transformers, current, voltage?

[Volter]

I am looking for transformers:
40V 700mA, 40V 700mA, 10V 700mA, the voltages cannot be below their values.
What determines the output current of a transformer?

[haklesup]

Mostly it's the gauge of the wire used to make the transformer.

If you try to get more than the rated current from a given transformer, it will try give it to you but eventually the undersized wire will get hot and burn through the insulation and then the transformer will fail.

The voltage as I think you already know is determined by the turns ratio between the primary and secondary sides. So is current for that matter but you asked about ratings not actual. Voltage rating is based on design and how much current would flow in the primary if the secondary were fully loaded. It is also dictated by the dielectric breakdown of the insulation on the wire. In most cases a transformer can be used at any reasonable voltage and still get the same ratio V at the secondary as long as the power or current is not exceeded.

[Robert Reed]

You neglected to describe the type of tranny , audio,switching, RF, line and so on.
Assuming you wete referring to line trannys, then as Hacklesup pointed out the more current, the lower the wire gauge has to be. However as the current rating is raised (while maintaining the same voltage) so is the VA rating. This requires iron - lots of iron. This is why a 10V x 10A tranny is much heavier than a 10V x 2A tranny.

[Bigglez]

I am looking for transformers:
40V 700mA, 40V 700mA, 10V 700mA, the voltages cannot be below their values.
What determines the output current of a transformer?

Are you following industry standards and using RMS values?

If those values are for DC after rectification we'd need to know
the rectifier configuration. Secondary current is quite higher than
DC output current. Perhaps 1.8 times for full-wave bridge and
capacitor circuits.

Transformers are designed to give the rated voltage out
at the rated current. So unloaded the output voltage will
rise. For commercial products this may be 20% for sub 20VA,
and perhaps half that for larger units. To get good regulation
requires more 'ingedients' - iron and copper. Both add to
weight and costs!

You haven't told us if this is an AC line application, and if so
whether its 60Hz only or 50 - 60Hz for worldwide use.

[Volter]

It's the output of the transformer that has to be 40V DC, 700mA, and 60Hz.
So, it the transformer is rated 120Vac/40Vdc at 2 amps than it should deliver the desired effect, correct?
Of course if I can find such transformer, most are 48Vdc, 2amps rated. I got some computer trans but they deliver only 12Vdc max.
Thanks.

[haklesup]

most are 48Vdc, 2amps rated

No transformer is rated for Vdc. A complete power supply maybe but a transformer is strictly an AC component.

Sounds like you will do fine with a 120VAC primary and 48VAC secondary. 2 A RMS menas that you will have no problem using less than half of that to get 700mA DC after rectification.

You loose a couple volts across the diodes in the rectifier and RMS rated voltage is actually less than peak to peak you might see on a oscilloscope. ignoring the math, you need a few extra volts AC above what you want in the final rectified DC for the rectifier and regulator to work.

How reguilated do you need this DC voltage to be? If it has to be very steady you need a rectifier (4 diode bridge rectifier), regulator and filter but if its not critical, a rectifier (single diode) and filter capacitor will be enough.

[haklesup]

most are 48Vdc, 2amps rated

No transformer is rated for Vdc. A complete power supply maybe but a transformer is strictly an AC component.

Sounds like you will do fine with a 120VAC primary and 48VAC secondary. 2 A RMS menas that you will have no problem using less than half of that to get 700mA DC after rectification.

You loose a couple volts across the diodes in the rectifier and RMS rated voltage is actually less than peak to peak you might see on a oscilloscope. ignoring the math, you need a few extra volts AC above what you want in the final rectified DC for the rectifier and regulator to work.

How reguilated do you need this DC voltage to be? If it has to be very steady you need a rectifier (4 diode bridge rectifier), regulator and filter but if its not critical, a rectifier (single diode) and filter capacitor will be enough.

[haklesup]

so now a simple edit results in a double post

How did I do that?

[Volter]

So many components involved! I forgot that the bridge needs some Vs across those Ds. This should make things easer than. I need to drive this NCP3065 and the output of this chip will deliver 0.7A.
Thanks very much.

[Robert Reed]

Just keep in mind that 40 volts is the manufacturers maximum stated value for input Vcc.

[Volter]

Like you said, I need to drive it with correct voltage and so I thought I could use LM317 but it is too much voltage for this IC and so I got to use LM5118 which is much more complex then the LM317.
Is there other way to drop down to 40V via LM317 regulator?

The transformer delivers 48V and across the bridge it drops 4.8V and so I need to drop additional 3.2V before I get to LM317, but how!
Thanks.

[Bigglez]

I could use LM317 but it is too much voltage for this IC and so I got to use LM5118 which is much more complex then the LM317.
Is there other way to drop down to 40V via LM317 regulator?

What is the required DC output voltage and load current?
Is the output voltage fixed or variable?
What is the input voltage? If the input is 120V AC, what
is the required range?

Depending upon your requirements the LM317 can be used
for outputs above 40V wrt ground, but not if the output range
includes ground.

[dyarker]

A 48 volt transformer puts out 67V peak after rectifier bridge (not counting diode drop.

A 36 volt transformer would be closer to your need for 40V regulated.

Cheers,

[Robert Reed]

Volter
Rather than type a long winded dissertation on the LM 317 vs voltage output, the easiest way is just Google up LM317 and down load the data sheet. It will tell you everything you want to know and more. Your one concern will be how much voltage you end up with across the 317 when circuit is up and running. Even at 700 ma, a dozen volts will require a fair sized heat sink.

[Volter]

Well, the load requires 39.6V and 700mA! To drive the output I need to use NCP3065 IC. This chip takes 40V input max.
LM317 can handle max load 37.5V with input rated at 40V and so I cannot use it!? And that is why I have to use this LM5118 that can handle the V and the I as well, it’s just complicated and so I will have tackle some details on this one. Especially what dyarker says that the

A 48 volt transformer puts out 67V peak after rectifier bridge (not counting diode drop.

Also, I can find 48V transformers but I'll see if I can find

A 36 volt transformer would be closer to your need for 40V regulated.

I'll try to post the circuit but that will be tomorrow.
Thanks.