I need help finding a transformer - Page 1
Forum section: electronics / beginners

author: FallenHero02
author_rank: Contributor
timestamp: February 24, 2020, 11:34:16 pm
content: I am building a 48V phantom power supply in Australia. I know what transformer I need but I have searched on numerous websites and am unable to find one. The transformer I need is a 25~30V AC 100ma transformer. If anyone can find one anywhere then that would be greatly appreciated.I've already drawn a schematic for the 48V phantom power supply I just need to find a transformer for it.

author: digsys
author_rank: Unknown
timestamp: February 24, 2020, 11:54:17 pm
content: Quote from: FallenHero02Is that 240VAC Primary > 25-30VAC Secondary at 100mA? Torroid or IandE? Mounting?

author: FallenHero02
author_rank: Contributor
timestamp: February 25, 2020, 02:19:29 am
content: That's the thing I don't know. There's an image of what I'm expecting it to look like attached. Also, I forgot to add that it should be a single phase. If it helps, I will be mounting it to the inside of a custom made box with some other electronics.

author: digsys
author_rank: Unknown
timestamp: February 25, 2020, 02:46:48 am
content: Hard to read that label, but that is very likely a common 240V > 28/30V transformer - something like -https://www.altronics.com.au/p/m2855l-powertran-30v-240v-150ma-centre-tapped-ei-core-transformer/There are several other electronic suppliers that would have something similar. IF it is a 15+15 output, just join them in series - NOTING Start-finish > start-finish !!!DON'T get the order wrong :-)

author: FallenHero02
author_rank: Contributor
timestamp: February 25, 2020, 04:37:02 am
content: I don’t think I understand what you mean. But that transformer looks close to the one I need. Is it possible to have that one at about 100ma? Also would exchanging a 100ma transformer for a 150ma transformer change the circuit?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: February 25, 2020, 05:04:23 am
content: Quote from: FallenHero02 on February 25, 2020, 04:37:02 am150 mA should work fine.

author: FallenHero02
author_rank: Contributor
timestamp: February 25, 2020, 05:07:43 am
content: If I’m going off of a design made in America that used a 25~30 V AC 150ma transformer is it bad that I would be using that transformer above in the link? (I’m in Australia)

author: digsys
author_rank: Unknown
timestamp: February 25, 2020, 05:32:22 am
content: As mentioned, having a slightly current spec is fine, and in some cases, "better", as it gives you more headroom. Heck, even double or more is fine, as long as it fits.The brown / blue wires (in your pic) indicate that it is an aussie transformer, that is our colour code for "mains". Using an aussie transformer in a USA product is MOSTLY fine as well, as long as the secondary voltage is the same. The ONLY time itmaybe an issue, is if the equipment was relying on the US 60Hz as a reference / or part of the design. If it is just rectifying it for DC, NO issues. We can't make a 100% ruling on that, as we don't know what it it going in to, AND the Input circuit, but it's not common.The ONLY other thing you CAN'T do (in MOST cases), is connect a USA transformer to aussie mains, as they are designed for 60Hz. MOST products are designed on a tight budget and don't allow for the lower frequency. What happens, is the losses are higher and the transformer can get very hot.So, for us to be 100% sure, we'd need to see the Input section of your "equipment". You will UNLIKELY blow anything up, it just won't work,and you'll have a new small heater:-)Edit: It won't over-heat, I made a mistake

author: FallenHero02
author_rank: Contributor
timestamp: February 25, 2020, 06:05:53 am
content: By equipment, do you mean the microphone, the computer/audio mixer or schematic/pcb?

author: digsys
author_rank: Unknown
timestamp: February 25, 2020, 07:08:09 am
content: Quote from: FallenHero02I meant - whatever the transformer is powering. I assume it is just to produce the 48V (DC?) you first mentioned, but I can't just assume that. So, if it is as simple as that, all is fine. It is unusual that one specifies an AC voltage, so was just trying to be sure.

author: FallenHero02
author_rank: Contributor
timestamp: February 25, 2020, 09:36:48 am
content: Thanks so much for your help. I asked you a lot of questions and you answered all of them

author: Zero999
author_rank: Super Contributor
timestamp: February 25, 2020, 09:53:46 am
content: 48V phantom power is such a low current, so a 15V transformer with a diode and capacitor voltage doubler circuit might do the trick.https://en.wikipedia.org/wiki/Voltage_doublerEDIT: You need more than a 30V transformer to get 48V out. Perhaps your circuit already has a voltage doubler? You might be able to change it to a voltage quadrupler. Pleas post a schematic.

