odd breakdown of a 78L05

[Thunderchild]

Ok so I designed this pic based battery monitor circuit, I used a 78L05 to provide 5 v to the pic, now i used a 2.2 uF output capacitor and no input cap banking on the vehicle battery to act as that.

Well I gave it to a friend with a 1956 Jaguar and he reported a problem, when first used it seeemed ok but after starting the engine and putting it on it was permanently showing high voltage, So I took it back assuming that spiky and generally high voltage (due to poor regulation on a car of that age) had damaged the pic via the analog input. well turns out that I'm wrong and that the 78L05 reg is what blew.

was it just because I omitted the input cap or are these regs (at least the ones i got) a bit fragile and not good for automotive use ?

I'll be trying again tomorrow and putting a scope on the car too see whats going on other than RMS values

 

[Nigel Goodwin]

Well for a start you should ALWAYS fit a capacitor on the input and output, it was most probably oscillating becasue of no input capacitor.

But even more so, it's a VERY rough environment, you need to provide substantial protection and filtering on the input.

 

[Thunderchild]

hm what sort of filtering ? LC ? add an inductor in series with the supply ?

 

[dougy83]

Possibly a series resistor and a shunt zener diode to keep spikes (due to alternator & maybe coils) below the input voltage rating of the 7805.

 

[Thunderchild]

I was thinking of an 82 R resistor at one point in series to take up some of the voltage more to help dissipate the power but maybe a good method of protection ?, my other concern is protecting the pic input (which so far is ok) without usetting readings, its my understanding that a zener will only conduct and have its nominal voltage with 5 mA running through it, I'm finding it hard to do that while not tampering with the input I am sampling

 

[bountyhunter]

The 78XX or 78LXX devices are extremely rugged, They are uncondiitionally stable and will not oscillate even without external caps, unless you drive a reactive load with them. I always use external caps to reduce noise, but they are not required for stability.

I'll wager it was blown out by a voltage transient. Car electrical systems have lots of them, and most electronic devices should be protected by at least a series resistor and power Zener if you want them to live, adding a cap across the zener also helps reduce the pulse amplitudes. A Tranzorb is even better than a zener for clamping protection.

 

[Thunderchild]

yes it was the very noisy power of a vintage car we regu;lation is obtained by opening and closing at high speed a relay, adding the cap fixed it as we reved the engine from nothing to max and it was fine but i may add extra protection

 

[picbits]

Use the LM2930/40 series - they are specifically designed for running on cars (load dump, transients and reverse charging). The only downside is you need to be careful of the output capacitor value.

Either that or filter/protect the input of the 7805 to buggery lmao

I've got a product out on the market that uses the 7805 100ma version and its used purely on cars. I've never had a failure but the input protection parts cost around the same as a bunch of 7805s (RC network with a zener and a transient supressor - oh and a fuse and some extra diodes just to be on the safe side)

 

[bountyhunter]

Use the LM2930/40 series - they are specifically designed for running on cars (load dump, transients and reverse charging). The only downside is you need to be careful of the output capacitor value.

Either that or filter/protect the input of the 7805 to buggery lmao

I've got a product out on the market that uses the 7805 100ma version and its used purely on cars. I've never had a failure but the input protection parts cost around the same as a bunch of 7805s (RC network with a zener and a transient supressor - oh and a fuse and some extra diodes just to be on the safe side)

They have stability problems, and there is no cap made that satisfies the ESR requirement of the output capacitor over the full operating temp range. The joke at NSC was you needed to use a 22uF film capacitor (which has virtually zero ESR) and add a series resistor of 0.5 Ohm and that was the cap that met the stability requirements.

It works, not elegant and certainly not small in size. You could also use a 22uF ceramic cap (X5R or X7R type) with a 0.5 Ohm resistor in series.

It's easier just to put your own protection on with s eries resistor and a Zener clamp.

Here's the stability curve: good luck finding a 22uF cap whose ESR will stay between 0.1 and 1 Ohm over the full temp range.

 

[Thunderchild]

I think the input cap sured the problem and maybe a protection zener along with a series resistor to take up spikes will do the trick, I could add a small inductor to really soak up spike but my 78L05 is now working with input and output on a vintage cars electric whos regulation is nothing like the modern systems so i think the 78L05 will do.

 

[bountyhunter]

I would put a good ceramic cap across the Zener to make sure and kill the high frequency hash.

 

[Thunderchild]

We'll see I should put a scope on it sometime