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.