Asking for review / validation : making a very simple AC transformer box - Page 1
Forum section: electronics / beginners

author: gwelavo
author_rank: Contributor
timestamp: July 24, 2023, 12:41:23 pm
content: Hello everyone,I decided to re-learn electronics from the basics. For that, I'm currently reviewing the 6.002 MIT course while reading Foundations of Analog and Digital Electronic Circuits. I'm also following Learning the Art of Electronics while reading The Art of Electronics. My end goals are 1. play with RF, I find the subject "magical" and 2. better understand the digital world as a Software Engineer.I didn't buy any kits for Learning the Art of Electronics, I purchased a DS1054z, a DP832, and a SDG 1025 a few years ago when I thought I would have the time to learn (I got 3 kids in the meantime, I think I'm good to go now).I managed to move forward in the last 2 weeks without much of a struggle even though I don't have access to a 6.3 AC transformer. I combined the 2 outputs of my function generator to produce a 60 hz signal with 10 khz noise on top of it with success (I know the amplitudes changed).Now I still find the exercise of making a box for such a transformer fun... So I bought this:https://docs.rs-online.com/714b/A700000008857720.pdfand everything on the bench in the attachments. I made a very crude schematic of how I want to plug the transformer with the fuse box and the switch. Iassumedthe phase is the best line to place both the switch and the fuse box.The transformer will be mounted on a 0.75 mm thick piece of steel, grounded, to secure it to the box.Is the plan sound ?If I understand things correctly, I will have access to 6.3 Vrms A.C. between COM and the other two plugs and 12.6 Vrms between the two plugs (read and green).Is that correct?Sorry for the dumb questions, this is still new to me.KevinP.S.: This is my first post on this forum, I don't know if I must introduce myself somewhere.P.S. 2: I fucked up when ordering the banana plugs... that's why I have a green and a gray instead of 2 reds and a black

author: tooki
author_rank: Super Contributor
timestamp: July 24, 2023, 01:45:12 pm
content: It’s fundamentally correct as you describe.However, I’d suggest a few changes:1. Use a double-pole switch and disconnect both line and neutral, since the French plug does not enforce polarity if it’s ever plugged into a non-grounded outlet (or, for example, plugged into a Schuko outlet.) That way it’s safe no matter what.2. Use four banana jacks and run each secondary wire separately. That will give you the flexibility to connect them in series to get 12.6V, in parallel to get 6.3V (with double the current), or to use them independently as two floating 6.3V sources.Alternatively, you can use a DPDT switch to switch two jacks between series and parallel.3. Get the right color banana jacks: I wouldn’t use green for anything except earth ground. You could use it as a third/fifth jack to earth ground.4. Add output fuses. A 1.6A fuse on each secondary should work nicely. Put the fuse on each secondary before any points where you connect the secondaries together.

author: gwelavo
author_rank: Contributor
timestamp: July 24, 2023, 03:10:15 pm
content: Thanks a lot for that!1. Good point, the switch already support it.2. Indeed, it will make more sense and be more "generic", I will order proper colored banana jacksAlso good idea about the DPDT, I think I already have one I can use. There would be 3 positions: independent, in series, and in //. I will also need to make some sort of a sticker for the front panel to document what's what (how jacks are connected depending on DPTD state).3.4. I didn't think about that. I have everything I need here.A follow up question:Would you use fast blow fuses or slow blow fuses?

author: tooki
author_rank: Super Contributor
timestamp: July 24, 2023, 05:11:42 pm
content: Quote from: gwelavo on July 24, 2023, 03:10:15 pm2. Maybe take a look at the schematics for some bench DC power supplies to see how they handle the series/parallel switching. Maybe with a multipole (3P or 4P)l 3-position switch you can have one switch that does it all.4. It depends on your load, but I don’t think it really matters here. We mostly want to protect against substantial overcurrent, where either type is fine.At really large overloads (e.g. 50x rated current) either type will blow practically instantly. At a medium overload (e.g. 3-10x), a fast-blow fuse blows faster than the slow-blow. But at very small overloads (e.g. 1.5x), the slow-blow fuse may actually blow faster than the fast-blow.The thing that perhaps speaks in favor of slow-blow is that if you ever end up doing anything with motors, they often take a spike of current to get going. So I’d probably put in a slow-blow and call it a day, but if you have a fast-blow in your drawer already, use that.

author: gwelavo
author_rank: Contributor
timestamp: July 25, 2023, 08:44:30 am
content: I think a DP3T is good enough for my use case, see the revised plan in the attachments. I should receive the correct banana jacks today. Once it is ready, I will share the end result.

author: tooki
author_rank: Super Contributor
timestamp: July 25, 2023, 12:36:10 pm
content: Quote from: gwelavo on July 25, 2023, 08:44:30 amNice.Looks like you don't even need a DP3T switch, just an On-Off-On DPDT.

author: tooki
author_rank: Super Contributor
timestamp: July 25, 2023, 12:36:51 pm
content: Oh yeah: one last thing I'd add, if not already present in the switch, is a power light!

