Choosing the right transformer for a power supply

[Paul92]

I just started finally building a little electronics toolbox for my projects and the time has come for a power supply.

I got a kit for a 0-30V 5mA to 3A power supply (https://www.amazon.co.uk/gp/product/B071F4M96P/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1). The question is what transformer to choose?

The kit says that it requires a 24V AC input source. I found an affordable transformer at farnell (I am going to connect it to an European 230V socket): http://uk.farnell.com/myrra/74032/transformer-flyback/dp/1501048?st=24v%20transformer

Now, the questions:

- From my poor knowledge of how transformers work, they are simply 2 coils on a core that resonate, and the parameters of the transformer are tuned by the number of twists in the coil, wire gauge, core properties. How can a transformer have a variable input voltage and guarantee a steady output voltage? I would have expected to be some sort of proportionality between the input and the output.

- The transformer from the link I found is rated at only 1.5A. The power supply handles up to 3A. This leads to 2 questions: if I try to draw more than 1.5 from the transformer, will it simply not handle it or will release the magical smoke? Also, if I try to add 2 such transformers in parallel, will I get 3A?

- Should I add anything between the transformer and the power supply and/or before transformer? I am thinking more about protection (some limiting resistor if the transformer gets fried if I try to draw too much? may some circuit breakers, rated at the maximum transformer current?).

[james_s]

The transformer you linked is a ferrite core transformer for a high frequency switchmode power supply. What you need for your project is an iron core transformer that will work on 50Hz, this will be a rather large chunky thing.

The power supply will actually draw somewhat more current from the transformer than it delivers on the output, without going into too much detail this is because when you rectify and filter AC into DC the resulting DC is the peak voltage of the AC which is higher than the RMS value, you don't get something for nothing so the current has to be higher. Yes you can parallel multiple identical transformers in most cases to increase the current capacity. If you overload a transformer it will overheat and eventually burn up.

[Audioguru]

I agree that you need a big 50Hz transformer not a tiny 100kHz transformer.

Guess what? The cheap power supply kit is a Chinese copy of a 15 years old defective Greek kit that is not reliable and does not work properly because many of its parts are overloaded. I helped fix the original Greek kit at www.electronics-lab.com where there are a few long threads about it. ebay and Banggood stopped selling this copy of the defective cheap kit.

The 24V 50Hz or 60Hz transformer has a voltage too low for an output from the kit of 30VDC at 3A but is too high for the 36V opamps used. My fixed circuit uses a 28V transformer and opamps rated for a 44V max supply. The 28VAC has a peak of 28V x pi/2= 39.6V which at an output of 3A is a power of 39.6V x 3A= 118.8VA. Therefore I recommend that at least a 120VA transformer is used. 120VA/28VAC= 4.3A, not 3A.

[IanB]

Now, the questions:

- From my poor knowledge of how transformers work, they are simply 2 coils on a core that resonate, and the parameters of the transformer are tuned by the number of twists in the coil, wire gauge, core properties. How can a transformer have a variable input voltage and guarantee a steady output voltage? I would have expected to be some sort of proportionality between the input and the output.

Correct. The output voltage will vary in proportion to the input voltage. So if the mains input voltage varies between 230 V and 240 V the output will go up and down correspondingly. If you live in the UK you should try to obtain a 240 V transformer (not a 230 V or 220 V transformer).

- The transformer from the link I found is rated at only 1.5A. The power supply handles up to 3A. This leads to 2 questions: if I try to draw more than 1.5 from the transformer, will it simply not handle it or will release the magical smoke? Also, if I try to add 2 such transformers in parallel, will I get 3A?

It is not obvious that that power supply can actually output 3 A (at least not without a heat sink on the regulator and a fan, and even then it's doubtful). I would suggest setting your sights lower and settling for 1 A.

- Should I add anything between the transformer and the power supply and/or before transformer? I am thinking more about protection (some limiting resistor if the transformer gets fried if I try to draw too much? may some circuit breakers, rated at the maximum transformer current?).

Probably not worth it. A fuse between the transformer and the power supply might be a good idea, but at such low power levels I would not bother.

[capt bullshot]

Audioguru pointed it out, but even before reading his post I knew from looking at the picture of the kit you've chosen that this thing will most probably end up in flames or smoke once you use it at more than 33% of its ratings.

That's an experience a beginner has to make once or more than once in his career, but it can be really frustrating. And todays money making driven market makes it even more frustrating since the specifiation of this kit is nothing but a bunch of lies. As a beginner one cannot jugde this because the lack of experience, but one may be blinded by the really low price tag. A decent transformer to supply this kit according to its ratings would cost a multiple of that.

