Power supply questions

[star882]

Why is the power transformer in a PC power supply is so small and light?
A 250W supply will need at least a 250VA transformer.
In realty a bigger one is needed, as regulators are less than 100% effcient.
The transformer is smaller than a 12VA one I bought at radio shack!

 

[Roy]

PC's use switching power supplies which don't use big step down transformers, I have a 32A switching PS that is about the size of 500 page book.

 

[star882]

A switching power supply works with a transformer to reduce the line voltage, a rectifer to turn this to dc, and a switching regulator to downconvert the unregulated voltage. I built one for the science fair 3 years ago. I fried the regulator with a short as i played around with electronics.

 

[star882]

Here is the picture

The quarter is for a size perspective.
The biggest transformer is the main power transformer.
The heatsink in the middle of the supply is for two SCRs. The transformer beside the heatsink is the driver transformer. The transformer next to the fuse is the aux transformer, which powers the computer's clock and the control circuits when the main power is off. The heatsink near the output wires is for the rectifer and regulator.

 

[B@rt]

hi star882,
your pic host doesn't support linking. I just copied the url to view it, and it looks nice, but it is BIG! Any chance on reseizing?

You can also use Image Station for your pics, it will allow you to link downsized pics...

 

[James S]

Your picture didn't show up yet so I can't tell if this is the case with your transformer, but the smaller power supplies that I've looked into and built in the past all used toroid style transformers. They tend to be more expensive, perhaps they are more difficult to wind? But they appear to handle more power in a much smaller package. check out part number PWR1118 over at http://www.bgmicro.com/ it handles almost 2 amps total and is only 2 3/4" in diameter.

Some of those flat looking ones are actually wound with a flat copper tape instead of just wire, which lets them handle a lot more current in a smaller space too. I don't know if those kind are used in regular computer power supplies but they are often used in high end power inverters or UPS systems.

-James

 

[star882]

Oops!
Go to http://www.geocities.com/star88232000/DCP_0324.JPG

 

[star882]

I am having technical problems with my web site.
Try http://www.geocities.com/star88232000/index.html?1031500905870
to see if it works.

 

[star882]

If it works, go to pictures, then computer power supply.

 

[CNC Dan]

Originally posted by star882:
Why is the power transformer in a PC power supply is so small and light?
A 250W supply will need at least a 250VA transformer.
In realty a bigger one is needed, as regulators are less than 100% effcient.
The transformer is smaller than a 12VA one I bought at radio shack!

<font size="2" face="Verdana, Arial">It probably is a switching power supply.
If they are lowering voltage, they typicaly retify the incoming line voltage and then use switching transistors to let small 'bursts' of current through to the output stage. These 'bursts' are then filtered with capacitors and inductors to smoth the output to smooth DC currrent.

The transformer you see might be a coil in the output stage.

 

[Evan]

I think a key reason switching supplies can use small transformers is that the operatate at high frequencies. The higher the frequency, the more power you can pump thru a transformer. So 60 Hz AC needs a big transformer for a few hundred watts. Airplanes use 400 Hz AC and can handle the same power in a somewhat smaller transformer. Switching supplies run in the range of 30,000 or more Hz and can pump a lot thru a very small transformer. However, at these higher frequencies, the losses in a laminated steel core get too high and ferrite cores work better. Until switching supplies came along, I only saw ferrite in RF circuits, like the loopstick or the tuning slugs in 455KHz IF transformers in an AM radio. A real engineer could probably tell you why ferrite works better at higher frequencies. I think the core works like a bucket with power stored into it by the primary, then drawn off by the secondary. If you bail faster, you can move as much water in a small bucket as a large one.

 

[carbonsparky]

Originally posted by Evan:
Switching supplies run in the range of 30,000 or more Hz and can pump a lot thru a very small transformer. However, at these higher frequencies, the losses in a laminated steel core get too high and ferrite cores work better. Until switching supplies came along, I only saw ferrite in RF circuits, like the loopstick or the tuning slugs in 455KHz IF transformers in an AM radio. A real engineer could probably tell you why ferrite works better at higher frequencies. I think the core works like a bucket with power stored into it by the primary, then drawn off by the secondary. If you bail faster, you can move as much water in a small bucket as a large one.

<font size="2" face="Verdana, Arial">Ferrite is kind of like having a bunch of iron beads bonded together with glass but each insulated from the next. Laminated transformers have iron or mild steel laminations which are insulated from each other, each lamination is stamped out of a sheet of metal.
The one main source of loss in a transformer is eddy currents. This is the current induced into a transformer or inductor core because of the magnetic field changes that take place when operating. The laminations found in motors and low frequency transformers and inductors help break up the eddy currents by not allowing current to flow from one lamination to the next. At higher frequencies the eddy currents paths become shorter and the laminations can not reduce them enough to be effective. Ferrite can do the job because the iron particles are small enough that the eddy currents do not have room to flow, even at the higher frequencies.

 

[star882]

One trick to do is to stick nails in a lizard, and connect the nails to 5v and ground. This will really show how much power they can crank out.

 

[B@rt]

Originally posted by star882:
One trick to do is to stick nails in a lizard, and connect the nails to 5v and ground. This will really show how much power they can crank out.

<font size="2" face="Verdana, Arial">

 

[star882]

I have moved this trick to lizard zapping.
Please try it, it is hilarious.

 

[lux0]

.

 

[JackBlades]

Originally posted by star882:
I have moved this trick to lizard zapping.
Please try it, it is hilarious.

<font size="2" face="Verdana, Arial">If you are serious about torturing animals, you are a JERK.