YS-U50SL power supply, 24 V 2.3 A

Another power supply someone sent me a link to, and one that I think could be interesting, especially for use in T12 soldering stations.


It isn't particularly powerful, but offers a very good power-to-size-to-specifications ratio, making it suitable for building compact devices, such as soldering stations. I once reviewed a very compact soldering station I built; with this power supply, it wouldn't have been much larger.

The packaging is as simple as it gets: a regular antistatic bag with a sticker listing the part number and output voltage/current.


One thing caught my attention while inspecting the packaging. The product page offers two versions, 12 V 4.5 A and 24 V 3 A, and also lists the power as 54 W. The figures add up for the 12 V 4.5 A version, but not for the 24 V power supply. When it arrived, I noticed the label said 24 V 2.3 A, which doesn't quite match the advertised 3 A.

But the product page did have a link to the original datasheet, which gives the correct specifications.
The datasheet is in Chinese, but it's quite easy to understand for the most part. For example, it shows that there are six versions of the power supply: 3.3, 5, 12, 15, 24, and 36 V. The 12–36 V versions can deliver up to 55 W.
Also, in addition to its continuous current rating of 2.3 A, the 24 V version can operate in the 0–2.5 A range, i.e. up to 60 W.


It connects via a two-pin input connector and a four-pin output connector. There is no provision for a ground connection. There is a pad near one of the mounting holes, but the capacitor that should be connected to it is missing.
The output voltage is fixed, and there's an LED near the output terminal block.


The assembly is neat, and some components are additionally secured with potting compound, though there's much less of it than in the previous power supply.


The power supply is compact, measuring 76.5 × 46 × 25 mm.


A drawing with all the required dimensions is available in the datasheet.


1. The input has a filter, as well as a 2 A fuse, a 10 mm 470 V varistor, and a thermistor.
2. The transistor is in a fully insulated package and mounted on a heatsink that looks brass-colored. The 82 µF input capacitor is adequate for a power supply of this rating.
3. The layout is dense but not overcrowded. The Y capacitor to the right of the transformer is easier to see.
4. The output rectifier is an MBR30150CT, more than adequate for this power supply. The output also has three capacitors: a pair of 680 + 330 µF before the inductor and one 330 µF capacitor after it. All the capacitors are rated for 35 V.


Most of the components are on the underside of the board. There are also some unpopulated spots, since the board is shared across the whole product line.


The soldering is neat and the board is clean. The only real thing to complain about is a chipped corner on one of the diodes, though I think that's down to poor packaging.


The datasheet includes a block diagram that's understandable even in Chinese, though it only gives a general overview.


Since I think this power supply is interesting and hobbyists may want to buy it for their projects, I drew up the schematic.


Last time I wrote about some odd design choices. Everything looks fine here, and I'll also explain the mistakes in the previous schematic.
1. The thermistor is correctly placed before the capacitor.
2. Zener diode ZD1 and resistor R022 provide overvoltage protection for the PWM controller's supply circuit. Last time, I chose the wrong resistor value, so this protection wouldn't have worked.
3. Zener diode ZD2 is connected in series with the optocoupler and protects against excessive output voltage if the feedback circuit opens or the TL431 fails. Last time, the Zener diode was connected in parallel with the optocoupler and had no effect on its operation.


Let's power it up.
A cold start takes about a second. The output is slightly higher than specified. No-load power consumption is 0.4 W, though the wattmeter may be inaccurate at such a low power level.


The first test checks the power supply's load capacity and how accurately it maintains the output voltage as the load changes.
The power supply delivered up to 3.1 A, after which the protection kicked in. The voltage stayed very stable, with only a slight deviation at the limit.


The datasheet includes graphs showing the allowable load depending on the supply voltage and ambient temperature (left), as well as the efficiency of different power supply models versus input voltage (right).


I measured efficiency at normal mains voltage over a range of 5 to 75 W, with 5 W increments on the graph's horizontal axis.
The result was about 88% efficiency from 20 to 75 W.


