Transformer vs Switching Power Supply

[redgear]

I need to power my micro and a relay from mains. I am confused between buying a cheap chinese power supplies and build my own step down supply using a transformer. Is building my own worth it?

Apart from the voltage and current ratings what are the things that differentiate transformers? Can I buy any cheap transformer off ebay? What are the minimum things to look for in a transformer to make sure it does not injure anyone or burn down the house when it fails.

Thanks

[Jwillis]

Construction.  A typical step down transformer would be electrically isolated between the primary side and secondary side. This means that the primary side is a separate winding from the secondary side. Where as an auto transformer is not electrically isolated . The neutral wire of the mains will be connected to the  load. Super cheap transformers may be auto transformers because they are cheap to construct.
Frequency . The transformer must be 60hz for your North American mains supply. Obvious but I thought I would add this anyway.
If you choose to build your own you will need rectification , filter caps and voltage regulation as for your requirements . This can sometimes drive up the cost.
Do you need a clean DC supply that a linear supply will offer or if your load is low than maybe a low cost SMPS wall wart would be adequate .

[MosherIV]

A pre-built psu is going to be cheaper than building your own.
Building one would for for learning.

What differentiates transformers :
Toroidial vs square.
Single or dual secondary

I would avoid ebay for any electronic components if you care about safety.

Not sure what to look for but I would protect the transformer/yourself by putting fuse on input and output

[Doctorandus_P]

There is lots of quality difference in the good old laminated iron core transformers.

If you're on the cheap it's well worth opening up old electronics stuff for parts.

When working with mains voltages, safety is always a concern.
An easy way for experimenting would be to have a plastic food container with lid. Make a few small holes in it for the wires, so mains voltage goes in on one side, and low voltage comes out on the other side. Make sure there is a decent separation of wires of these 2 sections. Did you notice the food container Dave put over the high voltage section in one of his latest trouble shooting video's?

[redgear]

Construction.  A typical step down transformer would be electrically isolated between the primary side and secondary side. This means that the primary side is a separate winding from the secondary side. Where as an auto transformer is not electrically isolated . The neutral wire of the mains will be connected to the  load. Super cheap transformers may be auto transformers because they are cheap to construct.
Frequency . The transformer must be 60hz for your North American mains supply. Obvious but I thought I would add this anyway.
If you choose to build your own you will need rectification , filter caps and voltage regulation as for your requirements . This can sometimes drive up the cost.
Do you need a clean DC supply that a linear supply will offer or if your load is low than maybe a low cost SMPS wall wart would be adequate .

Thanks. No any supply would do fine, switching supplies from reputed brands are expensive. Since it is a small circuit I thought I could learn and build one myself.

A pre-built psu is going to be cheaper than building your own.
Building one would for for learning.

What differentiates transformers :
Toroidial vs square.
Single or dual secondary

I would avoid ebay for any electronic components if you care about safety.

Not sure what to look for but I would protect the transformer/yourself by putting fuse on input and output

Thank You. Yes, I will fuse both the primary and secondary.

There is lots of quality difference in the good old laminated iron core transformers.

If you're on the cheap it's well worth opening up old electronics stuff for parts.

When working with mains voltages, safety is always a concern.
An easy way for experimenting would be to have a plastic food container with lid. Make a few small holes in it for the wires, so mains voltage goes in on one side, and low voltage comes out on the other side. Make sure there is a decent separation of wires of these 2 sections. Did you notice the food container Dave put over the high voltage section in one of his latest trouble shooting video's?

Thanks. No, I haven't watched the video yet. Will watch it now.

[Zero999]

A decent switched mode power supply, with an output which is isolated from the mains, will contain a transformer. It will work at a much higher frequency, than the mains, so will be much smaller.

I wouldn't recommend buying anything too cheap off ebay, as it could be unsafe. I've seen numerous teardowns of dodgy power supplies on YouTube. You're better off buying from a local supermarket, or electronics store. Hopefully they would have made the effort to ensure the products, they sell comply with local safety regulations.

[HB9EVI]

I can just affirm the previous statements:  many SMPS from ebay, ali & co are bare crap, without the neccessary safety and EMI components, so they are nothing but trouble.
I got several Meanwell over Ali which are genuine and well constructed, so if carefully chosen, you can get good products, but they cost an adequate price then.

[Jwillis]

I don't know what other peoples experience is with EBay but you can get good quality if you use common sense when making a purchase . Look what they are selling  .Ask questions and make a judgment yourself.
Much of what is available on Ali Express or Ebay is factory surplus and/or seconds that can be unbranded ,re branded , not packaged or older versions bought and sold by small outlets or brokers .
There are many small online electronics stores across North America that specialize in various electronic and electrical products that are very good and reasonably priced . And in many cases they know more about what they are selling than the traditionally distributors like Mouser or Digikey. Distributors like Digikey and Mouser are distributors  only and despite common perception they do not test what they sell. So they are not immune to defective or miss labelled products. Manufacturers test products by batch and not individually so some  times defective products do get to distributors .  The plus side the major distributors will except returns or replace defective products for free much of the time.

If your chose to build your own then I would suggest a basic linear supply . SMPS supplies can get complicated for the novice. There are plenty of circuits online that can be very inexpensive to build  depending on your requirements . In most cases you will only need 1 or 2 amps . Small iron core transformers can be less than $20 which would constitute the bulk of your cost. The rectifier ,filter capacitors , regulators , transistors and fuses are cheap. In many cases less than a couple bucks to only a few cents each . And if you have any questions many good knowledgeable people here are ready to help out .

