4 Essential Things to Know About Transformers

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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 there seem to be a few common misconceptions about transformers and the nature of how they operate. We’ve addressed several of these in past Tech Tuesday blogs, but never all in one place. So, to fix that, here are 4 essential things to know about transformers.

1. Wiring Transformers can be confusing

I have written a blog about this very topic. If you want the longer more detailed explanation, you can read it here. The short version is to 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 your common connection, and not 120Vac Hot as one might assume. Additionally, some transformers have multi-tap primaries, meaning there’s more wires than the typical four. Overall, pay attention to wiring diagrams on the transformer labels and datasheets and installation should go smoothly.

2. Don’t measure output voltage referenced to ground

Once again, we’ve already written about this. So, if you want all the details along with some good images and graphics you can read here. 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 it itself doesn’t have a reference point (aka: 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.

3. Output Voltage drops as transformers are loaded

Unlike the others, we haven’t addressed this in a blog yet. The gist of this is that all AC voltage transformers have their output voltage drop 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 all being said, it’s not entirely uncommon to see a transformer with a 24Vac secondary measure 27Vac unloaded, or 21Vac fully loaded. If your load needs a more precise voltage range, it’s recommended you instead use a switch mode or linear regulated power supply.

4. Transformer’s circuit breaker doesn’t protect your load

Not all transformers have them, but most have circuit breakers on their secondary. Many people have incorrectly assumed that because this circuit breaker is there, that they don’t need any additional protection for their load. While I can’t speak to any local laws and regulations, I 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 it.

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. So be sure to always add whatever load protection is required by your local authority.

And that’s it! The 4 essential things you need to know about transformers! I apologize for the longer read. Kudos if you made it this far. As always, if you have any questions on this, be sure to contact us at Tech Support!

Tym Moore
Tym Moore

Tym Moore is an Electrical Engineer who hails from the best state of Colorado.
He graduated from Colorado State University in 2017 and moved to Indiana shortly afterward to marry his now wife. Outside of work, Tym spends most of his time driving back home, and complaining about how flat Indiana is.

Be sure to give Tym a call for tech support. He will always do his best to find a solution for you.

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