Ripple was also measured at the maximum range. Below are the waveforms with no load and at currents of 1, 2, and 3 A, corresponding to 24, 48, and 72 W. Measurements were taken both directly at the output and with a C+C filter; there was almost no difference.
I have to say, I really liked the results. Even under overload, the peak-to-peak ripple is no more than 15 mV, and there are no spikes either.


The results are also pretty good with a slower timebase.


And, of course, another important test: temperature under different operating conditions.
The test was run in 20-minute stages at loads of 15, 30, 45, 60, and 70 W. After the 70 W test, I decided to increase the load current from 2.9 A (70 W) to 3 A, then to 3.05 A, but in the last mode the power supply went into protection fairly quickly.


Thermal images taken at the end of the 45, 60, and 70 W stages.


There were two possible reasons for the shutdown:
1. The thermal protection kicked in.
2. The overload protection threshold dropped slightly as the unit heated up, causing the power supply to shut down.

To check, I set the load current to 3 A almost immediately and ran it for another 20–25 minutes. The power supply remained stable, so the second explanation is more likely.


Without interrupting the test, I covered the unit with plastic film to impair cooling, lowered the current to 2.9 A to make sure the overload protection wouldn't kick in, and waited.


After 10–12 minutes, the power supply still showed no sign of shutting down. I could have continued the test, but honestly, I felt bad about killing a very well-made power supply, so I stopped the test there.


This is what things looked like after 20–25 minutes of testing at 3 A, followed by 10–12 minutes at 2.9 A with thermal insulation. What can I say—it was hot.


During the test, I also measured the temperatures of the main components: input bridge rectifier, input capacitor, high-voltage transistor, transformer, output diode assembly, output capacitors (before the inductor), and inductor.

Up to a load of 60 W, temperatures stayed within acceptable limits. Keep in mind that the power supply is rated for a maximum of 55 W, so 60 W (actually a little more) is already a 10% overload.
As expected, the temperature rose with the load, but the unit handled even 70 W without any problems.

The last two tests clearly show overheating. The second-to-last was simply at 3 A, while the last one was with reduced cooling.


As always, I checked how the output voltage changes with temperature.
1. Initial no-load voltage with the power supply cold
2. Voltage under load with the power supply severely overheated
3. No-load voltage with the power supply severely overheated.

Two things are clear:
1. As the temperature rose, the voltage increased by about 60 mV.
2. The voltage's dependence on the load remained very small, around 3 mV.

In my opinion, it passed the test, especially considering that the power supply was genuinely hot.


Of course, many people will ask what alternatives there are from established brands.
And there are some—for example, MeanWell EPS-65S-24: review.

There are a few differences: the EPS-65S-24 is slightly wider and has a rated power of 65 W, but its maximum power is about the same as the unit reviewed here, around 75 W. However, the EPS-65S-24 has a slightly more complex circuit design, including a separate optocoupler for the overvoltage protection circuit, and, of course, higher-quality capacitors.
And as far as I remember, the Mean Well has no thermal protection at all.


Conclusions.
Overall, I really liked this power supply. It easily delivers its rated power and can run for extended periods even with an overload of almost 50%. It has an input filter, its components are generously rated, and it includes protection against short circuits, overloads, and overheating. Its compact size and specifications also make it a good fit for T12 soldering stations.
I was also pleasantly surprised by the low output ripple and neat assembly.

The only significant issue, perhaps, is that the thermal protection doesn't work correctly. It seems to be set to too high a temperature. You could try adjusting it by reducing the value of the resistor connected in series with the thermistor.

Otherwise, it's a decent alternative to branded power supplies. Then again, this PSU is essentially a branded product too, not one of the many no-name clones, and it comes with a datasheet listing all the necessary specifications.
In short, it's a decent power supply with plenty of power headroom and low ripple.

As an alternative, I'd consider the Meanwell EPS-65S-24. In our stores, it costs about $12, excluding shipping. I've already described the differences above.

The store provided the coupon BGa26935, which brings the price down to $7.99 through August 15.

That's all for now. As always, I'll be glad to answer your questions and read your comments.