Converting 12v smps in to 15v

[sunil21]

For my new application I need a 15v/1amp supply. The place where I live I cant find one. What I found was a 12v/2amp supply.

This 12v one is with a TL431 on secondary. What I am trying to do is to convert this in to 15v(1amp). No intention to use more than 1amp but will be happy if I can get up to 1.5amp. Capacitors on the output side are rated 16v , I may have to change it

I have a few questions.

suppose normally if without TL431, if I change the secondary turns ratio to increase (or decrease) output to 15v(1amp). 12v/2amp is 24wats against 15v/1amp is 15watts.
will it work fine?

Is it okay to change the resistors around TL431 to change the output voltage to 15v (transformer turns not changed) and then can my application consume 2 amps ?


With this same thing, ignoring the components on the output side, I replace the transformer with another one to change output to, say 12v/3amp or 15v/2amp (suppose) do I need to change anything on the component side from ac 230v input to the primary winding of the transformer. what I am asking is that the circuit up to primary winding is good and have good rating and no need to alter?

one more question - Transformer calculations may be complicated - simply asking - Transformer (Ferrite smps ones) are usually rated in watts ?
or (the amps etc) will it go up/down according to the switching frequency ? duty cycle ?
just asking

[stj]

Re: Converting 12v smps in to 15v

i did his once, i changed a resistor related to the 431 and the optoisolator led.
i cant remember the details, but i was trying to use a tv supply as a battery charger

just make sure the output caps are rated higher than 16v - unless you want them to blow.

btw, the tl431 is the reference for the optoisolator feedback.
that sets your output voltage.
if you change the transformer it wont change the output because the 431/opto will just ajust the drive to compensate.

draw the circuit on the led side of the opto including the 431 and the related resistors.
maybe a good time to learn the magic of eaglecad
http://www.cadsoftusa.com/eagle-pcb-...e/?language=en

[selldoor]

Re: Converting 12v smps in to 15v

Too complicated for me can you get the 12v one for free? as if you buy it and
'have to buy parts to modify it, is one of these too expensive?
http://www.ebay.in/itm/Laptop-Adapto...item25951dcf7b

[stj]

Re: Converting 12v smps in to 15v

that's another way.
what is 550Rs (Rupies?)

[budm]

Re: Converting 12v smps in to 15v

http://danyk.cz/at_atx_en.html
http://polonai.se/smps/tosh/

[ammarben]

Re: Converting 12v smps in to 15v

change the resistors around TL431 to change the output voltage to 15v
you have 2 resistors change the one connected to the output
not the one connected to the ground
i have done this many time

[stj]

Re: Converting 12v smps in to 15v

great first post!

[selldoor]

Re: Converting 12v smps in to 15v

that's another way.
what is 550Rs (Rupies?)

Its around £5-£6 so quite cheap for us but I dont know how it relates to what is affordable in Kerala .Anyway the op may like the challenge but just put it in
as an easy alternative depending on what the conversion would cost.

[sunil21]

Re: Converting 12v smps in to 15v

Thank you guys for you suggestions.
As mr. Selldoor suggested there are adapters available on ebay worth buying. I already have an adjustable laptop adapter and a spare 19v adapter. One primary concern with this adapters are that they are mostly size - 7in x 3in or so.. and the unit is almost 1mtr away from the plug point and I may have to drag the unit sometimes. But with 12v/2amp etc it is like traditional adapters or like mobile chargers with everything in a box right behind the plug point, no concerns and only 1 or 2 meters wire to connect.

Also if we could alter things with changing 2 resistors or something and things work as we want; well - it is worth doing.

Okay I am asking one more question (so I don't have to start a new thread) - if I change the output to 15v by changing things around this TL431, can I draw 2amps of current?

[Agent24]

Re: Converting 12v smps in to 15v

You probably could draw 2 Amps but it wouldn't be wise to do it for too long. If you increase the voltage you will decrease the current available, if you want to keep the components operating at what they were specified for and the operating temperature expected etc.

I don't know how you would calculate exactly how much current you could get away with at 15v though.

[stj]

Re: Converting 12v smps in to 15v

it's probable that you couldnt do 2a at 12v - most ratings on psu's are lies.
DONT FORGET TO FIT 25V CAPS.