Hi all,

I'm trying to design a circuit in LTSpice with a voltage source and a switch that should be turned on only when the outputs of the voltage source reaches a peak (maximum and minimum --> see attached image) and it should be closed only for a very limited amount of time (and in order to let the tension be inverted). Now, in order to do so I was thinking of using a peak detector. Should it be a good idea or should I think of something else?

Thank you in advance!

 

Depends what you are trying to do. If you knew the maximum peak voltage you are trying to detect then you could use a simple voltage comparator and a potential divider to set the reference level.

Setting a one-shot pulse on detection can be done using the classic 555 timer in monostable configuration

 

Hi let,
Please post your LTSpice asc file so that we can run your sim.
E

 

Here is the .asc file

(don't pay too much attention to the parameters of the voltage source that controls the voltage controlled switch)

 

What exactly are you trying to do with the inductor and this circuit?
You can't place a switch in series with an inductor and shut if off without creating some very high voltages.

 

my problem is another one. In general, how can I set the switch to operate only when peak voltages are detected?

 

You can use a behavioral voltage source (bv) to operate the switch.

 

You can use a behavioral voltage source (bv) to operate the switch.

But with the behavioural voltage source I always have to set the parameter in order to detect the maximum voltage, that means that I have to explicitly tell the switch under which circumstances operate, is that right? My voltage peaks can be very random so I wanted to find a way in order to let the switch operate automatically (if it's possible....) that's why I was thinking of using a peak detector (or maybe a comparator...).

 

hi let,
Check the Help menu for the Cross function.
eg
.meas TRAN res FIND V(Vt) WHEN V(Vt) = v(ref) CROSS =1

E

 

My voltage peaks can be very random

If the frequency is constant it would perhaps be simpler to detect zero-crossings and operate the switch a quarter-cycle after each zero-crossing.

 

If the frequency is constant it would perhaps be simpler to detect zero-crossings and operate the switch a quarter-cycle after each zero-crossing.

sounds interesting! Practically speaking how do I do that?

 

sounds interesting! Practically speaking how do I do that?

You can make a zero crossing detector with a comparator. Just set your voltage reference to zero volts (ground) on one input, and put your signal voltage on the other input. Everytime the signal voltage goes to 0V the output will be high. The output will stay high as long as the input is positive, and is sometimes known as a sine to square wave generator.

The timing pulse can be set with a monostable 555.

 

Here's a way of using a comparator for sensing both positive and negative peaks, even if their amplitude is variable. The peak detector output can be inverted as necessary for triggering a 555 monostable:

V1, V2 and A1 are just for the simulation. R1 and C1 give a slight delay to one of the two inputs of the comparator. Their values are not critical.

 

Ok thank you very much, I will try if it works with my model!! Is it possible to do a similar model with Simulink too? Would it be easier?

 

For anyone interested, here is my version of a zero crossing detector. Instead of a square wave it gives a short pulse which can be monitored with a microcontroller.

Excuse the rubbish oscilloscope trace, just for illustrative purposes. You can see when the square wave crosses the zero point there is a short spike which is the output of the circuit. I have attached the pdf schematic and a multisim file (zipped) so you can simulate the circuit.

Additional:
You can simply increase the value of the capacitor to increase the length of the output pulse

 

Is it possible to do a similar model with Simulink too? Would it be easier?

I've never used Simulink, so can't say..