LM339 does not go to zero on out

[RenatoBR]

Hi all,

I built this simple circuit to discharge single cell NiMh, but on the end of the discharge (0.7v) the voltage on the base of the transistor does not go to zero and it continues draining current.
Is there a way to get zero volt on out of the comparator using 0.7v as a reference?

Rgds

Renato

 

[dick_freebird]

Add a base shunt resistor to Q1, 10Kohms or so.

 

[FvM]

The circuit can be expected to hold 0.7 V for a while, but would switch off Q1 below 0.7 V battery voltage. If not, something is defective.

P.S.: Base shunt resistor is basically better, but only mandatory, if you care about µA leakage currents.

 

[RenatoBR]

Put a 15k resistor on base of Q1.
The same problem:
Starting discharge: pin 1 of comparator 2.85v - base of Q1 0.82v
Discharge finished, battery bellow 0.7v: pin 1 of comparator 1.44v - base of Q1 0.73v.
Current draining: 130mA.


Renato

 

[xaccto]

output of the LM339 is open collector, the 2.2k to + would allow ~5mA, so thats good,
less than the max saturating current.
I would suggest the gain that is set about the comparator is the problem.
Look at the data sheet examples.
For me, i've always had problems when directly connecting the +- opamp type inputs to low impedance sources, so put some resistance and gain (f/b resistors) in the circuit.

 

[VSMVDD]

the problem is the soding great 4001 diode
you must use a schottky diode like a bat 85
not a silicon rectifier thats too slow
and will present slew to the slew ridden opamp
redesign around this
and post again
you can always add two in parallel to get the same current
most are 200ma bat85
foward current
**broken link removed**
maybe another barrier diode with a little more oomph
any diode where you have two
will introduce noise also
and this should be decoupled to ground or expect
and exponet responce as the circuit has gain
you circuit has nether bypass or decoupling caps
if this is a simulation forget it youll have too many invarient responces
on the vcc rail for one
if this is real i wonder why???
feedback is real in it also you need a cap after the load resistor before the diode
1nf
simply adding a 2.2v zener to the + input in series in enought to lower op+ by 1.7volts
adding a 470R shunt accross the led will also show when the battery nears its flat point
perhaps something that shows where the battery reaches 0 is easy also
using two leds
full :both bright
nearly emptyne or both a little or more
nothing :both off
using three as shuted leds
what i use is a digital pulsed r2r lader
then you can drain a battery in seconds
using transistors digitaly switched to drain it and resistors
simply
one 16 pin mcu 8 or 16 transistors depends on cells or use fets
and a bunch of resistors

simple pulse i/o the battery down in seconds

you get the same thing
using a fetlington like a vn66 and a simple 8 pin mcu to drive it with 6 resistors

 

[Audioguru]

Pin 1 of the comparator is an output. Pins 6 (-) and 7 (+) are its inputs.

The output of the comparator cannot be +1.44V. It must be +2.85V or very close to 0V.
Maybe it is oscillating from interference from the other 3 comparators in the LM339. They should be disabled.
Maybe it is oscillating because it doesn't have a supply bypass capacitor.
Maybe it is oscillating because it doesn't have hysteresis as mentioned in its datasheet.

 

[xaccto]

^??
The diode is simply used as a voltage reference for 0.7 V.
Though Using a 1n4001 is not a good choice, I would use 1N4148 or 1N914 and a higher series resistor.
Don't see what speed and needing a schottky has to do with it.......

 

[Audioguru]

Is the circuit built on a breadboard where all the connection wires are antennas for picking up interference?

 

[BABESH]

hi,
probably the reason could be that, immediate after switching off load, battery voltage wil shootup making greater than 0.7V reference, hence comparator is going High,
only solution to this is add large hysterisis to your comparator.

--babesh.

 

[FvM]

The reported measurements can only be explained by a bad circuit setup, involving large parasitic inductances. It's correct,
that using a comparator with negative feedback causes oscillations in most cases. If the battery would be connected very close to
the circuit, it most likely can stop oscillations by it's large capacitance. Apparently this isn't the case in your setup.

 

[RenatoBR]

Thanks to all,

I built the circuit in a protoboard, I'm going to build it in a pcb and after carnival I'll post the results.


Renato

 

[Tahmid]

Hi,
Try placing a 22k or so resistor from the transistor base to ground.
Tahmid.

 

[FvM]

Try placing a 22k or so resistor from the transistor base to ground

Read the post. Renatoa already did this.

 

[Tahmid]

Sorry, didn't read through.

 

[dick_freebird]

Might raise the 2.2K (R2) value so the comparator has an
easier time pulling it down. The only thing that'll cost you is
LED brightness.

You might want to check that transistor out, maybe it's got a
problem. Detach the base, ground it looking at base and collector
currents.

 

[RenatoBR]

Might raise the 2.2K (R2) value so the comparator has an
easier time pulling it down. The only thing that'll cost you is
LED brightness.

You might want to check that transistor out, maybe it's got a
problem. Detach the base, ground it looking at base and collector
currents.

Not only led brightness, less voltage on transistor base and less current on load resistor.

Now I know what's going on with my circuit.
When the battery voltage drops to 0.7 volts, pin 1 of the comparator drops to zero volt, but then the battery voltage increases fast to about 1 volt and the discharge start again.
What I need is a circuit that, when the pin 1 of the comparator drops to zero volt, it does not increase voltage again without first applying a reset, is it possible?


Renato

 

[FvM]

I know what's going on with my circuit.

Yes, others knew it before. They suggested hysteresis.

To be more specific, a functional NiMh battery does not immediately raise it's voltage by 0.3 V when disconnecting a moderate
load (except for a battery from the trashcan...). The voltage sensed by the comparator possibly does even with a good battery,
due to breadboard circuit design and long battery cables. But even with perfect assembly, the circuit would switch on and off or "can be expected to hold 0.7 V for
a while" as I mentioned above.

Adding a resettable hold circuit can be a solution. But it doesn't fit your original circuit very well, I think.

 

[BABESH]

RenatadoBR,
Google for "adding hysterisis to comparator" and you will get lot of infor. design as required for you.

Since u r ckt is in breadboard stage, u can exp with OPamp as well.

--Babesh.