14v/30VA transformer outputs 25 volts, why? - Page 1
Forum section: electronics / beginners

author: analogo
author_rank: Regular Contributor
timestamp: March 25, 2017, 09:11:38 am
content: I have a small motor that is powered via a DC jack. The print on it says 14v --- 30VA. The associated mains transformer should 240v AC into 14v DC. It also says so on its label.I tested the voltage on the DC output jack of the transformer and I read... 25v. What?How could it be? Maybe these 25v become 14v once a load is applied?And why does it say 30VA? All my other transformers state their max amperage (2.5A, for example).

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2017, 09:43:27 am
content: With a 30 VA transformer, the no load voltage is higher by about 20% than at nominal load. The exact number can vary a little. With rectifier and filter cap the DC voltage is close the peak value - thus a factor 1.4 higher than the effective AC voltage. The rectifier usually looses something like 1-2 V depend on the load - maybe even less at no load.There is also some variation in the line voltage - thus the 25 V DC are on the high side, but still within tolerances.With load the voltage will drop, but it would take a lot of load to get all the way to 14 V. More expect something like 16-18 V. It will be less if a smaller filtering capacitor is used.Noting transformer ratings in VA is not unusual, as the applies to a whole series with different voltages.Also note the AC current rating is simething like 2.1 A AC, but after rectifier and filter this would result in something like 1.2 A DC only.

author: mariush
author_rank: Super Contributor
timestamp: March 25, 2017, 10:30:51 am
content: The transformer outputs an AC voltage, and the maximum power is 30 VA. Think of it like this : if you connect an incandescent lightbulb designed to work at 14v AC from factory and rated or 30 watts, it will consume exactly 30 VA.Also, as you have determined, when there's nothing or a device using very little of that power connected to the transformer, the output of the transformer can be significantly higher. The smaller that power rating (the VA value), the larger that variation (to higher voltage values).Electronics use DC voltage, so you have to convert the AC voltage coming out of the transformer to a DC voltage. With regular AC voltage, 50 or 60 times a second voltage dips below 0 and then goes above 0, following a sine wave. If you use a bridge rectifier, that bridge rectifier basically flips those periods where the voltage goes down under 0 and makes them like the other periods where voltage goes from 0v to a high voltage:This is what a bridge rectifier does, at the top you have the AC voltage (the ~ pins on the bridge rectifier and the wires of the transformer) and on the bottom you have the Dc voltage (+ and - on the bridge rectifier, where - is also called ground)After rectification using a brige rectifier, the output voltage has a PEAK value equal to sqrt(2) x Vac - 2 x Vdiode or 1.414 x Vac - 2 x Vdiode where Vdiode is the forward voltage of an individual diode that forms the bridge rectifier, a value that's usually between 0.6v and 1v (depends on the diode and how much current goes through it)In your particular case, the 14v AC will be converted by the bridge rectifier in a DC voltage with a peak of aroundVdc peak = 1.414 x 14v - (2x0.7) = 18.3v DCand at low loads, as you have seen your transformer may output up to 30v, in which case that voltage will be rectified to Vdc peak low load = 1.414 x 25v - (2 x 0.7) = ~ 34vBut as you can see in the picture above , 50 or 60 times a second (the mains frequency in your country), the voltage coming out of the bridge rectifier drops down to 0 and then goes up again, and whatever you connect to the transformer won't like that.You can solve this by adding a capacitor right after the bridge rectifier, which charges with energy when the output goes up towards the peak dc voltage and then gives back the energy when the voltage from the transformer starts to go down towards 0... the capacitor basically fills those gaps.You can estimate how big of a capacitor you'll have to use with this formula :C (in farads) = Idc max / [2 x Frequency x (Vdc peak - Vdc min)] where Vdc min is the minimum voltage you want to have.So let's say you're in US where you have 60 Hz mains frequency. You know the peak DC voltage of 18.2v , and you know the transformer can output 30VA so ideally, the maximum DC current would be 30VA / 18.3v = 1.63 A but that's ideal, an approximation closer to reality would be Idc max = ~ 0.62 x Iac = 0.62 x (30va / 14) = 1.328 A - the constant 0.62 is a value that works well for a wide range of transformers.Now let's say you want to have always at least 14v DC at the output, and you figured that at most you'll have about 1.33A .. so you should use at least C = 1.33 / 2x60x(18.3-14) = 1.33/516 = 0.0025775 Farads or 2577 uF ... so some standard values you could use would be 2700uF or 3300uF rated for at least 34v because your transformer goes that high when there's no power used (so you'd go with 50v rated capacitors because 34v is awful close to 35v, the closest standard value near 34v)You could use larger capacitors and then the minimum voltage will be much closer to the peak voltage, or you could use smaller capacitors if you're sure you're not going to be close to 1.33 A , you can play with the numbers.In general there's a rule of thumb, saying you should design things for around 2200uF for each 1A of current, so if you do the calculations and determine you need much more than this value, it may be worth thinking about using a transformer that outputs a slightly higher AC voltage in the first place as it could be cheaper than installing a lot of capacitors.

author: Benta
author_rank: Super Contributor
timestamp: March 25, 2017, 10:33:48 am
content: If the motor and transformer belong together, it is very possible that the transformer is a "special" for this application.For driving motors, transformers with a "soft" characteristic is sometimes desirable to avoid too high start current. Also, such a transformer is usually inherently short-circuit proof.The downside is a relatively high no-load voltage, which drops to the correct level when the motor is running.

author: analogo
author_rank: Regular Contributor
timestamp: March 25, 2017, 06:16:48 pm
content: Thank you all for the detailed answers. I learned a fair bit today.As Benta correctly guessed, the motor and the transformer are from the same tool. I wanted to replace the transformer with my own 14v transformer.I have ordered a step-up converter that can output from 14v to 35v and max 6 amp. Once it will arrive I will do some tests and see if the motor starts nicely with just 14 volts. Should I expect some problems due to the fact that the converter is surely does not have a "soft characteristic"?BTW, what does "soft characteristic" mean?

author: Benta
author_rank: Super Contributor
timestamp: March 25, 2017, 07:36:52 pm
content: QuoteMy apologies, that's imprecise on my side, more like technical slang.For mains connected power transformers, it is in almost all cases desirable that the secondary voltage tracks the primary voltage.This means, that whether you draw a large or a small secondary current, the secondary voltage stays stable (provided the primary voltage is stable, which is mostly the case, at least in industrialized societies).This is done by minimizing copper and iron losses and having primary and secondary very tightly coupled magnetically.But in certain applications, it is desirable that secondary voltage varies with secondary current. Meaning high voltage at low current, but more importantly: decreasing voltage as current increases.The best known transformers of this kind are for arc-welding use.In your example, a transformer of this kind coupled with a matched motor can have a lot of advantages. Start current will be limited, overload current as well, even locked armature current (which is practically a short-circuit), so the transformer will protect the motor. And as I mentioned before, this kind of transformer can be made inherently short-circuit proof.A very simple design, that of course can be emulated with a lot of power electronics. Question is if it's worth it...