Floating vs Grounded in circuit. Problem and questions.

Thread Starter

mos_6502

Joined Dec 11, 2017
28
What is the difference from Grounded and floating in a circuit?

In my example circuit, if the S1 and S2 are open, the OUT point is always to zero volt also if I elimitate the R1??
 

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ericgibbs

Joined Jan 29, 2010
8,715
Hi mos,
Consider if the output was connected to say a CMOS logic input , allowing it to 'float' would be a bad idea.

E
 

Thread Starter

mos_6502

Joined Dec 11, 2017
28
Hi Ericgibbs,

yes, the output could be connected directly to a single Mosfet or to a CMOS circuit.

Could you explain to me why it is not a good idea to remove R1?
 

nsaspook

Joined Aug 27, 2009
6,256
Hi Ericgibbs,

yes, the output could be connected directly to a single Mosfet or to a CMOS circuit.

Could you explain to me why it is not a good idea to remove R1?
There a a couple of reasons. Here is one.

Your wiring is an antenna for electrical energy. With a very high input impedance even small amounts of transient electrical energy could result in logic voltage levels that could trigger gate inputs. With a low value damping resistor the electrical energy level needed for a logic level voltage signal increases greatly because that transient electrical energy will flow into the resistor path and reduce the voltage for the same amount of electrical energy while the proper signal source and easily supply the needed energy for correct voltage levels.
 

Thread Starter

mos_6502

Joined Dec 11, 2017
28
This is interesting.

But how is it possible to establish how much the potential could accumulate to activate the next circuit?What if the next mosfet has a fairly high threshold voltage?

Excuse the silly questions, but I don't have much experience with these things.
 

nsaspook

Joined Aug 27, 2009
6,256
This is interesting.

But how is it possible to establish how much the potential could accumulate to activate the next circuit?What if the next mosfet has a fairly high threshold voltage?

Excuse the silly questions, but I don't have much experience with these things.
Having a high threshold voltage helps but you still want to reduce the unwanted signal level. If the transient signal can be modeled as a current source then you can calculate the potential with X resistance.
 

BobTPH

Joined Jun 5, 2013
2,016
A MOSFET gate is a capacitor. If floating, it will stay at the same voltage it was at before. In your circuit, if the gate is high, and you open both of the switches, the capacitor will discharge through the resistor, turning the MOSFET off. Without the resistor, it will remain on for some undetermined length of time.

Bob
 
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