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Cheap way of getting 13V5 1A5 from mains

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Advanced Member level 5
Jun 13, 2021
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We want 13V5, 1A5 from the mains (100-265vac)

We dont want to bother with fans, so what say we put a 65w one of these..


..into a metal vented enlcosure with no fans, and just change the upper divider resistor so that we get 13V5 instead of 19V?
the feedback loop is likely to be so high in phase margin etc that we can get away with this?

We'd change it so the opto diode etc still got enough current etc.
(I guess as a flyback it may go into CCM at low mains, but we only need it for 200-265vac mainly)

We coudl even adjust the sense resistor so it didnt go into CCM. Since we only need 21W out of it.
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I don´t understand.
You say cheap. And you say 13.5V and 1.5W, this gives about 20W.
But then you choose a 65W supply.

I guess a 20W one (or 25w) should be cheaper.

And a 20W SMPS should have less than 3W power dissipation. So no fans needed.
(A fan is extra cost, extra power, an maybe is the first thing to go fail)

We could use a 30W unit, which is 85% efficient at 20W...this dissipates some 3W as you say......but you still need to get that heat out, and i'd rather use a 60W unit and no fan, to be sure of not overheating (because we dont want fans).
We cant find a 20W one off the shelf, and in any case, the power is 21W.

3W in a 30W supply with no fan coudl overheat something at high ambient....say 40 degc, we dont really know where the custoemr will put it, likely they dont least they dont want to have to specify an exact "max ambient temp".

So 60W and no fan sounds like it for us.


I thought "cheap" is the most important, because it is your first word in your post.

In a closed case, it does not matter whether there are
* 3W from a 20W supply or
* 3W from a 60W supply
of power dissipation. Both will cause identical temperature rise.

3W for a metal case should not be no problem. Vented or not.

Thanks, yes i would like to go cheap, but also the convenience of no fans is good, i thought because heatsinking is likely to be bigger in a 60w supply than a 20w supply, it would be more conducive to "no-fan".

There will be vents in the enclosire. (the total enclosure).

I was thinking of using one of these

...but its datasheet doesnt say what is its Abs Max input you know it?


* before it was 21W , then 65W, now 250W
* before it was 100-265V AC, now it´s 9...35V DC
* before it was isolated, now it´s non isolated.
* before it was cheap, now it´s maybe "4 x the cheap price"

Please give clear requirements.


Thanks, the requirement is still as in the top post, but that post was just wondering about changing the PSU compoments to get a different Vout, and i'm now thinking its not actually a good idea.

BTW, the overall requirement is for +13V5, 6A and -13V5, 1A5 from the mains. (ie +13V5(6A), 0V, -13V5(1A5) No fans because its dusty....we are allowed a case with vent holes in it though.

We are thinking of using 2 offline PSUs as follows

LM25-23B15: (for the -13V5, 1A5)

LCS100US15: (for the +13V5, 1A5)

...So the above two placed in a metal enclosure with vent holes, but no fans. Can you see any thermal issues with this? Mains input is 180-265VAC

An alternative way would be to use one of these to give a +13V5 rail...
LCS150US15: discussed, to give a 13V5, 150W rail.

..and then to add one of these to that rail to get -13V5 at 1A5
Which method do you think is best?

Then again, there's always the following alternate method....
Use one of these to give a 24V rail...

Then to get the +13V5, 6A, use one of these off the above 24V rail....

And to then get the -13V5 at 1A5, use one of these off the above 24V rail...
Which method overall do you think is best?....

Its a situation for customers who dont know much about power to fulfill their requirement when they dont want standard voltages (eg 5V, 12V, 15V, 24V...). Im actually amazed at the lack of adjustable offtheshelf PSU's......which is what prompted me to consider the desparate activity of changing components on standard offtheshelf PSU PCBs, as mentioned in the top post

Oh and BTW, what do you make of the above PSU.?..

...its 33mm by 23mm by 16mm and claims to be rated to 250W ?
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