PSU - Overload and underload
Load = What gets connected to the power supply's output lines.
Overload
Most people with a basic knowledge of electronics know that you must not overload a power supply. If you do, a fuse will blow somewhere, or maybe the power supply will sense the overload and shut down.
Overloads are generally in two categories:
1. To much (functional) load is connected; or
2. The load is faulty in a way that results in the load drawing much more power than normal.
Underload
But the power supplies used in vintage computers of the 1980's and 1990's typically (i.e. not always) also shut down if they are underloaded (not enough load connected).
So, if you have one of those power supplies, what is enough load to stop an unload condition ?
There is no single answer. Why? Because each make-model of power supply has its own 'minimum loading' requirements. And that is not something that is usually printed on the label of the power supply. Usually, it is in the technical document for the power supply.
Example #1 - ACME model 1234: Minimum loading is 7W on the +5V line.
Example #2 - Westinghouse model 78A: Minimum loading is 10W on the +5V line, and 3W on the +12V line.
So, no technical document/information for particular make-model of power supply can create a problem for someone. Imagine someone removing the power supply from their, say, XT clone, putting it on the table, and then wanting to test the power supply. They, for example, might connect a floppy drive to the power supply, and observe no output from the power supply. The question then becomes, which of the following is the case:
- No output because the power supply is faulty; or
- Underload: No output because the floppy drive is not an adequate load for the particular make-model of power supply; or
- Overload: No output because the floppy drive is what is at fault, overloading the good power supply.
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