Over Voltage Protection Crowbar Circuit

It's just that; with a battery applied, an overvoltage condition is highly unlikely to occurr, due to the clamping action of the battery, itself. A failure in the regulator doesn't result in overvoltage, it results in overcharging.

To protect against overcharging due to regulator failure, you'd be counting on the input fuse, as the battery looks just like a high capacity TVS.

OVP thresholds of the crude cowbar circuit are set higher than the circuit function can normally achieve. This would be a voltage higher than any battery you might attempt to charge with this normal output.

Rephrased - you gets what you pays for, and if you insist on buying crap, then crap ye shall get.

RL

The way to tell the difference is to put current into it for a while, then see if the car starts. Thses electronic chargers are smart enought to current and voltage limit. They are even smarter, in that they refuse to make any current below some terminal voltage. That sells more batteries.

went

output

What hazard? Current limiting? Its job is to sell suckers new batteries.

--

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

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

I stand corrected. Rephrasing: The problem is that putting current into a battery with a shorted cell is the same as a room heater, causing the good cells to overcharge and overheat.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

SCR

and

that

A grenading FET or inductor will, however, increase the chance of consequential damage and is thus best avoided.

--
Regards, Joerg 

http://www.analogconsultants.com/

In most of my fields any sort of grenading has serious consequences. Like Federales waltzing in for a throrough investigation. That is not something the user wants to have happen.

Could be, it depends. Usually on the amount of reservoir capacitance available downstream. If there is a cap downstream that holds enough energy to keep pushing an overvoltage for a while you need to crowwbar there. But then there should also be a fuse at that spot. Creating a dead short across a cap with a big SCR is a bit like Russian roulette.

In most cases the iso barrier only means one has to ferry the trigger signal across. This adds cost but sometimes that is well-invested money.

--
Regards, Joerg 

http://www.analogconsultants.com/

SCR

and

that

Presence of an inductor slows the rise of current in the fuse AND the SCR, adding more risk to the I^2*t margin. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    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.

Perhaps study it more carefully, and observe the 2nd page.

And I did say...

"Yep. I always like my own circuits >:-}

The fuse blower design was done before MOSFET's were common."

Did you miss the fuse blower discussion? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    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.

SCR

and

in

that

not

Actually, the I^2t term is agravated by peakiness in the current waveform, for a constant transfer of stored charge. If fusing is the limit, then keeping I^2t stress of the fuse and downstream components within the same ballpark makes coordination more predictable, ensuring that the fuse will blow first.

RL

SCR

and

in

that

not

But the *PHUT* sound will be more pronounced :-)

--
Regards, Joerg 

http://www.analogconsultants.com/

I'm not so sure about the Exide brand chargers sold by Kragen. One of the battery chargers in my picture tried to charge a battery with a shorted cell and cause the electrolyte to eventually boil. Just about everything in the radio vault had some form of corrosion damage. Maybe the newer variety of battery charger has a circuit to detect terminal voltage and refuse to charge anything below 10.5VDC or such.

went

output

The battery that boiled the electrolyte was probably not hot enough to start a fire. Spraying hot acid all over the wall didn't do much for the unistrut, coax cables, and connectors on the wall. I could have sued Kragen or Exide for not providing adequate protection on their chargers. It's possible that someone did sue, resulting in a redesign that would prevent any furthur such incidents. (I know, I'm guessing). That might explain why it refused to charge your totally dead battery.

Yeah, I guess that's the net effect, but that probably wasn't the intent. A trickle charge, until the terminal voltage increased, would have solved the problem, but that's apparently not a feature of the low end chargers.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

You need a combination of everything. Ckt below is based on data for 5A Fast Blo

and will clear ciruit in 10ms maximum. Check your blocking diode surge ratings.

Please view in a fixed-width font such as Courier.

. . . SD . .------|--/\/\-----+---+----+-|>|->>| 24VDC |-->>-+----->> TO LOAD . 5A F | | | | | . | | | SUPPLY| | . | R | -------- | . | [1R] | | . | | - - | . C | | /// | . +| | | . 7500uF === | | . -| | __ ------- . | |A _| TRIG| OVP | . | --- ---+----------------| DETECT| . | \ //G | | CKT | . | --- | ------- . | |K [470] | . | | | | . >----------+---+------+--------------------+-----> . | | . --- GND --- . /// /// . . . 2 . AMP SECONDS CALC: . . | . | . IPK| * . | * * . | * * . | * * . | * . | * * . | * * . | . ----+------------------------- . | . TIME -> . . . . T 2 . / 2 IPK . E = | (IPK exp(-t/RC)) dt= --- RC (1- exp(-2T/RC)) . / 2 . 0 . . T usually taken to be 50% current amplitude point T50 . . T50= RC LN(2) . . T50 must exceed fuse rated maximum melt time at IPK . which is 10ms for F-rated fuse . . 2 . IPK 2 . E = --- RC X 0.75 or C= 2 x E /(IPK x R x 0.75) . 2 . . Using Bussman TR1/MCRW5A Fast Blow with 300A interrupting . . rating at 125 VDC, E= 3 A^2 x s . . . Then select R= 1R so IPK is 50A minimum with allowance for other . . limiting effects such as ESR of C of addional 1R. . . . 2 . making C the larger of C= 2x3/(50 x 2 x 0.75)= 1600uF . . . and T50/(RLN(2))= 10ms/(2 x 0.7)= 7500uF . .

Maybe not in the gate, but at the anode? SCRs have a dI/dt limit to be observed, and this a capacitor-being-discharged event. It'd be a shame to blow both the fuse and the protective SCR in each event (but the SCR is likely to fail short, which is at least safe). Some surge protect schemes expend fuse and SCR together.

