7812 abuse

Whoops sorry! is there any budget for components of a protection circuit of any type? like, even 5 cents? I think it can be done for like 5 cent

You might try a signal diode in its ground pin, normally forward biased by quiescent current. Increases output voltage, but you can juggle the concept and check stability/transient implications on your own.

I've used lower voltage fixed regulators with zener+diode strings in the ground reference pins without serious issues in various projects.

You've got to keep in mind that -12 on the output pin of the regulator usually means -12v on a lot of other parts in the system, that don't like -12v. You should polarize your interconnection hardware properly or crowbar the sucker.

RL

Yes, originally I put 2 diodes in there, then realised the uncontrolled vol tage drop on the main 1A diode would produce much too much voltage variatio n to be acceptable. Diodes are affordable, but a different non-78 series re g is unlikely to be practical - I won't bore you with the story on that.

NT

That's a classic opportunity to use the PFET trick. Source to 7812, gate to ground, drain to output. The FET is off whenever the 7812 is unpowered. You might want to put a resistor to ground from the 7812's output for insurance.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

owered 7812 see -12v on its output pin. The -12v is from a lead acid, and y es I know they don't charge at 12v etc etc. I could put up with an occasion al failure, but not a lot obviously, it's a low cost consumer app. Has anyo ne tried this, or have insight into the expectable results? Moving to a muc h less common regulator isn't really workable on this one.

The module is a simple lead acid charger, the final connection to the batte ry is made by people outside of our control, so connector hardware won't so lve it. I could fit a fuse & crowbar diode, but that takes time to blow & t he 7812 still sees the -12v, just not for long. And a charger that fails to work because a fuse needs replacement is not a good feature here.

The rest of the supply can tolerate -12v, it's only the 7812 that is at ris k.

The 0v pin is being raised with resistors to get 13.8v out. A diode on that pin is already included, it's easy protection. But a diode on the output p in would work at 1A, thus drop much more than 0.65v & have an uncontrolled & uncompensated voltage drop. There lies the problem with a series diode.

So neither series nor parallel diode is working for this circuit. A discret e regulator is not impossible in principle, but a simple 7812 would be much preferable.

If I put a series diode on the 78's input side, current flow is restricted to going in the right direction with a reversed battery, though the termina l voltages would be way out. The voltage drop is affordable. I guess that's the way forward.

Thanks everyone.

NT

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voltage drop on the main 1A diode would produce much too much voltage vari ation to be acceptable. Diodes are affordable, but a different non-78 serie s reg is unlikely to be practical - I won't bore you with the story on that .

Yes. But you should know by now I'm an awkward bugger. Supply of PFETs can' t be counted on, so I avoid them whenever possible. This is a developing co untry product made with parts from supply channels you would not want to de al with. 7812s are no problem, 1A fets are.

However it looks like a series diode on the 78's input may be workable. I n eed to test it IRL - the 78 would see odd conditions but nothing outside it s voltage limits.

Thank you.

NT

I was thinking there should be a way to use a jellybean PNP transistor and a TL431 to detect the output rail swinging negative and connect the common terminal to the negative-going battery voltage instead when that situation is detected.

With the input rail at 0 and the output and common at -12 the 7812's own short circuit protection should kick in to handle it.

The topology is not readily apparent to me but it doesn't sound implausible. TL431s are about the most common part in the world after resistors and capacitors, they're in just about every broken SMPS there is too, f you have developing country constraints use them for everything! With a little cajoling they make serviceable op amp, integrators, active filters, switches, schmitt triggers, you name it.

Or rather if by "unpowered" the 7812 positive is completely disconnected nothing happens

The module is a simple lead acid charger, the final connection to the battery is made by people outside of our control, so connector hardware won't solve it. I could fit a fuse & crowbar diode, but that takes time to blow & the 7812 still sees the -12v, just not for long. And a charger that fails to work because a fuse needs replacement is not a good feature here.

