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I purchased a strain relief wire SR9C which has a small protection circuit for a laser diode. It says to contain a zener and a schottky diodes which protect the laser diode from ESD. By looking at the PCB board, I drew the following circuit (hopefully correct):

enter image description here

Also, I managed to recognize the schottky barrier BAT54 - L4, however, I do not know what's the zener diode part is, though it says that it is a 3.3V one.

Could someone explain me the function of each of the diodes in the circuit and what kind of protection they provide and to what extent. The detailed answer would be greatly appreciated.

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Due to the Schottky's low forward voltage drop, any (positive) ESD on the left side of your diagram will cause the Schottky to be forward biased, preventing back biasing of the LD. The Zener will clamp the forward voltage of the the LD to the Zener's breakdown voltage, in this case 3.3V, preventing over voltage across the LD.

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    \$\begingroup\$ Keep in mind that ESD in general can be either positive or negative. \$\endgroup\$ – Dave Tweed Dec 31 '14 at 15:38
  • \$\begingroup\$ @DaveTweed So, could I summarize the answers by saing: 1) Schottky diode protects from negative ESD and from reverse biasing the LD, and 2) The Zener diode protects from positive ESD and from over voltage? I assume that any positive voltage over 3.3V will be dissipated by Zener, and any negative voltage larger than the schottky forward voltage will be dissipated by the shottky diode. How to determine limitations? \$\endgroup\$ – Nazar Dec 31 '14 at 15:48
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    \$\begingroup\$ @Naz: After that, the limitations are primarily related to the total energy that each device can absorb safely. The energies associated with ESD tend to be fairly low, but if you applied the wrong power supply to the assembly, you could easily blow any of the diodes. \$\endgroup\$ – Dave Tweed Dec 31 '14 at 16:10
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I do something similar, but I use several (3-4) pn diodes in series rather than the zener. The laser diode is very sensitive to ESD and other over voltages. The Schottky is there to provide protection for reverse voltage across the LD and the zener to protect against too much voltage in the forward direction.

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  • \$\begingroup\$ Oh, I see. I recently burnt several components on the PCB board by accidentally reversing power supply polarity. If I had some kind of schottky diode on the power terminal this would save the board, right? Is this a general practice to use schottky for this application? How do I choose one to protect my board? \$\endgroup\$ – Nazar Dec 31 '14 at 15:39
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    \$\begingroup\$ It won't necessarily protect you from a really catastrophic condition like a reversed power supply. Let's say you have a 5 volt, 10 amp supply, and it gets applied (reversed) to the diode. Then the Schottky will limit at ~ 0.5 volts and dissipate 5 watts. This will destroy it. Then the LD goes. This sort of protection is against ESD, which has very high transient voltages, but quite low total energy and short duration. \$\endgroup\$ – WhatRoughBeast Dec 31 '14 at 22:36
  • \$\begingroup\$ @WhatRoughBeast Any suggestions or links that would talk about reverse power supply protection? \$\endgroup\$ – Nazar Jan 7 '15 at 12:58
  • \$\begingroup\$ @Naz - Piece of cake. Just add a rectifier diode in series with your circuit. The diode should allow current in the forward (laser on) direction, but will block current flow n the reverse direction, and the zener will take care of any leakage current. BUT. The rectifier must be rated for the laser diode operating current plus a margin, and the diode drive must be able to handle the added volt or so diode drop while operating. It almost certainly can, but you need to check. \$\endgroup\$ – WhatRoughBeast Jan 7 '15 at 13:20
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You'll want to use something faster than a Zener for ESD mitigation like a transient voltage suppressor (TVS). The zener isn't fast enough to react to the human-body-model discharge rate of an ESD event. TVS are designed with a very small depletion region which makes for a very fast recovery time due to their low load capacitance. Read the wiki on the avalanche effect for more details on this.

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  • \$\begingroup\$ That's interesting. What's the turn on time for a zener vs a forward biased pn diode, vs a laser diode? You might also note that a 3.3V zener is not working via the avalanche effect, but by tunneling. \$\endgroup\$ – George Herold Jan 2 '15 at 15:30
  • \$\begingroup\$ @GeorgeHerold if we are talking about ESD mitigation, the pn junction is reverse biased with respect to the external ESD event. The recovery time would be proportional to the parasitic capacitance. For a zener, this would be on the order of 10's to 100's of picofarads. For a TVS, this could be as low as single-digit picofarads. \$\endgroup\$ – cowboydan Jan 2 '15 at 19:59

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