Author Topic: Protection diode placement  (Read 17173 times)

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Offline DanielS

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Re: Protection diode placement
« Reply #25 on: October 06, 2015, 07:29:40 pm »
Anything is possible, but the odds of a voltage appearing on the output that is higher than the input seems pretty small.
Have you ever worked in a lab before? Accidents, such as input shorts or hooking things up backwards (either polarity or input-output) happen more frequently than most people would care to admit. I bet most people here with more than a few years of training/experience have fried over $100 worth of parts and equipment from such mistakes and accidents.

Accidentally reversing polarity on a linear regulator is a very easy mistake to do - it can be as simple as forgetting to put your multimeter back into voltage mode after measuring current before probing the regulator's input voltage. Bam! Dead LM317 if it did not have the reverse bypass diode. I fried quite a few in this or similar ways in my earlier days.
 

Offline Gyro

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Re: Protection diode placement
« Reply #26 on: October 06, 2015, 07:30:09 pm »
You could also look at automotive spec regulators - for instance the LM2940 which is fully specified for reverse battery protection to -15V and spikes of +/- 75V. Dropout voltage is typically 0.5V @ 1A. Quiescent current is higher at typ. 10mA though, maybe there are alternative parts that are lower.
Best Regards, Chris
 

Offline rs20

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Re: Protection diode placement
« Reply #27 on: October 06, 2015, 11:12:33 pm »
This. Barrel = ground is the standard; you shouldn't go against this standard unless you have a specific reason.

I thought the two prongs above the "J" was the barrel, and the long rectangle was the center?

On further research, you may be right. I can't tell anymore, that's a horrendous symbol if that's the case -- the rectangle looks so much like a sleeve/shield that slips around the barrel there. I'd never look at that symbol and think the rectangle represents a centre pin; it's rotated 90 degrees from the right place for it to be a centre pin! I much prefer this symbol:



Just to make up for being wrong, I'll point out that pin 2 above (and the corresponding pin in your schematic) is not a barrel pin per se, but a is-there-a-plug-plugged-in-detection-switch. Handy for switching between battery and DC power, for instance. When the plug is plugged in, pin 2 is floating, and pin 1 is connected to the barrel. When the plug is removed, pins 1 and 2 are shorted together.

Let's not get hung up on the schematic though, just make sure your real-life implementation is barrel-ground, centre-positive  :)
« Last Edit: October 06, 2015, 11:16:33 pm by rs20 »
 

Offline sentry7Topic starter

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Re: Protection diode placement
« Reply #28 on: October 06, 2015, 11:21:23 pm »
This. Barrel = ground is the standard; you shouldn't go against this standard unless you have a specific reason.

I thought the two prongs above the "J" was the barrel, and the long rectangle was the center?

On further research, you may be right. I can't tell anymore, that's a horrendous symbol if that's the case -- the rectangle looks so much like a sleeve/shield that slips around the barrel there. I'd never look at that symbol and think the rectangle represents a centre pin; it's rotated 90 degrees from the right place for it to be a centre pin! I much prefer this symbol:



Just to make up for being wrong, I'll point out that pin 2 above (and the corresponding pin in your schematic) is not a barrel pin per se, but a is-there-a-plug-plugged-in-detection-switch. Handy for switching between battery and DC power, for instance. When the plug is plugged in, pin 2 is floating, and pin 1 is connected to the barrel. When the plug is removed, pins 1 and 2 are shorted together.

Let's not get hung up on the schematic though, just make sure your real-life implementation is barrel-ground, centre-positive  :)


I was actually about to post my fix while you were writing! I made a custom symbol, and it looks like the one you found. The elongated rectangle in that other symbol is the sleeve. There is an inconsistency in the symbol and the part in my PCB program, hence the confusion. As far as the circuit is concerned, I think this is it... :phew: If there is anything majorly wrong with this scheme, let me know!
« Last Edit: November 26, 2015, 03:23:13 pm by sentry7 »
 

Offline sentry7Topic starter

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Re: Protection diode placement
« Reply #29 on: October 06, 2015, 11:39:39 pm »
By the way guys, thanks a billion, for the guidance here. If I had sent for my first boards with a layout like the original, I would surely have had some problems!
 


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