Discussion in 'Electronic Basics' started by max w., Feb 10, 2007.

1. ### max w.Guest

I see these terms often, but I haven't seen a description between the
two that I
am comfortable with. Or better yet an example an example of each.
Any help would be appreciated,
max w.

2. ### Tom BiasiGuest

First do you understand the properties of resistance and capacitance?
A load is the part of the circuit that is dissipating the power.
You say you see these terms often, where?
AC power generators, opamps?

Tom

3. ### max w.Guest

Yes, I understand the properties of resistance and capacitance.
I am trying to find out what they mean when it is applied to digital
electronics.
Thanks

4. ### Homer J SimpsonGuest

Pretty well self evident. If you are driving a long wire for example, co-ax
or the like, it is likely to have significant capacitance. You need to allow
for this.

5. ### max w.Guest

I understand that part. Let me explain it like this, and maybe my
conceptual idea is
wrong. A digital driver can drive so many pf. The input pin of the
capacitance. The total capacitance of the input pins cannot exceed the
strength of
have slows
the driven signal down. I assume this is the capacitve aspect. If this
assumption is
correct, what is an example of a resistive load on a PCB? (aside from
a resistor)
Thanks

6. ### John FieldsGuest

---
Since there's always _some_ series resistance inherent in the output
of any driver, the time it will take for the output to rise to a
certain voltage will depend on the resistance of the source (the

Assuming your circuit looks like this: (View in Courier)

VCC>------->\ <---GND
\
O
|
[Rs]
|
+----+--->Vout
| |
[RL] [CL]
| |
GND>----------+----+--->GND

If RL is >> Rs, then, since:

T = Rs CL

you can see that the time it takes for Vout to rise to a particular
voltage will increase as CL increases.

An example of a resistive load would be a coaxial cable terminated
with a resistive load equal to the source resistance and the cable
impedance:

VCC>-------->\ <---GND
\
O
|
[Rs]
|
GND>---------+ | |
| | |
| | |
| | |
| | |
| | |
| | |
GND>---------+ | +----->GND
|
[RL
|
+------->VOUT

7. ### mgGuest

For a back-of-the-envelope calculation, capacitances in parallel add
together. So, for example, if you had 2 loads of 15pf, the total would
be 30pf. Drivers have a characteristic output impedance. Transmission
lines (board traces) have a characteristic impedance. Loads
(receivers) have a characteristic impedance. Impedance is defined as
the ohmic resistance plus the net reactance, which can be either
capacity or inductive, or can equal zero. In the case where the net
reactance is zero, you would have a purely resistive load.

In situations where the source, line and load impedances are not
equal, there will be a reflection of the signal. In other words, when
the source sends a signal to the load, the signal will bounce back to
the source and distort the signal (i.e. make it less square). The
exception to this rule is when the length of the transmission line is
small compared to the wave length of the signal.

For information on impedance and impedance matching, you could do a
search on Google, or find a book on the subject.

8. ### John PopelishGuest

A resistive load passes current in proportion to the
instantaneous voltage. If there is voltage across the load,
it is passing current, regardless of time.

A capacitive load pases current when the voltage across it
changes, and in proportion to how fast it is changing. Any

So resistive loads draw current throughout the time when the
source applies voltage to it. The capacitive load draws a
spike of current when the source tries to step the voltage,
and slows the rise time of the step, if the source has
series resistance.

If the load is a resistance and capacitance in parallel,
then the source must supply both currents simultaneously.

If the load is a series combination of resistance and
capacitance, the capacitor blocks any DC current, and the
resistor limits the current that can be passed during any
fast rate of rise or fall, but extending the time the
current passes, after the step in voltage has stopped
changing. There will be and exponential decay of current as
the resistor drop shrinks, and the capacitor sees the fill  