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

likeing the SCR option right now

Would help to discharge the C thru the fuse: Please view in a fixed-width font such as Courier.

. . . SD . .------|----+---/\/\---+----+-|>|->>| 24VDC |-->>-+----->> TO LOAD . | 5A F | | | | . | | | SUPPLY| | . | R | -------- | . | [1R] | | . | | - - | . C | | /// | . +| | | . 7500uF === | | . -| | __ ------- . | |A _| TRIG| OVP | . | --- ---+----------------| DETECT| . | \ //G | | CKT | . | --- | ------- . | |K [470] | . | | | | . >---+------+---+------+--------------------+-----> . | | . --- GND --- . /// /// . . . 2 . AMP SECONDS CALC: . . | . | . IPK| * . | * * . | * * . | * * . | * . | * * . | * * . | . ----+------------------------- . | . TIME -> . . . . T 2 . / 2 IPK . E = | (IPK exp(-t/RC)) dt= --- RC (1- exp(-2T/RC)) . / 2 . 0 . . T usually taken to be 50% current amplitude point T50 . . T50= RC LN(2) . . T50 must exceed fuse rated maximum melt time at IPK . which is 10ms for F-rated fuse . . 2 . IPK 2 . E = --- RC X 0.75 or C= 2 x E /(IPK x R x 0.75) . 2 . . Using Bussman TR1/MCRW5A Fast Blow with 300A interrupting . . rating at 125 VDC, E= 3 A^2 x s . . . Then select R= 1R so IPK is 50A minimum with allowance for other . . limiting effects such as ESR of C of addional 1R. . . . 2 . making C the larger of C= 2x3/(50 x 2 x 0.75)= 1500uF . . . and T50/(RLN(2))= 10ms/(2 x 0.7)= 7500uF . . .

On a sunny day (Sun, 2 Dec 2012 10:33:46 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

Blo

will clear ciruit in 10ms maximum. Check your blocking diode surge ratings.

Nice

Blo

and will clear ciruit in 10ms maximum. Check your blocking diode surge ratings.

I deleted that post on Google- see follow-up with capacitor on other side of fuse -

You're exhibiting symptoms of the pervasive disease I seek to eradicate/prevent. It's known by various names... Tunnel vision. Not invented here. I'm too smart for my own good syndrome. Pervasive incompetence.

Infected engineers get defensive and tell you why you're wrong according to their limited view of the COMPONENT.

The guys you want doing your design reviews listen to what's being said in SYSTEM context and figure out how broadening their view might make their designs better. They look for learning opportunities, not arguments.

Power supplies are frequently subjected to conditions not spelled out in the spec. The spec may not say much about AC line transients. Inexperienced engineers may not pay attention to that at all. Same for load transients.

Pick a power supply. Grab the schematic and the designer. Ask, "what happens at this node when I rip the line cord out of the wall?" Most won't have even considered the possibility.

I'm not saying that it's hard to design a crowbar circuit that doesn't have the problem. I'm saying that, if you're not paying attention, it's easy to let a bad design get through. Crowbar misbehavior is not an isolated case as evidenced by other inputs to this thread.

You'll recall that I asked a question about the application. For a dedicated application, a less-than-optimal crowbar may not be an issue.

Here's the backstory that gets me so excited about power supplies.

I inherited a hardware group designing a computer workstation. One component was a custom power supply designed by a local power supply house. We'd seen prototypes. Our engineer went down the spec sheet running tests and signed off on the design. But when they got put into computers, we had random failures. The design firm denied responsibility because our engineer couldn't reproduce the symptom. This went on for months.

I finally gave up an took one home over the weekend. I returned on Monday with a test fixture. I invited the designer and his boss in for a demo. They brought their latest revision. I put a current probe on the transformer primary and a transient load synchronized to the switcher. By moving the load transient across the timing cycle, I could change the duty factor off 50% and walk the drain current right up the saturation curve. I gave 'em safety glasses and asked them to put their fingers in their ears as I embedded pieces of FET in the ceiling. I then asked how many more they wanted me to destroy before they got the message. The problem wasn't on the spec sheet. The problem wasn't on the schematic. It was on the layout. After all that, they still couldn't get it right.

I expect that if I'd suggested here that someone build a current transient tester capable of 10 amps in 10ns, I'd have been told I was an idiot. All I can say is that it's often quicker, easier, cheaper to run the test than to argue why it won't work. But I digress... Back to the story.

Purchasing fired 'em and contracted with a firm 500 miles away. Long story short, the first look at the new schematic showed it to be identical to the old schematic. I asked a few questions and discovered that the new firm didn't have the manpower for our design, so they hired someone. And who did they hire but the guy who was laid off from the first local firm. Didn't see that coming.

I threw a hissy-fit, so they hired a consultant "fixer" to clean it up. I met with the guy. Took me 15 minutes to decide that he exuded competence.

The new design came in on schedule, on budget, it worked and it flew through EMC testing with margin to spare.

The design was almost identical to the first one. But it was executed by someone competent and paying attention to the system details.

I wanted to hire him, but he wouldn't work that cheap. ;-)

Good times...

We've hacked home-made current transformers that let us measure 50 amp pulses, with risetimes of a couple ns. It's not hard.

--

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

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

SCR

and

in

that

not

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    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.

That can be ok because the whole unit has become a service case anyhow. Something on there must have catastrophically failed to cause this tripping in the first place.

--
Regards, Joerg 

http://www.analogconsultants.com/