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Hi Irfan,
I am actually building a few SLA chargers at the moment, both with LM317 and also a 5 amp version with LM338T which works in a very similar way. I have adjusted the schematic to show more clearly what it should look like in order to work correctly.
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For clarity, the green LED connection is shown not connected to the base of T1 as some people have mistaken that in the original.
R4 has been made 10 Ohms and ZD is now connected via a link to the battery side of R4 where it will sense the voltage on the battery terminal as is required. I notice that R2 is maybe a little low compared to the datasheet and I would increase it to the 240 Ohms suggested as original spec.
Setting up is now a matter of having the whole circuit powered up with no load and adjusting VR1 to give 14.2 volts (or what is suggested on the datasheet for your particular battery) at the positive battery terminal. To check the cut-off works, simply connect the LINK and if a fuly charged battery is now connected, TR1 will pull the adjustment pin low and the output from the regulator will fall.
With a battery under charge, the current flowing through R4 will give a voltage drop, as the battery becomes charged, so the voltage across the resistor is less and ZD will eventually conduct, switching off the charge current. The red LED will stay lit because it is being fed by the current from the adjustment pin but the drain on the battery is really tiny.
I have not actually built this circuit yet but have a few very similar that work well.
As Savio has noticed, the 8.2 volt value for ZD does seem a little low to get a switch-off at 14.2 as shown here. You may need to increase that and a quick guess would be 1.2 volt across the LED then another volt base-emmitter on T1 is only going to give you a little more than 10 volts at best before the zener kicks in and shuts the thing down. You may find it needs to be 12 volts to work to a full charge but as I say, I haven’t tested this yet.
Let me know how you get on with these changes.
I remember making something very similar to this a very long time ago! (OK I’m old
)The simplest rain drop sensor is a small strip of ‘Veroboard’, or even two strips of un-insulated wire wrapped on a piece of wood works just as well for experimentation purposes. The conductors become a resistor when a droplet of water bridges the gap between them. It’s the effect you get when you drop a cellphone in a puddle, things short out.
So to make a sensor work, all you need is that basic device and some means to detect it,.. a simple op-amp would do that.
Actually, NOT gates and Hex inverters are the same thing. Well almost!
A NOT gate is a buffer that inverts the output. A simple buffer does not invert and the NOT gate has a small circle on the output so imagine that as a little wheel that spins everything 180 degrees, it has one input and one output but putting just one such device in a chip would be hugly uneconomical so they include several.
So, a ‘Hex Inverter’ is just six NOT gates in a single package which of course makes better use of the space both in the chip, and also on the board.
Where a circuit has a couple of these NOT gates in series is often where a signal or pulse is required to have a small delay before it meets the next stage, which is only in the order of a few nanoseconds but enough maybe for another pulse of say synch or reset of a previous state to take effect. This is because everything you put in a signal path will give it some element of delay, however small. when you are dealing with high frequency or fast data rates, these delays can be useful or a complete nightmare so having a good dual beam oscilloscope when nothing seems to be working, often shows up where something turns up too early, or late.
In the particular circuit shown here, there needs to be some buffer but as non-inverting buffers tend to be electrically fragile, they have used two inverters in series (probably out of a 74HC04 or similar) for each line and the other two on the chip are simply not used.
I don’t know if you already solved this problem, but a quick look at the schematic shows that the zener diode is shown the WRONG side of the resistor R4.
That R4 value is stated as 100 Ohms, I would think that should be nearer 10 Ohms with the zener diode connected to a point closest to the battery lead. This connection then picks up the voltage drop across R4 and when the battery is charged, the diode conducts and switches on TR1. Where the original schematic shows this as connected to the side of the resistor that will always be the ‘fully charged’ voltage, which of course it isn’t, is it!
In the way this is shown, the green LED is merely connected from the output of the LM317 to 0 volts and is an indicator that the circuit is powered, not neccesarily charging, you should not connect the charger to the mains until AFTER you have connected the battery. Sparks look pretty but not in this circuit.
As any silicon device is the ‘fastest fuse on the planet’ I would suggest adding a fuse in the output lead rated at the maximum value for an LM317 which according to the datasheet is 2 Amps but not for very long! A 2 amp slo-blo fuse is cheaper than new chips in the event of a short circuit or a seriously dead battery.
The red LED should be on when T1 is switched on as the battery terminal voltage reaches the required potential. If you are charging a lead-acid battery to full capacity, this would be 14 volts approx depending on the battery charging requirements. if you wish to retain a ‘float’ charge, that voltage should be reduced to 13.2 volts
That voltage is set by VR1 with TR1 on the off state. To set this situation up, simply disconnect the zener diode and adjust VR1 for 14.2 volts at the junction of D3 – R4 (or 13.2 for float charging), then reconnect the zener diode (to the juncton ot R4 – battery terminal !) and with no load, TR1 will immediately turn on, the red LED will light and the output voltage will drop. Because there is little or no current flowing through R4, both sides will be at the same potential.
Now if you charge a battery, it will work correctly and indicate the state of charge.
I should point out that this charger is capable of 1.5 Amps and as a LA battery should be charged at 1/10 of the Ah rated, this charger will work for batteries up to 15 Ah if they are used in cyclic mode (heavy discharge) OR as a float charge for any LA battery.
