This is circuit Charger battery Lithium Variable current up to 2A by IC L200. I’ve always wanted a fully adjustable current DIY lipo charger (and recently higher output).
The L200 datasheet has an LM741 op amp design to achieve this. It works well but has a very non-linear action (worse with a log pot). So with James Hopper’s help I have tweaked this design to produce near linear current adjustment with a standard linear pot. In my version, current is variable from about 100mA – 2A. The circuit is slightly more complex but is nicer to use. Here it is in EAGLE and image formats below.
R3 is a current sensing resistor which feeds the LM741 with a voltage that varies according to current flow. This gets amplified by the op amp which tells the L200 to change current. I have used a 100mm length of 0.5mm insulated copper wire for R3 (= ~0.010 ohms). The precise value is not critical. However, it interacts with the gain of the op amp and affects the minimum charge current and overall current range. Although I have tested with a value half this size (0.005 ohms), R3 should be larger rather than smaller than the 0.010 shown (eg: between 0.010 and 0.100 ohm). A larger R3 will give a smaller starting current.
The gain of the op amp is set by the ratio created by R2 (470ohm), R6 (10k pot) and R7 (250k trimmer). It also depends on the value of R3 above. If you want a lower start current it is best to increase the value of R3 as suggested above. However, reducing R2 (say to 220ohms) will have a similar effect.
L200’s are internally limited to about 2A and this circuit makes an obvious correction if you go over this (you are likely to see current suddenly drop a bit as you rotate the pot over the 2A level). R7 is used to fine-tune the op amp’s gain and I recommend you adjust it so that max current can only get to just under 2A. If you can’t find a 250k trim pot then use a 100k trimmer with 150k resistor in series between R6 and R7 (eg: as shown in the first photo above). R7 does not require great precision so a single turn trimmer is fine.
The output voltage is set by the combination of R4 (1k) and R5 (5k trimmer). These values are suitable for 3 lipos (12.6v). A multi-turn trimmer is not essential but it does make it much easier to set the voltage accurately. A 10k trimmer is also OK.
The L200/LM741 design allows some leakage from cells connected while the circuit is not powered. This leakage is <20mA so is not very significant. Nevertheless, D1 prevents this loss, but more importantly protects the circuit from reverse polarity and any disasters that this may cause. It is therefore optional but I think strongly recommended. D1 would normally be a Schottky diode. It needs to be rated in excess of the circuit’s output current and voltage, and it is desirable to have a low a forward voltage drop and high reverse current capability. I’m using SMT versions which although small are not hard to use. Mine are rated at 3.5A, 30v, 0.35v and 535A. Not shown, but it would also be normal practice to protect the input from reverse polarity if it is being connected directly to a supply that can be reversed.
The L200 needs an input voltage that is at least 2v higher than the output to be able to yield the full rated output. You also need sufficient volts to cover the forward voltage drop of D1. With a 0.35V D1 drop, the input to the L200 needs to be 2.35V higher than the desired output (eg: 14.95v in for 12.6v out, perfect for a 15v supply…).
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