AC lab power supply - Page 1
Forum section: electronics / general

author: p-wave
author_rank: Newbie
timestamp: September 12, 2025, 04:51:34 pm
content: I need advice, how to implement ac voltage stabilization and protection system for my home lab, is there anyone who has done it? In which ways?After same research in the forum it seems that there aren’t many discussion about itThe number of instruments has now become important, as has their value, and I wonder what is the best way to protect them and reduce their possible variations in measures.The problem arises from the fact that the mains voltage in my area undergoes strong variations and on weekends it also reaches values of 256 volts acI ask for possible suggestionsThanks in advance for the helpp-wave

author: TimFox
author_rank: Super Contributor
timestamp: September 12, 2025, 04:54:16 pm
content: Have you looked into Solatmtransformers?Specifically, “harmonic neutralized” units with sinusoidal output (not clipped).

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2025, 09:37:52 pm
content: I have been using an online UPS for my workbench instruments.

author: p-wave
author_rank: Newbie
timestamp: September 13, 2025, 10:31:52 am
content: About resonant transformer I think this is an interesting solution that I had not considered, I’ll take a look on the specs, Thank you

author: EC8010
author_rank: Frequent Contributor
timestamp: September 13, 2025, 12:36:12 pm
content: I have a Claude Lyons Regavolt for my bench. It's averyheavy 4U rack-mount lump containing a buck/boost transformer good for 10A to the bench and a motor-driven Variac, itself driven by a germanium (!) error amplifier. It can periodically be heard twitching to hold mains voltage constant to about 0.5V . I rescued it from a skip in about 1986, fixed it (open-circuit wirewound resistor) in 1998, and it's powered the bench ever since. I did once try a constant voltage transformer but it did terrible things to mains shape that other mains transformers didn't like. Flat topped mains is essentially DC for a short period and saturates mains transformers, causing leakage. The motor-driven Variac solution sounds crude but actually works very well.I also have a two-stage mains filter preceded by a 300J 275V MOV to make the mains moderately clean. If needed, I have a TTi AC1000A low distortion power source that can correct the (flat-topped) mains to a sine wave. And if I suspect high frequency muck, I have a 500VA Topaz Ultra Isolator transformer that has Cps = 5fF for rejecting common-mode stuff. I have just discovered that Claude Lyons used to make a similar transformer; TRX.EDIT: I have just discovered that Claude Lyons still make their TS Series of motor-driven Variacs for stabilising mains voltage...

author: octillion
author_rank: Contributor
timestamp: September 13, 2025, 02:32:06 pm
content: Ferroresonant transformers (like Sola) have impressive regulation capabilities, but there are also some real down sides. As long as the frequency remains stable (which should be true of utility power, maybe not a small portable generator) the voltage regulation will remain stable over a wide range. They can also clean up severe harmonic distortion (if you feed a "modified sine wave" type square wave in, the output will be rather sinusoidal), but their output (even with a clean sinusoidal input) will have some distortion.The biggest disadvantage is probably just that; for a given kVA rating, they tend to be much larger and heavier than what you would expect of other transformers (like isolation transformers, Variacs). They are also much noisier, producing a buzzing noise that may be annoying if nearby out in the open.One interesting joint advantage/disadvantage is the built-in magnetic shunting means the output current is regulated and you can short the output indefinitely without damage. This could be advantageous if using the output on the workbench and you want some protection. The disadvantage is there is essentially no surge capacity, which means certain devices like motors, which otherwise fall within the VA rating, may not operate.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: September 13, 2025, 03:24:37 pm
content: I also vote for ferroresonant transformers, they are virtually indestructible.In addition to the acoustic noise mentioned previously, the other drawback is that they are inefficient, they have a steady state power consumption.

author: Bernd_2
author_rank: Contributor
timestamp: September 16, 2025, 05:39:25 pm
content: Hello,I use a Bluetti AC300 Powerstation with a 275 V MOV at the output to feed my lab equipment. It gives a clean sinus with 230 V and 50 Hz. The varistor serves to attenuate surges caused by nearby lightning strikes or when switching inductors.So my lab equipment ist protected from transients from the public grid and the current from the AC300 has significantly less THD as the current from the grid.Best RegardsBernd

