# Understanding transformers and power supplies... a few questions.

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#### Mull3t

So I get the basic gist of what a transformer with dual primaries and dual secondaries or a single primary and dual secondaries is. I have about the same understanding of what windings are in relation to the secondaries. I also understand that there is the concept of running the secondaries in parallel or in series or if you wanted to create a center tap. As I stated my understanding is quite basic. What I'm trying to wrap my head around is how to figure out what transformer to buy if you needed to give enough juice to a power supply. In my current situation that would be a bi-polar power supply. Do you get a transformer with two secondaries that are rated for higher than what your supply is regulating ie two 20VAC secondaries for a +/-15v supply. Or does that not matter?

Also, is there such thing as a transformer that has dual secondaries that are at two different voltages? ie one secondary at 5VAC and the other at 15VAC. If so, what is it called and where can you get one?

My other question has to do with power supplies. Let's say I build this bi-polar power supply and it can supply 100mA per each rail. I'm assuming this bi-polar supply can do that...

Sjöström Audio - SPS02 Sjöström Power Supply

If I wanted to power other components other than my main component that needs +/-15v could I do that? ie a component that just requires +15V. I might be answering my own question, but is the limitation based on how much current the component draws? So let's say component A requires 50mA from my bi-polar supply and component B requires 25mA. Then I have 25mA left over from the 100mA that the power supply can supply.

Thanks in advance for your help in helping me wrap my head around this.

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#### sofaspud

Do you get a transformer with two secondaries that are rated for higher than what your supply is regulating ie two 20VAC secondaries for a +/-15v supply. Or does that not matter?
There probably isn't an easy, or at least brief, answer to this question. Basically, the answer is yes. But remember that the secondary is rated for AC volts RMS, and the voltage after rectification and filtering will be the AC peak voltage. In other words, a transformer rated for 15VRMS at the secondary may be suitable, if its power rating and regulation are also adequate. Simply put, the transformer needs to have a great enough secondary voltage to maintain the DC operating voltage, allowing for things like diode drops, regulation overhead, peak current draw, etc.
Also, is there such thing as a transformer that has dual secondaries that are at two different voltages? ie one secondary at 5VAC and the other at 15VAC. If so, what is it called and where can you get one?
Sure there is. It's just called a dual secondary transformer. It's just that in audio (though not exclusively by any means), the two secondaries are usually equal, for building symmetrical bipolar supplies. They're available from most parts retailers.
If I wanted to power other components other than my main component that needs +/-15v could I do that?
You could. You're just paralleling loads. It can sometimes create problems, though. Since they're connected, the loads can "see" each other, and a circuit may be sensitive to that via the common supply lines.

#### Enzo

Here is a free bit of software you can download to help design basic power supplies. Might help, might not, but will be informative.

PSUD2

#### AndrewT

If you require a regulated supply then a good guide is Vac = Vdc reg.
If you require normal (not regulated) then a good guide is Vac ~ 70% of Vdc.

If you adopt a Salas Stule Shunt regulator then add a few volts AC to the required DC output. It needs more Vdrop across the CCS to get the CCS to work effectively.
This may also apply to all Shunt regulators.

#### Mull3t

Basically, the answer is yes. But remember that the secondary is rated for AC volts RMS, and the voltage after rectification and filtering will be the AC peak voltage. In other words, a transformer rated for 15VRMS at the secondary may be suitable, if its power rating and regulation are also adequate. Simply put, the transformer needs to have a great enough secondary voltage to maintain the DC operating voltage, allowing for things like diode drops, regulation overhead, peak current draw, etc.

So to put it another way... If I wasn't running a regulated power supply I might need higher AC volts RMS because of diode drops, overhead, and peak current draw. It looks like according to AndrewT's post that it depends on the type of supply how much VAC you need to start with.

Sure there is. It's just called a dual secondary transformer. It's just that in audio (though not exclusively by any means), the two secondaries are usually equal, for building symmetrical bipolar supplies. They're available from most parts retailers.
I know about dual secondaries and like you mentioned… I've only have seen them in symmetrical flavors. Can you provide an example of a non-symmetrical dual secondary transformer? Thus far in my research I haven't found anything.

You could. You're just paralleling loads. It can sometimes create problems, though. Since they're connected, the loads can "see" each other, and a circuit may be sensitive to that via the common supply lines.
So it looks like it's not worth it to have multiple components hanging off one supply. Then what do other DIYer's do? Use two or three transformers? or battery solutions for part of the whole build?

