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load dump protection (automotive)

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Scarr

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Hi all,

First let me just say that I am a novice, I try to muddle my way along but sometimes I need advice, so here I am.

I have two DC-DC power supplies one 3.3v 100mA and the other 3.8v 3A all from a car / truck supply (+12 - +24v) whilst my design appears to work sometimes the MCU locks and I think this is due to the load dump / spike problem.

Can anyone take a look at my attached schematics and advise on adding a protection doide to help things or anything else nice and simple.

Thx all

P.S. Please don't just say "zener diode" can you give me a SMT part number as I get a little more than confused when I type "zener diode" in google or even RS.
 

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Does it?

Hi, TMP

Thanks for the reply, I have checked C17 the positive terminal is going to the output of the inductor, is this not correct?
 
Scarr said:
Hi, TMP

Thanks for the reply, I have checked C17 the positive terminal is going to the output of the inductor, is this not correct?
If the correct output voltage is negative 3.8 then the cap is the wrong polarity.
 
Negative?

Am I missing something? why do you this it's negative (I'm probably going to regret asking that) if for some reason it looks negative or the schematic implies neg then its wrong as it is definitly positive.

Thx
 
Looks positive to me!.

BTW, don't post your pictures as huge JPG files, use GIF or PNG when they will be a LOT smaller, and lose no quality - JPG is for photos, not diagrams.
 
I can see why they both thought it was negative voltage. . . That's what it says on the schematic, look in the top right corner and you see -3.8v
 
fingaz said:
I can see why they both thought it was negative voltage. . . That's what it says on the schematic, look in the top right corner and you see -3.8v

That's because it's posted as a massive sized low quality JPG, and even at that on a higher resolution (or zoom to 100%) it clearly says +3.8V.

On this 17 inch monitor it reads -3.8V unless you zoom to 100%, but on my 19 inch at home it reads +3.8V without zooming (due to the higher resolution I run the 19 inch at).
 
I knew it would be my fault ;o)

OK, sorry about the picture size and the confusion it caused, and thanks Nigel for sorting it. now back to my question, lol

Any sugestions on a spike / load dump protection (probably a diode from what reserch I have done) criteria is

12v-24v input

3.3v DC-DC @ 100mA max
3.8v DC-DC @ 3A max (pulsed 2.5A GSM module)

Thx all.....
 
Last edited:
you can try an inductor between the input rail and the input capacitors. just use the same one you are using for the output inductor.
 
Bit tight on space

Good idea but the output coil is quite large and I can't really fit it on. Would having either several capacitors (varing value / type) or multiple small inductors help?

Thx
 
paralleling small inductors just makes a smaller equivalent inductor. just find or wind up one that has a few microhenries of inductance and can handle the input current.
 
Found this?

Hi all,

Found this little device a TVS

SMBJ24A

from what I can see this little beauty does all I want? But please if I'm barking up the wrong tree just tell me.
 
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