Adjustable AC to AC circuit - Page 1
Forum section: electronics / beginners

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 13, 2018, 11:21:18 pm
content: Sorry but another one.....What do people think of this AC to variable AC circuit? Any improvements? Is there a better way of doing the same thing?

author: Cliff Matthews
author_rank: Unknown
timestamp: February 13, 2018, 11:34:48 pm
content: Did you mean AC-to-DC? Flip the diodes around though..

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 13, 2018, 11:40:18 pm
content: Quote from: Cliff Matthews on February 13, 2018, 11:34:48 pmNo specifically AC to AC ie AC 30V in and variable AC voltage out. I can draw the equivalent AC to DC one off the top of my head but it the staying with AC that is more complicated for me.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 13, 2018, 11:48:43 pm
content: Quote from: cowasaki on February 13, 2018, 11:40:18 pmI think you're going to be disappointed.D1 and D2 mean that the output is DC. If you swap emitter and collector of Q1 it might even function as a variable DC supply, but only "might".

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 13, 2018, 11:54:53 pm
content: Quote from: Andy Watson on February 13, 2018, 11:48:43 pmYes, just looking at it now. It's not my circuit but one I found whilst looking for what I wanted. I discounted everything with a normal rectifier but didn't really look at that one enough it's not the kind of power supply circuit I'm used to. I'd blame google but it's my fault.Will have to keep looking unless someone can point me towards something. Basically a voltage amplifier circuit which has a variable "gain" of 0.01 through to 1?

author: Andy Watson
author_rank: Super Contributor
timestamp: February 14, 2018, 12:05:28 am
content: Quote from: cowasaki on February 13, 2018, 11:54:53 pmIt depends how much power you want and at what frequency. For a true, variable AC supply you are looking at "bi-polar" supplies - sometimes HP units will appear on ebay at almost affordable prices. If what you want is mains-frequency AC, you might be better looking towards a "variac" - plus a transformer if you want isolation too.

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 14, 2018, 12:12:36 am
content: Quote from: Andy Watson on February 14, 2018, 12:05:28 amI wanted to create a variable AC power supply to allow me to design and build my own DC power supplies without having the mains transformer sat on my bench and without having lots of different value transformers in stock to use as building blocks. I was also going to use it to test power supplies at lower voltages etc too. I considered a variac but they are very expensive for the use I would get and if I could just buy one 30v transformer and build it into a case with a small circuit and a pot that would be wonderfulIf I can't then I will just have to keep a selection of transformers. I thought that with the expertise on here someone might have an answer

author: Cliff Matthews
author_rank: Unknown
timestamp: February 14, 2018, 12:17:25 am
content: You'll never be sorry having one's always handy for blowing things up Photonic Induction Style!https://www.ebay.co.uk/itm/Metered-Variac-Variable-AC-Output-Transformer-2-Amp-0-250V-TDGC2-0-5KVA-/262622014680

author: Cliff Matthews
author_rank: Unknown
timestamp: February 14, 2018, 12:25:25 am
content: I wonder how high this will bidhttps://www.ebay.co.uk/itm/Zenith-Variable-Transformer-/302634592641

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 14, 2018, 12:35:06 am
content: Here, buy this.https://www.amazon.com/Variac-Variable-Transformer-Isolation-1000va/dp/B006NGC6HU#customerReviews

author: Cliff Matthews
author_rank: Unknown
timestamp: February 14, 2018, 12:38:13 am
content: Quote from: Wimberleytech on February 14, 2018, 12:35:06 amHe's on the other side of the pond mate.. that's 120v not 230v (besides shipping would be an arm and a leg..)

author: Cliff Matthews
author_rank: Unknown
timestamp: February 14, 2018, 12:44:59 am
content: Question: You're buying a Keithley 2100 ahead of the purchase of a current limiting split bench supply? I have a foggy picture here..

