How to increase capacitance of home made power supply?

[Syllith]

I've built a power supply out of a step down transformer (12.9V), rectified it with a 30A full rectifier, and added a couple 12000UF (20V) caps to make a bench test power supply. I'm a car audio installation tech and I want to use it for testing radios, amps, or anything really that runs off 12VDC. It works for the most part but there are a few problems with it.

First, I noticed the caps are filling up to about 16VDC, even though they're getting 13. I think this is because I don't have a bleeder resistor on there but I'd like to know specifically why they fill up to 16 in the first place? Why that number? What's the best way to limit this to 13V?

This brings me to my second problem. It drops to 9 volts with only a .85A load. This is clearly unacceptable. Not when my goal is more around 10A. Now of course this supply is unregulated at the moment and I could definitely just look up a schematic to solve this issue but I wouldn't learn anything that way would I? I want to know exactly how each component plays a part and how it affects the circuit. I was thinking about adding a bunch of smaller caps as well. I'm thinking these will fill quicker than the 2 large ones and perhaps help somehow? I've noticed many PC power supplies and such use many different sized caps. I know smaller caps help with higher frequency applications so I don't think adding them would hurt. I don't think this will help overall capacitance much though.

I'm trying to avoid relying on battery power. Do you guys have any ideas on how I can achieve my 10A goal without using a battery? I was thinking somehow a type of inductor would help, yknow, with it resisting changes in current and all that? If so, how would I go about figuring out what kind of inductor to add? Another idea was to increase the overall voltage of the supply to say, 30 volts DC, then regulating it down to 13. I'd have to change my caps out and I'd have to come up with a way to regulate it efficiently, so maybe a buck/boost converter? As long as it can handle the load I'm needing. I don't really need it to go any higher though so if it's easier to keep it at 13 thats cool.

The transformer also makes me suspicious. I went to an awesome electronics store in St. Louis that sells many many kinds of transformers. All of them though, no matter how big or small, were only around a few amps though. I don't know how much mine is specifically but it's probably a little under 2. But other high amperage power supplies use transformers even smaller than mine and they work just fine so I'm sure it's just my configuration.

So I've got quite a few knowledge gaps in this area so if you guys could help shed some light I'd really appreciate it. Thanks

[DTJ]

Syllith, the output capacity of your power supply is based primarily on the transformer. If your transformer is too small it will not be able to supply the required load.

In other words you need a bigger transformer (if you want to draw 10A continuously). Adding more capacitance will improve the ability of your power supply to cope with surges in demand but will not help in increasing the overall current supply capacity.

You also need to regulate the voltage otherwise it will vary as you load it up. Have a search around for schematics of DIY regulated power supplies for CB radios, they might be what you are looking for.

[IanB]

First, I noticed the caps are filling up to about 16VDC, even though they're getting 13. I think this is because I don't have a bleeder resistor on there but I'd like to know specifically why they fill up to 16 in the first place? Why that number? What's the best way to limit this to 13V?

It's because the AC voltage of a transformer is an average over each AC cycle. The peak voltage is about 1.4x more than the average, so for 13 V you would find a peak of 13 x 1.4 = 18 V. Now if you rectify the AC and use it to charge up a capacitor you will get the peak voltage stored in the capacitor.

If you want to limit the voltage to 13 V you will need a voltage regulator. However, building your own regulated 12 V 10 A power supply is more complicated than you can achieve with just the basic parts you have assembled so far.

Have you thought about using a re-purposed PC power supply? I don't know if that would work for audio applications, but it might. It would be cheap and simple if it did.

[mariush]

Transformers output an AC voltage and they have a VA rating (the maximum power they can provide) . You divide that number by the AC voltage to get the maximum AC current on the secondary of the transformer.
The transformer looks like it may be rated for 20 VA but let's be optimist and say it's rated for 50VA - if that's the case, the maximum current would be 50/12 = about 4.2A

When you convert the 12v AC voltage to DC using a bridge rectifier, you get an output voltage that's always positive, and the peak DC voltage is basically 1.414 x Vac  - 2 x Vdiode, where Vdiode is the voltage drop on each diode in the bridge rectifier (there's always two diodes active in the rectifier). This voltage is in the datasheet. For most rectifiers, the voltage drop is between 0.7v and 1.2v.. let's go with 1v for simplicity.

With small transformers in particular - but it's common for all transformers - at low power usage the transfomers will output more than the rated voltage... they could output 13v AC or even more instead of 12v AC ... so you have to keep that in mind when estimating the peak DC voltage. for example, in this particular case it wouldn't be wise to use capacitors rated for a maximum voltage of 16v after the bridge rectifier.

