# What is load capacitance

Discussion in 'Electronic Basics' started by Wong, Sep 17, 2003.

1. ### WongGuest

Hi,
What is load capacitance ? How does it affect the circuit ?
Thanks.

2. ### Dana RaymondGuest

Load capacitance is the capacitance seen by the device driving the load.
In the case of OPAMPs too much load capacitance can create problems because
as the device tries to increase the load voltage the capacitance will pull a
lot of current (remember, capacitance causes current to lead voltage).

So, when driving capacitive loads from an opamp you need to ensure that its
within spec or add a series resistor between them to limit that initial
inrush current.

Dana Frank Raymond

3. ### MantraGuest

First, a "load" is an impedance to which power from a circuit is
delivered (and either dissipated in the load or reflected backed to
the source circuit). Impedance is, very approximately, "The AC
equivalent of DC resistance with phase shifting added, both of which
change of over frequency". Examples could include antennas, loud
speakers, AC motors and appliances, lights, logic gate inputs, etc.

If you can assume only linear circuit components are used, you can
reduce *every* circuit, without approximation to just three
components: a power source (voltage or current), a source impedance
and a load impedance. The power source and source impedance
combinations are also known as Thevenin and Norton sources, thus every
circuit can be reduced to two components: a Thevenin/Norton source and
a load impedance. If you can't assume linear, you can still reduce to
three or two, but with certain limitations. Components like
resistors, capacitors and inductors are practically linear.
Semiconductors and vacuum tubes are definitely non-linear, thus there
are limitations.

If the load impedance has a primarily capacitive reactance, or
negative imaginary impedance, then you have capacitive load, which can
be modeled as a load capacitance. The load could also be inductive
(positive imaginary impedance), or resistive (zero imaginary
impedance). You can have combinations also forming
resistive-capacitive or resistive-inductive impedances. I'm assuming
you know complex numbers, or at least, trigonometry and right
triangles: impedance is always the hypoteneuse, resistance one side
and (inductive - capacitive) reactance the other side.

The specific effect on a circuit depends on what the circuit is. In
digital circuits a capacitive load acts to slow down pulse rise times.
In RF/microwave it creates a phase shifted reflection of incident
power back into the power source. In audio it can create a low-pass
filter effect. In power electronics (e.g. AC motors) it creates a
phase shift for starting a motor or for compensating power factor
shift caused by the motor. Sometimes these are intentional effects;
sometimes not. A common thread is that power is not fully used to its
theoretical maximum: only purely resistive loads dissipate any power;
"reactive" power is reflected back.

MM

4. ### SteveGuest

What is load capacitance ? How does it affect the circuit ?

geez... talk about opening a can of worms!

"load capacitance" is simply the capacitive element of a load.

For example, an amplifier circuit driving an 8 ohm speaker will
experience not just the 8 ohm resistive load, but some "inter-turn"
capacitance and some inductance, both due to the coil windings.

Capacitive loads can affect the rise and fall times, the frequency
response and phase of a signal etc. If the capacitance is large
enough, it could even swallow up the signal entirely and just give out
a DC level!

Capacitors present less of a load to high freqency than they do for
low frequency. Thus, a cap to ground acts as an "AC ground" and
effectivly becomes a low pass filter.

What is the circuit you are analysing? That would help people to give
a more precise answer.

nifty

5. ### WongGuest

What is the circuit you are analysing? That would help people to give
I am interesting on load capacitance value for a 32.726KHz crystal.
That frequency is feed to a microcontroller.
Thanks.