A transformer is an electrical device that transfers energy between two or more circuits through electromagnetic induction. Its primary function is to step voltage levels up or down while maintaining the same frequency, allowing equipment to safely operate at the required voltage. By isolating circuits and adjusting power levels, transformers make it possible to distribute electricity efficiently and reliably, while protecting sensitive control systems from overloads or mismatched voltages.
Here at Functional Devices, we get a surprising number of emails and calls asking about our transformers and PSH power supplies. Over time, we've realized many people hold some misconceptions about transformers and how they operate. While we’ve written previously about some of these, we’ve never addressed them all at once. So, to fix that, here are four essential things to know about transformers! If you still have questions after reading, don’t hesitate to reach out to our engineers.
1. Wiring Transformers Can Be Confusing
Wiring any electrical device can be confusing if you don’t have the proper training, but transformers can sometimes trip people up (pun intended).
Make sure you consult labels and datasheets before making connections as wiring a transformer might be more difficult than it seems. For one, the black wire on the primary is almost always the common connection, and not 120Vac Hot as one might assume.
Additionally, some transformers have multi-tap primaries, meaning there are more wires than the typical four. Overall, pay attention to wiring diagrams on the transformer labels and datasheets and installation should go smoothly. If you want a longer, more detailed explanation of how to wire a multi-tap transformer, you can read it here.
2. Don't Measure Output Voltage Referenced to Ground
If you take one thing away from reading this blog let it be this: Don't measure your transformer's output voltage while referenced to ground. Most transformers are isolated, meaning there's no electrical connection between the primary and the secondary. The result is that when the secondary isn't connected to anything, the output voltage can be literally any voltage when referenced to ground, since voltage itself doesn't have a reference point (this is also known as floating point voltage).
We've seen anything and everything measured on a transformer secondary ranging from negative voltage to 480V on a 24V output. This is no cause for alarm though. Instead, measure your output voltage referenced to the common connection on the secondary. This will give you a much more accurate reading and will be the voltage your load actually sees when connected.
So, if you want all the details along with some good images and graphics you can read here.
3. Output Voltage Drops as Transformers Are Loaded
All AC voltage transformers drop in output voltage as you load them. For example, on the 24Vac output of the PSH500A, at 1 amp of load, the output will be around 24Vac. However, when loaded to 4 amps, the output voltage drops to 21.1Vac. This is because of the internal resistance of the transformer. As more current is drawn from the transformer, it increases the voltage drop across the constant internal resistance, resulting in a lower usable voltage.
That being said, it's not entirely uncommon to see a transformer with a 24Vac secondary measure when 27Vac unloaded or 21Vac fully loaded. If your load needs a more precise voltage range, we recommend using a switch mode or linear regulated power supply instead.
4. A Transformer's Circuit Breaker Doesn't Protect Your Load
Not all transformers have them but most have circuit breakers on their secondary. Many people incorrectly assume that because this circuit breaker is there, that they don't need any additional protection for their load. While we can't speak to any local laws and regulations, we can still say with confidence that relying on a transformer's circuit breaker to protect your load is a bad idea. This is because load protection isn't the purpose of a transformer’s circuit breaker.
Most circuit breakers on transformer secondaries are there to protect the transformer, not the load. Usually, the circuit breaker is a slow-blow thermal breaker, meant to disable the transformer if it gets too hot. This means that voltage or current surges can still get to your load and damage it long before the breaker pops. Therefore, always add whatever load protection is required by your local authority.
FAQ: Additional Insights About Transformers
Here are some more frequently asked questions we receive about transformers.
Q: How do I determine the correct transformer size for my application?
A: Calculate the total load in volt-amperes (VA) by adding up all devices connected to the secondary. Select a transformer with a VA rating at least 20 to 25 percent higher than your calculated load to ensure reliable operation and account for start-up currents.
Q: Can transformers be used with both AC and DC circuits?
A: Standard transformers are designed for AC operation only. Using them with DC can damage the transformer or cause overheating. For DC voltage conversion, a different type of power supply or converter is required.
Q: Do transformer operating temperatures affect performance?
A: Yes. Transformers have rated temperature limits. Operating above these temperatures can reduce lifespan, decrease efficiency, and increase insulation wear. Always ensure proper ventilation and avoid placing transformers near heat sources.
Q: How do I know if a transformer is energy efficient?
A: Check the efficiency rating on the datasheet or product label. Higher efficiency transformers lose less energy as heat, lowering operating costs and improving system performance. Look for energy-efficient or UL-recognized designs for modern applications.
Q: Can transformers be mounted in any orientation?
A: Many transformers are designed for standard vertical or horizontal mounting, but orientation can affect cooling and performance. Always follow manufacturer instructions to ensure proper airflow and avoid overheating.
Functional Devices Is Ready to Transform Your Controls
Transformers are fundamental components in electrical and control systems, providing safe voltage conversion, circuit isolation, and reliable power delivery. Understanding the key misconceptions and characteristics we’ve laid out here ensures that your equipment operates safely and effectively.
For any questions about selecting the right transformer or PSH power supply for your project, Functional Devices’ engineering team is ready to help. Feel free to reach out for assistance.