Senior Moment!

Community Projects
1

Hi all,

I am experiencing a major senior moment, and the web isn’t helping. I have used op amps since dirt was a new invention, but I am having all sorts of trouble remembering how to approach this issue.

All I need to do is use a single channel op amp to generate a signal to indicate an over voltage condition in my off grid battery bank (12 volt nominal), and sound a little piezo buzzer if the battery voltage goes over 14.5 volts. This is needed because when the sun doesn’t shine and the wind doesn’t blow I have to run my small Kubota engine driving a large frame Lecee Neville alternator to recharge my battery bank. If I don’t pay close attention the voltage can go too high as I am manually controlling the alternator output so as not to overload the small engine (even at low amps this overcharging can happen). I have seen a Zener diode and resistor dividers both used to arrive at an input signal generator, and have built similar circuits in the past with success, but for some reason I cannot for the life of me remember what I did to arrive at a solution. I have a pile of single, double and quad op amps in my junk box, so I kept them thinking I had their parameters burned into long term memory. Apparently not.

We have just bought an Aims 4,000 watt pure sine wave inverter/charger which cost nearly a grand, and I sure as hell don’t want to expose it to overvoltage conditions if at all possible, even though the inverter has robust protection for such events. Only during periodic equalization cycles do I want my batteries (flooded lead acid) to ever see more than 14.75 volts. I have all this controlled from my house via hard lines driving relays in a distant (50+ feet) purpose built engine room/battery house, and have a battery voltage monitor driven with an LM3914 bar/dot driver hanging on the wall inside our house. I want a separated circuit for this warning device so as to isolate one function from the others.

Help! Thanks.

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http://www.cedarlakeinstruments.com/Sha … rvolt2.png

Not my best work, but that should do it for 6:30AM :slight_smile: Not tested though. Input is at R2 and output of IC1 (single supply op-amp) goes to rail if the Vin goes above the threshold set by the zener D1. R4 is for adjustment of incoming divider voltage. It’s roughly centered around 14V.

Would get better performance with an actual comparator or if I added hysteresis, but I did just wake up, so…

As I was drawing the schematic, I remembered that I had another circuit around that used a single transistor. I may have even tested that one…

3

That will work! I may break down and order a few actual comparators, but for what I am using this for a good op amp should work. Much appreciation lyndon. Now, all I have to do is dig around my diode drawer for the right one. Isn’t it odd how something so simple, that you’ve done before, can completely evporate from memory? Maybe it’s all those late hours spent trying to capture pictures of meteors, and worrying about getting that inverter set up. Who knows. All I know is I owe ya a big one!

4

After thinking about this issue for a few days I think I have arrived at a far more simple solution. I have a handful of extra 3914’s that are designed for just this application. All I need to do is replace one of the LEDs typically used as voltage indicators with a 3-5 volt peizo buzzer, and tune the output to sound off at the desired warning point. I had to order the low voltage buzzers off flea bay as all the ones I have are 12 volt. I tried one of those and it just barely squeaks. I could use an opto isolator to switch on a 12 volt signal, but that just adds parts, as would using a P channel mosfet.