Over Voltage Protection Crowbar Circuit

Put a fuse at the *output* and use the usual SCR/zener circuit.

Don't rely on the PSU input fuse. Too uncertain.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

I agree. The biggest I've done was for a 100-amp bought-in power supply. I used fabricated copper busbars for that. It made a satisfying thump on test.

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Very common. Bench power supplies should *never* be used to charge batteries without a suitably rated diode in series.

I've had it done to my bench supplies when I've been away. I better not find out who it was...

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)

Hey, we have the absolute majority in the house now :-)

--
Regards, Joerg 

http://www.analogconsultants.com/

Sounds like a couple of amateurs when it comes to computing I^2*t >:-} ...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.

I keep a bench supply at home and up in the cabin, mostly for charging dead car batteries. Most "modern" car battery chargers refuse to put any current into a zero-volt battery, but a bench supply will. I think the car supply places sell the "smart" chargers so that people will think their batteries won't charge, so the auto supply crooks\\\\\ people get to sell people a charger *and* a new battery.

I just crank the supply up to 15 or 20 volts, set the current limit (if it has one) to an amp or two, and connect it to the battery overnight.

--

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 
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Multichannel arbitrary waveform generators

Huh?

This works, and fast, done it many times. One has to make sure that the SCR triggers with gusto and is big enough.

--
Regards, Joerg 

http://www.analogconsultants.com/

I misconscrewed you as the one saying "inductor"... or maybe 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

That was my point, ideally you don't want to have the inductor in the path.

--
Regards, Joerg 

http://www.analogconsultants.com/

I don't follow your reasoning.

Is it the battery back-up, overvoltage protection method, or the power supply design you're questioning? The subject is crowbar ovp methods. You're point was the ovp condition occurring in a battery charging situation. I think was addressed.

Power supply compatibility in parallel redundant or backup applications is a subject in itself. Misapplication of products, or their immunity to such conditions can also be an interesting discussion, along with the sad state of the equipment and the amazement or frustration of the misapplicator.

Rest assured that you won't see a product that wasn't immune to the application of external voltages occupy or originate in my work area. This is, in fact, a crude test method to confirm the function of OVP circuitry, though alternative methods exist that may perform the same function automatically and with reduced hardware, in both design and in production test.

I do not, however, assume this capacity in any equipment that is unfamiliar to me, until proven otherwise.

The crowbar method of protection isn't always used, and it's external application in any black box situation should probably be evaluated.

Your battery charging or back-up situation isn't exactly a black box, so you should be able to get your mind around it, with a little effort.

There's no reason why a commercial bench power supply should smoke on application of external voltage, within it's normal output compliance, save the naivete of it's designer, or the complaisance of it's purchaser.

RL

What's wrong with this picture? Hint: All but one of the chargers are dead.

There were various possible failure modes, but mostly, they were from inability to distinguish between a depleted battery, and one that might turn into a hot battery acid belching volcano. While the chargers seem to be fairly well protected against charging a dead short, they're not so good at dealing with one shorted cell out of six cells. They just continue to pump power into the battery until the electrolyte boils. I'm not sure exactly what failed, but I did find shorted diodes in one charger, and a burnt hole where the controller chip had previously lived in another. The whole pile went to the recyclers.

I switched out all the above chargers and about 5 more with Xantrex/StatPower chargers and have lived happily ever after since. No more hot acid volcanoes.

Good conspiracy theory, but I don't think that's the reason. Litigation avoidance seems a better excuse. If there's any possibility of the charger starting a fire, or in any way damaging a customers equipment or facility, the charger should shut down.

Charging a nearly "dead" battery is a tricky borderline case, where the battery could be either severely depleted, or suffer from a shorted cell. The way it's usually handled is to trickle charge such a low terminal voltage battery. If it doesn't recover, then nothing is lost and no damage is done by the low current charge. If it recovers nearly normal voltage, then the charger will switch to a higher current charge.

--
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

Applying the crowbar to the output is the most effective way of reducing potential damage to the load - which is it's intended function. The end user would usually prefer that your power supply turn itself into rubble, before any harm can come to the load. Best to test the effectiveness of any protection method, as this has priority. A grenading fet would only pass single-fault abnormals in an enclosure (if isolation barriers are not compromised in the process) and is best avoided by design.

In many cases (though not in this simple buck example) the presence of an isolation barrier will alter design considerations for ovp protection.

RL

that

Presence of the inductor will not reduce the effectiveness of ovp crowbar protection that is located on the output terminals. Neither will a grenading fet.

RL

car

so

A charger is supposed to put current into a battery.

I had a zero volt battery, some light left on for a weekend or something. I went to Kragen Auto Parts and bought a charger, and it wouldn't even try. Zero output measured current. I took it back and they said the battery was destroyed and that I needed a new one.

