Best method for dropping supply voltage a volt or two for low power device?

Devices should generally be protected up to 16 volts in that invironment. Some charging specs go up to 15. 5 volts, but just over 14 would be more common. I do use diodes many times. How about diodes and a zener?

greg

Reply to
GregS
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.8-14VDC>------+
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Reply to
John Fields

What is the best method for dropping the voltage of a device that has a range of 8 to 12V powered by a car battery(so max, say 14V) to bring it within spec? The device uses from 100uA to 3mA depending on state.

I was thinking I probably could just use a diode or two and that should be fine but it would take about 4-6 to do it though and that assumes the battery is fully charged.

Seems like a regulator is overkill though(as I do not need regulation). I pretty much want the cheapest solution that will work.

I'm thinking of a simple zener regulator, say at 9V with a ~700 ohm current limiting resistor but this doesn't seem to work all that well over the whole supply range.

Basically I'd like something that will drop 2V when the supply is at about

14V and 0V when its at 8V. I imagine this is to much trouble and probably I won't find a better solution than just two diodes in series? Although this won't let me run the device if the voltage drops down past 10V ;/).

The biggest problem is that with low current draw(100uA) and high voltage(14V) it will drop, say, 1V, but at high current draw(3mA) and low voltage(8V) it will drop 2V ;/ So seems like I can't have my cake an eat it too ;/

The zener regulator also has a similar problem unless I want to waste a lot of power ;/

Any other methods I should look into that won't be to expensive? I thought about DC-DC converters but I'd like to avoid the additional cost and all the extra components.

(essentially the zener diode will work if I could somehow remove it from the circuit as the supply voltage decreased. Maybe a transistor could accomplish this?)

Thanks, Jon

Reply to
Jon Slaughter

Drink another cup of coffee, John ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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Tea, but yeah, that\'s atrocious!

I\'ll go make a cup and be right back...
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Reply to
John Fields

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10K resistor to a zener diode and regular diode in series regulating to 8.9V, that voltage feeding the base of an emmitter follower that puts out 8.2 Volts.

Mark

Reply to
Mark

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Neglecting the previous "solution" the circuit stays:

             8.2v
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Reply to
John Fields
[snip]

Sort of my thoughts, too... but it'll need some kind of transient protection... car voltages tend to be nasty :-(

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Hey, let's see what your buddy, Michael A. Terrell, can do. I claim the impotent piece of trash can't get to square one. You never see the sorry-assed piece of trash submit even a hint of a circuit, and then the pompous , pretentious, and woefully transparent fake has the gall to call stuff like this "minor circuit design." So let's see what /your/ boy can do- your fan club is comprised of largely the lowest quality dregs , vermin , and trash- could be wrong , but I f_cking doubt it...

Reply to
Fred Bloggs

He doesn't need precision voltage regulation, so something with a zener, and some transistors with high enough rating to take a 60V load dump will do, this one works down to 8.1V, a TVS at Vin will catch the HV short duration transients: View in a fixed-width font such as Courier.

. . . Vin . | . .----+----------. . | | | . | | | . | | | . [10k] | [470 . | | | . | |< | . +--| | . | |\\ | . | | | . | '---+------|-------> out . | | | . | [4.7k] | . \\| | | . |------|------+ .

Reply to
Fred Bloggs

Robert, Your present persona does not improve your reputation. You are very close to earning a TURD rating ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

The load has a maximum draw of 3 mA, so the 300 ohms will drop less than 1 volt under worst conditions, for 8 volts out with battery at 9 volts (pretty much a dead battery). At the other end, with 14 volts input, the zener clamps the output at 12 VDC maximum, with no more than 13 mW wasted in the resistor. It's a good idea to add a capacitor across the zener.

Paul

Reply to
Paul E. Schoen

I think 8V is the cranking voltage spec for most apps- definitely not

9V-has nothing to do with a dead battery.
Reply to
Fred Bloggs

A century ago, when I was designing ignition systems, "cold crank" was

4V ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

The 1908 models? Had they even discovered lead-acid or was it carbon zinc back then?

Reply to
Fred Bloggs

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BFM? ;)

.8-14V>---+------------+---------------+-----------+
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Reply to
John Fields

Time for another cup of tea, while you pick the shrapnel out of your forehead ?:-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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Nahh...

BFM
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Reply to
John Fields

MOV's and other transient suppressors have always troubled me... particularly when I have had some shrapnel come in my direction (*).

My preference is toward disconnect schemes.

(*) Once blew an alternator casing at 20K-RPM, but all the shrapnel missed me. An engineer at Ford was not so lucky... impaled in the chest, but not killed.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Y'know, I'd bookmark a link to a page or site that gives tips on how to build power supplies that'll work from typical auto voltage, and protect the small stuff on the output side from any mess on the input side.

Anyone got a url?

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