■ FIGURE 3. Linear regulator test data.
resistor (R4, 5, 6, 7 in parallel) gives us
very similar simulation results to the
hands-on experiment (Figure 3 again).
On both the breadboard and the
simulation, we forced the regulator into
current limiting by changing the load
resistor from 100 ohms down to 25 or
so — which I made from four parallel 100
ohm resistors. I chose an LM78L05 with
its 100 mA load current spec instead
of the more popular one amp LM7805
for demonstration. To force an
LM78L05 into current limit, we must
draw a minimum of 140 mA, so once
again we are taxing our 9V battery
(make a quick reading and remove the
battery ASAP). If all you have is a plain
Jane LM7805, you likely will kill the
9V battery long before reaching the
regulator’s current limit (of two amps)!
Although we ran the LTspice
simulation twice by changing the load
resistor, there are other ways to get
there. SPICE has a very handy
technique to sweep the voltage source
(i.e., our 9V battery equivalent) and
plot a time variable graph of the
circuit’s output. In effect, we can see
how the circuit behaves at power turn
on. To do so, change the spice
directive from “.op” (DC operating
point) to “.tran 100m startup” which
tells the simulation to start at zero and
run for 100 ms; the battery has been
changed to a voltage source that starts
at zero and ramps up in 50 ms. First,
using the 100R load, look at Figure 5.
Doing this with 25 ohms on the
■ FIGURE 4. Schematic for
NV_SPICE_ 21.as c.
output causes a current overload. We
observe the regulator changing to a
current-limited output, shown in Figure 6.
NV78L05 And NV7805
Our LTspice schematic uses a
simple voltage regulator symbol with
three terminals (Vin, Vout, and Comm).
Inside, the real IC part is quite complex
with more than a dozen transistors
and several diodes, as shown in Figure
7. This diagram was lifted from the
datasheet for the LM78L0x family, as
used in our solderless breadboard circuit.
The popular LM780x (a one amp
minimum output part) diagram is
shown in Figure 8. Both were
designed and fabricated in a bipolar
transistor process, although more
modern ICs will likely use FET
transistors instead. The real datasheets
for these commercial parts are
available from the Nuts & Volts
website ( www.nutsvolts.com) as part
of the download for this article.
Notice that I’ve “tuned” these
LTspice models a bit because some
component values were missing and
■ FIGURE 5. Graph plot for 100R load.
■ FIGURE 6. Graph plot for 25R load.
January 2009 55