Here is a free bit of software you can download to help design basic power supplies. Might help, might not, but will be informative.

PSUD2

I looked into this and it looks like it's made for a non-regulating supply.

If you require a regulated supply then a good guide is Vac = Vdc reg.
If you require normal (not regulated) then a good guide is Vac ~ 70% of Vdc.

If you adopt a Salas Stule Shunt regulator then add a few volts AC to the required DC output. It needs more Vdrop across the CCS to get the CCS to work effectively.
This may also apply to all Shunt regulators.

I'm thinking the Salas would be a good option. So then something like this would work? It has two 18VAC secondaries.
62054-P2S02 Acme Electric/Amveco/Actown | TE62054-ND | DigiKey

By the way how much does the output current matter. Also what does the Power - Max mean ? i.e. 15VA.

#### sofaspud

So to put it another way... If I wasn't running a regulated power supply I might need higher AC volts RMS because of diode drops, overhead, and peak current draw. It looks like according to AndrewT's post that it depends on the type of supply how much VAC you need to start with.
Yes, the type of supply is an important factor. In fact, the best approach is to work backwards - first determine what the load needs, then design in the regulators, filters, and rectifiers, and finally specify a transformer secondary to provide the required minimum AC output.
I know about dual secondaries and like you mentioned… I've only have seen them in symmetrical flavors. Can you provide an example of a non-symmetrical dual secondary transformer? Thus far in my research I haven't found anything.
There's this one, that has a 24VCT secondary and a 12V secondary, or this one, for the vacuum tube aficionados, has 3 secondaries.
So it looks like it's not worth it to have multiple components hanging off one supply. Then what do other DIYer's do? Use two or three transformers? or battery solutions for part of the whole build?
I'm not sure what other DIY'ers do. For production runs, a company can specify what secondaries they need and a transformer manufacturer will make them to order. Or a person can get 1-off transformer custom wound. Most can't afford that luxury. Some DIY their own transformers. Personally, I'm not (yet) that gung-ho; I'll scrounge units from old equipment. There's also nothing wrong with using multiple transformers. Batteries are also an option, if that's a practical solution.

#### Enzo

15VA means 15 volt-amps. At your level, a volt-amp is a watt, so a 15 watt transformer. If you have a transformer that puts out 15v at 1 amp, you have a 15 VA transformer. If you have one that produces 30v at half an amp, then that too is a 15 VA transformer. Or 5 volts at 3 amps. VOlts times amps.

You chose the transformer voltage based on what voltages you need. You determine how much current your needs are, then get a transformer that can provide that level of current. We would add more current to the rating so you are not using the thing right at its limit.

The software I linked may not cover adding regulators, but if you want to understand the relationships between AC voltages and DC and the action of filter circuits, it is useful.

#### Mull3t

Yes, the type of supply is an important factor. In fact, the best approach is to work backwards - first determine what the load needs, then design in the regulators, filters, and rectifiers, and finally specify a transformer secondary to provide the required minimum AC output.

So to clear things up the load is the "circuit", right? So if that's the case then let's walk through this. So I'm building a Joachim Gerhard Buffer for an upcoming ES9023 DAC project. The buffer requires a power supply that can provide +/-15v and 25mA per channel for a total of 50mA. So my supply or in this case SSLV1.1 needs to provide at least 25mA and +/-15v per regulator. So from my reading it appears the SSLV is pretty customizable with a few adjustments to get what you want. The challenge is finding the parts needed to get there, which should probably be saved for a more appropriate thread. The transformer then has to meet the needs of the regulator in order to perform well.

15VA means 15 volt-amps. At your level, a volt-amp is a watt, so a 15 watt transformer. If you have a transformer that puts out 15v at 1 amp, you have a 15 VA transformer. If you have one that produces 30v at half an amp, then that too is a 15 VA transformer. Or 5 volts at 3 amps. VOlts times amps.

Got it. So basically a 15VA transformer is using 15 watts of power per hour - or maybe not.

You chose the transformer voltage based on what voltages you need. You determine how much current your needs are, then get a transformer that can provide that level of current. We would add more current to the rating so you are not using the thing right at its limit.
So in the case of the SSLV it looks like I'd need more like 2x20VA because of voltage drop across the regulator(s). So another question I have is based around current needs. When the aforementioned transformer provides 417mA of total current then is that directly related to the current draw of each regulator? So if each regulator needs 100mA of current then this transformer would work? Better yet… how about this one? 62055-P2S02 Acme Electric/Amveco/Actown | TE62055-ND | DigiKey
It takes into account the voltage drop and also should have enough current to provide 200mA for both regulators.

