Opamp Circuit Help

Hiya Guys,
Well I've just about finished my new RE power board for the shed. I've madeup the K241 from Oatleys ( a wind genny controller) for my f&p genny and i've made my own 24 volt shunt regulator for the PV. Now the K241 has 2 led's, 1 for when the genny is charging and the second for when the genny is dumping the load. My PV shunt reg has a led to show when its dumping so now I just need a led to show when the pv is charging.
On reading up on opamps I reckon a low-side monitor would fit the bill, but I'm pretty green when it comes to opamp design, well on a new circuit I haven't encountered before. On reading a maxim app note(746) I reckon the below schematic would be the one to use. The app note does go on about high side switching but basically all I want when the PV start charging switch a led on simple.
The opamp can be powered from my nife bank (15 volts) and is totally isolated to my 24 volt batterybank. The resistor values are really only a guess as I dont have any formula to work with so if you guys could offer some help on this It will be appreciated.

Cheers Bryan

hi bryan,
Its close!, what current levels are we talking, whats the max/min [estimate would be fine]

Eric

Hiya Eric,
The idea is I intend to put this on the negative input wire from the PV and the wind genny will be on a seperate 200 amp shunt on the gennies negative lead. The PV will output a max of about 6 amps and the wind genny in a storm could output 20-30 amps. Next week when I finally get a digi camera I can take a pic or two of the new power board.

Cheers Bryan

G'day Bryan,
I would be inclined to run the Op Amp from the 24 Volt supply.

I would also make the OP Amp operate as a Schmitt Trigger so there is a fast transition when the current increases above a pre-set level.

The circuit attached is a differential amp as I guess you know.

At what current do you want it to trigger, ie. to turn the LED on?

I'll do some calculations and post a circuit later.

PS. if we are going to Adelaide, it will be next week. Have not made a final decision yet. I suggest you send me a PM re your availability.

Here's how I would do it.

The calculations and a few notes are in the .pdf (I could not up load the .xls).

However, you said above "wind genny in a storm could output 20-30 amps"

This could damage your battery and is possibly a fire hazard.

The old style windmills that have been on farms since Captain Cook was a boy have a lever that changes the angle of the vane so that it can be turned in windy weather thus reducing the speed of the prop. virtually to zero when necessary.

You could make a hi tech version to control the prop speed and thus reduce the genny's output.

PS> I could not find data for the TL0721. Do you mean the TL072?

If so, it is not suitable unless you have a negative supply voltage available.

The LM358 is available from JayCar, Altronics and Dick Smith. It's common mode range includes 0 Volt, so you don't ned a negative supply.

Hiya Len,
Eh mate I wouldn't worry about the wind genny putting a heap of amps as the genny controller will dump any excess load once the batteries are charged or hit the preset voltage. The whole point of the circuit is simply to light a led when the solar array starts charging. For this to happen the panels voltage needs to be above the battery voltage to charge. Currently I have a plasmatronics 24 volt 22 amp charge controller on there but I'm going to move that upto my shearing shed for the new 24 volt array up there. As mentioned before I havea 200 amp shunt and digital amp guage to show the wind genny amps and with the solar shunt regulator I made it doesnt show when the solar is charging , only when the preset voltage is reached a led comes on to show the dumpload working. Currently I have only 4 35 watt pv panels on the roof as 3 of the other ones finally died as as I'm running 24 volts and the panels are 12 volts I now have 1 spare. When I get the chance I'll throw my 175 watt 24 volt solar panel up then the maximum amps i reckon i'll get out of the solar will be around 6 amps on a sunny day.

Oh and I meant the TL072 the extra 1 in the schematic was a typo, anyway I thought you could run the tl072 of a single supply as I'm sure I've seen circuits before where it was done.

Cheers Bryan

bryan1 said:

Oh and I meant the TL072 the extra 1 in the schematic was a typo, anyway I thought you could run the tl072 of a single supply as I'm sure I've seen circuits before where it was done.

Cheers Bryan

Click to expand...

Yes, you can run any Op Amp on a single supply. But that is not the point.

If you want to use the TL072, you will need a negative supply voltage since, if used with a single supply, its common mode range excludes 0 Volt.

Whereas, the LM358 CMR includes 0 Volt so you don't need a neg supply.