author: gwelavo
author_rank: Contributor
timestamp: July 25, 2023, 01:49:56 pm
content: The light is built into the power switch and I will in fact use a on-off-on switch as it is what I already have. I remember having this switch when I built my first few kits... 25 years ago .

author: Jwillis
author_rank: Super Contributor
timestamp: July 25, 2023, 02:38:31 pm
content: Looks good. Well done so far. I didn't notice or see a reference of double shielded power cord for the mains supply. It would be an added safety feature and it would look better.

author: tooki
author_rank: Super Contributor
timestamp: July 25, 2023, 03:15:03 pm
content: Quote from: Jwillis on July 25, 2023, 02:38:31 pmWhy would you need any shielding at all on the mains cord, never mind double shielding?!? The primary place you see shielded mains cords is in audiophool setups...And shielding is not a safety feature anyway, it's to prevent noise from getting in (or out).

author: Jwillis
author_rank: Super Contributor
timestamp: July 25, 2023, 09:18:17 pm
content: Quote from: tooki on July 25, 2023, 03:15:03 pmSimple misinterpretation on my part. My apologies. I meant doubleinsulatedpower cord. 18AWG would be fine.

author: coppercone2
author_rank: Super Contributor
timestamp: July 25, 2023, 09:19:54 pm
content: oh yeah if you put banana jacks on it put a double switch in case its ever hooked up to split phaseshielding on the cord won't do anything unless you have a fully shielded run coming from a shielded generator (gas or mechanical isolator) or possibly inverter/battery. Probobly not the cleanest power but the contents would be stable/known. And then you want a shielded connector too.That's kind of expensive and specific. Might be useful on a ship or something that is built to have a shielded generator.Speaking of audio does anyone have a ten ton diesel generator powering their amplifiers?But if you have conduit runs, that are properly wired, it might do a tiny bit of filtering because the noise from the power pole wires might not get through the entire wiring network. At least certain frequencies, despite the fact that its hundreds of miles of wire. In that case a shielded cable might do some thing incredibly small. Like if its a long ass conduit run but you have a transmitter in the room, then the wire is not picking that up, and the conduit shields the main run. Very specific situation....So basically you should just spend the money you save on a shielded cable on higher quality banana connectors and internal wiring. You can do something like splurge on teflon wire or whatever. Shielded mains cable is the last thing you need. At least nice connectors might do something lol

author: coppercone2
author_rank: Super Contributor
timestamp: July 25, 2023, 09:48:02 pm
content: unless he means a double insulated extension cord, say rubber over something. Preferably with different colors so you can see damaged wiring (orange under black). That is a very nice feature to have in the shop, and some labs.

author: themadhippy
author_rank: Super Contributor
timestamp: July 25, 2023, 10:19:08 pm
content: Quoteyep,far too many times to remember.

author: tooki
author_rank: Super Contributor
timestamp: July 25, 2023, 10:20:37 pm
content: Quote from: Jwillis on July 25, 2023, 09:18:17 pmAh ok. Yeah, shielding and sheathing (insulation) are different things, as you know!Anyhow, in Europe, single-insulated mains cable isn't allowed at all, so you always get double-insulated, even on the dinkiest 0.5mm2(20AWG) mains cord for 3A Europlugs is double-insulated. (The only potential exception might be Christmas lights.)

author: tooki
author_rank: Super Contributor
timestamp: July 25, 2023, 10:24:26 pm
content: Quote from: coppercone2 on July 25, 2023, 09:48:02 pmIs it not enough to have the outer jacket (e.g. typically white, black, or gray for ordinary cable, or orange, red, or yellow for high-visibility outdoor cable) and the brown (line), blue (neutral), and striped green-yellow (protective earth) insulation of each individual conductor? You definitely see when the jacket is damaged through to the conductor insulation.

author: gwelavo
author_rank: Contributor
timestamp: July 26, 2023, 07:19:26 am
content: I put the cable I will use in the attachments, it is pretty standard in France and pretty much all the European countries I have resided in. For the complete reference it is 3G1,5 (3x15AWG) H05 VV-F (PVC isolation on the coper core and another layer of PVC on the 3 cables).Still waiting for the last components to arrive... in the meantime I'm looking at how to make clean front panels. Any advice? I thought about making a sticker but I'm not sure it will be really clean this way.

author: tooki
author_rank: Super Contributor
timestamp: July 26, 2023, 08:58:55 am
content: Quote from: gwelavo on July 26, 2023, 07:19:26 amYou mean labeling it?There are many ways. From simple labels made with a p-touch, to engraving and filling with paint, to laser etching. One method I’ve been meaning to try is one I saw not too long ago, of using a laser printer to print onto paper as a mirror image, then clear lacquer onto the panel and applying the laser-printed transfer and letting it dry before peeling off. (Unfortunately, most only show this onto wood, so I’m not sure if the absorbency of the wood is important to the process.) I’ve also seen a similar process using inkjets and aqueous acrylic lacquer.What I have also done is use the Avery laser nameplate labels. Use a really big one that can cover the whole panel (you can just get whole A4 sheets), laser print it, and then punch or cut the holes and trim it down to size.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 26, 2023, 12:16:58 pm
content: These days it is common to include a RFI filter on the mains input of power supplies. You may not need it.. You can purchase them cheaply. They come with the proper X and Y capacitors. I prefer the ones with the three prong plug.This is what I am talking about. Not this one but a new one. Or possibly one from a discarded electronic component you have. There are better ones that are bigger and do not have the three pronged plug.https://i.ebayimg.com/images/g/nLAAAOSw~KpgCgzO/s-l1600.jpg

author: gwelavo
author_rank: Contributor
timestamp: July 26, 2023, 03:35:34 pm
content: Good idea, I will definitely use this for the next appliance. This one is for learning, I guess the less filtering at input the better the learning experience will be.