So for your 12v AC transformer, you'll get a DC output with a peak DC voltage of 1.414 x 12v AC - 2 x 1v  =  ~  15v
The peak AC current can also estimated with the formula  I dc = 0.62..0.7 x Iac (the constant varies usually with how big the transformer is), so by going with the 50VA / 4.2A estimation, such transformer may only be able to output 2.6A .. 3A

Anyway, you figure the peak voltage but the output will be all hills and depressions, matching the number of cycles in your mains AC input ... if you're in US where it's 60 Hz frequency, you'll have 120 hills and depressions in your DC output.
That's where the capacitors come in , after the bridge rectifier, to charge up when the output is near the peak DC voltage and fill up the depressions in the output, making the difference between minimum and maximum DC output much smaller.

There's another formula which can estimate how much capacitance you need based on what's the minimum voltage your want to have in your circuit all the time :

Capacitance (in farads) = Current (in Amps) / [ 2 x AC Mains Frequency x  (Vdc peak - Vdc minimum)]

From this formula you can also go the other way around, if you know capacitance you have you can estimate the minimum DC voltage for a particular peak current.

Ayway, for  our  50 VA / 4.2 A fictional transformer, and assuming you're in US where you're dealing with 60 Hz mains frequency, and you want the output voltage to always be at least 13v, then you can put the numbers in the formula :

C = 4.2 A / [ 2 x 60 x (15v - 13v)] = 4.2 / 240 = 0.0175 Farads or 17500 uF 

That's the least you need to have always at least 13v at 4.2A... at lower currents voltage willl be higher than 13v.. for example at only 0.5A of power usage the capacitors could be always filled so much that you'd always have about 14.5-14.8v at the output.. and that's why you need additional voltage regulators to take this DC voltage that varies within a narrow range (13v .. 15v)  to a stable 12v DC.

 

[P90]

Use a decent size PC power supply with a couple large super capacitors on the 12 volt output...

[Cliff Matthews]

That transfo... what was it pulled from? The winding you are using is just one of several.. Agile Magnetics appears to wind custom stuff in 100VA, 250, 500, etc.. sizes and this model (RMS-12) looks to be 250VA. I have just 3 questions:

- What test's have you done on resistance and continuity between all the winding's (I count at least 13 wires here..)
- If the front bottom is the primary, why does it have 5 wires? (120/240v primaries often require use of 2 winding's in parallel)
- Can you measure the other 3 secondary winding's with a real meter? (surely that can't be the only meter you have..)

On a guess, the 2 red winding's carry the bulk of the output (perhaps audio PA), the yellow could be for pre-amp, and the grey winding you are using is good for maybe 500ma of panel lamps, etc.. (hence the sag with 850ma of load).

[P90]

That transfo... what was it pulled from? The winding you are using is just one of several.. Agile Magnetics appears to wind custom stuff in 100VA, 250, 500, etc.. sizes and this model (RMS-12) looks to be 250VA. I have just 3 questions:

- What test's have you done on resistance and continuity between all the winding's (I count at least 13 wires here..)
- If the front bottom is the primary, why does it have 5 wires? (120/240v primaries often require use of 2 winding's in parallel)
- Can you measure the other 3 secondary winding's with a real meter? (surely that can't be the only meter you have..)

On a guess, the 2 red winding's carry the bulk of the output (perhaps audio PA), the yellow could be for pre-amp, and the grey winding you are using is good for maybe 500ma of panel lamps, etc.. (hence the sag with 850ma of load).

"Tests"
"Windings"

It's plural not possessive, so lose the apostrophes, or wait... is that apostrophe's... lol 

[Cliff Matthews]

That transfo... what was it pulled from? The winding you are using is just one of several.. Agile Magnetics appears to wind custom stuff in 100VA, 250, 500, etc.. sizes and this model (RMS-12) looks to be 250VA. I have just 3 questions:

- What test's have you done on resistance and continuity between all the winding's (I count at least 13 wires here..)
- If the front bottom is the primary, why does it have 5 wires? (120/240v primaries often require use of 2 winding's in parallel)
- Can you measure the other 3 secondary winding's with a real meter? (surely that can't be the only meter you have..)

On a guess, the 2 red winding's carry the bulk of the output (perhaps audio PA), the yellow could be for pre-amp, and the grey winding you are using is good for maybe 500ma of panel lamps, etc.. (hence the sag with 850ma of load).

