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
-- View in Courier: .8-14VDC>------+ . | . [1K5] . | . +------>8.2V . | . [LM4040-8.2] . | .GND>----------+------>GND JF
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
Drink another cup of coffee, John ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
-- Tea, but yeah, that\'s atrocious! I\'ll go make a cup and be right back... JF
t
le
It it
the
he
sh
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
--- Neglecting the previous "solution" the circuit stays:
8.2v / 14V>---[R]--+-----------+ | | [LM4040-8.2] [LOAD] | | GND>--------+-----------+and there are four cases to consider:
The LM 4040 will stay in regulation if the current through it is between 100µA and 15mA, so we need to use the first case to set up the criteria for the resistor selection.
8.2v 15mA--> / 3mA--> 14V>-----------[R]---+-----------+ | | [LM4040-8.2] [LOAD] | | GND>-----------------+-----------+That way, with the supply at max, if the load current goes to 100µA the extra 2.9mA will get shunted to ground through the reference and its current then will rise to 14.9mA, which is within spec.
So, for the resistor we have:
E 14V - 8.2V R = --- = ------------ = 389.3 ohms I 0.0149A
The closest 1% on the low side is 383 ohms, so the current through the resistor will be:
E 14V - 8.2V I = --- = ------------ = 0.01514 amperes R 383R
since there's 100 µA in the load, the reference will be carrying:
Ir = It - Il = 0.01514A - 0.0001A = 15.04mA
which should be fine.
On the other end we have the fourth case, which is worst case with a low source voltage and a high load current:
8.0v 3mA--> / 3mA--> 8V>---------[383R]---+-----------+ | | [LM4040-8.2] [LOAD] | | GND>-----------------+-----------+Now, since the source has dropped to 8V, the reference won't be drawing any current.
Also, since the load is spec'ed to draw, in this case, 3mA with 8V across it, then it must look like:
E 8V R = --- = --------- = 2667 ohms I 0.003A
and the circuit reduces to:
8V E1 | [383R]R1 | +--->E2 | [2667R][R2] | GNDso we can write:
E1 R2 8V * 2667R E2 = ------- = -------------- = 6.995V ~ 7V R1+R2 383R + 2667R
Of course we dont know what the load looks like with 7V across it, but suffice to say that with an 8V source and a resistor in series with the load it would be _impossible_ for the load to see 8V across it.
So, basically, the circuit fails when the source voltage falls below:
E2(R1+R2) 8V * (383R + 2667R) E1 = ----------- = --------------------- ~ 9.15V R2 2667R
Which isn't too bad considering that a 12V lead-acid battery's output should never be allowed to fall to less than 10V. ;)
But maybe there's a better way?
I'm thinking about a rail-to-rail input and output opamp:
8-14V>---+------------+ | | [57k6] | | | +-----------|+\\ | | >--+-->VOUT | +--|-/ | [LM4040-8.2] | | | | +---|----+ | | GND>-----+------------+------->JF
Sort of my thoughts, too... but it'll need some kind of transient protection... car voltages tend to be nasty :-(
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
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...
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] | . \\| | | . |------|------+ .
Robert, Your present persona does not improve your reputation. You are very close to earning a TURD rating ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
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
I think 8V is the cranking voltage spec for most apps- definitely not
9V-has nothing to do with a dead battery.
A century ago, when I was designing ignition systems, "cold crank" was
4V ;-)...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
The 1908 models? Had they even discovered lead-acid or was it carbon zinc back then?
-- BFM? ;) .8-14V>---+------------+---------------+-----------+ . | | |K | . [57k6] +---|----+ [Z18V] | . | | | | | | . | +--|-\\ | | D . | | >--+ +------+--G NCH . +-----------|+/ | | | S . |K | | |K | | . [LM4004-8.2] | [RL] [Z15V] [1KR] | . | | | | | | .GND>-----+------------+----+----------+------+----+ JF
Time for another cup of tea, while you pick the shrapnel out of your forehead ?:-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
-- Nahh... BFM iuO gcS kF iE nT g Besides, The MOSFET doesn\'t start conducting until a few volts after the 18V Zener avalanches, and then it just dumps the transient to ground. If the load dump goes negative then the MOSFET\'s substrate Zener will kill it. Maybe... You tell me, you\'ve got a lot more experience with car electrics/electronics than I do. JF
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
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?
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.