- [Instructor] Lets take a look at my solution for
the challenge to protect my thermostat from
a short circuiting power supply.
If one of the supplies failed and turns into a short circuit
current from the other supply would
flow backwards through it.
That's a reverse current which
I can easily stop with a diode.
If I place a reverse current protection diode between
the two power supplies like this, it'll block that
reverse current coming from the right supply,
from flowing through the short circuit on the left
to keep my thermostat running.
However, if it was the power supply on the right that failed
instead of the one on the left, that diode wouldn't help.
Because current from the working supply on the left
would flow straight through the broken supply on the right.
To fully protect this system from short circuiting
power supplies, I need two diodes to protect against
reverse current, one for each supply.
When the supplies are functioning correctly,
both of these diodes will be forward biased,
so current will flow from the power supplies
to my device more or less freely.
If either one of the supplies fails and internally
short circuits, the diode protecting it will become
reverse biased and block current from flowing backwards
through the short.
The other power supply can fully support the thermostat,
so the system continues to function.
Since I'm placing these protection diodes in series
with the power sources, I want to minimize the amount
of supply voltage that I loose to the forward voltage
drop across the diode.
Which makes this a good time to use Schottcky diodes,
which have a low forward voltage
and a large reverse breakdown voltage.