What (0603 footprint) Zener diode (ESD protection) for a Moedex board

LEDBE
I

Inkidu

Newly Enlightened
Joined
Feb 15, 2010
Messages
134
being limited to a 0603 package does restrict your options. however.... does it need a zener?

Conventionally, to protect a circuit or device, you would put a zener across it, with the zener's cathode at the power supply rail that you were trying to clamp to a safe voltage. In this case, the led can handle the extra positive voltage from the ESD event (i.e. it will conduct enough to limit the voltage rise). The only real concern is the negative voltage.

With a zener connected in the usual way (zener cathode to led anode), the application of a negative voltage would cause the zener to be forward biased like a regular diode, and limit the reverse voltage applied to the led to 0.7v or so. Not bad. Should be safe for the led. However, it does raise the question of whether a zener is needed at all. Why not a plain rectifier diode? Maybe a schottky, since it has a lower forward voltage?

If you feel compelled to use a zener, pick one with a high enough zener voltage that it won't be conducting during normal led operation. Since the led can have a Vf as high as 4.88v (page 4 of datasheet),

looking at the Digikey selection guide... there are very few zeners designed for 0603 pads available in cut tape (small quantities).
The 641-1303-1-ND is not actually a zener, it's a transient suppressor, which is fine for ESD use. It doesn't conduct at 5v, and starts to conduct at 6v, so it's a good match in terms of voltage.

For the sake of protection, I think I'd put a 1000pF ceramic cap (0603) across the zener and led too. It'll help absorb some of the charge, limit the voltage, and allow more time for the zener or regular diode to start conducting in the forward direction. Couldn't hurt. 🙂

Steve K.

edit: regarding the direct drive.... if there is any chance of hooking the power up backwards and blowing out the protective diode, I'd suggest a diode in series with the power wire. Maybe a schottky to limit the voltage drop?
 
Last edited:
Thanks for the reply
I would imagine it isn't so easy to talk to a newbie

While I understand these things might not total be needed I have to buy solder paste anyways so a a little over a dollar more won't hurt me (still paying for shipping)
I am already soldering from scratch so none of this is much more work.
I ordered the diode
http://parts.digikey.com/1/parts/1616420-diode-array-esd-prot-1ch-5v-0603-cpdu5v0u-hf.html

And the capacitor

http://parts.digikey.com/1/parts/62380-cap-1000pf-50v-ceramic-x7r-0603-cc0603krx7r9bb102.html

So I think I am protecting for (reverse) current with the diode and I am protecting for voltage with the capacitor (voltage sink in a way)

I assume I put the cap where the 0603 footprint for NTC resistor would be?
Thanks for the help.
 
Did I not get that quite right, (reverse current) the diode just allows the power to flow through it instead of the led when there is too much power?
 
Last edited:
both the zener diode and the cap are there to protect the led from excessive voltage, although the cap can only absorb brief pulses. Honestly, the zener diode has a limit to how much it can absorb too, and this is a result of its fairly small size.

The zener (which isn't really a zener, but let's just ignore this for now...) will have to be wired up with its cathode connected to the positive power wire, which connects to the led anode. It will never get to work as a zener, because the led will conduct current before the applied voltage gets high enough to cause the zener to break down and conduct. The intent here is to prevent a damaging reverse voltage from being applied to the led, and this just relies on the fact that the zener diode acts like a regular diode when forward biased.

I'm not clear on how that circuit board is wired up. It looks like the two sets of 0603 pads at the top of the photo are supposed to connect to the led anode and cathode, but that's just a guess. The ideal location for the cap and zener diode would be right at the pads where the wires are attached, assuming that there is room (and assuming that all transient voltages and ESD would be coming from the power wires).

Since you did bring up the subject of protecting the led from current, I should mention that the led isn't protected from excess current. Were you planning on adding a resistor in series with the battery, or is the battery limited to a safe current level?

Steve K.
 
Steve K said:
I'm not clear on how that circuit board is wired up. It looks like the two sets of 0603 pads at the top of the photo are supposed to connect to the led anode and cathode, but that's just a guess. The ideal location for the cap and zener diode would be right at the pads where the wires are attached, assuming that there is room (and assuming that all transient voltages and ESD would be coming from the power wires).

Since you did bring up the subject of protecting the led from current, I should mention that the led isn't protected from excess current. Were you planning on adding a resistor in series with the battery, or is the battery limited to a safe current level?

Steve K.
Click to expand...
Thanks for the reply
I assumed the same as far as the placement (upper portion) of the 0603 pads
The picture isn't very helpful there is some faint trace lines but that's it.
The PDF isn't much more help it basically gives the dimenisions that are already given
I am not very fluent in French. I might just wait a couple days and hope I can make out the traces when they arrive.
I will have extras of this board I figured someone in the states might want a couple. Steve if you want one I'll send out one free of charge for all the help.
Just PM me.

As far as current/direct drive, I was planning on starting my 2900mA Redilast at a low charge and I will see how it works.
The led does have a fairly high Fv (typ. 4.1)
While not ideal I think I won't do any harm.
Comments?

I am still very much a novice (only one true mod,Jayrob mag, under my belt) and I am trying to do this on the cheap.

Thanks for any help.
 
Inkidu said:
I will have extras of this board I figured someone in the states might want a couple. Steve if you want one I'll send out one free of charge for all the help.
Just PM me.
Click to expand...

thanks for the offer, but I don't think I have a need/use for it.

Inkidu said:
As far as current/direct drive, I was planning on starting my 2900mA Redilast at a low charge and I will see how it works.
The led does have a fairly high Fv (typ. 4.1)
While not ideal I think I won't do any harm.
Comments?
Click to expand...

I'm not familiar with Redilast... is that a battery?
If it's a battery with a significant internal resistance, it might work. For instance, the little watch batteries or 9v batteries have enough resistance that their output voltage will drop a lot if you hook them up to a significant load. The info I can pull up on the interwebz suggests that this is a battery that could provide more than the 700mA that the led is rated for. I'd certainly try things out with one or two ohms of resistance between the battery and led. That ought to keep things fairly safe (make sure that the resistor(s) are rated for the power).

Also.. note that the Vf goes down as the led warms up. With the battery connected, the led will warm up, the Vf will drop, and the current will increase.... and might increase quite a bit!

Start with a couple of ohms in series with the led. Later, if you are feeling adventurous, you can remove the resistors and see how long it takes to let the magic smoke out of the led. 🙂

regards,
Steve K.
 
Thanks for the reply
The cell, most likely made by Panasonic, is sold over at cpfmarketplace. Side note he carries a newer 3100 mAh cell now. I just ordered one. I think I might need all the capacity I can get because of the high led fV. i.e. voltage drop will be less with a cell with more capacity
http://www.cpfmarketplace.com/mp/showthread.php?235778-Redilast-Li-ion-Battery-sales!-High-quality-Made-in-Japan-Korea!

Here is a PDF of the 2900 mAh cell

http://www.lighthound.com/assets/panasonic18650.pdf

Thanks for the info, sounds like a small resistor would be a good idea.
 
Last edited:
I just got the cap and diodes in the mail and holy smoke I better pull out the magnifying glass to see these things.
Quick ?
The spec sheet says "Marking Code: Cathode Marking Code: band & E05"

http://www.comchip.com.tw/cms/UserFiles/QW-A7007-CPDU5V0U-RevC596801.pdf

I assume that the black striped end seen in the PDF spec sheet is the cathode end?
And that end per Steve K. instructs should on be on the anode pad i.e. backwards.

I would assume the direction of the cap shouldn't matter????

Thanks for any help.
 
You must log in or register to reply here.

Latest posts

Top