where to find low voltage isolation transofmers? ie 1:1 + experiment

[darron]

can anyone suggest a source for a 50hz low voltage transformer that can put out 12VAC for a power supply (so around 200mA)?


i bought a small audio transformer tonight to play with, not expecting it to work. specs were:
Impedance: 3kΩ CT / 3kΩ
Freq. Response: 300Hz-3kHz ±2.5dB
Rated Power: 450mW
Primary DC Resistance: 220Ω ±10%
Secondary DC Resistance: 270Ω ±10%
Fly Lead Length: 7mm
http://www.altronics.com.au/index.asp?area=item&id=M0222

i fead it the output of a 12V transformer (which read as around 16V with this tiny load). the secondary was still around 10V which wasn't bad. i loaded the sucker down with 55 ohms to make my maximum approx. load (which i didn't expect to hold with the 450mV power rating) and it went down to about 1VAC.


unless someone tells me not to, i might try something a little beefier:

Impedance: 10kΩ / 10kΩ
Freq. Response: 20Hz - 30kHz
Insertion Loss: -0.5dB
Weight: 63g
http://www.altronics.com.au/index.asp?area=item&id=M0710


i notice the second one has a much more appropriate frequency response, will that make a big difference?



i can't find anything low voltage in any catalogs really except for telephone and audio transformers.


also, do you think it would introduce noise using these unshielded type transformers (at low voltage) inside a pedal power supply? without going into a massive argument, does anyone have an opinion on the virtues of 6 or so of these in a power supply as compared to a daisy chain cable?



thanks for your input guys.. i look forward to learning a bit more about transformers from you all (:

[PRR]

> a 50hz low voltage transformer that can put out 12VAC for a power supply (so around 200mA)? ... i bought a small audio transformer tonight

You mean, a "power" transformer? (Altronics calles them "Mains" and "Toroid" transformers.)

> a 50hz low voltage transformer that can put out 12VAC for a power supply (so around 200mA)?

You do not say, but I assume, this plugs into your 240V wall-outlet.

12.6V / 240V 300mA Centre Tapped Transformer
http://www.altronics.com.au/index.asp?area=item&id=M2852L  - $8.95

> 12VAC around 200mA

Ass-uming 240V input, this is 0.010A at 240V. 240V/0.010A= 24,000 ohms, 2.4 Watts.

The M0710 is, IIRC, rated for perhaps 0.450 Watts at 300Hz. The 50Hz rating would be (50/300)^2 or 0.028 times that, 0.012 Watts. No wonder it was saggy.

Most low-price "audio" transformers are NOT designed to take full power at 50Hz. Some might pass 20Hz but at VERY low level before gross saturation.

And the in/out isolation is rated 50V or so. It might stand more, but I would not bet my life or gear on it.

And yet you are looking for 1:1 transformers? You are feeding 50Hz at 12V AC?

Then the only obvious answer is a two-secondary transformer, 240V:12V:12V, and tape-off the 240V side.

Altronics does not list a low-price dual-secondary part. There is this but it's 10 times more than you need:

Transformer Toroid 240/12+12 30VA
http://www.altronics.com.au/index.asp?area=item&id=M4912A   - $28.65 (ouch!!)

Ahhh...... they list them as "PCB" transformers.

Transformer PCB 5VA 12+12V
http://www.altronics.com.au/index.asp?area=item&id=M7064A  - $10.50
Sec. V: 12+12
Sec. Parallel: 0.416A
Sec. Series: 0.208A
(ignore the numbers in the photo)

This will easily carry 12V:12V at 200mA.

[darron]

thanks for getting back to me on this one PRR (:




You mean, a "power" transformer? (Altronics calles them "Mains" and "Toroid" transformers.)
'power transformer' is what it's for, but i actually did mean that i bought an audio transformer (:



> a 50hz low voltage transformer that can put out 12VAC for a power supply (so around 200mA)?
You do not say, but I assume, this plugs into your 240V wall-outlet.

i meant low voltage at the primary also. so the output would be about 12VAC to feed a regulator anyway, and the input could be anything from say 6VAC to 24VAC from a transformer. i don't want to feed mains into the box, and there's no point in feeding it switchmode DC because then i'd need to oscillate it to pass through the isolation transformer.




The M0710 is, IIRC, rated for perhaps 0.450 Watts at 300Hz. The 50Hz rating would be (50/300)^2 or 0.028 times that, 0.012 Watts. No wonder it was saggy.

Most low-price "audio" transformers are NOT designed to take full power at 50Hz. Some might pass 20Hz but at VERY low level before gross saturation.