So I hacked a charger: an old DSL power transformer, a diode, and a belt sander as a current-limiting resistor. After a couple of hours it started up, and the battery was fine.

Now I keep a bench power supply around.

--

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

(...)

Sure, but how is it suppose to tell the difference between a depleted but chargeable battery and a battery with a shorted cell? Without some manner of ESR measuring algorithm, they look the same to a commodity charger. The problem is that putting current into a shorted cell is the same as a room heater.

went

output

Then the battery charger controller did it's job. It protected itself from a potential hazard. If it were a more sophisticated charger, it might have started with a trickle charge, which should have brought the terminal voltage up to the point where the charger would be willing to charge it.

Incidentally, Kragen is where most of those dead chargers were purchased. Returning it was the right thing to do.

I had a similar problem on a mountain top radio site, where the available resources tend to be rather limited. So, I dragged the battery out to my truck, and put it in parallel with truck battery using jumper cables. I did NOT run the engine. Judging by the sparks, there was quite a bit of current flowing. After about 2 minutes, the terminal voltage was high enough that the Schumacher(?) marine charger would agree to charge it normally. A trickle charge would have been better, but I didn't want to wait.

I like the belt sander as a resistor trick. However, it won't work with a variable speed controller or solid state switch in the belt sander.

Don't forget a series protection diode.

--
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

I see some problems with that circuit. The IRLM6401 is only rated at 12V and the gate only +/-8V. Of course this is apparently for a low voltage battery, probably 6V, considering the 3.3 ohm load. M1 seems to be connected so that its internal diode normally conducts, although the 0V biased gate would turn it on. I think there should be a resistor on the gate. The shunt regulator DZ1 is set to 5.2V, so I see this may be for a 5V logic supply. I see that a voltage over about 5.8V will start to shunt the Vgs of M2 and turn it off when it becomes less than about 0.6 to 0.95 V gate threshold. But it still does not seem to provide any voltage regulation to the load, and with

0.05 ohms or each MOSET at 1.5A loses 150 mV. If the voltage is only marginally high, or if the load increased, there could be about 2A and 1 or 2 volts on the MOSFET which would exceed its power dissipation of 1.3W.

I don't see what is so great about this circuit. It does not have voltage regulation or current limiting and it does not have any hysteresis to keep it from entering a destructive linear range or possibly oscillate. The SCR crowbar with a fuse (or maybe a polyswitch device) is more robust and far simpler.

Maybe you or someone can explain the merits of your circuit.

Paul

There is not that much difference between charging a battery with a bench supply, or charging a large capacitor in a circuit. I have a large capacitor that I put across my bench supply so that the circuit I'm using it on can draw current surges much higher than the current limit. I haven't had a problem if I leave the capacitor connected and turn off the supply.

Paul

You're sounding a bit snippy, but let's see if we can wrap our heads around it.

I never mentioned backup. overvoltage protection method, or the power

Overvoltage protection IS part of the power supply design. The subject is crowbar ovp methods.

My point was that people use power supplies in ways that don't show up on a spec sheet. Having a stiff voltage supply hooked across the output is a proxy for one type of potential application. It's important for designers to consider what conditions might occur and do whatever is reasonable to prevent failure of the supply or destruction of the DUT. Transient conditions inside the power supply are part of the analysis.

In particular, it's easy to design a circuit that crowbars the output when it reaches 28V. What's not so obvious is when the output is held at 24V by a battery or a big bank of caps, or back EMF from a motor when the input power glitches. Anything that triggers the SCR while the internal power supplies are dropping out can cause problems.

Well, misapplicators don't realize that they're misapplicators. It's the designer's job to protect them from themselves. If you do nothing but meet the numbers on the spec sheet, you're not doing enough.

Not clear exactly what you mean. If you are also testing shutdown transients due to power line fluctuations with external voltage applied, I'd agree.

It's not about you. It's about designing robust products. Users are very creative with unintended applications. My experience from design reviews is that designers tend to have tunnel vision. A skilled designer with ability to see the big picture is rare.

I used to build test equipment for a living. In many a design review, I witnessed a circuit that probably met spec, but would have been a failure in the hands of a typical user. Hanging a SCR across the output without serious consideration of all the ways it could get triggered would qualify in that respect.

Again, it's not about me or my application. It's a caution that an SCR crowbar can be very effective or a ticking time bomb if you're not paying attention. Designers sometimes don't pay attention.

We all like to think that we're the world's greatest engineer. But the sad fact is that half of us are below average. The cutoff point for incompetent is well above average.

As sentences with no content go, that's a good one. I'd have to agree. Power supplies are invulnerable unless they aren't.

Putting current into a short produces no heat in the short.