[SOLVED] compare two voltages and select the maximum one...

[flote21]

Hello guys,

I need to build an analog circuit without any uP, FPGA or programmable logic device, to compare to input voltages, and provide to the ouput the maximum one. Anyone can tell me any recommendation to do it?

Thanks in advance for your help!

 

[KlausST]

Hi,

feed both analog inputs to a comparator.
feed both analog inputs to a SPDT analog switch.
Control the switch with the comparator output.

Klaus

 

[crutschow]

What are the voltage levels and are they AC or DC?

 

[schmitt trigger]

A pair of diodes.
Each anode connected to one of the voltages.
The two cathodes tied together and then to the load.

EDIT: with a dual-diode package, you can do it with a single device.

 

[crutschow]

A pair of diodes.
Each anode connected to one of the voltages.
The two cathodes tied together and then to the load.

EDIT: with a dual-diode package, you can do it with a single device.

As long as the diode drop to the load is not a problem.

 

[E-design]

We need to have more information if the resultant voltage needs to power something, or if it is only for monitoring purposes.
If it is just for monitoring, the diode drop can be compensated with an op-amp, provided the voltage levels is within the limits of the op-amp.

 

[flote21]

Hello everyone!
It is to monitoring the DC voltage provided by a PTC thermistor.
Thanks for the ideas

 

[E-design]

You perhaps can use this as a starting point. Depending on the values of the PTCs, the input voltages (VG1, VG2) derived from them may need buffering by using two additional opamps.

The attachment shows only one circuit, but illustrating a DC and variable signal example.

 

[flote21]

Hi it looks a nice solution. however I dont understand very well the finality of the SPDT, because without it, I can get the same results:





Greetings

 

[KlausST]

Hi,

I think there is a misunderstanding.

You said "select" ... for me this means you don't generate a new signal, but choose either the one or the other.
The load current then is supplied by the input signals.

With your circuit you generate a new signal that is the higher value of both input signals.
No input signal is loaded..

Klaus

 

[FvM]

Both solutions are feasible, also the #9 circuit is loading the input with 100k pull down resistor. Problem of this thread is lack of specification, e.g. tolerable input current and voltage error, expected output impedance.

Apparently flote21 has found a solution according to his implicit requirements, so the thread should be marked as solved.

 

[E-design]

I dont understand very well the finality of the SPDT, because without it, I can get the same results:

The switch was just to show that with a fixed DC input, the circuit compensates for the diode drop. In 8 V, out 8 V.

 

[crutschow]

Here's a simple max value circuit that should do what you want.
It's output is the highest of the input voltages and it has a high input impedance.
The diodes are inside the feedback loop so have no significant effect on the input-output voltage offset.
In your case you only need a two input circuit with two op amps.

 

[flote21]

Hello!!
thanks for all the answers. I liked also the last one to remove the offset of the diodes.