Hi there

I've been playing with all kinds of circuits to limit the current when
charging a flat lead acid battery. I've been able to construct a 'dumb'
one with a HEXFET and quad opamp in a lowside configuration
thanks to great advice from the power electronics guru Bob Blick
himself. I also came up with a nice little high-side circuit driven from
a PIC12C508 and a quad opamp.

>From an adaptability point of view, I decided to go for the PIC
solution.

Background :
----------------
Supply is (Vsup) goes to the drain (D) of an IRF530. The source
(S) is connected to the positive side of the battery. The gate (G) is
pulled low with a 4k7 resistor.

To switch G, it is pulled high with a PNP tranny switch by a NPN,
which is switched by the PIC. As the current from D to S increases,
so does the voltage drop over DS because of the (low) internal
channel resistance. Thus the voltage on the positive battery terminal
dips slightly (150mV @ 1.5A typical) when it supplies current. By
measureing this drop, you can know what the current is.

To get this voltage to a more measureable level, you divide Vdrain,
as well as Vsource with similar voltage dividers to get the levels
down to a volt or two. This difference is then amplified by one
opamp in a LM324. The amplified level is compared by two other
opamps. The one will indicate 'overcurrent', and the other will
indicate 'undercurrent'. These outputs are fed to the PIC.

By now all the Bob Blicks of this wolrd will have wondered how
the heck I keep the FET switched on hard. I use two pins as a
charge pump to supply the gate tranny with a voltage 5V higher
than Vsupply. It works well.

To limit the current, I use a 4 bit (16 resolution) pulse train with the
high time variable from 0 (no current) to 15 full current. If the
'overcurrent' input is high, I decrease the high time. If the
'undercurrent' is high, I increase the high time. Every now and again
I invert the two charge pump pins to keep the gate switching supply
up.

When the current direction reverses, the PIC sees it as a very low
load, and swithes the FET on hard.

The Challenge :
---------------------
To get a gate switching frequency between 20kHz and 30kHz using
the 4MHz internal oscillator. This is not trivial and will require a good
few assembly tricks. With non-optimal C code I've been able to get
around 1.5kHz, but (as you can immagine) the circuit screams like a
banshee when you draw a few amps. The FET stays cold even at
loads up to 1.5A!

I AM NOT INTERESTED IN A BATTERY CHARGER CHIP.
It defeats the object of the challenge, and is, above all, 3X the
cost. There is also a diode in the return path which is a killer in low
voltage operation.

I will post the full solution with schematics, source code, and full
credits on my web page.

--
Friendly Regards

Tjaart van der Walt
mailto:tjaart@wasp.co.za

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