Linear power supply mains frequency what to do? - Page 1
Forum section: electronics / projects

author: jord4231
author_rank: Regular Contributor
timestamp: May 09, 2025, 10:04:15 am
content: Hi All,I've have a large 13.8v Linear power supply RS-50A. It's Astron branded 220v~250v version 50/60hzI noticed that at idle it's drawing around 1amp so at 240v 50hz = 240watts! The transformer core gets to around 45c (no load)I should note I disconnected the secondary to make sure I had no rectifier shorts etc (no problems here)So I decided to try it on a inverter that I can vary the frequency and voltage on. I set it for 240v 60hz and it's idle current dropped about 80watts to 160w. (this is not a viable long term solution)If I run it at 220v 50hz the current is much closer to running it off 60hz around 90w or so a significant drop.Yes I know SMPS are much more efficient but I do want to use this supply SOI have a few choices, What would you do?1. Run it on 240v 50hz and don't worry about it.2. Run it on a variac or some kind of step down transformer to knock 20 or so volts off the line voltage, bring it down to 210-220v so it Draws 80w or so less at idle(I'm aware this will likely make it more inefficient and its already very inefficient)3 Something else?I don't want to replace the transformer in this unit (its a new supply that I was not anticipating to have this problem with)Thanks for reading

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 09, 2025, 10:06:15 am
content: You're measuring the apparent power (VA) not the active power (W) otherwise it would have probably melted by now.They are not "real" losses.

author: jord4231
author_rank: Regular Contributor
timestamp: May 09, 2025, 10:10:04 am
content: Thanks for the reply. Yes that is a good point. I wonder how that will show up on my electricity bill?So I should just run it and forget about it?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 09, 2025, 10:11:30 am
content: Quote from: Ice-Tea on May 09, 2025, 10:06:15 amI don't know the environment temperature, but apparently the transformer gets quite warm even without a load.Maybe it's just a crappy transformer that works very near to it's saturation voltage, but it's also possible there is an internal short in the transformer between one or a few of it's windings. In such a case there is not enough voltage over the shorted path to create a significant current, but the losses can be high enough to heat up the transfomer.But there is indeed also a big difference between VA and Watts. 240Watts would need a big heatsink to get rid of it without overheating.

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 09, 2025, 10:14:18 am
content: For domestic users, it's not taken into account (industrial users are billed, though). It's still inefficient, but it's not as bad as you first thought. To figure out the actuall loss/consumption you can use a socket power meter likehttps://meters.uni-trend.com/product/ut230b-eu/but most of them are pretty poor when measuring idle (small) lossed.

author: jord4231
author_rank: Regular Contributor
timestamp: May 09, 2025, 10:16:00 am
content: I had the service tech in the USA run the same supply up at 240v 60hz and he was reporting a very similar wattage to what I was seeing.He also said those transformers run warm its normal for them. It is quite a large transformer apparently "good for 100a" I asked what its VA rating was but never got a straight answer.

author: jord4231
author_rank: Regular Contributor
timestamp: May 09, 2025, 10:34:22 am
content: Ok I'm starting to get my head around the reactive power measurement a bit better I think.I have a cheap TP link plug that monitors wattage and its giving me around 80W so I guess thats really more like it!This might be a better measurement than using the fluke meter on the current setting I guess due to the reactive power I'm reading

author: Benta
author_rank: Super Contributor
timestamp: May 09, 2025, 11:07:52 am
content: It's not only reactive power, but also current distortion which will skew your measurement. Most of the cheap socket power meters are not able to handle this well, for a precise measurement you need and old-fashioned rotary power meter.The problem on linear supplies is that they only draw current at the top of the voltage sine wave (to charge the storage caps). This gives an immense crest factor (look it up).

author: jord4231
author_rank: Regular Contributor
timestamp: May 09, 2025, 11:20:40 am
content: Thank you I will look it up!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2025, 12:45:00 pm
content: If the current is going down with an only slightly reduced voltage from 240 to 220 V, this is points to core saturation. A shorted turn would go roughly linear down with the voltage. Also the test with 60 Hz points to that direction. Low frequency and a high voltage is the point where core saturation is limiting. The magnetization current is still mainly reactive power and not real power.80 W of power with no load is still quite a lot.