And the in/out isolation is rated 50V or so. It might stand more, but I would not bet my life or gear on it.

yeah, i did a quick calulation in the shop before buying it and knew it wouldn't hold up, but was hoping it might not sag so far and that i could feed it a higher input voltage. just an experiment anyway...

as said, i was planning on feeding a low voltage, so 50V rating is happy. they probably use that rating for the phantom powered balanced lines.

on your suggestion, i won't try the beefier 20Hz one then (:



using a dual out (+12v +12v) transformer isn't a bad idea. i was also thinking of back to back transformers as that's the trick to make a cheap mains isolator, but i'd do it backwards. your idea sounds a lot better though

in either case, it seems a waste and shame to have the high voltage coming back into play. do you think having the primary (240V) floating would introduce any extra noise?


altronics, awesome as they are, are a bit limited. i also looked through the massive RS catalogues.


i also considered ramping up to 120V and using R.G.'s mentioned 'spider' multi-tapped transformer.... but again, would rather not create mains potential if i can avoid it...




i have no experience, but would it be practical for me to wind my own? 12VAC or so going in and then maybe 5 secondaries (to handle say 2A max) ? if anybody can suggest which gauge of wire i should use and how many winds on the primary then i can play around until i get the secondaries right (:





thanks again

[R.G.]

I toyed with this a bit before. It's a seductive idea: make lots of isolated low voltage outputs by using cheap, small audio transformers instead of a multi-secondary mains transformer.

Here's where I got to:
Most small audio transformers do not have either the frequency response or the power handling you need to run them at line frequency. This is a restatement of PRR's analysis, which was correct.

Small, cheap audio transformers are designed only to go down to 300 Hz or so. This makes the overall transformer about 1/5 the mass of a transformer of equal power at 60Hz, 1/6th the mass of one for 50Hz. And they're designed for low power. They typically are rated for 100-400mW.  400mW at 9V is 44.4mA, and these are RMS numbers. By the time you rectify and filter, you're only going to get maybe 25-30ma of this as usable DC.

The sneaky thing is that this is rated power at the lowest specified frequency. So they're only good for, say, 400mW at 300Hz. The power they can transform drops dramatically as you get under their low frequency rating. In addition, the voltage you can put on the primary without saturating the core drops linearly with frequency. So this quickly gets to being fighting the laws of physics.

My take on it was to use an ordinary audio transformer and run it from a square wave driver at higher than AC line frequency. The laws of physics start working for you. The power rating of a transformer and the voltage it can stand without saturating go up with increasing frequency. So if you run that 400mW at 300Hz transformer at 3kHz, it suddenly can transform 4000mW. It can withstand a bigger primary voltage and possibly give you enough of a step up to run a linear regulator after it.

You'll need that because the 6kHz ripple frequency coming out of this thing will be unendurable. It takes a very good design in terms of layout, switching speeds, wiring, and radiation to get this to be usable. Took me a long time to get it to be usable. And by that point, it was too complicated to be simple to do any more, so I got distracted by other bright, shiny things. 

The obvious thing to do is to run the frequency even higher, get it above audio, etc. etc. I've toyed with that, and there may be something there for us. But the requirements on not having this thing spit out RF interference get tough, and you can no longer use cheap audio transformers, because the core loss and non-optimal winding (for power transfer) in cheap audio transformers make them much less useful. Several companies in fact make these; they're sold as DC isolating power modules. They're about $20 and up each.

It's a good idea, and soem good thought went into it. The practicalities proliferate on you.

[darron]

thanks for your input R.G.



so it seems not so simple in the end, and definitely more trouble than i was looking for. i suppose the main point was that i was hoping someone would actually sell exactly what i want, and i'm surprised that it can't be named....


what about PPR's suggestion for a pcb transformer and using the two secondaries? would that create noise problems having the primary floating?




mmmmmm......

[PRR]

BTW, one problem could be predicted from specs:

> Primary DC Resistance: 270 ±10%
> Secondary DC Resistance: 270 ±10%
> i loaded the sucker down with 55 ohms ... it went down to about 1VAC.

You have a 540 ohm source and a 55 ohm load. Counting on thumbs, loaded voltage is 1/10th of input voltage, 90% of power coming in vanishes as heat.

Power systems, you have the generator power and then it is all losses. Efficiency is important (though depending on electric-bill impact.... 6VA iron can be inefficient and you won't notice). Most audio is about taking a small interesting signal and a large boring DC supply, amplifying. Transformer loss has a very different meaning. In many cases the actual Power Transfer is under 10%... amplifier inputs don't need much power, and if an extra penny for a high-hFE transistor allows a less-efficient dollar-cheaper transformer, well duh, let silicon do the work.

> do you think having the primary (240V) floating would introduce any extra noise?

Don't put the non-connected pins next to your audio. If it works, cut them flush, insulate WELL, and mount that side away from audio.

If you were building a brainwave monitor you might also put a grounded shield over the pin stubs. That won't be necessary here.

That cures electrostatic fields. The magnetic field is the same with or without the 230V winding.

I do not understand why you need this at all? Surely your input 12V AC is isolated from wall power connection?

> 12VAC or so going in and then maybe 5 secondaries

Ah.

