Electronics > Beginners
Help me design a PSU
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AngraMelo:
Efe,

I know it can be a pain to source components when you dont have a digikey/mouser type of vendor in your country, believe me, Im in Brazil and here sucks as well.

Now, for the more experienced guys to help you (and I do not include myself in the middle of them). You do need to specify somethings.
If you want something around 15V, your 48V transformer probably wont do (or at least wont do with a simple design).

Usually, the hard things to get for us are ICs and other specialized things. So you probably have access to general use diodes, resistors and transistors.
So with that in mind we need to get another thing straight. You are trying to learn with the project as well as achieve your goals, now, do you know a little bit about transistors and opamps? If you dont building a PS using an opamp to control de voltage and cascading transistors to get the current you need might be the right solution for you. If you dont have access to an opamp using a simple zener that will set up the max Voltage limit and using a pot to vary that voltage (and still using the cascading transistors) might give you what you want.
It seems that you can follow and build circuits from a ready made schematic and what you are looking for is learning more about how the things work and starting to customize the circuits for what you need. Unfortunately asking for help here will not give you that. When you ask for help to build a simple PS you will get a ready made schematic and thus not learning what you are actually wanting to lear which is design the circuit yourself.
The guys here are incredibly helpful and knowledgable but you kind need to ask the right question.

My suggestion is: If you know how to build LM317`s (and other simple voltage regulator ICs) circuits, if you have a good idea how to use transistors and opamps then you need to add more features to your PS to make the project more challenging. If you dont know how to use transistors and opamps I strongly encourage you to built a couple of simple PS`s using them, even if it doesnt have a proper V regulation, and then starting asking questions about how to tweak the circuit to get the current limiting, proper regulation, less noise...

Efe_114:

--- Quote from: AngraMelo on December 19, 2018, 01:53:48 pm ---Efe,

I know it can be a pain to source components when you dont have a digikey/mouser type of vendor in your country, believe me, Im in Brazil and here sucks as well.

Now, for the more experienced guys to help you (and I do not include myself in the middle of them). You do need to specify somethings.
If you want something around 15V, your 48V transformer probably wont do (or at least wont do with a simple design).

Usually, the hard things to get for us are ICs and other specialized things. So you probably have access to general use diodes, resistors and transistors.
So with that in mind we need to get another thing straight. You are trying to learn with the project as well as achieve your goals, now, do you know a little bit about transistors and opamps? If you dont building a PS using an opamp to control de voltage and cascading transistors to get the current you need might be the right solution for you. If you dont have access to an opamp using a simple zener that will set up the max Voltage limit and using a pot to vary that voltage (and still using the cascading transistors) might give you what you want.
It seems that you can follow and build circuits from a ready made schematic and what you are looking for is learning more about how the things work and starting to customize the circuits for what you need. Unfortunately asking for help here will not give you that. When you ask for help to build a simple PS you will get a ready made schematic and thus not learning what you are actually wanting to lear which is design the circuit yourself.
The guys here are incredibly helpful and knowledgable but you kind need to ask the right question.

My suggestion is: If you know how to build LM317`s (and other simple voltage regulator ICs) circuits, if you have a good idea how to use transistors and opamps then you need to add more features to your PS to make the project more challenging. If you dont know how to use transistors and opamps I strongly encourage you to built a couple of simple PS`s using them, even if it doesnt have a proper V regulation, and then starting asking questions about how to tweak the circuit to get the current limiting, proper regulation, less noise...

--- End quote ---
yes, i think due to my poor english i couldnt express myself right. the title shouldnt be "help...." everyone was so helpful and helped me a lot even tough i couldnt pick the title correctly . i know op-amps transistors, ADCs, DACs, MOSFETs,capacitors inductors and more but i am not even close to being proffesional i dont have such experience. yes i cant find special ICs but i can find jellybean op-amps transistors, but everything i can find are fake, generally they are tend to not work i have to buy 5 to get 2 working , I think for the final design i will buy all the components from farnell and wait 30-40 days shipping and pay extreme shipping and probably a whopping-120%-tax (i tried to buy 100 euro hakko 888d and i had to pay 250 euro in total ) i think i will go with 2n3055 darlington pair,  a fast op-amp, some stabilizing caps , a CC sink circuitry for constant load.
AngraMelo:

--- Quote from: Efe_114 on December 20, 2018, 06:09:43 pm ---
--- Quote from: AngraMelo on December 19, 2018, 01:53:48 pm ---Efe,

I know it can be a pain to source components when you dont have a digikey/mouser type of vendor in your country, believe me, Im in Brazil and here sucks as well.

Now, for the more experienced guys to help you (and I do not include myself in the middle of them). You do need to specify somethings.
If you want something around 15V, your 48V transformer probably wont do (or at least wont do with a simple design).

