Arduino Forum

Using Arduino => General Electronics => Topic started by: Lavan on Dec 11, 2015, 09:20 pm

Title: Zener Diode Protection
Post by: Lavan on Dec 11, 2015, 09:20 pm
Hi There!..

Recently.. one of my LM317 got shot and fried my arduino circuit.  I was using LM317 to convert 16V to 5V.  This time I am thinking of adding a zener protector to safeguard my circuit in such senarios ,  my circuit current consumption is max 50ma @ 5V.   I am planning to use attached circuit.  My understanding is even if the LM317 gets short and voltage output is 16V, current will be ~ 160 ma after the resistor R3.   So my question is what should be the wattage resistor(100R) R3  and wattage zener diode ?  and how to calculate the same?

Thanks in Advance

Title: Re: Zener Diode Protection
Post by: DVDdoug on Dec 11, 2015, 09:35 pm
Power is calculated as Voltage X Current, so 5V x 0.16A = 0.8W.    ...A one or two watt Zener should work.

It's more common to use a fuse instead of a resistor.   You don't get a voltage loss across the fuse (better voltage regulation) and if the regulator shorts-out and the fuse blows, you'll know it.

Title: Re: Zener Diode Protection
Post by: Lavan on Dec 11, 2015, 10:28 pm
If I want to replace the resistor with fuse, should I go with 100ma fuse?
Title: Re: Zener Diode Protection
Post by: septillion on Dec 11, 2015, 11:18 pm
If you only need 50mA a 100mA slow fuse should be fine. It's by the way more common to place the fuse near the input of a circuit.

Also, it's recommended to place a diode across the input and output of the LM317 (of course with the cathode connected to input, not the other way around :p )
Title: Re: Zener Diode Protection
Post by: Wawa on Dec 12, 2015, 01:51 am
Zeners are generally bad at overvoltage protection.
TVS diodes (super zeners) are commonly used for that.
A normal zener will blow before the fuse does.
I doubt that a fuse/zener/TVS is fast enough.
Google "crowbar circuits" (zener/thyristor).
Better still is a good supply.
For <$2.00 you can get 5volt switching (micro) buck converters, free shipping.
Leo..

Title: Re: Zener Diode Protection
Post by: TomGeorge on Dec 12, 2015, 06:28 am
Hi,

Why don't you use a LM7805, and forget about the resistors, just use the appropriate bypass caps.
The tag of the LM7805 is gnd, so you won't need to insulate the tag from the heatsink.

Tom.... :)
If you use the circuit you have posted, how will you know if the 317 has failed, the zener will just clamp the output.
Use a 1W zener and put a 250mA fuse in place of the 100R resistor, now if 317 fails and output goes to 16V, for an instant more than 250mA flows and blows the fuse, output goes to zero, device is saved.
Title: Re: Zener Diode Protection
Post by: Paul__B on Dec 12, 2015, 11:22 am
(http://forum.arduino.cc/index.php?action=dlattach;topic=365247.0;attach=146801)


Nope, useless!

