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DAFlippers wrote:
+1 (assume the protection circuit works).
The authors of AofE describe how the AN-D11 protection circuit can malfunction given certain conditions (strike of off-line 240VAC pops depletion-mode MOSFET gate oxide due to dv/dt into Cgs)

The authors then extended AN-D11 to solve several problems.

Cambridge University Press

Electronics Physicists

The Art of Electronics

3rd Edition

by

http://www.cambridge.org/us/academic/subjects/physics/electronics-physicists/art-electronics-3rd-edition?format=HB

(page 362)

5.15 Instrumentation amplifier

5.15.5 A riff on robust input protection

Figure 5.81. Bulletproofing the amplifier frontend with slew-rate control, diode clamping, and input-current limiting.

 

"Dare to be naïve." - Buckminster Fuller

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valusoft wrote:
Q2. Would I also be correct in suggesting that the protection circuitry comprising Q1, Q2 and R1 should be duplicated in the V2 line to afford the claimed protection?
That's what a Bourns TBU does.

http://www.bourns.com/products/circuit-protection/tbu-high-speed-protectors-hsps

http://www.bourns.com/docs/Products-General/Bourns_LVDS_Input_Protection_PortNote.pdf

 

"Dare to be naïve." - Buckminster Fuller

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gchapman wrote:

DAFlippers wrote:
+1 (assume the protection circuit works).
The authors of AofE describe how the AN-D11 protection circuit can malfunction given certain conditions (strike of off-line 240VAC pops depletion-mode MOSFET gate oxide due to dv/dt into Cgs)

The authors then extended AN-D11 to solve several problems.

Cambridge University Press

Electronics Physicists

The Art of Electronics

3rd Edition

by

http://www.cambridge.org/us/academic/subjects/physics/electronics-physicists/art-electronics-3rd-edition?format=HB

(page 362)

5.15 Instrumentation amplifier

5.15.5 A riff on robust input protection

Figure 5.81. Bulletproofing the amplifier frontend with slew-rate control, diode clamping, and input-current limiting.

 

 

Thanks. All I have to do now is find a copy... laugh

 

Ross McKenzie ValuSoft Melbourne Australia

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If it's really a high voltage current limiter, then perhaps it's relying on the 324's internal protection to keep the output voltage within range while it keeps the current low enough for the internal protection to deal with. Rather like using a megaohm or so in series with an input pin to sense mains frequency. But then, why not just put a few megaohms in stead of the fets?

 

I have to admit, I haven't looked at a 324 datasheet to see if the chip even has internal protection. Aren't these made of BJTs instead of FETs? Then you're probably ok as long as you can keep the current down to what the junctions can deal with.

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

Last Edited: Sat. Jan 14, 2017 - 02:17 PM
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valusoft wrote:

Thanks. All I have to do now is find a copy... laugh

 

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Now, THAT answers a few QUESTIONS!

The largest known prime number: 282589933-1

It's easy to stop breaking the 10th commandment! Break the 8th instead. 

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Ross:

 

When I ordered AOE3 from Amazon I was offered a $50 credit towards the purchase if I took out a amazon visa card. $80 book and free shipping for $30. Can't beat that with a stick. Haven't used the card since.

 

Rick

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Torby wrote:
I have to admit, I haven't looked at a 324 datasheet to see if the chip even has internal protection.
LM324K has internal protection (ESD)

Torby wrote:
Aren't these made of BJTs instead of FETs?
Almost totally BJT


http://www.ti.com/general/docs/datasheetdiagram.tsp?genericPartNumber=LM324&diagramId=SLOS066W

http://www.ti.com/product/LM324/datasheet

 

"Dare to be naïve." - Buckminster Fuller

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@Rick... there is always this last one also.

 

http://www.ebay.com/itm/The-Art-...

 

@Brian. Thanks. Do H&H mention the negative input channel?

 

Ross McKenzie ValuSoft Melbourne Australia

Last Edited: Sat. Jan 14, 2017 - 10:26 PM
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Torby wrote:
But then, why not just put a few megaohms in stead of the fets?

  • Noise

http://www.ti.com/product/LM324/datasheet/parameter_measurement_information#SLOS0666841 (Noise-Test Circuit)

  • Input bias current (adds to input offset voltage)

http://www.ti.com/product/LM324/datasheet/specifications#SLOS066262

 

An alternative is to use a telecom PPTC resettable fuse :

http://www.littelfuse.com/products/resettable-ptcs/telecom.aspx (PolySwitch Telecom Resettable PTCs)

but those don't act fast so would still need protection from di/dt (slew rate limiter and/or voltage clamp)

There's a subtle defect with current limiting because the internal ESD protection takes a very short time duration to become active (a very brief over-volt); better to add external ESD protection with some current splitting (current shared between external and internal ESD protection) and limit current (PPTC or a resistor) to protect the external ESD proection.

Some TVS are on extended leads such that the TVS is raised off of the PCB (don't smoke the PCB composite and/or lift nearby traces)

Or, reasonably sink heat from the TVS's lead frame into the PCB.

 

Sometimes a megaohm resistor is required; evaluate all currents (differential amplifier bias, external TVS leakage, MCU leakage)

Littelfuse

SP724 Lead-Free/Green TVS Diode Arrays - 3pF, 8kV, SCR & Diode Rail Clamp Array

http://www.littelfuse.com/products/tvs-diode-arrays/general-purpose-esd-protection/sp724-lead-free-green.aspx

...

The SP724 offers very low leakage (1nA Typical) and ...

...

 

Some of the CMOS amps and comparators have improved common-mode voltage range and power supply voltage range such that can replace some BJT implementations.

But, there are still impressive new BJT and JFET amps to match the impressive new ADC.

 

"Dare to be naïve." - Buckminster Fuller

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valusoft wrote:

@Brian. Thanks. Do H&H mention the negative input channel?

 

Rinse and repeat what's on the +ve.

#1 This forum helps those that help themselves

#2 All grounds are not created equal

#3 How have you proved that your chip is running at xxMHz?

#4 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand." - Heater's ex-boss

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Paulvdh wrote:

So if you want an "high" input impedance ( > 10M ohm) then what's wrong with some good old fashioned resistors?

Perhaps because a 10Mohm resistor combined even with stray capacitance could form a performance debilitating low pass filter where not needed.

Ross McKenzie ValuSoft Melbourne Australia

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@Brian. That is what I thought. Thanks. Not that I am about to rush out and build this or even need it. But I was roaming through my pdf collection and came across this one and it tweeked my "I don't think so" filter.

 

 

Ross McKenzie ValuSoft Melbourne Australia