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2018, 01:30:40 am
content: The best option for a variable mains frequency AC supply is a real Variac followed by a step-down transformer to trade off voltage for more current, and provide isolation. The step-down transformer should have a RMS secondary voltage of about 20% more than the max DC output voltage you want to give you enough adjustment range and *MUST* have a RMS secondary current rating of at least 1.5 times the max DC current you will draw from after a bridge rectifier.If you want 30V RMS max, a 230V 1A Variac feeding a 230V to 30V stepdown transformer would be good for 7.66A RMS out - if the stepdown transformer secondary was rated for more than that. That means you could use it to test a 5A linear PSU design without overloading it.Therefore you don't need a very large and expensive Variac. A 1A one is usable, a 2A one is better and anything over 5A is probably overkill.N.B. put a circuit breaker or fuse on the Variac output to match its wiper current rating - Variacs tolerate brief overloads well up to a few times their rating, but sustained overcurrent will burn out the wiper and probably destroy the winding as well. Ideally, put an analog ammeter in series with the Variac output with a redline marked at its rated current.If you were in America, another option would be an old Lionel train controller, as before the modern electronic ones, they were isolated 0-18V Variacs.

author: james_s
author_rank: Super Contributor
timestamp: February 14, 2018, 02:21:59 am
content: A small variac may well be cheaper and easier to find than a vintage Lionel train controller, those are collectible.A variac or variable autotransformer is a handy device to have around.

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 14, 2018, 06:52:22 am
content: Quote from: Cliff Matthews on February 14, 2018, 12:44:59 amI have a current limiting bench power supply already. In fact my bench equipment is pretty good but mainly aimed at my current experience of digital electronics. So my scope has a logic analyser etc. My current bench power supply is current limiting but reasonably rudimentary and a better one is on the cards depending on what I find/decide on.Basically I did A level electronics 30 years ago but pretty much my only interest has been computers and therefore mainly digital. I have designed and built computer add-ons and repaired computers at a component level. I designed a micro controller based camera trigger system but found the sensors (which were mainly analogue a real challenge). My only real analogue experience relates to analogue circuits within computers such as the odd audio amp or sensor which I can test and work out what is wrong before fixing. I have learnt about power supplies and designed my own but the first thing you do is convert to DC so again little AC experience. I do however want to start looking at this area hence my current questions and the purchase of the Keighley. I'm probably better at the software than the hardware at the moment but again I want to address this and learn!There will be a lot more questions but if it's an area I know about I will help by answering other peoples.

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 14, 2018, 06:57:42 am
content: Following this post it would appear that there is no easy solution. So I’ll just keep a selection of transformers and look out for a variac at the right price. I did look for a variac first but all the ones I found at a reasonable price were 120volts!

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 14, 2018, 07:36:49 am
content: Quote from: Ian.M on February 14, 2018, 01:30:40 amThanks.

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 14, 2018, 07:47:00 am
content: Quote from: james_s on February 14, 2018, 02:21:59 amLast time I looked I couldn't find anything but just looked again and here we are:https://www.ebay.co.uk/p/3-Amp-Variac-Variable-Transformer-500va-Max-0-240-AC-Volt-Output-Regulator/2145067828?iid=272778720447&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D49130%26meid%3D55c103bca7a44afcbba77e994f949d7f%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D253063627227%26itm%3D272778720447&_trksid=p2047675.c100005.m1851ORhttp://www.ebay.co.uk/itm/Enclosed-Variable-Autotransformer-Variac-for-Bench-Mounting-1ph-6A/332174705926?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D49130%26meid%3D3d006da0370f4cbcbbbfac0e10dde570%26pid%3D100005%26rk%3D5%26rkt%3D6%26sd%3D322928711825%26itm%3D332174705926&_trksid=p2047675.c100005.m1851Not sure what it's going to be like but basically it appears to do just what I need although being able to limit the current would have been useful. I'm off to work now so will have another look later. If I can add something after the variac to limit the current that might work too but again I'll have to look later.