Alas, I can't give a recommendation for a decent kit suitable for a beginner. Such a kit should contain not only the bare electronics, but also the the transformer, heat sink, enclosure and other stuff to complete the project. I've built a few kits when I was a beginner some decades ago, and since they were good and complete, I was able to build it and get it working as expected. Later on, I've learned how to design my own circuits and what the correct component choices would be.

So far remembering the good old times. Still don't have a recommendation for you, sorry.

[exe]

I have the same kit. I supply it from a small 9V 30VA transformer. Of course this limits output voltage and current (I'm careful not to output more current than transformer can deliver). But the thing is, if you don't need 30V output you can get a smaller transformer (for smaller voltage).

PS heatsink is a must. Be sure to understand how linear power supplies work, how much they produce heat, what would be the appropriate heatsink size. Plus, the enclosure should provide enough cooling.

[Kleinstein]

That kit is not reliable working with a 24 V transformer and 3 A s also pushing things hard.  With a 24 V transformer one would have to upgrade some of the OPs and might get something like 24 V out. With the supplied transistor maximum power is more like 50-80 W. So more like 2 A max would be a sensible limit. The rectifier and filter cap is also more suitable for 1-5-2 A.

With the supplied OPs something like a 18-20 V transformer would be the maximum and thus maybe 15 V as a maximum voltage, maybe a little more at light load.

For a simple rectifier and filter cap input section one would usually get an AC current that is somewhere between 1.5-2 times (the larger number for large toroidal transformers and large fitler cap) the DC current. So a 2 A DC rating would need a 3-4 A AC current rating to be on the safe side.

A fuse between the transformer and the circuit is a good idea, especially with a circuit that might fail from overload. One might get away with a suitable fuse on the primary side, though this can get difficult.

[Audioguru]

The original Greek kit used a small 3300uF main filter capacitor that produced a lot of ripple when the output voltage is high with a fairly high output current. The Chinese kit also has a small main capacitor. I recommend 3.6 times larger at 12000uF.
Since a single output transistor needs liquid nitrogen to cool it then two output transistors in parallel with emitter resistors to balance them are recommended.
Many of the resistors get too hot and should be larger, then they will not fit on the small Chinese circuit board.

[james_s]

Seems like it would be simpler to just derate it to say 0-15V 0-1.5A rather than make all these modifications in attempt to make it meet the published specs.

[exe]

Yep, "smoothing" cap after bridge is too low for claimed current (also, I guess, current ripple is too high for it). I'd suggest ~2000-3000uF for each ampere of current. Bigger cap will put more stress on transformer.

I also saw suggestions for transformer to have 50-100% more VA than output needs (although, never seen any calculations). Justification: transformers are rated for continuous output. With bridge rectifier and a beefy cap peak currents are high and transformer works only for a fraction of time (when output voltage is higher the cap's voltage). May be my explanation is hard to follow, but you can use this online simulator to see current and voltage waveforms: https://www.changpuak.ch/electronics/power_supply_design.php .

Another problem is inrush/startup current, but that's another story

[Ian.M]

The average DC current you can draw without exceeding the transformer's RMS secondary current rating depends on the rectifier topology, and whether or not it directly feeds a reservoir capacitor.  See Hammond  Manufacturing Ltd (transformer division)Design Guide for Rectifier Use for derating factors.   The derating factors can be confirmed by SPICE simulation if you prefer a more analytic approach.

When you intend to run a regulator circuit close to its limits, another important consideration is the voltage range of the unregulated DC bus.   Typically, its necessary to allow for a mains supply variation of +/-10% from its nominal voltage (though some regions have asymmetric tolerances specified in their standards for mains supply), which will result in an approximately +/-10% variation in RMS secondary voltage and thus DC bus voltage.  Transformer regulation is another issue - a transformer specified for 15% regulation, will have an output voltage 15% higher unloaded than at full load, so its easily possible for the unloaded voltage with 10% high mains supply to be 40% more than the fully loaded voltage with 10% low mains supply that one has to design to if you wish to avoid regulator dropout.  This makes it extremely difficult to design a lab grade linear regulator circuit for an output voltage more than 50% of its max rated input voltage, unless you add a pre-regulator.   

A Zener clamped capacitance multiplier may be worth considering - it will follow the troughs of the ripple when the output is significantly loaded, which will vastly reduce the influence of the ripple on the main regulator, and the Zener clamping the base of the capacitance multiplier's pass transistor can be selected to limit the maximum unloaded voltage to keep it within the main regulator's input voltage rating.