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 18, 2025, 05:19:04 pm
content: for basic a filter :here is link below board of industrial suppressor for heavy duty printers ...pretty clear idea ....https://i.ibb.co/Z1kh517t/supp.webpas it 90x and for 208 , not 120 , i do modify some for 120 , decreasing MOV ,GDT, changing capacitors as it may wear out , adding TVScheap UPS usually have MOV only, rely on disconnection over surge ... but not fast enough , have some incident as equip fry (lucky not my), ( lighting hit nearby small transformer an it exploded )combination of everything in the filter then UPS , would be probably best reasonable approach for a home lab under 1000W load at max ...( additionally I'm thinking to connect 2 transformers back - forward from dead 1500W UPS's for isolation ... just thinking about )stabilizing: search for Constant Voltage Transformer (CVT) as it most trade name would be

author: adam4521
author_rank: Regular Contributor
timestamp: October 11, 2025, 09:14:59 am
content: Those Sola/ferro-resonant transformers are fascinating -- never come across those! I also discovered that the motorised variac type of regulator must be selling into some kind of popular market -- there are astonishingly low-priced ones available on Amazon -- 500VA for £22.99 delivered, complete with input/output voltage displays (KEMOT SER-500). Must be something wrong with it...Anyway, for my own part, I'm not looking for a 'continuous' supply, but rather a closely regulated one that I can use just for test purposes, hence interest in this thread. The core of my plan is to use a second-hand audio amp (I have a Yamaha AX-496, 85W RMS per channel) to drive a 230/12V toroidal transformer in reverse, connected to an outlet. I will feed a pure 50Hz sinewave into the amplifier.I was hoping to use a Wein bridge network around the amp to make the whole thing self-exciting, but when I've tested that network in LT Spice I'm finding my coupling into the amplifier is loading the oscillator too much, so it doesn't work properly. I don't want to use a general purpose/bench signal generator due to the considerable risk of mis-placed button push sending HF through the amp (the Yamaha apparently has output power capability out to 100kHz!).My new plan is to make a single purpose LF sine generator with some ability to trim amplitude and frequency. Thinking of checking out a technique in the Horowitz and Hill book of 'sine from square' which uses a tracking 8 pole filter that you can get in an IC. I will possibly follow with an op-amp with a negative feedback network via a small step-down transformer (1.5W) that will feedback from the output and improve the apparent regulation of the whole system.I've looked into 'pure-sine' inverters, decided that I didn't want the switching noise. However, not having done this before, I'd welcome any hints and ideas.

author: rikkitikkitavi
author_rank: Contributor
timestamp: October 11, 2025, 12:17:43 pm
content: Quote from: p-wave on September 12, 2025, 04:51:34 pmSince you are flagging Italy, and it is a part of the European union mains voltage (1N) is 230+-10%, ie 207-253 V. 256 is outside permitted range. Or your meter is not showing the correct value.Have you notified your grid operator ?As for variations within the tolerance band, there have been some pretty good replies.I presume frequency is stable.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 11, 2025, 02:08:16 pm
content: Quote from: adam4521 on October 11, 2025, 09:14:59 amBeware! I blew up a perfectly innocent 100W amplifier doing the same thing. The problem is that the transformer puts a lot of shunt capacitance across the amplifier that can make it go unstable, oscillate, and destroy itself. A zobel network may help. If I were you, I'd measure impedance against frequency of the proposed transformer up to 300kHz before connecting it to the amplifier.Definitely don't try to wrap Wien bridge round the power amplifier. Make a low level oscillator that works, then connect it to the amplifier and turn the volume up.Also beware that most electronic equipment is rather a nasty load, drawing large current pulses. Your 85W won't go far.

author: adam4521
author_rank: Regular Contributor
timestamp: October 11, 2025, 07:25:42 pm
content: Thanks for those warnings! I'll try and characterise the transformer model with some measurements and you have also got me thinking about the X capacitors that are present on most 'modern' loads - maybe they could also cause a problem. Damping - noted, and test with small amplitude to begin with! Yes, SMPS loads are very peaky - the amplifier could easily run out of puff if I put much of that on it.

author: coppercone2
author_rank: Super Contributor
timestamp: October 11, 2025, 07:34:46 pm
content: Quote from: EC8010 on October 11, 2025, 02:08:16 pmis this a torroidal transformer that did this?I think type of winding might be very important. The 'wound over each other' transformers have the highest capacitance. Speakers also often have a PTC in series with the winding, in addition to any filter, which might clue you in that the amplifiers that are sold with them might not be that robust