The rest of the supply can tolerate -12v, it's only the 7812 that is at risk.

The 0v pin is being raised with resistors to get 13.8v out. A diode on that pin is already included, it's easy protection. But a diode on the output pin would work at 1A, thus drop much more than 0.65v & have an uncontrolled & uncompensated voltage drop. There lies the problem with a series diode.

So neither series nor parallel diode is working for this circuit. A discrete regulator is not impossible in principle, but a simple 7812 would be much preferable.

If I put a series diode on the 78's input side, current flow is restricted to going in the right direction with a reversed battery, though the terminal voltages would be way out. The voltage drop is affordable. I guess that's the way forward.

Thanks everyone.

NT

+++++++++++++++++

Lead acid batteries like to have a negative voltage coefficient of

So diodes can be useful for that.

Crowbar idea:

Not quite as cheap as a single didoe. almost though.

with in unconnected they seem to behave like zeners.

-12v? do you mean 7912?

--
  When I tried casting out nines I made a hash of it.

on that pin is already included, it's easy protection. But a diode on the output pin would work at 1A, thus drop much more than 0.65v & have an uncontrolled & uncompensated voltage drop. There lies the problem with a series diode.

put a dioode parallel with the 7812 -then the 7812 will onlt see -0.6V and the resistors will see the rest. (so you might need a more powerful resistor)

but using a 7805 (or LM317) instead of 7812 would mear a higher resistance to ground and thus less power dissipated in this fault condition

???

--
  When I tried casting out nines I made a hash of it.

on that pin is already included, it's easy protection. But a diode on the output pin would work at 1A, thus drop much more than 0.65v & have an uncontrolled & uncompensated voltage drop. There lies the problem with a series diode.

put a dioode parallel with the 7812 -then the 7812 will only see -0.6V and the resistors will see the rest. (so you might need a more powerful resistor)

but using a 7805 (or LM317) instead of 7812 would mean a higher resistance to ground and thus less power dissipated in this fault condition

???

--
  When I tried casting out nines I made a hash of it.

My initial thought too but it soon became obvious that the abuse mentioned in the subject line was in regard of accidental reverse connection of a 12v LA battery to a 1A rated voltage regulated battery charger based on the 7812.

Presumably, the requirement is for a charger that won't damage a small (3 to 15 AH) 12v SLA battery by charging it beyond the absolute maximum terminal charging voltage rating of 13.8v (preferably set to 13.65v[1] when floating such battery packs 24/7/52 for years on end). A vented flooded cell 12v LA battery (such as SLI or deep discharge marine/leisure types) could be serviced by a basic mains transformer/rectifier charger or a simple solar panel source without lasting harm[2].

I'd say the most pragmatic solution to his reversed connection hazard

*is* the use of the basic blocking diode between the battery and the regulator output terminal. Not only does this protect the 7812 from reversed connections, it also eliminates a discharge leakage path when the 7812 is deprived of its input voltage (power cut or loss of output from a solar panel) when connected to a correctly polarised battery load (all the more important when a 1K2/240 ohm resistor network is used to elevate the common pin by 2.4v to obtain, after allowing for a diode drop of circa 0.65v, a terminal charging voltage of 13.75v (give or take the effect of diode current rating , the condition of the battery, its type and the ambient temperature - there are *so* many variables to contend with, a simple series diode for reverse polarity protection is as much sophistication as you'd need in the circumstances, anything more and you're looking at a far more sophisticated solution than can be satisfied by a 7812). [1] The usual 13.8v per 12v SLA battery setting used as an initial default charge voltage setting by UPS manufacturers is chosen simply because it offers the maximum autonomy figures in the sales brochures from a fresh battery pack for the first 12 to 18 months (at least long enough to get past the basic 12 month warranty period after which you're advised that the battery pack, despite its very high cost, is merely "A Consumable", to be replaced every 3 years or so).

