transformers, current, voltage?

[Bigglez]

What is the required DC output voltage and load current?

Well, the load requires 39.6V and 700mA!

Good, we are making progress!

Is the output voltage fixed or variable?

Unless you can articulate (in words or numbers) exactly what
you want there's no point in posting here for help.

[Volter]

It is fixed! You might read that through out the post. No changes in the voltage. It would be called a leanear or steady load, correct?
Thanks.

[Volter]

How do I insert an image if I cannot attach anything?

[Bigglez]

It is fixed! You might read that through out the post. No changes in the voltage. It would be called a leanear(sic) or steady load, correct?

So you are saying that the load requires a specific voltage,
and always draws the same current?

What if the voltage changed by one percent up or down?
Does the current also change? By what amount?

It's not a linear load. A linear regulator is a circuit technique
that is quite a bit different from switch-mode power supplies (SMPS).

[Volter]

That is the role of the NCP3065 to provide the required current at that volage. The voltage does not change since the load is driven by the current. The voltage is just to turn it on to deliver the I.

[Bigglez]

That is the role of the NCP3065 to provide the required current at that volage. The voltage does not change since the load is driven by the current. The voltage is just to turn it on to deliver the I.

I have no idea what you just wrote. Anyone?

[Bob Scott]

I have no idea what you just wrote. Anyone?

Yes Bigglez. Volter seems to have a bit of trouble explaining and he omits vital stuff like whether the transformer output is AC or if it's a wall-wart with DC output.

It helps if you read the data sheet. It's a LED driver IC. You don't need anywhere near 40VDC.

http://www.onsemi.com/pub_link/Collateral/NCP3065.PDF

[Volter]

You guys! If you read my post you would find that I need 40Vdc! That is the requirement that I need to follow!
Hopefully that helps.
A 48V secondary, is that RMS value, and so the peak is then 67V, and so should the filter cap based on the peak value? That is after the D bridge?
Thanks.

[Bigglez]

If you read my post you would find that I need 40Vdc!

I have, and here's what I have found:

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?

It's the output of the transformer that has to be 40V DC, 700mA, and 60Hz.

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!

Well, the load requires 39.6V and 700mA!

It is fixed! You might read that through out the post. No changes in the voltage.

That is the role of the NCP3065 to provide the required current at that volage. The voltage does not change since the load is driven by the current. The voltage is just to turn it on to deliver the I.

Sorry to say I'm totally confused by your posts...

[dyarker]

From spec sheet for LM317:
Maximum Input-Output Voltage Differential - 60V

Your output will be 40V, therefore your input can be 100V. Go ahead and use the LM317. However I would still recommend a 36V transformer. The peak voltage will be right around 50V loaded, after the diode drop that leaves about 5V for the LM317's overhead. The LM317 will run cooler, and you'll waste a lot less energy.

-OR-

If the object of the exercise is to light as series string of LEDs, just use the LM 317. In fact the light I'm typing by is 12 white LEDs in series. Power is from a 48VDC regulated switch mode wall wart. Positive goes to anode of LED string. LED string cathode goes to regulator INPUT. Regulator OUTPUT goes to fixed resistor, to common (negative side of 48V supply). The value of the resistor sets the current. Set LM317 for 1.2V out. You wanted 0.7A (700mA) so resistor should be:
1.2V / 0.7A = 1.7 Ohms, odd value, parallel a couple standard values to get
or
use a standard 1.8 Ohm resistor and set LM317 to 1.26V out.

Cheers,

[Bigglez]

From spec sheet for LM317:
Maximum Input-Output Voltage Differential - 60V

Really?

[Bob Scott]

You guys! If you read my post you would find that I need 40Vdc! That is the requirement that I need to follow!

OK Volter. I do try to help. Are you actually building a circuit or just doing a design?
Is this homework?
Who gave you the requirements?
Please explain in detail what you are trying to accomplish. How many LEDs? CAn you post a general ciruit idea so that we know what you are trying to do?

[dyarker]

That's what it said. When I went looking I was expecting to see a maximum input value, but there wasn't one, just max in out differential and it said 60V. See:
http://www.fairchildsemi.com/ds/LM/LM317AHV.pdf

Volter, check data sheet for manufacturer of LM317 you actually have. Anyway 40V out plus 40V differential is 80V, still enough.

PS Biglez the max dif is the same but you circled 117, 317 is further down the page

[Bigglez]

Biglez(sic) the max dif is the same but you circled 117, 317 is further down the page

Really? Are you actually looking at the LM317 datasheet or the LM317AHV

[Bigglez]

Anyway 40V out plus 40V differential is 80V, still enough.

The abs max from input to output, or input to adjust is 40V.
This is a process limit and can't be exceeded.

If the LM317 is used as a current source, with a load side
sense resistor feeding back a voltage proportional to current,
the output to adjust voltage will be Vref (1.2V typ).



In that special case the input can be 40V higher than the
output (of the LM317).

If we only knew what the OP is trying to do with this
project, it would have been completed by now...