Car Batteries

And why not? It's a perfectly good constant voltage drop over its operating range. I've use them that way over the 20+ yrs I've been in the business. ...lew...

consider a 7805 regulator

this will give you nice stable 5VDC at up to 3 amps

Thanks everybody. Great answers!!! I will follow up on the ones that I understand (e.g., buy this or that product).

Thanks again,

Ivan Vegvary

The 78xx are 3A in TO-3 packaging.

Not so. Current limit doesn't change with package.

Why not a voltage regulator or better yet - a 12V DC to 6V DC switcher. There are lots of power supplies like this about the size of a hand or less.

Martin

Mart> I'm showing my age!!

The 78xx series are only up to one amp depending on packaging.

Try LM350 adjustable regulator - Guaranteed 3 amps. (with sechematic)

Richard

[ ... ]

That's what MG did with the MGA. Two small 6V batteries, on either side of the driveshaft, accessed through a removable panel behind the driver's and pasenger's seats. There was a short jumper cable which curved over the driveshaft to connect them in series.

Of course, the car still ran positive ground, which was a bit of a nuisance. :-)

Enjoy, DoN.

Because a Zener only STARTS to conduct above a certain voltage. Zener regulators are GENERALLY shunt regulators. The load is in series with a low value, high power resistor, and the zener goes across the load in such a manner as to short excess voltage to ground, causing the voltage drop across the resistance to absorb/regulate to the zener resistance. Diodes in SERIES are generally used for minor voltage adjustment (not regulation) with the forward drop of various diodes generally running in the 0.64 volt range for silicon diodes, Schottky diodes drop significantly less, Germanium diodes significantly more.

See:

if you don't want to take my word for it.

By the way, what business are you in?

25+ in the automotive, and almost as long now in computer electronics.

With a REAL GOOD heat sink. The common 7805 is considered a ONE AMP device. Some 7805P devices are rated at 1.5 amps.. This is with nominal heat sinking. Bare they are a 500ma device. 3 amps is definitely running the "ragged edge" A Linear Technologies LT323 is a 3 amp unit. An LT1003 is a 5 amp unit. It is obsolete.

Actually, it does.

The MAX on a 78xx is 1 amp. Smaller packages are rated lower - down to the 78Lxx parts in TO-90 and SO8 packages(100 ma).

The LM350 would be my choice.

Richard

either one is fine - just make sure it is mounted to some aluminum to provide a heatskink. The 78XX is cheap - around ten cents the last time I bought a bunch, the LM350 costs a little more - and a fixed regulator is a lot easier for someone to use who doesn't do a lot of electronics - it has an input terminal, and an output terminal, and a ground terminal, nothing to mess up. They do come in various voltages, but 5V is very common/cheap/easy to get, and will be fine for a 6V car to run gauges. You may wish to measure current drawn by the gauges though - I think 1 amp should be enough. And, the regulators are so cheap you could put one on fuel gauge and another on the temp gauge if it is electric and not mechanical.

For most uses, 78xx is indeed generally regarded as a 1-amp part. Fairchild even calls it that, but notes that this depends some on conditions. It is not a max rating by any means.

STM calls it a 1.5 amp part, but the specs are the same.

Fairchild was the original designer and mfr of the 78xx. It may have been Jim Widlar's last design at Fairchild before moving to LM-land (National).

Excluding 78Mxx and 78Lxx, the same die is used regardless of package. The current level at which it goes into limiting is not tightly specified and can be as much as 2.2 amps -- regardless of package. It can also be considerably less in some parts of same partnumber but different lots or different mfr.

The 78xx also has internal temperature limiting, so part temperature can also cause it to shut down. That is a function of package, but also of (Vsupply - Vout), heatsinking and ambient temperature.

There are certainly cases where the thermal limit would govern: e.g. full-MIL temp range (up to 125C), minimum dropout voltage and junction-to-case thermal resistance could establish a max current due to thermal limiting that is lower than the current limit would be under other condx with the same part.

I don't know enough about Ivan's app to have a preferred approach.

I'll bet you're very good with autos and computers -- but don't quit your day job to design power elex!

A Zener would indeed provide a fairly constant drop and they are sometimes used that way. This wouldn't be a regulator, just a drop that is fairly constant over a reasonable range of load currents. The output would not be regulated, but it would be reduced by a fairly constant amount.

Might be a radio noise issue with an AM radio. Some of the "simple switcher" chips are very easy to use, though. I've built little switchers by epoxying parts on the back of a wallwart -- no circuit board at all, just haywire things and then glop 'em good.

An obsolete regulator would be appropriate for an obsolete car, no?

AM radio???

Certainly a switching supply would be more efficient than a linear.

Well for about 25 years I worked for a private electronics research and development Co that did about 95% work for the millitary. We were one of the two bigest in the Infrared surveliance business. After a big cut back in that business I designed and built special equipment and interfaced some regular equipment to computers for a "well known" university whose football coach is also very well known :-) for another

10 yrs and moved to NM to do similar work for UNM for 8 more. Is that sufficient background to give you some idea of my experience. ...lew...

Don Foreman wrote

Thanks Don. :-) ...lew...