Output current

The output current of a transformer is the electrical current that is available on the secondary side (i.e. at the output) of the transformer. It depends directly on the load connected to the transformer and on the design and capacity of the transformer itself.

Technical basics

A transformer works on the principle of electromagnetic induction and transfers electrical energy between two or more windings (primary and secondary winding). The following applies:

Is=Up⋅IpUsIs=UsUp⋅Ip

• IsIs: Output current (secondary current)
• UpUp: Input voltage (primary voltage)
• IpIp: Input current (primary current)
• UsUs: Output voltage (secondary voltage)

This formula is based on the assumption of an ideal transformer without losses.

Factors influencing the output current

1. Load (load): The higher the connected load, the higher the output current.
2. Transformer power: The rated power (in VA or kVA) limits the maximum permissible output current.
3. Voltage ratio: A transformer with voltage reduction (e.g. 230 V to 12 V) supplies a higher current at the output for the same power.
4. Efficiency: In real transformers, losses occur (copper losses, iron losses) which slightly reduce the actual output current.

Security and borders

• Overload protection: Transformers are equipped with fuses or circuit breakers to prevent damage caused by excessive output current.
• Heating: A permanently high output current can lead to overheating and damage to the windings.
• Short-circuit current: In the event of a fault, the output current can increase extremely – short-circuit protection is therefore essential.

Note : The lower the output voltage for the same power, the higher the output current.