author: coppercone2
author_rank: Super Contributor
timestamp: July 26, 2023, 09:40:07 pm
content: Quote from: tooki on July 25, 2023, 10:24:26 pmI dunno its just a thing they doAnd since its two layers you can have one that is more prone to tearing but has superior resistance properties while maintaining full safety. Overall it might lead to a longer life span. I think its usually rubber over something else.

author: gwelavo
author_rank: Contributor
timestamp: July 26, 2023, 09:59:26 pm
content: Progress is slow, I'm trying to figure out how to place the elements. I think I'm close. I will add more space between the banana jacks for labeling and move the selector switch close to the power switch next. The box is not wide enough to line up the banana jacks, I triedI tried the toner transfer trick on the ABS panels without success. I managed to copy Cosmo text to the panelsI ordered some transfer paper for Laser printers to see if I can do better, my warhammer supplies might be useful afterall.I'm now experimenting with multi-colored 3d printed panels. The fit is pretty good and the cutouts are perfect. I will try a combination of white and red as I just burned the black spool.If nothing works, I will try to engrave black painted acrylic with my shaper origin. I never ever tried to cut something else than wood with it though.

author: gwelavo
author_rank: Contributor
timestamp: July 28, 2023, 12:08:19 pm
content: It is alive!I'm still waiting for the transfer paper to make a better front pannel. That will not stop me from using it and continue Learning the Art of Electronics.Parallel and series modes are working fine. I attached what I see with the scope. 7.5 Vrms is within tolerance without loads.Now there is something weird happening when the outputs should be independent. I don't really know if it is normal. You can see the outputs on the multimeter between the 2 reads and blacks jacks. I don't know what I should expect but I see 4 V across the jacks.What do you think?

author: Ian.M
author_rank: Super Contributor
timestamp: July 28, 2023, 12:26:01 pm
content: Connect a 10K resistor between the two red terminals and re-measure. I expect the 4.1V will drop by more than an order of magnitude, in which case its probably due to capacitive leakage, and you can calculate the leakage current by ohm's law. You can confirm its not an insulation defect by a power off resistance check between the two windings.N.B. unless the transformer is specifically designed and constructed for it, don't expect a high insulation voltage rating between secondaries. I wouldn't be happy running your transformer with more than 50V between them even though its primary to secondary isolation rating is 4KV RMS!

author: gwelavo
author_rank: Contributor
timestamp: July 28, 2023, 12:47:20 pm
content: I checked and indeed, the voltage dropped to 8.8 mV with a 10 k resistance between the two red jacks. There seems to be no insulation defect as shown in the pictures. If I'm not too dumb the leakage current is around 880 nA. Thanks for insights.

author: Ian.M
author_rank: Super Contributor
timestamp: July 28, 2023, 01:05:19 pm
content: Assuming the interwinding capacitance is between the outer layer of one secondary and the inner of the other, they are effectively in series with the capacitance between them. I estimate the capacitance is around 200pF, reasonable for the transformer size and construction.A simple calculation from Zc=1/(2𝜋C) gives 187pF, but that ignores the voltage drop across the end layer of each winding. Depending on the number of turns per layer and total turns, the actual capacitance could be a factor of two greater.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 28, 2023, 03:54:43 pm
content: Another measurement worth making: AC voltage from the red or black to protective ground. You will be surprised!

author: Ian.M
author_rank: Super Contributor
timestamp: July 28, 2023, 04:16:00 pm
content: Yes, and again, see what difference the 10K makes, and if its different for the two secondaries when they are independent.You've still got enough room on that panel above the power switch to add a binding post for Mains PE ground, which is useful to have if the circuit you are working with needs a ground reference, and you said earlier you had a spare green binding post . . .

author: gwelavo
author_rank: Contributor
timestamp: July 28, 2023, 05:29:46 pm
content: Interesting. I will do the measurements tonight. Just to be sure I understand correctly, in order to reproduce figure 2L.10 of the book (see attachments), I just need to make sure the "output" is plugged to earth (either the one of the function generator or the earth from my power supply, they are the same).In case I have a circuit that depends on earth, I will then need to connect either the black or the red jacks to earth in order to avoid the difference of potential.

author: EPAIII
author_rank: Super Contributor
timestamp: July 30, 2023, 04:26:03 am
content: If you are going to go that route, why not get one with the fuse holder also built in. One part gets all of it: cable entry with disconnect ability, filter, and accessible fuse. And you only need to make one mounting hole in the box.Quote from: Wallace Gasiewicz on July 26, 2023, 12:16:58 pm