overvoltage protection (5V /12V DC)

I like the polyfuse+transzorb thing. We've done it thousands of times and it always works.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Tyco has been expanding their line of protection circuits (PolyZen) that put the two together, so that the dissipation in the transzorb heats the polyfuse and makes it switch more predictably.

Cheers

Phil

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

I've done it but it's expensive and only works for a range of currents. High-voltage/current polyfuses aren't available and even the higher end of those are PTH only (though I agree, SMT parts are pretty bad).

I looked at polyzens but IIRC the "zen" isn't really a zener, rather some sort of SCR device. They didn't have very nice reset characteristics, IIRC. Power had to be completely removed. It's been a while since I looked at them and I could be wrong.

They're supposed to be visiting us tomorrow so I'll ask again. Well, that is if anything is open tomorrow. The entire state is closed down (an inch of snow is a disaster, ya' know) and I doubt that anything will be open until it melts (maybe Thursday).

Yep. Some who lurk here can't fathom that the solution is just change to another style FET... their heads are just so overloaded with hate that they can't see >:-}

Absolute safety is garnered with a crowbar scheme... blow fuses so rapidly they light up like flashbulbs. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

with =

the =

here is a =

2 =

do a bit of =

gs.)

Put a easy to read label on everything.

Awesome!!! Hmm (searching on web) A through hole device would make the hack into an existing product easier. We can butt the two together with a bit of heat shrink though.. Thanks, George H.

We color coded the plug and jack. (There's always someone.) George H.

We build a lot of fairly low-power stuff, powered by a wall-wart or a customer-provided DC supply, often 24 volts. The radial polyfuses that we use are values like 60V/0.1A, 60V/0.5A, 60V/1A, 30V/2.5A. And we use fairly monstrous (600W, SMB package) transzorbs. Both are cheap, like 50 cents for the pair.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

What I hate is bad circuit design.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

It was just thrown out as an extension of a scheme I used in a LiIon battery charger control (ASIC) illustrating that body-to-body connected FET's can block bilateral current flow... in an IC you can make that structure as 2 devices sharing a common body well.

However the concept also causes cancer in those who bottle up smug hateful thoughts in their bodies... and good riddance to them >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

On a sunny day (Tue, 28 Jan 2014 17:58:55 -0500) it happened "Maynard A. Philbrook Jr." wrote in :

Yes diode works, I accidently put in an AC wallwart into the DC input of something expensive I made here, it worked, but only so so, small capacitor, huge ripple.

Over - and reverse voltage protection: power zener + fuse, SRC crowbar usually works too.

The MOSFET polarity reversal circuit is actually pretty useful in low-voltage circuitry, where you can't stand the diode drop. Since the drain goes to V_in, you're protected against polarity reversals out to the full drain-source breakdown voltage. In low noise applications, you're much better off spending the 0.7V on a cap multiplier.

The OVP issue is a bit thornier. If it's just a matter of somebody plugging in the wrong wall wart, the transzorb and polyfuse is probably good. It's simple and reliable, and also quick, unlike a TL431 + MOSFET but I'd as soon have another MOSFET in there somewhere as well, just to reduce the power dissipation in fault conditions.

Some folks have a tendency to run instruments on stacks of RV batteries to get rid of ground loops, and one of those can make a real mess if you plug it in wrong.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Low power is easy but fifty cents is a lot of money for something that's not supposed to be used. ;-) OTOH, I use some 6000W TVS diodes - about a buck-and-a-half at million quantities. As was pointed out, these also do the reverse voltage connection.

We're apparently in different businesses. We have a few products that have sold in the thousands, but most ever sell in the hundreds. Our median product sales price is probably something like $2K, and parts cost is a small part of that. Reliability, in all senses of the term, is worth spending a few bucks on.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation

Different business, for sure. Our products sell in hundred to two hundred range (some less) but a low roller is 10K/yr and 1M/yr isn't unusual. I do the advanced platform work so my designs are only made in runs of 10 to 100. These are used as pre-production models, customer demos, and the basis for RFQs, so have to represent production designs to the customers. I can't use parts that wouldn't be used in production.

[there's an obvious difficulty]

A fuse plus a high-current zener DOES work, but the fuse (and in case of lightning-stroke events, the hard-to-solder zener/transzorb/sidactor) both need to be diagnosable and replaceable. PTC 'polyfuse' is a bit crude, and you need to size it carefully or regulation suffers.

You could also opt for keyed connectors (not a nice option, though; you'd have to key every wallwart for a different plug, and document it all).

You could try to fit each item with an IEC-standard AC socket, and let the power strip be your surge protection (obviously this means AC power inside the box).

And my personal favorite is to use AC-out wallwarts (for isolation ony) then in the powered box, rectify by voltage doubling (two capacitors, two diodes) and use DC/DC converters to provide regulated internal power. There's no reasonable way a DC supply could overvoltage the rectifier, and (if you use 24VAC intermediate) there's lots of DC/DC converters for telecom use at 48VDC nominal input.

As Lasse said...

but remove M1 since you didn't ask for reverse protection.

Pick M2 to handle your max input voltage, and re-scale resistors around the TL431 to suit your desired trip point. ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Back in the 1970s, we were getting complaints of malfunctions in our main product, which had two boxes, connected by a pair of color-coded coax cables.

Had an argument with my business partner - he didn't believe that the customers were mixing the two cables up, but that would explain the symptoms.

More to humor me that from conviction, he agreed to change one cable to use TNC connectors, the other cable continuing to use BNC. The complaints stopped.

Joe Gwinn