author: bostonman
author_rank: Super Contributor
timestamp: May 09, 2025, 12:59:47 pm
content: QuoteI'm slightly confused. I know what VA is versus W. Watts is a resistive load and voltage/current are in phase whereas VA is taking into account the phase shift between voltage and current.The unit is drawing 1A, is this not the amperage regardless of it being VA?I'm just thinking current is current that's flowing through the line. Yes, I agree, 240W would have the power supply glowing, but I always thought apparent power was the inefficiency calculated due to efficiency losses such as maybe a motor where 7A is going in to make 1hp (rounding off the math), but it's only generating 375hp (approx. 50%).

author: Benta
author_rank: Super Contributor
timestamp: May 09, 2025, 01:28:55 pm
content: Quote from: bostonman on May 09, 2025, 12:59:47 pmIf the supply voltage is 240 VAC and the current is 1 A, shifted 90 degrees in phase, the real power is 0 W, but the apparent power is 240 VA.

author: bostonman
author_rank: Super Contributor
timestamp: May 09, 2025, 01:34:23 pm
content: I guess my confusion is: how would the person know a phase shift exists without looking at both waveforms?Unfortunately I would have made the same mistake and thought it was 240W.

author: Benta
author_rank: Super Contributor
timestamp: May 09, 2025, 01:37:23 pm
content: Quote from: bostonman on May 09, 2025, 01:34:23 pmThe person wouldn't have any way of telling with just a voltmeter and ammeter.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 09, 2025, 01:44:42 pm
content: I have repaired a few Astron PS. They are quite good and the transformers are very nice. The 45 C temp is just warm. If you use it to power something substantial and the transformer does not get hot, the unit is just fine. These are very good linear supplies and built well within specs, unlike other brands I have seen.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 09, 2025, 02:33:59 pm
content: Quote from: Benta on May 09, 2025, 11:07:52 amThat's probably why mains always looks slighly clipped in the oscilloscope, thousands of caps getting charged just in the very top of the wave

author: bdunham7
author_rank: Super Contributor
timestamp: May 09, 2025, 03:31:01 pm
content: Quote from: jord4231 on May 09, 2025, 10:04:15 amIt's likely just fine as-is. Will you be using it occasionally, often or always-on? And when it is on, what is the normal expected load like? With linear PSUs operating at more voltage than necessary, sometimes adding primary resistance improves their operation. If it were mine and it was always on, I might try adding a large 1 or 2 ohm power resistor in series with the primary. This will not decrease the no-load magnetizing current very much, but under load it will decrease the primary voltage and increase the rectifier conduction angle, both of which will reduce the heat produced in the PSU itself. Of course those resistors will dissipate some power so you'd have to take care of that.Keep in mind that under full load this PSU probably draws 10ARMSor so. So 1A of magnetizing current under the worst conditions (high voltage, low frequency) isn't really out of line.

author: jord4231
author_rank: Regular Contributor
timestamp: May 09, 2025, 10:28:13 pm
content: Thank you, It will only be on as need 4-5 hrs at a time a few days a week. So shouldn't be too much of a problem

author: bostonman
author_rank: Super Contributor
timestamp: May 11, 2025, 12:20:43 pm
content: QuoteJust to clarify, and maybe for others too that are reading about apparent power, is the 1A being measured "fake".I'm uncertain if this was measured with a wall power meter or a multimeter in current mode. I'm assuming it was measured with a wall power meter and it was interpreted/calculated incorrectly.Would have an inline multimeter in current mode measured correctly?My concern is that I've forgotten about this and taken incorrect measurements the few times I've cared to measure overall current through something.

author: bdunham7
author_rank: Super Contributor
timestamp: May 11, 2025, 01:14:47 pm
content: Quote from: bostonman on May 11, 2025, 12:20:43 pmNo, the current is not fictitious, it is really there. A power meter like a Kill-A-Watt, a TRMS current clamp or a TRMS reading multimeter should all measure the current correctly. Same for the voltage. The part that can be "fake" or incorrect is the power. Power depends on more than the RMS voltage and RMS current. The power factor of the magnetizing current of an unladen transformer is similar to the velocity of an unladen swallow--it depends on the specific circumstances.

author: JoanBS
author_rank: Contributor
timestamp: May 11, 2025, 03:15:02 pm
content: This consumption is abnormal or has been measured incorrectly.According to the Astron website these are the specifications of the RS-50A model:- CONTINUOUS DUTY: 37 Amps- ICS*: 50 Amps- DIMENSIONS: 6 x 13¾ x 11"- SHIPPING WEIGHT: 46 lbs- FUSE: GMC 10A- POWER TERMINALS: 1/4"-20 Screw Type- AC CURRENT DRAW: IDLE: 316mA/ FULL LOAD: 8.2A- FAN COOLED: NoThis refers to models with a supply voltage of 105-125VAC, so for 220-240VAC the consumption should be considerably lower (approximately half), which could be easily checked with a TRMS clamp meter.

