Circuit Peak Detect Issue with OpAmp

indeed, what you say is what I see happening. I have tried a 10M hysteresis. Had some unusual results. Will get my oscilloscope and measure what is happening in the real world. I have noted a RESET by holding the switch down and consequently draining the capacitor, will take about 3 sec..

I will post results in the next day if I could add some hysteresis and still maintain the ability to read a drop in voltage on the - pin relative to the peak value on the + pin...to the order of a 3-7mV..

...and your reply was NOT hazy at all!!

Is there a reason why your using this schematic? I know there are many others out there, some even by manufacturers. I saw one post where they've noted that you should have a temperature sensor if your using high charging rates... Which would seem logical and safe...

http://www.edaboard.com/ftopic132547.html

Otherwise, I'd check the components.

Thanks for your thoughts. The add-ons such as a clamp switch, better current source, thermo protection, timer will come later. Checking those links out...always never can have enough of those!

I have been tackling this issue of a simple peak detect and hold charger for some time and this schematic has been on the to-do-list for a while now. Surprisingly, there is not much out there on the WWW regards a "working" voltage peak detector system for NiCads without using a dedicated IC or programmed micro-controller. I assumed this one works, as the author has made a prototype. I also have run several simulations before deciding to spend some time on a breadboard and the sim looked promising.

What I can say from my playing with the circuit in the real world since I posted my question, the peak detect portion of the circuit is inefficient along the length of the "+" OpAmp input pin trace. I have removed some voltage leak issues and have also come to find that a clean and steady DC source is vital.

I have a battery on charge now at 300ma and will see what happens...hope it won't fart and wake the missus up

find that a clean and steady DC source is vital.

Notice that the last thing the designer did was to add the de-coupling cap (C6) on the OpAmp. Of coarse he may have fitted it at the beginning but only added it to the diagram towards the end.

I don't know anything about charging batteries, so, ignore this if it's rubbish. It seems to me that there is an inherent problem with the "method". Charging stops when any one of the cells in the battery reaches full charge. I'm struggling to get my head round it but I think that this effect combined with the "memory" effect in NiCads will cause a vicious circle in which the weakest cell in the battery is the one which determines the end of the charge and discharge cycles, and gradually gets worse. Shouldn't there be a discharge period at the start of the charge cycle.

Thanks for that...I am aiming towards charging a single battery. I think that is the smart option, considering the many issues charging two or more batteries.

I will be adding a discharge of battery option to the circuit prior to start of charge. I want to get the main idea work first.

Happy to note that I think I have fixed the many issues the circuit has. Will see what happens when I charge a dead flat battery. Will post my idea based on this circuit should everything work as it should. A fully charged battery did turn off the charge as the voltage of the battery dropped 2ms relative to the peak.

Mind you though, after isolating the "+" and "-" OpAmp input pins from each other, treating the OpAmp strictly as a comparator (thanks RedFred), stopping OpAmp input pins from leakage into the battery, reducing leakage from the caps....and on and on I go.

Will post my success or failure tomorrow regards my amended circuit to the one I had issue with. Hope the battery won't fart on me

I honestly thought that the original circuit I linked to was a working one, as he made a prototype that claims to work and every man and his dog references this as a look on the WWW for those who want to make a peak voltage detect NiCad charger.

Hi CraziestOzzy.

Of course, the op amp behaves this way as it acts as a comparator. When the voltage at "-" reaches exactly the voltage at "+" the output of the op amp changes its state. If you want to change its state when the voltage at "-" is somehow lower than the voltage at "+", then you have to add a voltage divider at the "+" input of the op amp. In this way the voltage at "+" will be a few millivolts lower than before and when the voltage at "-" reaches this new, lower voltage the op amp will change its output state (high). Concerning this voltage divider, I recommend you that the resistor which is connected to ground must have a large value (e.g. 1M or so) in order not to discharge fast the capacitor of the detector. Also, I would recommend you to add some hysteresis on the op amp (I always put some hysteresis in such comparator circuits), although it may not be necessary in this case.