Usually, the hard things to get for us are ICs and other specialized things. So you probably have access to general use diodes, resistors and transistors.
So with that in mind we need to get another thing straight. You are trying to learn with the project as well as achieve your goals, now, do you know a little bit about transistors and opamps? If you dont building a PS using an opamp to control de voltage and cascading transistors to get the current you need might be the right solution for you. If you dont have access to an opamp using a simple zener that will set up the max Voltage limit and using a pot to vary that voltage (and still using the cascading transistors) might give you what you want.
It seems that you can follow and build circuits from a ready made schematic and what you are looking for is learning more about how the things work and starting to customize the circuits for what you need. Unfortunately asking for help here will not give you that. When you ask for help to build a simple PS you will get a ready made schematic and thus not learning what you are actually wanting to lear which is design the circuit yourself.
The guys here are incredibly helpful and knowledgable but you kind need to ask the right question.

My suggestion is: If you know how to build LM317`s (and other simple voltage regulator ICs) circuits, if you have a good idea how to use transistors and opamps then you need to add more features to your PS to make the project more challenging. If you dont know how to use transistors and opamps I strongly encourage you to built a couple of simple PS`s using them, even if it doesnt have a proper V regulation, and then starting asking questions about how to tweak the circuit to get the current limiting, proper regulation, less noise...

--- End quote ---
yes, i think due to my poor english i couldnt express myself right. the title shouldnt be "help...." everyone was so helpful and helped me a lot even tough i couldnt pick the title correctly . i know op-amps transistors, ADCs, DACs, MOSFETs,capacitors inductors and more but i am not even close to being proffesional i dont have such experience. yes i cant find special ICs but i can find jellybean op-amps transistors, but everything i can find are fake, generally they are tend to not work i have to buy 5 to get 2 working , I think for the final design i will buy all the components from farnell and wait 30-40 days shipping and pay extreme shipping and probably a whopping-120%-tax (i tried to buy 100 euro hakko 888d and i had to pay 250 euro in total ) i think i will go with 2n3055 darlington pair,  a fast op-amp, some stabilizing caps , a CC sink circuitry for constant load.

--- End quote ---

Awesome! So you have a pretty good idea of what you are doing!! Im sure you will find plenty of help here! Good luck, buddy!!
Im in the exact same situation as you, building a couple PSs and having to deal with huge taxes and a lack of good components.
If you dont mind me asking, where are you from?
rstofer:
No matter what marketing says, rail-to-rail op amps can't go to 0V and still source or sink current.  They just can't.  Think about the active device that would be pulling down toward the 0V rail.  It has to have SOME voltage drop.

So, instead of having 0V as the lower op amp supply, we use -1V or so.

The problem is the spec and that is the death of most of these projects.  A) It's very hard to get to 0V.  Change the spec to "approach 0V" and accept 1V or 1.5V minimum.  B) Relatively high voltage and current spec (particularly current) when there is no possible use for such high values.  Somebody sees it written somewhere and suddenly 30V 5A becomes a thing.  Sure, the commercial guys can do it but they can get transformers wound any way they want and they can design in switching relays.  You just shouldn't drop 60V at 3A across the  pass transistor but the math is what it is!  Whatever the input capacitor voltage is, minus whatever the output voltage is, times the current, is turned to heat in the pass transistor.  Note that the heat goes up as the output voltage goes down!

In the days of CMOS, 100 mA is pretty high current.  1A is completely over the top.  Now, it's true, I'm not building audio amplifiers for a parade ground but those would have dedicated supplies anyway.  Around the digital bench, 1A is a LOT.  We also don't need variable voltage, 3.3V, 5V, +-15V covers just about everything.

Yes, when you are learning about analog circuits, an adjustable supply is nice.  But it doesn't need to go above 15V or above 1A.  I might build such a thing with an LM317 and call it a day.  BTW, I would pay a lot of attention to Figure 3 of this datasheet.

https://www.st.com/resource/en/datasheet/lm217.pdf

Don't overlook 5V 2A wall warts.  I use them all the time!



spec:
The ultimate barrier to getting true zero volts from a PSU without a negative rail, is leakage current. The output transistors leak current quite a bit and the killer is that the leakage current increases exponentially with junction temperature- take a look at the data sheet for your output devices.

Quite often with many PSUs, you are dissipating a lot of power in the output transistors, so the junction temperature is high and leakage current becomes a real problem.

But, bearing in mind that a PSU is a power amplifier, there is another aspect. If you have no way of loosing the leakage current, or you have no current drain on the output stage, you get into a no-mans land where the changing characteristics of the output transistors can cause problems.

The solution is quite simple though:
[1] Either generate a negative supply (2 x caps, 2 x diodes)
[2] Or use a power diode and a constant current sink, of around 20mA (power diode, 1 x small signal transistors, 1 x medium power transistor, 2 x resistors)

In general, a minimum constant current drain is good news as it also provides some current sinking in addition to current sourcing, which is handy for some loads.

Of course, the ultimate is a current sink capability equal to or more than the maximum source output current capability of the PSU (similar output stage to an audio or servo power amplifier). The LT1118 three terminal regulator, does this.

https://www.analog.com/media/en/technical-documentation/data-sheets/1118fd.pdf
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