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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
Is it safe to assume that all standard transformers, both step up and step down (such as a small project transformer rated 120VAC to 12.6VAC at 3 amps , or an NST that changes 120VAC to 4,000VAC at 30mA) ...are rated with the voltage as open circuit voltage, and the current rating as short circuit current?
The reason I am confused is because I have been told the NST can't supply their rated 30mA at full voltage of 4kV, but my stepdown transformers CAN supply their rated 3amps at 12.6 volts.
Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
No. A few types of transformers (including NSTs, doorbell transformers, most transformers used inside "wall wart" power converters) use internally current limiting by shunting part of the magnetic path between primary and secondary windings. This allows them to safely be operated when short circuited indefinitely without overheating. Because of the high leakage inductance, these transformers have relatively poor voltage regulation under varying current loads.
However, MOST other transformers use considerably tighter primary:secondary coupling. These transformers cannot withstand short circuiting for long without major overheating. These transformers also have better voltage regulation (less voltage variation) under load current changes, and are rated for the given output voltage while supplying the stated output current.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The difference in the two types of transformer is the leakage inductance, but that is almost never specified directly, and can only be inferred from the type and the ratings.
With a power supply transformer, the leakage inductance is minimised, to make the supply as "stiff" as possible. Current through the transformer is intended to be limited simply by the load not drawing too much current. The transformer has low enough losses so that the temperature rise under o/c and rated load conditions stays below a safe level. If the load draws more than the rated current, the transformr will deliver it, and the temperature will rise further, ultimately burning insulation or melting solder connections. Intermittent use at 2x or even very intermiitent at 3x rated output current is quite usual - in our circles at least.
With an NST, the leakage inductance is designed to a specific fairly high value by adding an iron path between the windings, to limit the current into a short circuit load. Invariably, the transformer has enough surface area so that heating under both o/c and s/c conditions does not cause excessive temperature rise.
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Transformer's Power Ratings