Yeah thanks...hysteresis may not be needed with the amended circuit, will see what happens. I will look into the voltage divider, I was actually looking at throwing in some gain...but again will wait and see.

I treated the OpAmp in the end as having two individual inputs...a comparator if you like. Treating the OpAmp as such ended up in a complete rehash from the original circuit I had issues with.

UPDATE

yet another good answer, RedFred got one too ...the voltage divider worked a treat in the end...once I figured out the source of my problem. Also added a few goodies as explained in reply below with new circuit diagram The capacitor and its behind the scenes interaction with the OpAmp were to blame. Seems that the capacitor was being fed an extra 10mV from the input pin of the OpAmp and maybe also reaching a rather "saturated" state.

To make a long story short, I have resolved the issue regards to OpAmp not behaving as intended.

The peak detect capacitor required for detecting and holding the peak battery is at fault, using the circuit provided in my opening post.

Stripped down to its bare essentials, the capacitor was always reading HIGH once it had charged relative to the opposite input pin of the OpAmp (I did not know that this was happening until now of course) . On application of voltage to the stripped down circuit, the capacitor would start out LOWer on the - input pin than the other + input pin. After the capacitor had fully charged, voltage would be higher on the - input pin than the opposite + input pin...not good.

Not good, as a drop in voltage on the + input pin (to be a lower voltage than the - pin) is what is required to switch the OpAmp to my desired state of LOW. Hence the problem of why my use of the original circuit would always eventuate in the OpAmp mysteriously switching.

I have my doubts that in using the original circuit, that a battery ever reached a charged state.

See my fix...simple, but sure was a pain to get there....

The 1N4148 diode "isolates" the capacitor feeding into the - pin from the present battery voltage that is fed into the + pin. Also the forward voltage drop forces the - pin to read LOW, even when the capacitor is fully charged. The output from the OpAmp now reads HIGH and does not switch LOW until the + pin reading the present battery voltage drops a few mV below the voltage stored at the - pin.

The resistor setup is used to calibrate the voltage for the + pin to read several mV above that of the - pin. This is calibrated only after the capacitor has fully charged.

Also, an isolated power supply is needed for powering the OpAmp with the feed into the input pins via the charging battery. I added a current limiting resistor and a voltage divider prior to the input of the OpAmp circuit (not shown here) and will show my complete circuit shortly. I have discarded this popular circuit I originally had a problem with (that is always reference to on the WWW but not pursued) and have made something entirely different, but inspired by the author I mentioned in my opening OP.

I also changed the OpAmp to a JFET TL2062 with accurate results

Thanks for the help too...much appreciated and hope I had my - and + in the right order!!!

Good work, CraziestOzzy. However, the resistors (R) network -that you added in the circuit- doesn't seem like a voltage divider. (Maybe it is not presented on your drawing correctly.)

Maybe, another solution could be to replace the 1N4148 diode with a schottky diode which has only 0,3V voltage drop (instead of the 0,7V voltage drop of the 1N4148). Another way to have no voltage drop at all, is the use of an "active positive half-wave rectifier" (using the second part of the LM358) but, in this case, it is not recommended because of the 220μF capacitor (which will exist at the output of the op amp).

yeah, thanks...I used a 19TQ015S schottky with a 200mV drop at 30 degrees celcius...was better than the 4148. But for the basic model I sketched above, I use a 4148 diode, which works.

OUCH...you are right...silly me. the middle resistor (trimpot) is supposed to go to the + input pin of the OpAmp....oops. Will correct that shortly, as I now have a working circuit. Charged three batteries and shutoff each time. Need to go down to a 100ohm trim pot for the final adjustment and solder onto a board, as I can only switch the OpAmp to LOW after a drop in 7 mV....any adjustments on a "wishboard" below that is impossible.