"Tests"
"Windings"

It's plural not possessive, so lose the apostrophes, or wait... is that apostrophe's... lol

I was never good as a writer, but I did manage to learn Spanish over-sea's (it's possible my English is messed-up now, but now I'm too old to give a crap..)

[P90]

That transfo... what was it pulled from? The winding you are using is just one of several.. Agile Magnetics appears to wind custom stuff in 100VA, 250, 500, etc.. sizes and this model (RMS-12) looks to be 250VA. I have just 3 questions:

- What test's have you done on resistance and continuity between all the winding's (I count at least 13 wires here..)
- If the front bottom is the primary, why does it have 5 wires? (120/240v primaries often require use of 2 winding's in parallel)
- Can you measure the other 3 secondary winding's with a real meter? (surely that can't be the only meter you have..)

On a guess, the 2 red winding's carry the bulk of the output (perhaps audio PA), the yellow could be for pre-amp, and the grey winding you are using is good for maybe 500ma of panel lamps, etc.. (hence the sag with 850ma of load).

"Tests"
"Windings"

It's plural not possessive, so lose the apostrophes, or wait... is that apostrophe's... lol

I was never good as a writer, but I did manage to learn Spanish over-sea's (it's possible my English is messed-up now, but now I'm too old to give a crap..)

LOL
I'm just busting balls...    LOL.

[Kalvin]

If you are using only 12V, why don't you use a car battery or any other 12V battery which can supply enough current. Just hook the battery to the battery charger and you have a nice power supply capable of providing enough juice. Use some fuses in order to provide extra protection should you accidentally short-circuit the battery. Buy some cheap current and voltage displays from eBay or from your favorite electronics shop, and you have a set of instruments for monitoring the load current and the battery voltage.

[Kalvin]

* removed the suggestion of using the supercapacitors, as the large supercapacitors may be too dangerous for the beginners. *

[Syllith]

Thanks for all the info guys. I'm not sure what the transformer came out of. It was purchased at Gateway Electronics in Saint Louis. They had shelves of them and I picked one that looked decently beefy and suited my voltage requirements. I cannot find any specs online, or even a picture, or the same transformer I have.

https://www.jaycar.us/12v-30v-100va-6a-multi-tap-type-2170-transformer/p/MM2015

I'm in the US but if I gave this 120, would the 30V secondary output somewhere around 15V? If so, would this still maintain the 6A? 6 would be perfectly fine for my needs. With the .85A I'm getting now, I'm able to power and amp and play a sub (very) quietly before it cuts out.

[Syllith]

Use a decent size PC power supply with a couple large super capacitors on the 12 volt output...

Absolutely this would work, but this is solely for learning. My goal is to tell myself "I wanna build this thing to these specs completely with my own parts" and then execute it. I plan on wasting tons of money buying random parts I may or may not use, as long as the final product is what I set out to build. I'll find a use for the stuff I buy sooner or later

[Syllith]

If you want to limit the voltage to 13 V you will need a voltage regulator. However, building your own regulated 12 V 10 A power supply is more complicated than you can achieve with just the basic parts you have assembled so far.

I do have a lot more parts than what I've shown so far. I have every value resistor, maybe 15 types of diodes, various IC's, many transistors, maybe 40 different caps, and all kinds of goodies. And anything I'll need I can go to Gateway Electronics in St. Louis where I can get anything I need.

I don't have very many high current parts though, or any inductors, but again I can just go purchase them as needed. But for the most part, I have the means to build pretty complicated circuits. Do you, or anybody else, have any good schematic suggestions on a regulator that may suit my needs?

[P90]

You can always salvage a good beefy tranformer and many parts from an old car battery charger, look at local goodwill store or craigslist. There's your learning to built from parts...

[Cliff Matthews]

What are the voltages of the other secondary windings? You certainly have 250va of transfo there, but getting the expected amperage depends on knowing the characteristics of what you have. Do you have at least a $10 multi-meter to report back some results? Keep in mind if the primary windings are 240/120v, you'll want to in-phase parallel 2 of them or you'll be getting only half capacity.

[Syllith]

You can always salvage a good beefy tranformer and many parts from an old car battery charger, look at local goodwill store or craigslist. There's your learning to built from parts...

Already beat you to it lol. This is how this whole idea got started actually. The transformer was actually shorted internally so it drew 5A at idle with no load, but I tried my best to bring it back to life.

[Brumby]

My goal is to tell myself "I wanna build this thing to these specs completely with my own parts" and then execute it. I plan on wasting tons of money buying random parts I may or may not use, as long as the final product is what I set out to build.