[John B]

Buying switchmode power supplies of ebay is a lucky dip, just like buying ICs and transistors. Unless you're sure that the seller is selling a legitimate brand, why bother taking the risk? IME, common problems include fake current ratings, premature failure, and generally questionable design decisions. Thankfully I've never had a dangerous failure with one of them.

[ledtester]

How much are you willing to spend on a power supply?

Also - you said you need to power your micro and relay from mains. What voltage(s) do you need?

[redgear]

A decent switched mode power supply, with an output which is isolated from the mains, will contain a transformer. It will work at a much higher frequency, than the mains, so will be much smaller.

I wouldn't recommend buying anything too cheap off ebay, as it could be unsafe. I've seen numerous teardowns of dodgy power supplies on YouTube. You're better off buying from a local supermarket, or electronics store. Hopefully they would have made the effort to ensure the products, they sell comply with local safety regulations.

I can just affirm the previous statements:  many SMPS from ebay, ali & co are bare crap, without the neccessary safety and EMI components, so they are nothing but trouble.
I got several Meanwell over Ali which are genuine and well constructed, so if carefully chosen, you can get good products, but they cost an adequate price then.

I don't know what other peoples experience is with EBay but you can get good quality if you use common sense when making a purchase . Look what they are selling  .Ask questions and make a judgment yourself.
Much of what is available on Ali Express or Ebay is factory surplus and/or seconds that can be unbranded ,re branded , not packaged or older versions bought and sold by small outlets or brokers .
There are many small online electronics stores across North America that specialize in various electronic and electrical products that are very good and reasonably priced . And in many cases they know more about what they are selling than the traditionally distributors like Mouser or Digikey. Distributors like Digikey and Mouser are distributors  only and despite common perception they do not test what they sell. So they are not immune to defective or miss labelled products. Manufacturers test products by batch and not individually so some  times defective products do get to distributors .  The plus side the major distributors will except returns or replace defective products for free much of the time.

If your chose to build your own then I would suggest a basic linear supply . SMPS supplies can get complicated for the novice. There are plenty of circuits online that can be very inexpensive to build  depending on your requirements . In most cases you will only need 1 or 2 amps . Small iron core transformers can be less than $20 which would constitute the bulk of your cost. The rectifier ,filter capacitors , regulators , transistors and fuses are cheap. In many cases less than a couple bucks to only a few cents each . And if you have any questions many good knowledgeable people here are ready to help out .

I don't know what other peoples experience is with EBay but you can get good quality if you use common sense when making a purchase . Look what they are selling  .Ask questions and make a judgment yourself.
Much of what is available on Ali Express or Ebay is factory surplus and/or seconds that can be unbranded ,re branded , not packaged or older versions bought and sold by small outlets or brokers .
There are many small online electronics stores across North America that specialize in various electronic and electrical products that are very good and reasonably priced . And in many cases they know more about what they are selling than the traditionally distributors like Mouser or Digikey. Distributors like Digikey and Mouser are distributors  only and despite common perception they do not test what they sell. So they are not immune to defective or miss labelled products. Manufacturers test products by batch and not individually so some  times defective products do get to distributors .  The plus side the major distributors will except returns or replace defective products for free much of the time.

If your chose to build your own then I would suggest a basic linear supply . SMPS supplies can get complicated for the novice. There are plenty of circuits online that can be very inexpensive to build  depending on your requirements . In most cases you will only need 1 or 2 amps . Small iron core transformers can be less than $20 which would constitute the bulk of your cost. The rectifier ,filter capacitors , regulators , transistors and fuses are cheap. In many cases less than a couple bucks to only a few cents each . And if you have any questions many good knowledgeable people here are ready to help out .

How much are you willing to spend on a power supply?

Also - you said you need to power your micro and relay from mains. What voltage(s) do you need?

Thank you everyone
I am planning to build a simple linear supply myself.

[Siwastaja]

Instead of choosing between a dodgy Ebay switch mode supply, or decent mains-frequency transformer, you'll have the most obvious choice of buying a modern, decent switch mode supply. I'd recommend looking at Meanwell products available at most distributors. They are maybe 2-3x as costly as the Ebay crap, but safe and reliable, and still not too expensive.

A big question is do you need regulated voltage. If you do, the traditional mains transformer + linear regulator is highly inefficient and also expensive. If unregulated voltage is acceptable (for example, in audio amplifiers), it's already much simpler.

For regulated output voltage, switch mode is almost always the only sane choice.

[redgear]

Instead of choosing between a dodgy Ebay switch mode supply, or decent mains-frequency transformer, you'll have the most obvious choice of buying a modern, decent switch mode supply. I'd recommend looking at Meanwell products available at most distributors. They are maybe 2-3x as costly as the Ebay crap, but safe and reliable, and still not too expensive.

A big question is do you need regulated voltage. If you do, the traditional mains transformer + linear regulator is highly inefficient and also expensive. If unregulated voltage is acceptable (for example, in audio amplifiers), it's already much simpler.

For regulated output voltage, switch mode is almost always the only sane choice.

Yes, I need regulated voltage. Meanwell was the first supply I looked at. The local supplier I usually buy from said he is out of stock for meanwell items. He has Hilink and Mornsun in stock. Another forum member told me both are decent brands when it comes to performance and safety but the Mornsun will be more reliable when than Hilink as the hilink ones are made for IOT. I'm just confused between the two.