Sine wave power inverter

[sougata]

Dear Sam,

These are the interfacing circuit to do the following in order of your doc.

1. Mains Sense Circuit: Comes from a small step down transformer. The rectified DC voltage is an average representation of the Mains Input Voltage. AN0 is used to sample this pin and decide the status of the mains input. That is within range or outside range. If out of range switches to inverter mode. While in inverter mode used to sense mains restore condition. So this is part of a periodic ADC routine.

2. Battery Sensing Circuit: Used to sense the battery voltage for deep discharge cut-off and charging state change (Boost/Trickle/Equalize). So this is also used in both the inverter/mains-standby mode. Used AN1

3. Charger Circuit: An inverting amplifier. Note that the same shunt (generally in the low side negative battery terminal) is used to measure battery charge and discharge current. The ADC onbaord 16F72 is 8 bit. If a signed AD was performed by offsetting the signal then you loose one more bit and have 7 bit AD for the current sense. So to channels are used with two amplifiers. This is negative battery dischargei.e., current is being put into the battery, thus the inverting amplifier. Phew......

This goes to AN4 and is used only when charging the battery.

4. Load Sensing Circuit : Same as above but for discharge. So a non-inverting amplifier. connected to AN2. Used for overload protection.

5. Output Voltage Sensing: This either comes from a sense winding on the inverter transformer or a small step down transformer (better) connected to the filtered output of the inverter output. This is the voltage feedback for correcting voltage under different load/battery condition. Average DC represents the average output voltage. No in cycle distortion correction possible. (Not within the scope of 16F @ 20MHz, Needs a 18F @ 40MHz)

6. Power Supply Circuit : Your PIC runs on 5 Volts.

7. Sync Pulse Circuit: PORTB.0 INT based zero cross over sensing. Can measure line frequency. Can detect abrupt mains failure (UPS mode). Can do phase angle control on charge circuit. Can detect mains restore. Important for grid connected inverter, where you need to sync your timebase to the mains.


Assuming the above circuits have been ripped from common inverter design prevalent in India. I don't know why people are still stuck with the 16F72 when there are newer introduction like PIC18F1330. I could upgrade my design. Who would buy it ??

This design is far from complete. You need to design the driver stages. Then decide on the software. In India most designs run on software PWM using the compare module. I don't know why ? But they are commercially successful. .... Well I am not.


P.S. - Sorry Sam, I cannot make the code public. (People will come to know about my bad programming skill). I only dream of making an open source sinewave inverter, in PBP. Can't find time. Cause I need to come up with a fresh new design to avoid any legal obligations on my old design.