That is a healthy attitude - but be careful.  You can end up with tons of 'stock on hand'.  Just look around these boards for people whose 'collection' of stuff is an embarrassment, if not shame.

I'll find a use for the stuff I buy sooner or later.

... famous last words.   

By all means learn - and mistakes are a good way to do that, as long as they don't involve injury or destruction of property. (Blowing the side out of a TO-220 package is allowed ... and inevitable).

[P90]

You can always salvage a good beefy tranformer and many parts from an old car battery charger, look at local goodwill store or craigslist. There's your learning to built from parts...

Already beat you to it lol. This is how this whole idea got started actually. The transformer was actually shorted internally so it drew 5A at idle with no load, but I tried my best to bring it back to life.

Looks like you went to town with the mdf and cable ties... lol... ahhh, reminds me of the not so good old days of installing car audio... lol.
I usually make sure a circuit works before committing to countless wire clamps and ties...   

[Syllith]

What are the voltages of the other secondary windings? You certainly have 250va of transfo there, but getting the expected amperage depends on knowing the characteristics of what you have. Do you have at least a $10 multi-meter to report back some results? Keep in mind if the primary windings are 240/120v, you'll want to in-phase parallel 2 of them or you'll be getting only half capacity.

Woah there, you may be on to something. The transfo is at work right now and I'm at home getting drunk so I cannot do any readings right now lol. During testing though it spit out exactly what it was rated at. 8.8, 12.9, 30something, and 189. I feel I am only getting half capacity because all the other transfos were rated around 2 something and this one was unlabeled but size-wise, it was bigger than most. .85 seems too little. 1.7A seems a lot more likely. My buddy and I had trouble hooking it up because of all the wires. Maybe 13 or so. Finally, we saw a solid blue wire and a white/blue wire. We connected these and that's how we got our voltages out of the secondaries. What you're saying is I should send the same 120vac to the other primary coil (in phase of course), and that will give me the same voltages but higher amperage?

[P90]

My goal is to tell myself "I wanna build this thing to these specs completely with my own parts" and then execute it. I plan on wasting tons of money buying random parts I may or may not use, as long as the final product is what I set out to build.

That is a healthy attitude - but be careful.  You can end up with tons of 'stock on hand'.  Just look around these boards for people whose 'collection' of stuff is an embarrassment, if not shame.

I'll find a use for the stuff I buy sooner or later.

... famous last words.   

By all means learn - and mistakes are a good way to do that, as long as they don't involve injury or destruction of property. (Blowing the side out of a TO-220 package is allowed ... and inevitable).

I have bins and buckets of "I'll use this someday" hardware and parts. And the problem is the buckets keep multiplying...

[Cliff Matthews]

Regarding those primary windings. The left side of this small pic shows a link used for 240v. You would remove it and instead, parallel the 2 positive phases and  the 2 negative phases (if I phrased that right..) to take advantage of the full power output of this little beast.

That's why measuring the DC resistance of all windings is important. Of course, grouping and continuity between windings is super important (since you don't want to create a dangerous auto-transformer out of this...)

[Syllith]

Looks like you went to town with the mdf and cable ties

Oh yea, of course. I like to make my installs look good, even if it's just a stupid experiment. It actually worked just fine outside the box but after assembly it failed. It's the same setup, so I'm assuming the blown transformer finally gave out. Now it's just a heavy paperweight. Or a door stop. Oh well, it looks cool lol.

[P90]

Looks like you went to town with the mdf and cable ties

Oh yea, of course. I like to make my installs look good, even if it's just a stupid experiment. It actually worked just fine outside the box but after assembly it failed. It's the same setup, so I'm assuming the blown transformer finally gave out. Now it's just a heavy paperweight. Or a door stop. Oh well, it looks cool lol.

You forgot a quick coat of shellac on that MDF, it maked it pop and looks great!   After using a roundover bit on the edges of course...  :>)

[Brumby]

I have bins and buckets of "I'll use this someday" hardware and parts. And the problem is the buckets keep multiplying...

I'm too late, then.