This arises out of the battery manufacturer's own recommendations where it's true enough in the case of putting them on charge and forgetting to disconnect them from said charger for a week or three. If you want to see more than a pathetic 3 or 4 year life out your UPS battery pack investment, you'd be well advised to reduce this 13.8v setting to 13.5 or

13.6 volts and sacrifice maybe as much as 10% of the initial autonomy rating - don't worry, you'll be quids in some three years down the line when instead of only getting 30% of the brand new AH rating, you'll still be seeing something nearer to 80%. [2] A bit of overcharging (gassing charge) is actually beneficial provided you can make up the water loss. Counter-intuitively, as I've discovered to my cost, one of the worst possible "Abuse"es you can apply to a cheap 'n' cheerful SLI battery, kept in splendiferous isolation from all forms of vibration and acceleration forces, is a regimen of rock steady 13.8v float charging 24/7/26 (it won't survive 24/7/52).

It seems such kind and gentle treatment is the recipe for a very shortened service life compared to the more arduous conditions they've been designed to cope with in road (and off-road) going vehicular service. Who knew? Who the feckin' Hell knew!? :-( Well, *I* know...

*Now*! Take heed and allow your batteries to "Live long and prosper." :-)

--
Johnny B Good

On Wednesday, October 24, 2018 at 8:47:22 PM UTC-4, snipped-for-privacy@gmail.com wrote :

wered 7812 see -12v on its output pin. The -12v is from a lead acid, and ye s I know they don't charge at 12v etc etc. I could put up with an occasiona l failure, but not a lot obviously, it's a low cost consumer app. Has anyon e tried this, or have insight into the expectable results? Moving to a much less common regulator isn't really workable on this one.

Use a cheap NPN, collector to BATT (-) output, emitter to chrgr COM. Put pw r diode in series with BATT(+). Then no current is drawn from battery in ei ther case when circuit is off. Reverse battery drops all the voltage across transistor CB, eliminating any stress on the 7812. Transistor operates in saturation, so cheap TO-220 without heat sink is sufficient.

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ed voltage drop on the main 1A diode would produce much too much voltage va riation to be acceptable. Diodes are affordable, but a different non-78 ser ies reg is unlikely to be practical - I won't bore you with the story on th at.

can't be counted on, so I avoid them whenever possible. This is a developin g country product made with parts from supply channels you would not want t o deal with. 7812s are no problem, 1A fets are.

I need to test it IRL - the 78 would see odd conditions but nothing outsid e its voltage limits.

One could block current that way, but the unknown & variable voltage drop o n the output tr would make the thing unserviceable. 78s aren't like opamps, you can't use external feedback to set the output voltage. It's fixed at 1

2v +/- 0.6v. The charger circuit addresses that wrong voltage but not in a high gain closed loop way, so extra variation is not compensated for.

NT

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it could be disconnected or it might be connected. We don't control what fo lks will do with it so it needs to survive them all.

NT

unpowered 7812 see -12v on its output pin. The -12v is from a lead acid, an d yes I know they don't charge at 12v etc etc. I could put up with an occas ional failure, but not a lot obviously, it's a low cost consumer app. Has a nyone tried this, or have insight into the expectable results? Moving to a much less common regulator isn't really workable on this one.

attery is made by people outside of our control, so connector hardware won' t solve it. I could fit a fuse & crowbar diode, but that takes time to blow & the 7812 still sees the -12v, just not for long. And a charger that fail s to work because a fuse needs replacement is not a good feature here.

risk.

that pin is already included, it's easy protection. But a diode on the outp ut pin would work at 1A, thus drop much more than 0.65v & have an uncontrol led & uncompensated voltage drop. There lies the problem with a series diod e.

crete regulator is not impossible in principle, but a simple 7812 would be much preferable.

ted to going in the right direction with a reversed battery, though the ter minal voltages would be way out. The voltage drop is affordable. I guess th at's the way forward.

I'm not really following that one :)

NT