Small iron needs 20 or 30 turns per Volt. Each winding is therefore about 300 turns. You won't manage this (sanely) unless you remove the bobbin from the iron. Iron is often shellacked. Attacking it with a paring knife leads to bloody thumbs and bent laminations (bent reduces the iron magnetic properties; and you never get all the lams back in). People do this. It seems crazy to me.

> R.G.'s mentioned 'spider' multi-tapped transformer

If available in your land, this would seem to make most sense. IIRC it is 8(?) 11V windings plus a line-winding. Feed your input 12V to one winding, get 7 isolated windings. Since each winding has only 1/16th of total area, you can not pull full current from all windings without large sag. (If one winding is rated higher than the rest, that should be input.)

I hate high-frequency work, but 12VAC into a transistor buzzer reduces the turns/Volt to 1 or less (depending on core and frequency). If you play with the deadly-end of a CRT tube TV, the 1V heater on the HV rectifier is fed from one turn on the H-sweep coil. PC power supplies work the same and need only a few turns to make 3.3V DC. The small turns means low resistance and little sag; it is often good-enough to regulate one output and tolerate the few-tenths sag on all the others.

Hard questions for easier solutions:

Do your pedals really need 200mA? A bunch of classics are closer to 1mA. Anything that can be run on a battery is probably closer to 10mA than 200mA.

Do your pedals really need isolated power? You are going to tie all the signal grounds together. At least half your pedals use the same power polarity. The whole board is only a few feet wide, no large loop-area.

> small audio transformers do not have ...the power handling you need to run them at line frequency.

"Frequency response" has a different meaning in power systems. Low inductance which would suck the bass out of a weak amplifier just means the generator works trivially harder.

It's not just cheap. Hammond 125ESE, a 15 Watt universal tube output tranny, is fairly expensive (far more than 15VA of power iron) yet will not stand full 15W at 10K impedance below 130Hz. The waveform becomes very distorted, saturation. If the source is Ellsworth Dam, the megawatt generator will overwhelm the weak iron and force a near sine wave output; a poor 6550 bottle would not. (BTW the 125ESE only claims 150Hz, and will do much better when worked at 5K or 2.5K effective impedance.)

[davidallancole]

They used to use isolation transformers in bathrooms for the razor plugin.  Could you find any of those?

[darron]

so winding one myself, even for fun, doesn't seem to practical....


i don't want to start building oscillators...



do i NEED that many isolated outputs? no... not at all (:       the other day i built a power supply which i posted in the pictures thread... i was very happy and the thought was to daisychain the power. i'm usually happy with this but i thought "what about an ANAL person's power supply?" and bought an expensive hammond enclosure for it 

200mA is the sort of current i'm thinking of for a LM317T without a heatsink, to shut itself off before you can unknowingly start to sag the voltage down. if you're just running the one effect, as designed then 50mA each is usually safe unless you want to run a delay or something....


you're right, my 12v AC input is of course isolated. i made sure it had an AC input instead of DC so that if anyone ever used it they would almost have to plug in a transformer supply.




i just thought... another advantage of the spider supply (yes, it has 8 outputs) is that you could probably use two neighbour windings in series to get an 18V output as well (:

They used to use isolation transformers in bathrooms for the razor plugin.  Could you find any of those?

there's the idea.... finding that exact sort of transformer probably costs a lot, if bulky, and is not PCB friendly, but i started this thread hoping that someone out there made plain and simple low voltage power isolation transformers and it's pretty sad and surprising that it doesn't exist! something like a bathroom one but made for the tiny current/voltage we draw and appropriately sized....

[davidallancole]

I don't know very many (common) circumstances that require isolating 12 Vac.  Thats probably why you don't see that type of transformer, not that my thoughts control the electrical/electronics industry.

[darron]

not that my thoughts control the electrical/electronics industry.

...yet


it seems there's a market for almost every other useless piece of mojo? (:

[R.G.]

...yet
it seems there's a market for almost every other useless piece of mojo? (:

This niche has not yet been exploited it seems. The thing is, power is usually distributed at power line voltages, for an assortment of technical reasons. So the vast majority of low frequency transformers are designed with one end matching power line frequency and voltage. But you could make a new useless piece of mojo if you want.   

Designing a 12Vac to 12Vac transformer is pretty simple. I can run through the numbers for the design if you want. The big problem is actually making the things. You're left with hand winding or finding a custom transformer shop that will make them for you. This last probably means making 1000 of them to start with, or otherwise paying a lot for the smaller run.

As an example, when I first came up with the Spyder transformer, I did go ask several transformer makes to quote making them for me. I got a few "no quote" responses, one "sure, those will cost you $500 each" and one "We can't make them in your quantity, but our custom shop will talk to about lots of 500 at a time." They weren't being snotty - it's just economics. To make this go, you'd need to go locate a transformer shop willing to do it for time and materials pricing, and order as many as you could stand to buy to get the per-unit cost down. Notice that it does not matter what custom transformer you're doing this with, the answer will be the same. I just recently did this same exercise for another custom transformer design, with the same results.