author: Manul
author_rank: Super Contributor
timestamp: May 11, 2025, 04:35:24 pm
content: 1A magnetization current is quite high. In a real world phase shift will not be 90 degrees, primary has resistance. So there is power loss (and heating to 45C). What is resistance of the primary?I wonder what would happen if you run 1A DC (assuming it was accurate RMS value at AC) to the primary and compare the heating? I believe ideally it should be very similar. If there is significantly less heat at DC, this would point to shorted turns or core losses, in other words, coupling related effects.

author: bostonman
author_rank: Super Contributor
timestamp: May 11, 2025, 10:45:35 pm
content: QuoteMaybe I misunderstood something. The power was calculated based off 1A of current, so I thought the OP was using one of those plug in meters to measure it. Further down, it was determined 1A couldn't have been correct.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 12, 2025, 12:13:23 am
content: using a toroid would have been better for a psu ... laminated xformers are not fully or really efficient, core losses, heat ... crude designs

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2025, 12:19:33 am
content: Quote from: bostonman on May 11, 2025, 10:45:35 pmAs I understand it the power that he calculated was incorrect, that was VA not W. He then used an actual power meter and got correct numbers. And when he reduced the voltage to 220VAC the indicated power dropped a lot and presumably so did the current.

author: radiolistener
author_rank: Super Contributor
timestamp: May 12, 2025, 03:59:41 am
content: Quote from: jord4231 on May 09, 2025, 10:10:04 amReactive power typically isn’t accounted for on residential electricity bills unless you're a large industrial consumer. Residential meters usually only measure active power, which affects billing. However, it's important to verify that it's indeed reactive power, as the transformer might be malfunctioning due to a short circuit in the windings or being designed for lower voltage and saturating at 220V.You should also check if the transformer overheats when running no-load. If it’s getting excessively hot or causing other issues, it could be operating incorrectly or with incorrect parameters. In that case, it’s recommended to perform a detailed check or consult a qualified people.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2025, 06:49:08 am
content: The magnetizing current is mainly reactive power. Here it looks like the core is approaching saturation, as seen from the significant higher current at 240 V compared to 220 V. In this case the magnetizing current is no longer sine shaped, but gets higher just before the zero crossings. One the positive side the reactive power is normally not metered for private users - just the relatively small losses (maybe 5%) related to the current.At the reduced voltage the no load current is no longer that far off the number given for the specs. Chances are that with 60 Hz it would go down even more. It normal that no load current for a transformer is quite sensitive to the voltage.A series resistor to reduce the voltage is not a good idea: it adds real power losses, especially when loaded. It would make more sense to have a series inductor. This would also reduce the voltage, but with little real power. Ideally the inductor would saturate at higher load currents, so that the voltage would not drop too much when the supply would have to provider high power.

author: jord4231
author_rank: Regular Contributor
timestamp: May 17, 2025, 01:42:37 am
content: Well this has been a great learning experience for me.Just for anyone interested. I have used a differential probe and current clamp on a scope to analyse the Power factor, Real power,Apparent power , Reactive power and Phase angle.Thank you everyone for the replySee attached screenshot

author: bostonman
author_rank: Super Contributor
timestamp: May 18, 2025, 01:10:16 pm
content: I assume yellow is the diff probe me measuring voltage (230V in your country?)?Also, as did I, my captures included the somewhat flat spots near the zero crossings on the current probe measurements along with non-rounded peaks.What is the reason for the odd shapes? I know it has to do with the inductance, but not sure how to interpret what is occurring.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2025, 04:43:23 pm
content: The flat tops can come from laoding mains with simple rectifers and filter caps without PFC. These take high currents near the peaks and this way flatten the peaks. The linear power supply in quitestion could be one of the bigger such loads.