The software I linked may not cover adding regulators, but if you want to understand the relationships between AC voltages and DC and the action of filter circuits, it is useful.
I'll have to take a closer look. I think it's time to read Electronics for Dummies because of a lot of this is getting way beyond painting by numbers -- which I don't mind of course. I love how this hobby always provides a challenge.

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#### sofaspud

So to clear things up the load is the "circuit", right? So if that's the case then let's walk through this. So I'm building a Joachim Gerhard Buffer for an upcoming ES9023 DAC project. The buffer requires a power supply that can provide +/-15v and 25mA per channel for a total of 50mA. So my supply or in this case SSLV1.1 needs to provide at least 25mA and +/-15v per regulator. So from my reading it appears the SSLV is pretty customizable with a few adjustments to get what you want. The challenge is finding the parts needed to get there, which should probably be saved for a more appropriate thread. The transformer then has to meet the needs of the regulator in order to perform well.
Yes, first of all "load" is just whatever you're sending power into. For a power supply, it's the circuit to be supplied, for an audio amp it is a speaker, etc. For a stage within the audio amp you could say it's the following stage, so it is kind of a "referenced to what" concept and I should have been more clear.
I introduce that concept because we can apply it here. The transformer must supply a load we'll call the rectification and regulation circuitry. We know that the output from regulation will be ±15V @ 25mA (to be fed to another load). There will be losses, so the transformer must output more than that ±15V @ 25mA, and designing the supply involves selecting components to meet all those needs. While minimizing unnecessary losses through overcompensation.
Got it. So basically a 15VA transformer is using 15 watts of power per hour - or maybe not.
No. It can output 15VA (watts) continuously.
A watt/hour is really a different topic.
So in the case of the SSLV it looks like I'd need more like 2x20VA because of voltage drop across the regulator(s). So another question I have is based around current needs.<snip>
15VAC@25mA is only 0.375VA (x2=0.750VA). 15VDC@100mA is only 1.5 watts (x2=3W). I can't see you needing a transformer over 10VA total. Current draw always depends on the load, not the supply. Admittedly, I didn't view the transformer submissions, nor am I certain of your application. For specifics, a separate project thread may be a better place.
I'll have to take a closer look. I think it's time to read Electronics for Dummies because of a lot of this is getting way beyond painting by numbers -- which I don't mind of course. I love how this hobby always provides a challenge.
It's past that time. There is a good web page describing all the steps. I've linked to it before, but I couldn't find the link or the page or a saved doc. I do have a good article by Steve Ciarcia that I'd be willing to email to you. It's a >200W single-output linear supply project but the design can be scaled bigger or smaller - it's his thorough description of design choices that's valuable.

#### AndrewT

Shunt regulators are very different from Series regulators.
A Shunt regulator draws the same current ALL the time.
This is a continuous LOAD on the transformer and the rectifier and the smoothing.
The maximum continuous AC current of a transformer for CONTINUOUS duty supplying a resistor is VA/Vac.
10VA and 20+20Vac is 10/40 = 250mAac into a resistor.

The maximum continuous DC current from a Capacitor input filter is approximately HALF the AC current rating of the transformer.
i.e. that 10VA transformer when feeding a capacitor input filter can support a maximum CONTINUOUS DC current of ~125mA.

If your load is a dual polarity single channel of 25mA, then the CCS in the Shunt Regulator MUST pass >25mAdc for each chanel and for each polarity.
A two channel amplifier as the LOAD for a SSLV must have the CCS set to >50mA.
It is recommended regularly that the Shunt part inside a Shunt regulator MUST pass sufficient current to maintain a low output impedance.
The CCS also "wastes" a bit of current that is not sent to the output. If I allow 5mA for waste and 50mA for the "Shunt" current then the CCS consumes 25mA per channel plus 50mA for the Shunt plus 5mA of "waste" for a total load current of 105mAdc from the capacitor input filter.
That is ~ 84% of the maximum capacity of the transformer. The transformer will run warm. That warm transformer will heat up the internals of the regulator.

I would not use a 10VA 20+20Vac to power a pair of 25mA loads from a SSLV.
I would aim for >=15VA. If I were to increase the voltage to 22Vac then >20VA would be about right.

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