# current sensing

Discussion in 'Electronic Design' started by kell, Jan 16, 2006.

1. ### kellGuest

Looking for a little help with a simple voltage regulator I'm building
for an antique vehicle with a DC generator.
The armature puts out a current that goes through a blocking diode in
the regulator, into the positive terminal of the battery. I need to
limit the current to 10 amps.
If I just string the b/e junction of a bjt across a sense resistor,
then the resistor would have to dissipate 6 or 7 watts during current
limiting. I don't want to use a bulky sense resistor and develop that
much extra heat so I'm looking for a scheme that will use a sense
resistor with a lower voltage drop across it. Any ideas?

P.S. I have to put the sense resistor between the armature and the
battery positive because the ground connection of the voltage regulator
will carry only the field current, not the current from the armature.
The charging current from the armature goes through the chassis and the
ground strap attached to the battery's negative terminal.

2. ### Rich GriseGuest

Go to your local auto recycler (you may have to shop around a bit) and try
to find a voltage regulator from the same vintage and model of car, clean
its contacts, and slap it in.

Good Luck!
Rich

Have a look at: http://www.allegromicro.com They have three terminal current
sensing devices that will output to 5V! Nice package...

4. ### kellGuest

I just thought of something very simple.
With a pnp transistor's b/e junction set across a sense resistor placed
in the current path into the battery's positive terminal, I can bring
the pnp transistor's base down a few tenths of a volt by connecting it
to a voltage divider across the battery, effectively reducing the
voltage needed across Rs to turn the transistor on.
Anybody see any problems with this?

5. ### Guest

See

http://www.nationa.com/an/AN/AN-23.pdf

The LM105/205/305 are all obsolete, but they used this technique.The
problems are the tolerance on the base-emitter voltage (around 200mV)
and the temperature dependence (-2mV/C).

6. ### Guest

use an opamp, or comparator.

A linear discrete amp is another possibility.

NT

7. ### DavidGuest

Or feed the current carrying wire through a few turns of wire to develop
the control voltage (ie. CT)

--

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General Manager
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8. ### ehsjrGuest

It doesn't reduce the rsense wattage, and the divider
isn't a divider - the top resistor is shorted.

Draw it - you'll see the problems:

+ -----+---[Rsense]---+---+---> To Batt (+)
| | |
| | [Rdivider]
\e | |
PNP |------------+---+
/c |
| [Rdivider]
| |
- -----+------------------+---> To Batt (-)

You could use a .01 ohm 3W as the sense resistor,
with a comparator circuit if you must avoid a high
wattage resistor.

Ed

9. ### Winfield HillGuest

ehsjr wrote...
A comparator or opamp that works at the +Ve rail is good,
but it's possible to work with common discrete transistors
and low-value sense resistors, if two transistors are used...

.. + -----+--+-- Rsense --+----> To Batt (+)
.. | '-----, |
.. Rc | |
.. | c\| |
.. \e |----'
.. PNP |---+--e/| NPN
.. /c |
.. | R
.. | |
.. - -----+-----+---------------> To Batt (-)

10. ### kellGuest

Ed, clip that conductor that shorts the upper resistor and you'll see
Anyway, I just thought of something a little better and just as simple.

11. ### kellGuest

it's possible to work with common discrete transistors
I guess I'll have to get your book, look up the circuit you just posted
and read about it so I can get my naches (Yiddish for brains) around it.

12. ### RickGuest

I'm wondering why you want to limit the generator output to 10 amps....

13. ### Jasen BettsGuest

doesn't seem substantially differnt to this:

. + -----+----- Rsense --+----> To Batt (+)
. | |
. Rc |
. | |
. \e '
. PNP |---+---|<----'
. /c |
. | R
. | |
. - -----+-----+---------------> To Batt (-)

which was what I was about to propose.

14. ### kellGuest

Look into antique generators and you'll find out why.
It's not an alternator.
It's an ANTIQUE GENERATOR. ON AN ANTIQUE BIKE.

15. ### kellGuest

Sorry I yelled Rick.

Win: It looks like the tempcos of the transistors cancel!

16. ### Tony WilliamsGuest

I vaguely recall a low-drop high-side current sensing
circuit along the lines of the ascii sketch below.

Rs
V+ -------+-----/\/\----+---> Iout
| |
R1 R2
I1| |I2
e\| Vbase |/e
pnp|----+----|pnp
/| |\
| |
+------+ +----> Vswitched
| | |
\| | |/
npn|----+----|npn
e/| |\e
| |
0v-------+-------------+----

Iout*Rs = I1*R1 - I2*R2

The relative values of R1 and R2 were adjusted so
that Vswitched changed logic state at a defined Iout.

I haven't done any sums yet to find out the optimum
value for Vbase (probably with respect to V+), etc.

17. ### Derek PotterGuest

Stability. The threshold depends on VBE which has a temperature
coefficient of -2mV/C. Also the bias voltage is derived from the 12V
which is probably not too stable. A better solution is to use a PNP
long-tailed pair. This gives a good CMRR without needing presicion
resistors round an op-amp

18. ### Fred BloggsGuest

What kind of generator is that- it looks like the field coils and
armature are fed in parallel from Batt(+) and the current limit opens
the high side armature/field feed while placing resistance in series
with the field coil low side return to GND. The low side of the armature
is permanently connected to GND. It looks like the old time current
regulators routed a portion of the field coil current through the
armature current sensor magnet for hysteresis. Your circuit proposals
are shunting the armature current away from the battery and doing
nothing to limit generator output. Part of the current limit is to
protect the generator against overcurrent and your scheme does nothing
about that. It would be a shame to burn up that relic. You can use an
LM185 to make a precision high side current sensor if you want to stay
away from GMR or Hall Effect. But this isn't going to do you any good
unless you use it attenuate that field coil current.

19. ### RickGuest

Gee, maybe if you would have said it was for a BIKE in the first place I wouldn't have

I can read and know the difference between an alternator and a generator.

Got a real nice book on automotive generators (from 1925), by the way...

20. ### Fred BloggsGuest

Go monolithic reference, then even a cheap pot works with this:
View in a fixed-width font such as Courier.

View in a fixed-width font such as Courier.

..
..
.. .---------------------. R1
.. | | 1.24V - (Vled-Vbe)*--
.. | | R2
.. ----+-----[Rs]------+-------> I= ----------------------
.. | | | RS
.. | | |
.. | | |
.. | [R1] |
.. | LM185 | |
.. --- | |
.. / \ -------------+ ---
.. --- | \ / led
.. | | ---
.. [R4] [R2] |
.. | | |
.. | | |
.. +---------------' |
.. | |
.. | |
.. | |
.. >| |
.. |-------------------+
.. /| |
.. | |
.. +---> to comp [R3]
.. | |
.. [Rc] |
.. | |
.. --- ---
..
..