author: Benta
author_rank: Super Contributor
timestamp: May 18, 2025, 08:36:42 pm
content: Quote from: jord4231 on May 17, 2025, 01:42:37 amVery nice plot, and very informative and typical of linear power supplies. Let's take a closer look.First, it seems to be running idle ( = no load). For a 550 W output power supply that will probably pull around 1 kW at full load, a quiescent power of 72 W is realistic, though not very good.I'll ignore the power curve in your plot, it's just a calculated waveform.Now, the mains voltage curve. The flattened-off tops certainly indicate other big linear/non-PFC power supplies in your area, but it's not from your supply (the current draw is too low at this point).Second, the current trace. That tells us a lot.1: the peaking is from charging the storage caps on the secondary side, as the rectifiers only conduct at the voltage peak.2: It's not in phase with the voltage peaks. Why? Because the transformer magnetizing current is shifted 90 degrees and overlaid on the output current reflected to the primary. That causes the apparent time shift.So what happens when you start loading the output? The current peaks will increease significantly (high crest factor) and shift towards the voltage peaks. At full load they will be almost in phase.Try it out.Cheers.

author: 5U4GB
author_rank: Super Contributor
timestamp: May 18, 2025, 09:07:25 pm
content: Quote from: jord4231 on May 09, 2025, 10:10:04 amIt's OK, there are plenty of vendors on Aliexpress who will sell you small plug-in miracle devices to fix this :-).

author: Benta
author_rank: Super Contributor
timestamp: May 18, 2025, 09:24:40 pm
content: Quote from: 5U4GB on May 18, 2025, 09:07:25 pmVery helpful and constructive :-(

author: bdunham7
author_rank: Super Contributor
timestamp: May 18, 2025, 09:50:57 pm
content: Quote from: jord4231 on May 17, 2025, 01:42:37 amCan you take the same measurements using 220V input?

author: jord4231
author_rank: Regular Contributor
timestamp: May 18, 2025, 11:35:54 pm
content: Thank you for the detailed reply 🙂I will conduct a few more experiments at different line voltages and loads the supply and repot back.I have found a 20-25c temp increase of the transformer core at 240v over 220v.At 220v it idles around 55c and 240v it sits around 75c after about 3 hours.The quiescent current is about 25w more at 240v I guess this is turning to heat.I may add a fan to the case as I feel the ventilation is not enough.It will be interesting to see what the core temp does under a heavy load.Looking at the transformer secondary on the scope it doesn't seem to clip the secondary seen at the filter capacitor rises slightly fom 21.4 to 22.8v. from 220v to 240v also the transformer hum increasesI will report back in the next few daysAgain thank you all

author: jord4231
author_rank: Regular Contributor
timestamp: May 22, 2025, 08:38:42 am
content: Ok as promised I have attached a few more measurements.After around 3 hours run time @ 240v 50hz the core is aprox 75c - 167fAt @ 220v 50hz its around 55c 131f and hum is less as expectedMeasurements as follows220v 50hz NO LOAD220v 50hz 20A DC Load230v 50hz NO LOAD230v 50hz 20A DC load240v 50hz NO LOAD240v 50hz 20A DC load

author: jord4231
author_rank: Regular Contributor
timestamp: May 22, 2025, 08:46:45 am
content: Next same thing at 60hz (ran off an inverter to achieve this) 50hz test done with variacPlease note in the 60hz tests I adjusted the Amps per div220v @ 60hz 20a load230v @ 60hz 20a load240v @ 60hz 20a load220v @ 60hz NO load230v @ 60hz NO load240v @ 60hz No loadOrder changed from 50hz tests above sorry!Conclusionrun it on the mains 240v 50hz and forget about it OR get a 240-220v step down transformer to save 20c of heat on the core (hardly think its worth it)Running it of 60hz is not a viable option I was using an inverter to do.I'm thinking whilst 75c is hot for us its probably ok for the transformer. I will add better ventilation to supply lid to keep the capacitors in the supply cooler.Pic of transformer included in postUnder a 3 hour load test the transformer ran about 10c cooler under load than idle.. InterestingThank you all

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2025, 09:49:39 am
content: Quote from: jord4231 on May 22, 2025, 08:46:45 amYou could use a much smaller, say 3A, 240V:24V step down transformer, connected as an autotransformer to convert 240V to 220V.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 22, 2025, 10:25:01 am
content: You dont have to pay for reactive power.

author: jord4231
author_rank: Regular Contributor
timestamp: May 22, 2025, 10:32:50 am
content: Quote from: Terry Bites on May 22, 2025, 10:25:01 amTrue at this stage I'm more concerned about the transformer temp but I do think its okThanks for the comment