[Mechatrommer]

to determine how much capacitance you need, use formula..
C = (I * t) / V

C = capacitance needed.
I = current you've specify
t = 1 / f. f = frequency of transformer. if you have full bridge rectifier, multiply f by 2.
V = how much voltage sag you can accept

http://electronics.stackexchange.com/questions/36992/capacitor-sizing-on-a-voltage-regulator-output

for example:
you want 10A and no more than 5V of voltage sag. f = 60Hz but you have full bridge rectifier. f = 120Hz. hence t = 1 / 120 = 8.3ms

so punch in the formula:
C = (10 x 8.3ms) / 5
C = 16.6mF

if you got enough capacitance built but still experience more sag than you anticipated, it means your transformer is lacking no matter how much further capacitance you will add. fwiw...

edit: last but not least. you'll need an oscilloscope for this job to see voltage sag/droop behaviour. Vdc multimeter will give you confusing result because of mentioned problems, ie you cant see Vpp with a DMM, it usually indicates Vrms or Vaverage.

[Cliff Matthews]

Thanks for all the info guys. I'm not sure what the transformer came out of. It was purchased at Gateway Electronics in Saint Louis. They had shelves of them and I picked one that looked decently beefy and suited my voltage requirements. I cannot find any specs online, or even a picture, or the same transformer I have.

I'm in the US but if I gave this 120, would the 30V secondary output somewhere around 15V? If so, would this still maintain the 6A? 6 would be perfectly fine for my needs. With the .85A I'm getting now, I'm able to power and amp and play a sub (very) quietly before it cuts out.

Awesome store you have down there! If only it were 8 hours and one border crossing closer.. 

[mpicker21]

I am like you. I want to do it all from scratch to learn and appreciate what makes something tick. I recommend figuring your windings out, as others suggest, but be aware that 10 amps like you mention is pretty beefy. Transformers that big are going to cost a big chunk of money. To help avoid overspending you might want to do some back of the napkin math to determine your needs. Hopefully the one you have will suffice. If it is in fact a 12.9v transformer then use a 12V regulator like the LM7812. I learn a lot from data sheets. I'm not sure where you are skill-wise but if you look at example circuits in data sheets you can get starting points for some more research. Look at figure 13 of this data sheet for increasing power of an LM7812. https://www.fairchildsemi.com/datasheets/LM/LM7812.pdf

Good luck.

[Kalvin]

One could get a transformer for a car battery charger. Some models are quite beefy giving out 10+ Amperes and they are pretty cheap.

[Cliff Matthews]

In an adjacent transformer thread user Seekonk (in the US) seems to offer-up some transformers (maybe PM? Paypal is always a friend). https://www.eevblog.com/forum/beginners/where-to-find-a-transformer/msg1059659/#msg1059659 They are 24v but with flexible primaries, so it seems you could get 6v and 12v out too. You'll find a lot of good people on this forum, best site for a tech by far!

[P90]

One could get a transformer for a car battery charger. Some models are quite beefy giving out 10+ Amperes and they are pretty cheap.

Now why didn't I think of that?       

[SL4P]

Just one more comment...
The chunky bridge rectifier you're using is rated at 30A - but that is probably with an 'infinite' heatsink, or forced cooling. 
Sitting on the bench like that, i'd be surprised if it could handle more than 2-3A continuously.
12V x 3A = 36 Watts dissipated

[BradC]

12V x 3A = 36 Watts dissipated

If that rectifier was dropping 12V he'd be getting nothing out. Try 1.2V (2x0.6V diodes) x 3A.
In reality it'll be worse than that under a heavy load as those diodes tend to have a greater Vf as the current wicks up, but it won't be dissipating anywhere near 36W until you are approaching the 30A limit. It will want a good heatsink however.

[P90]

12V x 3A = 36 Watts dissipated

If that rectifier was dropping 12V he'd be getting nothing out. Try 1.2V (2x0.6V diodes) x 3A.
In reality it'll be worse than that under a heavy load as those diodes tend to have a greater Vf as the current wicks up, but it won't be dissipating anywhere near 36W until you are approaching the 30A limit. It will want a good heatsink however.

You mean that crusty MDF it's sitting on isn't a good heatsink?    lol

[Cliff Matthews]

Just one more comment...
The chunky bridge rectifier you're using is rated at 30A - but that is probably with an 'infinite' heatsink, or forced cooling. 
Sitting on the bench like that, i'd be surprised if it could handle more than 2-3A continuously.
12V x 3A = 36 Watts dissipated

Just 2 diodes are on for each phase and Vf on power rectifiers is around 1.0v, so I would calculate 2watts dissipated per amp. Your instincts are spot-on for the 3A upper limit, since 6watts would have it very hot indeed. It has about the same surface area as a 10watt ceramic bathtub resistor but as a semiconductor, breakdown starts around 150C. http://www.mouser.com/ds/2/169/kbpc25005tw_thru_kbpc2504tw-270328.pdf
BradC posted similar too. 