author: digsys
author_rank: Unknown
timestamp: February 25, 2020, 10:23:04 am
content: Quote from: Zero999True, but he did originally ask for 25-30VAC, so at 30VAC > 42VDC and with aussie mains, that could be heaps higher :-)Also, equipment running at 48VDC, willusuallyhave a MUCH higher tolerance to Voltage variations, unlike say 3V3 or 5V systems. It's all a "best guess" given the imited info ... WORST case, unless we get more detailed info, he's only out under $9 clams :-) I very much doubt he'll want to go down the "design and make an add-on" PCB routeWe'll see

author: FallenHero02
author_rank: Contributor
timestamp: February 26, 2020, 03:30:50 am
content: Here is the schematic I have made so far without the transformer in there. The transformer will connect to the right side. I have based it off of this videoHe has two schematics in his for the 48V power supply and the phantom power. I have merged these two together and will be turning it into a PCB. I was hoping that I would have a wired transformers because it would be easier to use it with through holes.Also here's the parts list of the product:Parts used:2x 1n4004 diodes4x 10v zener diodes1x 24v zener diode1x 220pf 50v ceramic cap2x 10uf 63v electrolytic caps2x 22uf 63v (or higher volts) non-polarized electrolytic caps2x 100uf 63v electrolytic caps4x 1000uf 63v electrolytic caps2x 22R ohms 1watt carbon resistors2x 10R ohms metal film resistors2x 6.8K ohms metal film resistors5x 10K ohms metal film resistors1x 100K ohms metal film resistors1x BD139 transistor2x BC546 transistorMale & female XLR panel mount connectorsVero Board25~30VAC 150ma transformerPower Switch3AG fuse holderABS jiffy box 158x95x53mm

author: digsys
author_rank: Unknown
timestamp: February 26, 2020, 04:49:26 am
content: I watched most of that video (just skipped boring bits :-) ), and have a few concerns ...A 30VAC supply won't provide enough voltage (at nominal AC) PLUS the headroom needed for the zener regulator. As I mentioned, it works out to 42VDC. BUT, as I said, the microphonesshouldstill work fine, I couldn't find any tech specs that were useful. What DOES concern me (a little) is that the presenter made no mention of the lower voltage / or even measured it ... maybe he knew that the lower V wasn't an issue and didn't bother? OR he's not too switched on with electronics. This is just an observation.The next worry is, allowing potential makers to connect 240VAC wiring inside a MIC box, without ANY instruction on Insulation / isolation / protection and all the possible dangers. Having grown up in the music industry, making amps etc, I saw enough incidents of musicians being electrocuted (none died thank goodness) from their mics !! In nearly all cases, it was bad amateur wiring. Next common was liquid spills. WORST practice was bands disabling Earth leakage because their equipment kept tripping it !!You can still find 24VAC plug-packs, and even 24VDC plug-packs using transformers. AT least then the "dangerous" part is sealed and built to safety standards. You can still leave the rest of the circuit as is, even leave the electrolytic filter caps in. Any reason you wouldn't have the plug-pack outside?Heck, I'm certain, you could use a 24VDC S/Mode plug-pack and not see any hum / noise. Naturally, make sure it is a reputable brand. They have strict "noise" figures. Again, you could leave the rest of the circuit as is. It will even help with final filtering.If you do decide to build it as he's shown, pay SERIOUS attention to the AC mains wiring.

author: mariush
author_rank: Super Contributor
timestamp: February 26, 2020, 07:34:38 am
content: Any particular reason why you would not just use a transformer with 2 x 18v secondaries, or 2 x 24v secondaries, or a 48v transformer?Then just use a linear regulator rated for 60v or more to stabilize to 48v.

author: FallenHero02
author_rank: Contributor
timestamp: February 27, 2020, 10:27:28 am
content: I was wondering, in the thumbnail of the video you can see that he has added used a switch and something else on top. I was wondering what is the part on top for and what wires would you connect to those parts?

author: digsys
author_rank: Unknown
timestamp: February 27, 2020, 11:40:26 am
content: Quote from: FallenHero02That's a mains switch and mains fuse, again with glaring mistakes. Brown wire (Active = live) from mains Input goes to the switch first (preferably NOT the common which is usually the center tab). Then a brown wire (supply yourself) goes from switch common TO THE FUSE BOTTOM !! NOT the way he has it - ie Blue wire (Neutral) going to the fuse TOP !! Then after the fuse, to the transformer. The reason you solder "live" to the bottom is so - when the fuse blows, and someone unscrews the top (forgetting to power off), the fuse is NOT LIVE. Also, Insulate ALL mains connections, either with heatshrink or tape. You don't want ANYTHING mains voltage BARE in that box !!Don't you know anyone local who can check this for you? That guy seriously worries me !

author: FallenHero02
author_rank: Contributor
timestamp: February 27, 2020, 11:53:50 am
content: Yeah, I've been in constant contact, but I can get another person's opinion if necessary.

author: digsys
author_rank: Unknown
timestamp: February 27, 2020, 12:31:52 pm
content: Quote from: FallenHero02Great. Definitely worth it when working with mains !!

author: FallenHero02
author_rank: Contributor
timestamp: February 27, 2020, 10:42:07 pm
content: what type of fuse should I use for it? His parts list says to use a 3AG fuse but I was told by a friend that it would be best to use a fast blow fuse. I mostly need help with finding out what current I would need.

author: digsys
author_rank: Unknown
timestamp: February 27, 2020, 11:18:55 pm
content: Quote from: FallenHero02Well, 30VAC x 0.25A ~ 8W + Inrush / magnetizing / capacitor current, you could get away with 100mA slow-blow. Definitely don't need 3AG ! I'd go for either 1AG or 500mA (or less) slow-blow. At that low power, I doubt it is critical, but 3AG is way over-kill. Fuses are cheap .. even a 0.2AG may be enough, but not necessary.