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2018, 09:44:44 am
content: The 6A Ravistat one looks like its decent quality. Seethe manufacturer's specs. For UK bench use, I'd mod it to add a 6A panel mount circuit breaker (or 5A if you cant find a 6A one) on its output if it doesn't already have one, and add a IEC C13 socket for its output so you can easily hook it up to various adapters e.g. aQuicktest QT1or a trailing BS1363 13A socket, or your step-down transformer for linear PSU design testing. To minimise the metalwork you may choose to use a trailing lead mounted IEC C13 socket. I wouldn't advise using its binding posts for anything other than monitoring its output voltage on a DMM - keep them screwed down so they aren't an immediate touch hazard.Limiting the current from any AC PSU except a fully electronically controlled four quadrant bench PSU is *HARD*. Its easy to trip a breaker on over-current but to limit the load current without distorting it and maintain voltage regulation up to the current limit requires fast acting feedback to the output voltage control, which cant be achieved with a mechanical control like a Variac. Even if you motorise it, mechanical inertia will limit its response speed to a significant fraction of a second.A poor-man's alternative for AC current limiting is adim bulb testerwhich uses a series incandescent bulb to limit the current utilising the x10 resistance increase of a tungsten filament between cold and its normal operating temperature. Either use a low voltage one on the output of your step-down transformer, possibly utilising 24V truck bulbs, or a mains voltage one on its input. Only use genuine incandescent filament bulbs - no LED or CFL bulbs! Its advantageous to have a couple of bulb sockets wired in parallel for more choice of limiting current by fitting different combos of bulbs. You can test the effective limit by putting a heavy resistive load on your Variac (a 500W to 1.5KW oil filled heater or bar fire or radiant heater or water heater would be appropriate, but a fan heater would not as the fan wouldn't run well at reduced voltage and the element might overheat) with the dim bulb tester in the feed to it, and cranking up the output voltage while monitoring the input voltage and current. If you are careful not to exceed the 6A output current rating of the Variac, an electric kettle filled with cold water will do for brief testing, though for a 3KW kettle, you wont be able to draw more than about 4A into the Variac before you reach its max output current rating.

author: Cliff Matthews
author_rank: Unknown
timestamp: February 14, 2018, 11:12:40 am
content: On the UK eBay site, I see lots of good prices on 120v variable transformers (in a land of 230v that's odd..). You could buy one and front-end it with a cheep as cheese, universal 120-240-120v power convertor. These are an auto transformer like a variac but fixed as 2:1 ratio. Hard to add anything more to what Ian has said though...Isn't this forum just great?

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2018, 11:25:55 am
content: I wouldn't go for a 120V Variac + a stepdown transformer to feed it for general bench use anywhere in the world that uses 220V-240V AC. Its just too useful to have the full output voltage range.Even in North America a 220V Variac is a better choice if you can source one at a price you can afford. There's no problem running one from 110V (with output of half the nominal dial setting) and when you do need a full 220V (+10% to 20%) you've got the option of either running it from a 220V circuit or it may even have taps that let it boost to over 220V from a 110V feed, at somewhat reduced output current capability, when boosting above double the input voltage. Seehttps://www.eevblog.com/forum/repair/refubishrepair-(and-modify)-general-radio-variac-v5h/

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 14, 2018, 02:10:45 pm
content: Quote from: Cliff Matthews on February 14, 2018, 12:38:13 amGranted. But I was making a broader point: this is a case for buying your test equipment than building it.

author: not1xor1
author_rank: Frequent Contributor
timestamp: February 15, 2018, 06:44:19 am
content: Quote from: cowasaki on February 13, 2018, 11:40:18 pmCheck if you can find Elektor June 1991, "Variable A.C. Power supply". (Sorry I can't provide a link)The circuit uses power BJTs as variable resistor across a diode bridge.You might take inspiration from that.

author: cowasaki
author_rank: Frequent Contributor
timestamp: February 15, 2018, 07:55:36 am
content: Quote from: not1xor1 on February 15, 2018, 06:44:19 amI will check this out and see what I can make of it. Thanks

author: Cliff Matthews
author_rank: Unknown
timestamp: February 15, 2018, 11:09:22 am
content: That reminds me of the AC pre regulator in the Keithley 2001... MarcReps has a nice video on it (same bridge concept but uses a MOSFET instead of BJT).https://www.eevblog.com/forum/testgear/9-minutes-of-te-fun-with-marco-reps-on-keithley-2001-repair/

author: paulca
author_rank: Super Contributor
timestamp: February 15, 2018, 12:00:14 pm
content: I have one of these:https://www.ebay.co.uk/itm/Motor-PWM-Controller-Kontroller-Regler-Steuer-Steuerung-Schalter-Dimmer-AC-220V/172930179230?hash=item284371209e:g:Xi8AAOSwq8BZ5FIRAlthough mine is 40A and smokes occasionally when I run a 13A boiler off it.It's a bad habit.