[Syllith]

Huh, so it appears one of my caps has died. I did nothing. I played a radio for maybe 3 minutes and unplugged it. Came back to work the next day and the second cap was shorting the whole thing. Removed it and I'm back to normal, but off the 1 cap. What could have happened? Just old/bad caps?

Also, for the final product, don't worry about heat. I will make sure it is properly dissipated. I'm just kinda rigging it up before I put it in a case. I don't know how much crap I'm going to have to fit in there but when I know I'll factor in space for heatsinks.

[Cliff Matthews]

They did look a little 70's vintage. If rated at 25v, that size looks to be around 10,000 uF. Sometimes they dry out, sometimes old ones need to be reformed. http://www.nmr.mgh.harvard.edu/~reese/electrolytics/#reform

[mariush]

Regarding bridge rectifiers.

That's why you see in higher end computer power supplies two GBU / GSIB /  style bridge rectifiers with a heatsink between them



Here's for example a 800v 25A bridge rectifier GSIB2580 : http://uk.farnell.com/vishay/gsib2580-e3-45/bridge-rectifier-25a-800v/dp/9549757

You can see they can do that 25A up to 100C and then they derate.
So basically you can parallel two of them and place heatsink between them and this way you also get the benefit of keeping the two bridge rectifiers about the same temperature and the other benefit (and main reason you'd parallel two) is that current through each lowers, and therefore the voltage drop on each diode should be smaller as well.

So with this particular diode I linked to , the forward voltage is estimated at a bit over 0.9v at 10A and it goes to 1v at 20A ... so let's say you want 20A to go through, with one diode you'd dissipate 2x1vx20 = 40w, with two rectifiers you'd dissipate  2 x ( 2x0.9x10) = 36 watts.

And by the way, for your particular project it would be fun to design your own "bridge controller" using an idela bridge controller like LT4320 : http://cds.linear.com/docs/en/datasheet/4320fb.pdf, which you can still buy in DIP package if you can't do surface mount. Basically, you combine it with four mosfets and your big capacitor and it rectifies up to 72v AC and tens of amps (30A is realistic). You could  even use 4 to-220 or something through hole for mosfets, you just have to follow the datasheet and pick mosfets suitable for working with this controller and based on the maximum current you want.

The benefit is that instead of losing about 2v x current watts in heat, the losses are basically very small, they depend only on internal resistance of the mosfets.. for typical mosfets they'd be equivalent of 0.1v drop or less so instead of 30w or something like that, you're looking at 1-2 watts of losses.
ps. and of course, another added benefit is that now your peak dc voltage will be about 1.8v higher compared to bridge rectifiers, so maybe your capacitors won't have to be as big to keep the minimum voltage to your desired amount
If you're thinking of doing it, here's the DIP package and in the details below you'll find two links to user guide/additional documentation and example circuit (but it's really simple to use) : http://www.digikey.com/product-detail/en/linear-technology/LT4320IN8-PBF/LT4320IN8-PBF-ND/4693752

[eephys]

For what it's worth, I think you have the Agile Magnetics transformer hooked up backwards.  The red and gray windings should be the primary (input) ones - in series for 240 volts, in parallel for 120 volts input.  That bundle of wires at the bottom of the transformer are the secondaries (outputs).  There should be solid color ones at the outer ends of the winding with solid/stripe ones at various tap points.  There may be multiple secondary windings, some with center taps or taps at other voltages.

See this page for more info:  http://www.delabs-circuits.com/cirdir/electric/tables/tabl0007.html

I was working recently with a similar transformer made by the old Regional Manufacturing Specialists (RMS), now Agile Magnetics, learned about the color coding of the wires.  The transformer was also marked RMS-12 and was rated for at least 750 VA.

[Syllith]

Really now. I'll try hooking it up differently. I thought it was odd the primary was on the bottom.

[Cliff Matthews]

For what it's worth, I think you have the Agile Magnetics transformer hooked up backwards.  The red and gray windings should be the primary (input) ones - in series for 240 volts, in parallel for 120 volts input.  That bundle of wires at the bottom of the transformer are the secondaries (outputs).  There should be solid color ones at the outer ends of the winding with solid/stripe ones at various tap points.  There may be multiple secondary windings, some with center taps or taps at other voltages.

See this page for more info:  http://www.delabs-circuits.com/cirdir/electric/tables/tabl0007.html

I was working recently with a similar transformer made by the old Regional Manufacturing Specialists (RMS), now Agile Magnetics, learned about the color coding of the wires.  The transformer was also marked RMS-12 and was rated for at least 750 VA.

