120AC to 12VDC Power Supply without transformer?

[asm_]

Since my recent discovery of switching mode power supply, I have been thinking about how I can take advantage of this new technology... new to me at least.

Therefore, instead of using a car battery to power my ICE charger, I am considering building a 12V power supply without a transformer. However, since I have no former electronics education background, I am having hard time figuring out how to proceed.

It appears to me that one cannot simply run A/C power through rectifier and directly into the switching power regulator. Since most switching power regulator can only take input up to 40V DC. Is there an easy way or common way of converting 120V AC down to say around 40V without using a transformer? The expected current draw on the 12V side needs to be at least 15A up to 40A.

Any input would be helpful.

~ASM

[Comatose]

In AC-DC switching, you usually want a transformer. it won't have to be a big heavy thing like it would running from 60hz, but transformers still make life a lot easier, not to mention MUCH safer. With a non-isolated topology like buck, you run the risk of electrocution if something goes wrong (from a 120v source)

What you're looking for is called a "forward" converter. There are a large number of controller chips availiable for them. 40A may take some doing.

[Chippie]

the transformer provides the necessary isolation between input and output...120vac to 12vdc...

Using a transformer to achieve this is easy, achieiving it without is easy too..but it aint safe.

A thyristor could be used to reduce the 120 down to 40 vdc, by switching the scr on at the appropriate time in the cycle...but to achieve a current of the magnitude you state, it would need to be a hefty device.

[Chippie]

What you're looking for is called a "forward" converter. There are a large number of controller chips availiable for them. 40A may take some doing.

As an example, TL494 or SG3524 could be used in a switching app..The transformer of such a design supply 40 amps would need some careful construction invovling the use of copper tape as opposed to conventional wire...Imagine trying to wind ECU thick enough to handle 40a ?

Interestingly enough, there is a design I've seen on the web for such a unit...see the attached..Hope its of use...

http://www.arrl.org/notes/1816/templates/ see the foot notes at the bottom of the page for corrections!

[asm_]

The only reason I want to stay away from transformer is becuase they are usually bulky and heavy. Especially when the current requirement goes up. I know that transformer offers electrical is isolation. But if it's possible to use a smaller transformer to achives isolation, it would be ok.

I am pretty sure it can be done, as most computer ATX power supply does not have huge transformer while they can supply up to 550 watt.

~ASM

[Comatose]

As the frequency you're operating at goes up, the size of the transformer necessary goes down. Standard ACis 60hz. Switching regulators usually run at 53,000hz, or 150,000hz or higher.

[vintage1]

1/. You want a transformer of some sort for mains isolation.

2/. What you want is a 110v line to 12v DC switched mode power supply. Many of these are available. In your PC for a start. Also in low voltage halogen lighting units.


I suggest obtaining one of these, and putting a shottky bridge rectifier on its output with a resonable electoltyic. My guess is it will give you about 14v mnoredaltely well regulated at whatever power its rated for. I have one that does 200W built in a wall somewhere...

[Andrew McGregor]

Simplest solution is to go buy a 12V switchmode. A PC power supply is pretty cheap, can be jumpered to start up when you apply power (normally the PC's motherboard links the PSU to the switch), and can be had with ratings up to 1000W without much trouble. Most electronics stores have smaller switchmodes if a 40A PC supply is overkill.

Don't bother building these things, they're hard to design, harder to test, and dangerous if you get them wrong. Small switchmode DC-DC converters, like a BEC for a model, on the other hand, are quite good fun.

Search for the PC power supply mod, it's simple and cheap.

[simingx]

The only reason I want to stay away from transformer is becuase they are usually bulky and heavy. Especially when the current requirement goes up. I know that transformer offers electrical is isolation. But if it's possible to use a smaller transformer to achives isolation, it would be ok.

I am pretty sure it can be done, as most computer ATX power supply does not have huge transformer while they can supply up to 550 watt.

~ASM

They don't have a big transformer, they have a small transformer :D
The reason these can output 500 watts is that they first rectify the mains to DC, then switch it through a half bridge converter at 50 or so kHz. This way the required magnetics are much smaller.

Anyway, most "cheapo" PC power supplies cannot supply anywhere near their "rated" output... the switching transformer in those units is way too small!

Look for used industrial power supplies, they can usually be found pretty cheap. Are you going to blow up your $150 charger to save $50 on a PSU? :)

You can usually get away with a smaller power supply than is "recommended" if you don't use the charger at full power (i.e. not charging the maximum number of cells at maximum current).

[asm_]

Anyway, most "cheapo" PC power supplies cannot supply anywhere near their "rated" output... the switching transformer in those units is way too small!


You are indeed correct about that. I recently tried to use a PC power supply and it failed to hold its 12v power rail at 3amp. The package clearly marked 10A for 12v. :mad: What really happened was the voltage quickly drop to about 10.5 volt under heavy load. I think most motherboard has voltage regulator to compensate for the cheap power supply that most computer use. On the other hand, an decent PC power supply that cost about $90 can easily hold its own for whatever current draw needed by the ICE charger.

This project of mine is not really mean to save money, else I would have just spend $50 and buy a 12v 20a P/S from hobbypeople.net. I am interested in this for the learning experience. So, if any one can point me to the right direction, perhapes a book that deal specificly with modern switching power supply design, it would be greatly appricated.

Thanks

~ASM

[Ricardo RW]

I suggest obtaining one of these, and putting a shottky bridge rectifier on its output with a resonable electoltyic...At the output of a PC PS? Is this to improve filtering of the output wave form, ripple?

[Bruce Abbott]

I recently tried to use a PC power supply and it failed to hold its 12v power rail at 3amp. The package clearly marked 10A for 12v. :mad: What really happened was the voltage quickly drop to about 10.5 volt under heavy load. Your PSU probably can deliver 10A at 12V, but you need to change the feedback resistor network so that it regulates 12V only. Most PC power supplies only regulate the 5V rail (sometimes a combination of 5V and 12V) so the 12V output will sag under load.

Modifying the circuit is not difficult (you probably only have to remove one resistor and and add or change one other) but you have to carefully calculate the correct values, otherwise the PSU may go overvoltage and blow up! A less effective but safer solution is to load up the 5V rail with a power resistor or light bulb (around 2~3A load is usually enough).

[simingx]

Sometimes the problem is not that the PSU can't regulate properly without a load on the 5V.. it's that the switching transformer is grossly undersized for it's "supposed" output...