Hi, I’d like us to analyse this circuit together, which simulates a ‘hard reset’.

The input Vi is a button that is held down for approximately 5 seconds.
The output Vout1 remains high for the entire duration that the button is pressed.
The input out1Filt to the second comparator is an RC circuit with a time constant of 4.2 seconds che si carica tramite R4 e C1//C2, which is compared with a resistive divider (set to 0.7) on the "+" input of the second comparator.

In theory, I would like Vout2 to switch after RC = 4.2 seconds.

R5 is there to allow the capacitors to discharge over the next few minutes.

The comparator is chosen at random; we can assume it is ‘ideal’ – I just want to understand the circuit.





1) Look the input out1Filt .. after 3.2 seconds it reaches 13V... but why doesn’t it reach 15.2V (0.63 × 24V) after R*C = 4.2 seconds?

2) In theory, the discharge (controlled by R3 and D1) should be much faster than the charge, but that doesn’t seem to be the case from the drop in input out1Filt

So, is the circuit not working as it should, or have I just got the dimensions wrong?

 

hi andrew,
It appears that the V(outfilt) never exceeds the V(thresh)...??
E

 

hi andrew,
It appears that the V(outfilt) never exceeds the V(thresh)...??
E
View attachment 367371

That's true... but how does it switch the second comparator, then?

 

Look the input out1Filt .. after 3.2 seconds it reaches 13V... but why doesn’t it reach 15.2V (0.63 × 24V) after R*C = 4.2 seconds?

The out1Filt voltage is limited to 1/2 * V3 + 0.7V ~ 12.7V because of the discharge path.

In theory, the discharge (controlled by R3 and D1) should be much faster than the charge, but that doesn’t seem to be the case from the drop in input out1Filt

The discharge path is kind of useless. As soon as out1Filt goes below ~ 12.7V the diode D1 in the discharge path closes and C1/C2 are discharged by R4||R5.

 

The discharge path is kind of useless. As soon as out1Filt goes below ~ 12.7V the diode D1 in the discharge path closes and C1/C2 are discharged by R4||R5

I don't understand this sentence; you say it's useless, but then you say D1 in the discharge path closes, so is it necessary(?)
Can you explain again?

Once the discharge path is removed, it reaches the threshold, but I still don’t understand why the output V(out2) doesn’t switch.. it seems to be following its own course

 

The out1Filt voltage is limited to 1/2 * V3 + 0.7V ~ 12.7V because of the discharge path.

OK, so it should be limited to 12.7V… but why there are 13V in the simulation (red line)?