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 11, 2025, 07:41:38 pm
content: What do you want to make exactly? If it's an lab AC power supply, then using an audio power amplifier followed by a transformer is a pretty decent setup. It's also used in the GW-Instek Mains voltage test gadget that Dave made a video about recently.If you only want to stabilize the mains voltage in your lab due to fluctuations on your mains net, then the good old setup with a variac and a servo motor that automatically adjusts it is probably good enough. You can also use a biggish toroidal transformer and add a bunch of low voltage windings and relays to switch between them.You can also make an easy differential transformer setup. Assume you have a 6V 20VA mains transformer (so suitable for 3A on the low voltage winding). By putting that 6V winding in series with your mains voltage, you can either subtract or add (Depending on how you connect it) those 6V from your mains supply, and this is good upto the 3A current, which will be 650VA on the mains voltage side.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 11, 2025, 08:52:39 pm
content: Quote from: coppercone2 on October 11, 2025, 07:34:46 pmIndeed it was. Toroids tend to have quite high capacitance compared to a similarly rated EI transformer.As Doctorandus_P points out, the solution all depends on what your actual requirements are...

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 12, 2025, 06:56:13 am
content: Capacitive loads on audio amplifiers can be a problem (I.e. Capacitive loads on an opamp can also be a problem). Apart from a zobel network. Do you think that limiting the bandwidth of the audio amplifier to 100Hz or so is a good option? This can be achieved by putting a small capacitor over the resistor in the feedback network.Seems to be a good idea. See page 62 of the pdf below where a 6.2pF capacitor was added to an opamp.https://www.ti.com/lit/ml/slyp716/slyp716.pdf

author: EC8010
author_rank: Frequent Contributor
timestamp: October 12, 2025, 08:23:54 am
content: Both op-amps and audio power amplifiers are applications where performance is defined largely by feedback. However, what's inside the feedback loop is still important, so I think it's dangerous to make many generalisations. If you had a circuit diagram for the power amplifier, you could simulate it in SPICE and see whether it objected to having its bandwidth reduced. That would be a guide. I would be inclined to limit to 10kHz rather than 100Hz as the amount of capacitance needed to limit to 100Hz might be enough to make the amplfier unstable. The O/P's amplifier is 85W, which is probably enough that it's a discrete component design and therefore a diagram might be available.

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2025, 02:11:15 pm
content: Another more clever but more complex method is to use a power amplifier to drive a transformer but with a lower voltage output, which is placed in series with the AC line. A feedback loop from the AC output is now compared with a 60 Hz oscillator phase locked to the input AC line and the difference is used to drive the transformer to produce a purified full power AC output. This design has the advantage of requiring only a fraction of power from the power amplifier and transformer.The make "line stabilizers" that work this way but they are hard to describe for purposes of finding them.Quote from: Doctorandus_P on October 12, 2025, 06:56:13 amA Zobel network is more about preventing parasitic oscillation in the output stage, which has its own local feedback loop.QuoteNo, and using the feedback loop to control bandwidth can cause other problems. A better method is usually to add phase lead, possibly by adding a resistor (zero) in series with the frequency compensation capacitor, but there are lots of ways.QuoteThat example adds 6.2 picofarads of capacitance to cancel the effects of parasitic capacitance at the inverting input.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 12, 2025, 02:39:58 pm
content: Quote from: David Hess on October 12, 2025, 02:11:15 pmI have a TTi AC1000A Low Distortion Power Source that works in that way. It effectively restores flat-topped mains to sinusoidal. What it doesnotdo is to regulate amplitude, and it inherently cannot regulate frequency. So what use is it? It gives a level playing field for pre-compliance EMC testing (where your darling will also be supplied with sinusoidal power).

author: coppercone2
author_rank: Super Contributor
timestamp: October 12, 2025, 08:04:59 pm
content: it might also let a cheap scope work on cheap generator power

author: EC8010
author_rank: Frequent Contributor
timestamp: October 12, 2025, 08:14:09 pm
content: Quote from: coppercone2 on October 12, 2025, 08:04:59 pmThat's an interesting thought. But a cheap digital 'scope almost certainly uses a switched mode power supply and doesn't really care about the waveform because it immediately rectifies the input. But an analogue 'scope (CRT) and elderly analogue kit would be rather fussier.