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2025, 11:21:33 am
content: How big is the transformer?Looking at the oscilloscope output, something isn't right.The standby losses are higher than what would be expected.The iron losses will dominate the standby power, since most of the power is lost in the core, because the current and therefore I2R losses in the copper are low.Here's the data sheet for a 500VA transformer. It specifies the iron losses of 16.6W. In reality, standby losses will be more, due to the small copper losses and quiescent current of the linear regulator, but not double.https://docs.rs-online.com/7788/A700000013598161.pdf

author: jord4231
author_rank: Regular Contributor
timestamp: May 22, 2025, 11:31:16 am
content: I have no exact rating unfortunately the manufacture would not tell me, However the supply is rated for 37a continuous and 50a peak on the DC side secondary is centre tapped 17v x2So I'm guessing its about 500-750va it is quite largeInterestingly the manufacture states 120v version idle draw is 37watts. That is very close to my 60hz measurements at 240v

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2025, 12:42:03 pm
content: Quote from: jord4231 on May 22, 2025, 11:31:16 amAs it's close to the manufacture's specification, then it's obviously fine.Is it old? Perhaps modern industrial transformers are more efficient, with the recent drive to reduce wastage.

author: jord4231
author_rank: Regular Contributor
timestamp: May 22, 2025, 01:19:07 pm
content: I guess so, No its a new supply unfortunately. An old design I guess.I think it's just terribly inefficient at 50hzI guess one day if and when the transformer fails I'll pop toroidal one in. Until then I guess I'll add some more ventilation to the lid to try and keep the heat out of the box for the sake of the electrolytics

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2025, 06:34:35 pm
content: The no load current went down quite a bit with 60 Hz or the reduced voltage. This points to a transformer relatively close to saturation and for this reason more idle current. The current waveform shows quite some peak just before the zero crossing, just like expected for approaching saturation.So the way the transformer is designed, it does not like the high voltage low frequency case very much and thus a bit higher no load power than normal. It should still be no real problem.In a linear supply the other losses are likely still way more important, unless there is quite some pre-regulation.

author: jord4231
author_rank: Regular Contributor
timestamp: May 22, 2025, 11:04:22 pm
content: Quote from: Zero999 on May 22, 2025, 09:49:39 amThis is what I think I'll do I have a transformer that has a second primary of 30v that should knock me down to 210-212v this will get be where I need to be on the cheapThanks for the comment

author: jord4231
author_rank: Regular Contributor
timestamp: May 23, 2025, 09:27:10 am
content: Update. I have made up a buck transformer with a transformer I had laying around from an old AV receiver.The Astron supply now sits at 210v under load and is much quieter now (very slight hum).I will report back on the temp in a few more hours.I suspect this will be the permanent solution with the supply drawing about 35watts at idle now quite and improvement over the original 63wattsI would like to thank everyone who contributed to this thread and I hope that someone else may find it useful one day also.I have learnt a lot!

author: jord4231
author_rank: Regular Contributor
timestamp: May 23, 2025, 10:56:45 am
content: Update on transformer core temp.Now 45c after 3 hours at 210v that's 30c less down from 75c at 240v

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2025, 04:17:53 pm
content: Quote from: jord4231 on May 23, 2025, 09:27:10 amWhat does the output of the power supply look like? Is the ripple any worse?

author: jord4231
author_rank: Regular Contributor
timestamp: May 24, 2025, 11:26:23 pm
content: Just measured the ripple on the output with and without the bucking transformer on the line 4.8 ~ 5mv ripple, No significant difference found

author: MF-jockey
author_rank: Regular Contributor
timestamp: May 25, 2025, 04:35:08 pm
content: I have extensive professional experience building small and large transformers. The pictures in reply #35 clearly show that the core is reaching saturation.Either the core material is not good enough, or the number of turns in the windings is too small.If there is still some space between the coil and the core, additional turns can be added later and connected in series with the primary winding. The winding direction must be observed. This can also be done with stranded wire, but the insulation should be suitable for higher temperatures (>100°C). Since these turns are located on the outside of the coil, they have good air cooling and can be one step smaller in cross-section than the original winding. The secondary voltage naturally decreases accordingly with an increase in the number of primary turns. This reduction also occurs with ballast elements such as resistors or coils, but it is then highly load-dependent. This is not the case when the number of primary turns on the core is increased.The additional turns should be designed for at least 20V so that the transformer runs better at 240V/50Hz. First, thread a few test turns of thin wire and measure the voltage of this winding at 220V. Then calculate how many turns you need.