555 Circuit used in battery desulfator

[Raff]

Hi everyone,

I have been experimenting with lead acid battery desulfators as I have quite a few old batteries around here that wont take a charge. I have built 2 so far for friends and another for myself, it so far has worked surprisingly well I must say.

Something about the circuit has me confused. Just how does the 555 timer circuit work? Just how does the capacitor charge?  It isnt like any 555 circuit Ive ever seen before. Having said that, it does eem to work. 

Thanks,
Raff.

[c4757p]

The 555 timer is a very simple circuit with just a couple comparators and a latch. I'm sure you can figure it out if you just draw that circuit with the 555 internal circuitry in place.

555 Schematic

[Raff]

Hi c4757p,
Thanks for the reply. I drew up the circuit as you suggested using the internal diagram of the good old 555, but it still doesnt makes sense to me. The cap is connected at one end to Vcc, the other end to pins 2 and 6 which are connected together and inside the 555 are inputs to op amps, which I would imagine are a high impedance. The only path to deck that I can see is via the 270k resistor, and then the internal transistor on the 555 discharge pin which is also a high impedance. Strangely the 15k resistor (which has one end to Vcc) is grounded by the discharge pin to deck as well. I just cant wrap my head around how it is supposed to work. Sorry to be a pain.

regards,
Raff

[jeroen74]

Quick analysis, could be wrong

When the circuit starts up the voltage across C3 is zero volts. Hence, TRG and THRES are at VCC level. This ends the interval, turning on the DIS pin, connecting it to ground, slowly charging the capacitor through R4. Then TRG/THRES fall below the other level, which starts the interval, turning off the DIS pin, and it's becomes high impedance. The capacitor is then discharged through R2.

The confusion I guess arises from the fact the capacitor is referenced from VCC, not the usual GND, so all the polarities are swapped.

[Raff]

Hi jeroen,

I think youve got it there, I can see how that would work now many thanks. Its just Ive never seen a 555 wired that way before. I think the part I missed is that as the cap is initially discharged when power is applied, it appears as a short circuit placing Vcc on pins 2 and 6 and thus starting the process up so to speak.

Regards,
Raff