No offense intended here..  This forum is typically hesitant giving novice members potentially harmful advice. 

Since this member has already achieved 12v from this transformer, we know at very least 10:1 ratio's improperly connected could expose someone to more than 1000 volts...

This is why the member was previously asked if they even had a voltmeter (other than the one shown..) and if any resistance and continuity tests were done.

[Cliff Matthews]

Really now. I'll try hooking it up differently. I thought it was odd the primary was on the bottom.

Syllith some members here are concerned for your safety..  Can you please report any un-powered measurements you've taken?

[Syllith]

Thank you for the concern, but don't worry. My job is working with electricity. While it's very different than what I'm doing here, it still makes me very conscious of shorts and what my circuit is doing. I've been using a 3.5 amp fuse on my power wire and when I test, I move it to the center of my shop and power it from a distance with a switch. I have yet to short it out. I have also not made any changes to it at all yet. I'm doing my research before I go further. Anything I'm unsure of I will ask about first.

I suppose next I'm going to do some resistance tests on the coils. If they have a low resistance, they will not be hooked up. Since I have no information on my transformer other than the output voltages, this is all I can do, unless you have another way you'd like to recommend on how to hook it up correctly? It's spitting out all the correct voltages that it has labeled on the side, but I still believe .85A is too little. I feel it should at least be around double that. Is it possible there is another coil that needs to be energized as well? I do believe it has 120V and 240V input primaries but I will have to get back to you on that when I get back to the shop.

[Cliff Matthews]

Good to hear you have a healthy respect on this and didn't hook a secondary into the mains AC    I have some 120v 250va transformers with a DC resistance of ~1.5ohms (with AC it will not be resistance but reactance/impedance or whatever..).

Measure the resistance of the primary used originally and to any wires with continuity to that, with the objective of finding the highest resistance across the group and how it is tapped (milli-ohmmeter's work better than most DVM's for this). Do the same for any other winding groups with continuity between them. Since it was stated it has 13 wires in total, do not assume all the other wire groups are secondary outputs - there may be other primary winding groups. Also, check to see if any wire has continuity with the frame (or nothing at all, for a shield).

If you run into some really low resistances, you could power up as before and carefully measure the AC across the other wire groups. Additional isolated primary windings, meant to be paralleled at 120v or more will likely carry some healthy potentials, so be careful. Keep your meter fixed on the highest range, even if it is auto-ranging (it's faster). Don't give up, this stuff takes a little time to figure out. Maybe keep in mind, transformers this size are worth a good chunk of change.. Document what you get, by groupings and note the wire color.

** Good reading with a table of VA ratings and winding resistances can be found here:
http://sound.whsites.net/xfmr2.htm#s113  It's certainly better than reading my drivel.. 

[eephys]

I just learned that the RMS-12 marking on your transformer (and mine) indicates a particular method of insulation and has nothing to do with the power rating.

[Syllith]

Slow day at work today, so I have time to mess around.

The transformer is labeled on the side (from the store, not Agile Magnetics) as 8.8VAC, 12.9VAC, 18.3VAC, and 169VAC. It mentions nothing of input voltages but I'm thinking it may have 240V and 120V. I will need someone to help verify that.

Here are my DC resistance test results:

Bottom wires:

Blue and blue/white - 4.5 ohms
Brown and white/black - 3.2 ohms
Brown and white/brown - 3.8 ohms
White/brown and white/black - 0.9 ohms

Top front wires:

Red pair - 5.5 - 6 ohms
Grey pair - 4 ohms

Top back coil wires:

Orange pair - 3.6 ohms
Yellow pair - 20 ohms

Right now, I have the blue and white/blue wire hooked up to 120V mains. On the top wires, I get the voltages corresponding to the label on the side. The grey pair give me the 12.9 VAC I'm looking for. Can you confirm if there is another way to wire this to increase output amperage?

[Syllith]

Oh, and another thing. When I was taking my readings, the bottom wires seemed to be solid, but the top wires were jumping around a little bit. I waited a while for them to stabilize and those are the numbers reported in the previous post. Can someone explain why this might be?

[Cliff Matthews]

So front bottom, front top, and top back are in separate continuity groups? You should include voltages measured across those too.

Also, depending on input line voltage stability, I'd expect up to 3v variations on yellow, that's likely the listed 169v winding.

[Syllith]

Yes. The wires I listed are the only readings I can get. Any other combination will result in no connection.