What you want is a "crowbar".  The 5.1 V Zener will trip the SCR if the voltage exceeds about 5.7 V, completely shutting down the regulator.  If the regulator has failed, damage to it by this action is entirely irrelevant.  Do note however, that the actual design problem in the circuit above, is the use of a variable resistor.
(http://static.electro-tech-online.com/imgcache/6222-scr_overvoltage_crowbar.gif)
Title: Re: Zener Diode Protection
Post by: Wawa on Dec 12, 2015, 12:07 pm
Do note however, that the actual design problem in the circuit above, is the use of a variable resistor.
+1
A trimpot it not perfect. If you try to adjust the pot while the Arduino is connected, contact bounce will create 15volt spikes on the output.
Leo..
Title: Re: Zener Diode Protection
Post by: MarkT on Dec 12, 2015, 05:23 pm
That circuit is very sensitive to contact bounce in the trimmer - the recommended circuit
has a capacitor across the trimmer which both prevents this and reduces ripple amplification,
go look at the datasheet "Typical Application" circuit (Figure 9.2 in the T.I. datasheet).
Title: Re: Zener Diode Protection
Post by: Watcher on Dec 12, 2015, 05:28 pm
Any recommended part values for the  "crowbar" above?
Title: Re: Zener Diode Protection
Post by: MarkT on Dec 12, 2015, 06:45 pm
Any recommended part values for the  "crowbar" above?
I'm not sure the R or C are needed - just make sure the zener leakage at the
working voltage is well below the minimum trigger current of the SCR.  Note that
SCR gate will be a volt or so above ground so the zener voltage will be a bit less
than you might think - check the datasheet for the SCR in question (it just has to
be big enough to handle the current, so most devices will be fine).
Title: Re: Zener Diode Protection
Post by: raschemmel on Dec 12, 2015, 06:53 pm
CROWBAR (https://www.google.com/search?q=LM317+crowbar+circuit&espv=2&biw=1468&bih=900&tbm=isch&imgil=QnUojt4mCRm8xM%253A%253BvZQZzn5Rm69ApM%253Bhttp%25253A%25252F%25252Fwww.radio-electronics.com%25252Finfo%25252Fcircuits%25252Fscr_overvoltage_crowbar%25252Fscr_over_voltage_crowbar.php&source=iu&pf=m&fir=QnUojt4mCRm8xM%253A%252CvZQZzn5Rm69ApM%252C_&dpr=1&usg=__dXNY-B0BfJTEaZhu2ji735THC7M%3D&ved=0ahUKEwjf6aLm8NbJAhUQ_WMKHULZBqAQyjcIMg&ei=9l1sVt-WN5D6jwPCspuACg#imgrc=oRNi8GWoI36ENM%3A&usg=__dXNY-B0BfJTEaZhu2ji735THC7M%3D)

@ Paul,
That's funny. I was about to paste the same crowbar circuit you used until I scrolled down and saw you had already used it....
Title: Re: Zener Diode Protection
Post by: MarkT on Dec 12, 2015, 07:09 pm
Actually I found the site that crowbar circuit came from and it admits the C prevents the crowbar
from firing with short pulses, which is simply wrong - the whole point of the circuit is to prevent
the output voltage exceeding the breakdown voltage of the load, short spikes are just as able
to destroy as long spikes.

Just the zener and SCR are needed I conclude.  In fact I'd add a small resistor in series with the
SCR gate to limit the current through it, perhaps 100 ohms or so.  However I'd also then check
the circuit functions as expected with a 'scope to back up my intuition in case I've missed something.

For really demanding applications I would suggest a high speed comparator, flip-flop and MOSFET would
be better to respond really fast (SCRs are not exactly snappy switches).
Title: Re: Zener Diode Protection
Post by: Wawa on Dec 12, 2015, 08:09 pm
Remember that there is a cap across the supply line (C2), and maybe more downstream. They should kill spikes.
A small cap on the gate might be needed if the crowbar circuit is used in a switching supply (HF more hash).
I would replace the zener/gate resistor for a 100ohm trimpot (gate to wiper).
And pick the zener 1-1.5volt less than supply.
Then you can trim the crowbar firing voltage, and have some gate series resistance.
Leo..
Title: Re: Zener Diode Protection
Post by: TomGeorge on Dec 13, 2015, 12:07 am
Hi,

Capacitor C is there to stop triggering when the supply powers up.
Going from 0 to Vreg has in some instances the same gate trigger effect as Vreg to Vunreg when failure occurs.

