What is my transformer output with no load?



So we've done a couple of different projects using this transformer in the past. And we've actually gotten a common question of why is it sometimes the output of the transformer isn't exactly what I'm expecting. And so we're going to go over that really quick today. And there's a couple of reasons why that is.

So first of all, this is set up in parallel, so we're supposed to be getting a nominal 12 volt output. However, if you look over here, you can see that the RMS using my multimeter is saying 13.5. And then the RMS using my oscilloscope is measuring 13.9 with an above expected average of about 400 millivolts, which explains why there's a little bit of difference in there. However, the important thing to note is that with neither of them, am I getting the 12 volts that I'm expecting.

So there's two main reasons that this comes up. The first thing is probably the most obvious. In the datasheet if we look at it, we see that it is specified to give 115 volts in and 12 volts out. So let's use our multimeter here and just check the primary side of this transformer and hopefully not shock myself in the process.

You notice that it's not a 115, it's 118.5, 118.8. It's kind of bouncing around. So it's anywhere between a 115 and a 120 volts incoming. And so since if you have any theory behind a transformer, it's all about the turns ratio. If you have a higher input, and you have the same turns ratio, you're going to have a higher output. So that is the first and most obvious reason why but you think, okay, that's only a couple of volts off. A couple of volts on the input shouldn't yield that much of a difference on the output, if we're seeing about a 1 to 10 ratio of input to output, and that comes to the second thing. So let me hook this back up really quick...down...Put those on backwards.... I'm fully expecting you to cut this part out.

Now that it's hooked back up, and we're looking at this again, we still have the 13.5 and the 13.8 volts. What is the difference between the way this is working right now and the way that it's going to be used in real life? There's no load. Both of these are very high impedance measurement sources. So you're not having much of any load. I mean, they're both designed to have the minimal amount of load possible. Whereas this is rated at a certain load.

So as we look at it, that's again, we can go to the datasheet. And this is for the VPS24-5400, you're expecting a 12 volt output in parallel configuration with 115 volts in if you have a 10.8 amp load. Since this is rated for 130 volt amps, that means that when this is fully loaded, you should get a 12 volt output. And that takes us to another point in the datasheet where it's talking about the voltage regulation in which says it's 25% typical at full load to no load. So that's where you're looking at it and saying, okay, whereas right now we have no load, we wouldn't expect to see more than a 25% difference between how between the output now versus if we had that full 10.8 amp load.

Now, I think we've gotten spoiled because most of our laptops, if you look at the power supplies, it says, you can give anywhere between 100 and 240 volts out or input, and it'll give exactly 14 volts output in DC beautiful. And if you want, I mean, I know that triad man magnetics makes things like that. But you're not going to get that same voltage regulation, you're not going to get that rectification, you're not getting the whole system with just the transformer. So when you're using a transformer, you have a lot more realistic expectations of what you're going to get out.

I think the biggest thing and the biggest answer to that question is, you're going to get a difference in output on the transformer and the only way to approach that is to look at the datasheet, see what the rating is on there, what you should expect, and then if you're dealing with your transformer, and it's outside of those specs, outside of what the datasheet says, then that's a cause for concern. That's like, okay, something might be wrong with the transformer. But other than that, just make sure that you know exactly what you're expecting, you know what your load is, you know what the acceptable range of that load is, and you should be good.

So hopefully that's helpful. Hopefully it clears up a couple of those questions that people had about transformers and why you get that variation on the output. If you enjoyed this tutorial, give it a like, subscribe to our channel and we will catch you in the next one.

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