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6A 5V Regulator

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cduma

Tue Dec 22 2009, 05:10PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

Lets assume that the input voltage is 13.8V

I was told that if I take the 5V output from a 7805 to the base of a 2N3055 and apply 13.8V to the collector then the voltage across the emiter and ground will be 5V but, the current capacity will be much higher.
Is this true?
Will the 2N3055 operate as low as 5V? If not than what will?
How much current will be drawn from the 7805?

On the same note, how much current can be drawn from a standard 9V battery? Intermittent duty 3hr max with direct supervision.

[Edit: Double post]

Proud Mary

Tue Dec 22 2009, 05:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The LM338 variable voltage regulator is as cheap as chips, can handle 5A continuous and 7A peaks, and will go down as low as 1V2. But don't skimp on the heatsink.

I'm not sure what you mean by a "standard" 9V battery.

cduma

Tue Dec 22 2009, 05:55PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

By standard I mean average run of the mill 9V battery. An alkaline duracell or something like that.

At this point I am not looking for solutions. I just want to know if it will work. Normally I would just use multi-sim but, I lost my copy and can find another on "the bay"

klugesmith

Tue Dec 22 2009, 05:58PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

cduma wrote ...
Lets assume that the input voltage is 13.8V
I was told that if I take the 5V output from a 7805 to the base of a 2N3055 and apply 13.8V to the collector then the voltage across the emiter and ground will be 5V but, the current capacity will be much higher. Is this true?

No. Search keyword: emitter follower. [edit] Not surprisingly, wikipedia does poorly on this subject. Overloaded with small-signal formulas, and not a word about DC bias (which is key to your problem).

Will the 2N3055 operate as low as 5V? If not than what will?
How much current will be drawn from the 7805?

You seem to have a good practical grasp of voltage and current, so it's time to learn the DC circuit model for NPN bipolar transistors. Easy to find on the internet, but IMHO a tutorial would be out of place in this thread or this forum.

On the same note, how much current can be drawn from a standard 9V battery? Intermittent duty 3hr max with direct supervision.

Have you started by, uh, reading the datasheet? Go to duracell.com (or equivalent for other major brands) and follow links for OEM technical data.

cduma

Tue Dec 22 2009, 07:12PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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I concur my good fellow!
I couldnt make any sense out of anything on that wiki page. Thanks for the hint on DC bias. I will continue my search for knowledge there.

I cant seem to find the max current of a 9V battery on any of the sites. They contain a plethora of charts detailing voltage drop over time but, no max current anywhere!

[Edit: Double post]

Bjørn

Tue Dec 22 2009, 09:23PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The information you need is in the datasheet (1.7 Ohm):

Remember that several 4HV members have had 9V alcaline batteries explode so it might be a good idea to avoid stressing it too much.

klugesmith

Tue Dec 22 2009, 09:28PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

cduma wrote ...

I cant seem to find the max current of a 9V battery on any of the sites. They contain a plethora of charts detailing voltage drop over time but, no max current anywhere!

Must confess that I already knew that, but the datasheets -do- give numbers for the internal resistance.

For alkaline batteries, a load which would theoretically discharge them in less than 2 hours
( a load greater than C/2, about 1.4 A from a AA size cell or 300 mA for 9V battery )
is well off the data sheet. Total amp-hours will be a small fraction of the Ah value with normal loads.

Initially you can extrapolate from the datasheet. But if your application is intermittent loading
at multiples of C, I think you'll need to make your own measurements.

Can use a quartz wall clock, powered by the battery under test, to objectively measure how long
it takes for your test load to exhaust the battery. For 9V battery you'd want a voltage divider
or regulator to feed the clock.

mikeselectricstuff

Tue Dec 22 2009, 11:13PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
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Posts: 253

for 13.8 to 5V at 6A you really should use a switching regulator - a linear will be burning a ridiculous amount of heat.

brtaman

Wed Dec 23 2009, 12:13AM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
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Posts: 247

9V batteries are weak little things when it comes to current, you are looking at ~<50ma reliable draw (maybe 60-70max). I believe their general capacity is around 600mA, but that is of the top of my head, someone correct me if I am wrong.

This is for the alkaline, I do not know of the current limitation using nimh 9V battery, I do suppose a larger current draw can be expected, but not much more.

Mattski

Wed Dec 23 2009, 03:18AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

mikeselectricstuff wrote ...

for 13.8 to 5V at 6A you really should use a switching regulator - a linear will be burning a ridiculous amount of heat.

Quite right, about 50W in fact.

Look on a site like Analog Devices to find a chip that will do the job. Or look around on digikey/mouser/etc. The datasheet or a separate app-note will show you pretty much all you need to build a regulator using the chip. Something like this is pretty good, although it's 5A not 6A.

Course it might not be as fun to use these ready-made voltage regulator chips, if you really wanted you could design one yourself.

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