Tom....... :)
Title: Re: Zener Diode Protection
Post by: Paul__B on Dec 13, 2015, 12:29 pm
I would replace the zener/gate resistor for a 100ohm trimpot (gate to wiper).
In the context, that suggestion is an absolute scream!
     :smiley-lol:       :smiley-lol:       :smiley-lol:       :smiley-lol:       :smiley-lol:       :smiley-lol:       :smiley-lol:
Title: Re: Zener Diode Protection
Post by: Watcher on Dec 13, 2015, 12:41 pm
Paul__B: Do you have any specific part values to suggest for 5V MCU supply protection?
Title: Re: Zener Diode Protection
Post by: Lavan on Dec 13, 2015, 05:47 pm
Thanks all for your responses.  Good to know about crowbar circuit protection.  Also there was a recommendation to use 7805 instead of LM317, is that means 7805 is short circuit protected?  i.e., even if its failed, there are no chances of input = output?
Title: Re: Zener Diode Protection
Post by: raschemmel on Dec 13, 2015, 06:11 pm
If you don't exceed the Maximum electrcal specs and you use a heatsink , it will shutdown the output if there is a short on the 5V lines.
Title: Re: Zener Diode Protection
Post by: raschemmel on Dec 13, 2015, 11:25 pm
Quote
would replace the zener/gate resistor for a 100ohm trimpot (gate to wiper). 
Quote
In the context, that suggestion is an absolute scream!
@Paul,
Forgive my ignorance but I'm not seeing the humor in this. What is your point ?
Is it that less than 100 ohms might not be enough current limiting for the zener ?
Or is it that there is no need for adjustment in this application ?

Please enlighten me .
 
Title: Re: Zener Diode Protection
Post by: Paul__B on Dec 14, 2015, 03:39 am
Forgive my ignorance but I'm not seeing the humor in this. What is your point?
Whilst regulators can fail of themselves, at least two people agreed with me that the trimpot is the weakest link in the use of a variable regulator, failure of contact of whose slider will necessarily cause over-voltage.

You then suggested a crowbar circuit using - a trimpot!

Just sayin'.
Title: Re: Zener Diode Protection
Post by: Wawa on Dec 14, 2015, 04:02 am
Two completely different things.
While adjusting the pot of the voltage regulator, contact bounce can/will spike up the voltage.
While adjusting the crowbar pot, nothing bad will happen.

Zeners come in selected voltages only 3.9volt, 4.7volt, 5.1volt, etc.
Some finetuning could be needed when 5.5volt is ok and 6volt is not.

As suggested, ditch the LM317 use the 7805.
No crowbar circuit needed.
Leo..

Title: Re: Zener Diode Protection
Post by: raschemmel on Dec 14, 2015, 04:55 am
Quote
You then suggested a crowbar circuit using - a trimpot!   
You are mistaken

A- The crowbar circuit I linked did NOT have a trimpot
B - I didn't suggest a trimpot

My only post was Reply#11. Read it. Look at the link. No trimpot.
Title: Re: Zener Diode Protection
Post by: Paul__B on Dec 14, 2015, 06:43 am
Ah!  Sorry, my apologies - it was of course Wawa who suggested the trimpot, wasn't it?

But you were the one who then queried my derision - slight glitch (sic.) in my reply.
Title: Re: Zener Diode Protection
Post by: Watcher on Dec 14, 2015, 06:52 am
Quote
As suggested, ditch the LM317 use the 7805
Why is  7805 preferable in this case instead of  LM317?
Is it just because it is not adjustable, or is there another reason?
Title: Re: Zener Diode Protection
Post by: raschemmel on Dec 14, 2015, 07:31 am
I questioned your post because you quoted ME instead of Wawa.
Title: Re: Zener Diode Protection
Post by: Wawa on Dec 14, 2015, 08:01 am
Why is  7805 preferable in this case instead of  LM317?
Is it just because it is not adjustable, or is there another reason?
Every Arduino uses a fixed regulator.
Easier to use, less parts that can go wrong.
A micro doesn't care if the voltage is 4.75 or 5.25volt.
Leo..
Title: Re: Zener Diode Protection
Post by: Lavan on Dec 14, 2015, 05:57 pm
Seems LTC4360 is the robust protector.  Not sure if any of you already used in your projects.  Any comments?

http://www.linear.com/product/LTC4360#overview (http://www.linear.com/product/LTC4360#overview)
Title: Re: Zener Diode Protection
Post by: raschemmel on Dec 14, 2015, 06:15 pm
This is a DIP package (TMS112)  (http://www.mouser.com/catalog/specsheets/7550.pdf)