How to wire this Toroidal safely  - Page 1
Forum section: electronics / beginners

author: Dutch RC
author_rank: Contributor
timestamp: September 14, 2017, 09:36:33 am
content: Hi,I mistakenly bought 3 cheap ass Chinese power supply kits, they take 24v AC and output 0-30v 3A DChttps://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html?p=ZW051057416242016088&rmmds=searchNow I thought about trashing them because I couldn't find a cheap powersupply that provides 24vac until I spotted this Amplimo Toroidal @ my dad's place.It's a Amplimo 71012 and I can't find a datasheet for it.I'd say wire blue to yellow and I get 24vac 12.5AThis one has 2 blue 2 grey 2 red and 2 yellow. Does that mean it's double the output like 4*12v or am I missing something important.To prevent my house from burning down I thought I'd ask before I try. If you want me to measure something or more pictures please let me know.Regards,

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 14, 2017, 10:05:01 am
content: You take all the wires with the same color together: red on red, yellow on yellow etc.. to get 24VAC you join blue and yellow together; red and grey you'll use to connect the power supply circuit; actually this toroid is quite overdimensioned for your needs, but it works, if you don't mind its heavy weight.The explanation for that kind of wiring is the high current, to keep the diameter of the wire thin, they took 2 wires in parallel for each secondary.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2017, 11:00:14 am
content: You don't have to use both secondary windings, as the PSU only has a 3A output. In any case, use a fuse, both on the primary (2A slow blow) and on the secondary (5A slow blow). There should be a power switch and if it's in a metal case, it needs to be earthed.That kit has issues. The supply voltage to the op-amps, exceeds their maximum ratings and it's unlikely to give 30V out, without any ripple, with 24V in.Check out the following threads for more information.https://www.eevblog.com/forum/beginners/im-confused-about-power-supply-for-op-amps/msg933475/#msg933475https://www.eevblog.com/forum/beginners/bangood-psu-enhancements/msg897563/#msg897563

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2017, 11:08:45 am
content: Quote from: HB9EVI on September 14, 2017, 10:05:01 amIt is about right if he is trying to make a triple-output power supply with the three kits.

author: Yansi
author_rank: Super Contributor
timestamp: September 14, 2017, 11:14:20 am
content: Youre not gonna make three PSUs if you have only two independent secondaries.Also as was correctly stated, the design of the PSU is the "classic" internet rubbish, that exceeds voltages of the opamps, and is likely to not be able to give 30V without any ripple.24V AC in is good for maybe 20V dc out. (do not forget the fluctuations of mains voltage).Usually, 30Vdc power supplies are supplied with about 36Vac.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 14, 2017, 11:25:42 am
content: Quote from: Hero999 on September 14, 2017, 11:00:14 amare those links working for you? I'm just getting the forum index.Would be interested to see the schematic; I'm always wondering what those 'miracle' kits from Ali, Bang & Co. make go. Got this year an Audioamp-module from such source on the bench: 250mV and 180mV offset on the speakers, so I think I have some idea how that 'regulated psu' circuit is looking.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2017, 11:28:39 am
content: Quote from: HB9EVI on September 14, 2017, 11:25:42 amI've just corrected them. Try again.

author: Dutch RC
author_rank: Contributor
timestamp: September 14, 2017, 12:15:32 pm
content: Quote from: drussell on September 14, 2017, 11:08:45 amThanks all, and it's indeed my plan to power 3 kits from this one. I'm not planning to go above 12/15 volts on them. So I hope I'm good with this one. At least the price was right.

author: Brumby
author_rank: Unknown
timestamp: September 14, 2017, 02:04:31 pm
content: Might be this one....http://www.elak.be/product/details/velleman/electronic-components/30012/M6404949It has black for the primary - but otherwise looks good.Unfortunately, there's no datasheet I can find - but there are some specs.

author: Dutch RC
author_rank: Contributor
timestamp: September 14, 2017, 03:40:01 pm
content: Yup I found that one too,I'll be connecting the colors together, haven't found any other with seperate wires.

author: Dutch RC
author_rank: Contributor
timestamp: September 14, 2017, 03:54:54 pm
content: No fires, works fine with colors together and yellow with blue connected.I thought you could get away with lower AC voltage then DCI've seen another kit that needed 24v AC or 30V DC.