Here are the results from my voltage test:

Blue and white/blue:
Red pair - 171VAC
Grey pair - 13.2VAC
Orange pair - 18.6VAC
Yellow pair - 8.8VAC

The brown and white/brown wire reported almost the same exact same voltages.

Connecting the brown and white/black seems to have upped all the voltages by 6% or so. Perhaps this is the combination I need to get the most amperage? The grey pair is now reporting 15VAC. This seems perfect for my purposes. A little overvoltage is actually welcome.

I think the primary is on the bottom and the 4 paired wires are all just output and the bottom wires are just different inputs.

[Cliff Matthews]

Also, up close can you identify where any magnet wires are entering closest to the core?

Here: https://ludens.cl/Electron/trafos/trafos.html on the last photo, the primary winding is the 2 left side terminals connected to the winding closest to the core.

[Syllith]

No, unfortunately I cannot. It is covered by plastic that's been heat pressed to the transformer housing. I cannot see inside.

[Cliff Matthews]

So it seems you have in-phase paralleled Blue and blue/white (4.5 ohm) with the Brown and white/black (3.2 ohm) winding then?
If so, that would explain a 6% increase, but now the Blue and Blue/white is not carrying much current.. I would only parallel identical windings without access to view the core entry points.

Have you left this powered up (but unloaded) for more than 15 minutes? (without much more than slight warmth on the core?)

[Syllith]

When I was building the tone generator for it, it was my main power source. It wasn't drawing much current but it ran for a long time. It was nice and cool. I powered a kenwood radio and a single speaker for about 5 minutes one time and it was fine as well. Just not a lot of current to keep the cap charged up.

[Syllith]

So I think I may have found the best combination for this transformer. Connecting the brown + white/black wire (the one that seems to produce the most current) and used the yellow pair (8.8VAC) and that actually bumped it up to a solid 14V at idle and with a car stereo playing a single speaker at full volume, it only dropped to about 12V or so. This is more what I was expecting from this transformer

But... It's still not enough. I'm starting to realize that even though the transformer is physcially large, it has 4 different outputs and at least 3 inputs. There's a lot going on and it doesn't seem to carry nearly as much current as I original thought.

So in that case, I think I'm either going to find another one that fits my application, build my own, or custom order one (probably from Agile Magnetics). Depends on the cost. It's worth calling for at least a quote though. Unless anybody knows of a transformer that'd be able to provide about 5A at 12-24 volts. I don't mind if it's over 12 because I can just regulate it from there. This way I can make it variable.

[Cliff Matthews]

**Can you measure the core size to get an estimate on VA rating?

$25 can buy something more efficient (and not so heavy to ship..)
Meanwell RS-150-12 (11.4-13.2v adj) or the RS-150-15 (14.25-16.5v adj)
http://www.mouser.com/ProductDetail/Mean-Well/RS-150-15
http://www.mouser.com/ds/2/260/RS-150-SPEC-806251.pdf

[Cliff Matthews]

That 8.8v winding sounds like it's doing its job, since without a load you would see about 10VAC (~14vdc rectified). Another way to get to where you want to go is doing a $10 "slow boat from China order" on a 300w DC-DC Buck converter. Then you can use the grey, the orange, or even the grey+yellow as input to the converter (pre-adjusted to a typical 13.8v) with a cap on both sides to keep the audio loose and lively. http://www.ebay.com/itm/300W-DC-DC-CC-CV-Buck-Converter-7-40V-to-1-2-35V-8A-Step-down-Power-Module-/261530423651

[Tcucore]

I've been using a 3.5 amp fuse on my power wire and when I test, I move it to the center of my shop and power it from a distance with a switch. I have yet to short it out. I have also not made any changes to it at all yet. I'm doing my research before I go further. Anything I'm unsure of I will ask about first.

you should be aware that power at the primary must equal power at the secondaries (cumulative).  This means if you are sourcing 3 amp at a 12.5V secondary (for example) with a 125V primary, the current drawn from the primary is reduced by a factor equal to the voltage reduction from primary to secondary.  Use the following expression to determine a reasonable primary fuse, otherwise your 3 amp fuse my never trip during a short.

(Primary Power = Secondary Power )  therefore (Primary Voltage * Primary Current) = (Secondary Voltage * Secondary Current).  Use a slow-blow so the initial inrush doesn't blow it when you turn your supply on.

Additionally, I have a Triad power transformer I've never used.  It's rated for 10A with a 20V secondary.  If you are interested, I'll mail it to you at cost for shipping.  I'm currently using it as a book-stop on my shelf.