I am exploring the possibility of a Picaxe controlled 12V sealed lead-acid (SLA) battery charger, so need to provide a regulated voltage to a low impedance load. The scheme suggested in a previous thread ("Driving low impedance with PWMOUT" seems to employ the power transistor (MOSFET or bipolar) in linear mode.
Would it be possible/better to use a variable duty cycle PWM input to the MOSFET which would then avoid the power which needed to be dissipated since it would cycle between fully on (low resistance) and fully off (infinite resistance).
Clearly there would need to be some smoothing of the ADC feedback to the Picaxe, but I suppose that this could be a simple resistor between the MOSFET drain and ADC input and a capacitor from ADC input to ground.
What about the smoothing of the main power to the battery being charged? Perhaps not necessary at all with a PWM frequency of perhaps 5kHz?. I notice that an inductor is commonly used for such smoothing (Buck converter?), but wouldn't a simple capacitor to ground achieve the same?
My proposed scheme is:
1. Set PWM duty cycle to 0 initially
2. Let everything settle for a few seconds
3. Measure the output voltage (suitable divided to ensure <5V)
4. Increment duty cycle by +1 or -1 if measured voltage lower or higher than desired (14.5V?)
5. Pause for perhaps 100ms to let things settle again
6. Return to step 1
To terminate (or continue trickle charging at 13.6V) when the battery is fully charged, I propose setting and holding PWM to 0 once every minute or so to measure the battery voltage for comparison to a 14.1V threshold.
I would appreciate any opinions as to whether my thinking is basically sound or not.
Would it be possible/better to use a variable duty cycle PWM input to the MOSFET which would then avoid the power which needed to be dissipated since it would cycle between fully on (low resistance) and fully off (infinite resistance).
Clearly there would need to be some smoothing of the ADC feedback to the Picaxe, but I suppose that this could be a simple resistor between the MOSFET drain and ADC input and a capacitor from ADC input to ground.
What about the smoothing of the main power to the battery being charged? Perhaps not necessary at all with a PWM frequency of perhaps 5kHz?. I notice that an inductor is commonly used for such smoothing (Buck converter?), but wouldn't a simple capacitor to ground achieve the same?
My proposed scheme is:
1. Set PWM duty cycle to 0 initially
2. Let everything settle for a few seconds
3. Measure the output voltage (suitable divided to ensure <5V)
4. Increment duty cycle by +1 or -1 if measured voltage lower or higher than desired (14.5V?)
5. Pause for perhaps 100ms to let things settle again
6. Return to step 1
To terminate (or continue trickle charging at 13.6V) when the battery is fully charged, I propose setting and holding PWM to 0 once every minute or so to measure the battery voltage for comparison to a 14.1V threshold.
I would appreciate any opinions as to whether my thinking is basically sound or not.