1 week Ago By Steve Bush
Without making it – so bare that in mind – the circuit might be the improved circuit. The TLVH431 (labelled ‘LMV431’ in diagram) will work with 1.24V across its anode and cathode, and regulates to get its control input to 1.
24V. This circuit relies on the combined forward drop of the red led and the TLVH431 to be less than 3.0V – other colours of led might/will not work, and the set voltage might need to be increased to 3.
1V rather than 3.0V to get the red version working. The two resistors forming a potentiometer to the right of the TLVH431 should be chosen to regulate the top point to 3V when the mid-point is at 1.
24V, which will deliver a sharply-regulated 3.0V to the opto-isolator’s led and its 390 resistor, giving a constant current through the opto led, dropping slowly below 3.0V.
Putting the red led inside the feedback loop means that the red led will transition quite sharply from on to off as available voltage from the battery goes downwards through the region of 3V. The resistor in parallel with the led is needed to keep the TLVH431 operating when the red led’s voltage drop is significantly reducing its current flow – it needs at least 100μA, and this should be available all the way down to 2.5V from the battery (so 12k max, I think).
As the max recommended current through this device is 70mA, all of the circuit will be well within spec. However TLVH431 seems to be hard to get hold of. LMV431, on the other hand, seems much easier to get, but its abs max is 30mA, and max recommended is 15mA, making the circuit possible, but marginal if the circuit is operated when on charge (it is not intended to be charged when operating as part of the isolation will be lost).
Using a blue led in the ( ) instead of the red led and the two silicon diodes was an idea from yesterday. This still seems possible, but would have to be tried as blue leds seem to start conducting at 2.4V, and have a soggier characteristic (higher equivalent series resistance) that red leds.
Whether that blue equivalent series resistance is higher than a red led equivalent series resistance plus the equivalent series resistance of two silicon diodes I do not know – I can look at the V-I curves, but that does not reveal the visible dimming effect in the circuit. If anyone tries a blue led instead of the red led and two diodes, please reveal all in the comments. The soggy red circuit is good enough for me 🙂 *’New Improved’ is ironic.
It just made me smile to type it in a click-baity way while pretending to be a proper blogger.