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2017, 04:02:49 pm
content: Quote from: Yansi on September 14, 2017, 11:14:20 amYou can do it fine as long as two of them share the same ground potential. If you want them to be stackable (usable in series) then you need them separate, obviously, but he could put two on one that use the same ground and have the third floating off the other set of windings in case he needed higher voltage / negative voltage, etc.Quote from: Yansi on September 14, 2017, 11:14:20 amQuote from: Dutch RC on September 14, 2017, 03:54:54 pmThat particular kit may be poor and he may well want to use something better but 24 Vac is about 33.5 Vdc once it is rectified and filtered so it should be darn close to being enough headroom for 28, 29, maybe 30 volts DC out if the circuit were decent.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2017, 06:20:52 pm
content: You'll be lucky to 25V out of this thing, at full load. Q4 & Q2 drop a total of around 2V and the output stage of U2 drops around 3V.That's the least of the issues. The TL082 is only rated to a maximum total supply voltage of 36V, yet this circuit gives it 38V or more, especially at light loads, when the voltage on the transformer will be higher than the rated value. I Googled the data sheet for a 300VA toroidal transformer.The first hitstates a regulation factor of 7.1%, so that's an open circuit voltage of 24×1.071 = 25.7V, giving 25.7×root(2) - 1.2V = 35V on C1, now note that pin 4 is connected to a negative voltage doubler and a 5V zener diode, giving -5V, making the total supply voltage 40V. This is assuming the mains voltage isn't also on the high side.One fix is to use a lower voltage transformer, a lower voltage negative power supply and an op-amp rated to 44V, but that won't get rid of the issue with ripple on the output, when the voltage setting and current drawn is high.

author: Dutch RC
author_rank: Contributor
timestamp: September 14, 2017, 10:53:04 pm
content: I'm aware this one isn't a good powersupply, I found out after it shipped. It was recommended by a friend who helped me with the search for a cheap power supply.Then I noticed that it needs AC instead of DC so the DC laptop adapters I bought were useless and I started a search for a cheap AC transformer, I was about to trash them because I didn't want to spend €75 on a transformer, When I was going through my dad's workshop I spotted this old toroidal and decided to give them a try when they arrive.If they don't fit my needs I'll have to search for something cheap and decent, I need at least 5v 1~1.5A and 12v 1~2AAnd maybe something else to power, therefore I bought 3.And 3x €60 is not an option for me right now. Just spent my hobby money on a Rigol DS1054Z which will arrive on Friday I hope.Hope this explains my choices for the ones wondering why I went for these cheap ones, my friend told me they're okay and I'm still learning.

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2017, 02:43:39 am
content: The negative rail doesn't need a lot of current, and its trivial to get a negative output from a cheap LM2596 DC-DC converter module off EBAY. Seehttps://iw0ffk.wordpress.com/2014/09/19/negative-voltage-from-lm2596-dc-dc-ebay-module/N.B. when determing the maximum input voltage in this configuration you *MUST* take it relative to the negative rail the module will generate.Therefore you can use your DC output laptop adaptors - just delete the bridge rectifier and D6 above, connect the laptop PSU across C1 and the LM2596 module to feed a negative voltage to C3.However its essential that the laptop PSUs *DONT* have grounded outputs as there appears to be a current sense resistor R7 on the low side of the supply, and if the laptop PSU is grounded that resistor would be shorted out if any other ground is connected to the load e.g if you simply use a scope probe.

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2017, 08:22:53 am
content: Quote from: Ian.M on September 15, 2017, 02:43:39 amThose are both good suggestions.The easiest way to ensure the laptop PSU isn't earthed, is to get one with a two pin mains power connector. Of course a continuity test can be used to check one with a three pin connector, but as far as I'm aware all laptop PSUs with an earth, have the output earthed.Quote from: Dutch RC on September 14, 2017, 10:53:04 pmHere are the recommended modifications. Of course it may not be practical to implement all of them but the more the better. The TLE2141 or MC34071 are recommended here for the op-amp, but I'd be more inclined to use the OP07 and omit RV1 & R10.

author: cdev
author_rank: Super Contributor
timestamp: September 15, 2017, 01:31:50 pm
content: If a transformer is not well marked, you need to keep phase in mind, so you combine windings in a phase aware manner so you wont be canceling out your signal (or shorting it out, if you are using two windings in parallel ..You can test them with a low voltage first.In schematics they use a dot.The last time I needed to do this, I used a low voltage AC signal from my cheap sig gen. It worked well.Quote from: HB9EVI onYesterdayat 04:05:01