op amp 101:
The LM324 is an operational amplifier, a common analog component.
op amps are very powerful analog amplifiers that can be used for many functions.
They can amplify AC or DC voltages from less than 1X to over 1,000,000 times.
They can act as high input impedance "buffer amplifiers", which can monitor a high impedance circuit without loading it down, and give a low impedance copy of that voltage that can drive a relay or led directly.
op amps are also useful to detect minute differences between two signals and switch from lo to high as the voltages cross each other, this is called the comparator mode.
Thousands of other circuit variations allow op amps to be the most versatile analog component.
I use the LM324 as a buffer and comparator in this circuit.
The battery/charger output voltage is divided by 100 as it passes through the 100K,950, and 100ohm pot. 200V will produce 2V which can be calibrated with the 100 ohm pot to be very accurate.
The divided voltage is passed through one of the 4 operational amplifiers used as a buffer amp. This duplicates the divided voltage on the buffer output with up to 20 ma of current available, while not loading the high resistance divider, as the buffer input impedance is way up in the megohm range.
The setpoint voltage which needs to be very stable, is held stable by the two precision voltage reference ICs which hold the voltage supply for the setpoint pot at exactly 4.88v even with the 12V supply operating over a 9-13V range(fan power).
The setpoint pot is a 2k 20 turn pot so the adjustment is smooth and repeatable over the full 270V operating range. The setpoint is also buffered with another of the op amp section.
The two buffered voltages run through identical 10k resistors to a third op amp which compares them and amplifies the difference by over 1,000,000, so only a few microvolts of difference will send the op amp output either to the + supply voltage or to ground.
The battery volts when charging starts will be lower than the setpoint, so the op amp output will be at the max + voltage, until the battery gets to just a few microvolts higher than the setpoint, at which point the op amp output will switch to ground.
The DPDT relay is what switches the 700ma CC supply AC power.
If the op amp output is high (Battery volts < setpoint), the transistor is turned on, but the relay is off, as the power for the relay coil is passing through a NO set of the relay contacts, so even though the circuit is ready to hold the relay on, we must "latch it on by momentarily completing the coil circuit by shorting across the NO contacts which will activate the relay and latch it in the on state. This allows both CC supplies to work in parallel. Once the battery volts gets to the setpoint, the op amp output will switch off, turning off the relay as the transistor switches off.
Once to CC drop-s to 350ma, the battery voltage will drop slightly, causing the op amp comparator to go high, but the relay in the off state has disconnected the path to ground through the contacts and cannot re-activate.
Hope that clarifies how it works.
Last edited by Mike Dabrowski 2000; 03-22-2010 at 12:23 PM.
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