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Forum AC to DC Power Supply Design
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  • energy_power
Related

AC to DC Power Supply Design

Former Member
Former Member over 8 years ago

Afternoon all,

 

Late on a Friday, I am clearly making some sort of simple error but I am blind to it.

 

Please see the circuit below.  I have soldered it up and it isn't working.  I get 36VDC on what should be the regulated 12V rail and a little over 1VDC on the 5VDC rail.

 

Any ideas, please?  I should add that if I remove the transformer and supply 18VDC from my bench PSU then everything works fine, even after the 36V spike which is over the LT3501 max so I *think* the regulator is fine.

 

The number above the transformer is a UK Farnell product code.  It is a 2x 9V transformer with the secondaries wired in series to give 18V.

 

Cheers,

 

Simon

 

PS: Context; it is a power supply for a rPi which is controlling, among other things, some 12V RGB LEDs.

image

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Top Replies

  • pro100alex
    pro100alex over 8 years ago +1 suggested
    Value cap C9 is very low, so you get +18V with big ripple.Try to change it at least 1000uF.
  • mcb1
    mcb1 over 8 years ago +1 suggested
    In the datasheet for the converter, the diodes across SW1 and SW2 are Schottky diodes https://www.vishay.com/docs/89071/b360a.pdf In your circuit you have indicated they are 3A 40v 500mV, so the first…
  • s2000gt
    0 s2000gt over 8 years ago

    You might try limiting your input current on your bench supply to see if you can duplicate the symptom with the transformer.

    250mA rating on the transformer may be too low for this circuit.

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  • pro100alex
    0 pro100alex over 8 years ago

    Value cap C9 is very low, so you get +18V with big ripple.Try to change it at least 1000uF.

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  • jw0752
    0 jw0752 over 8 years ago

    Hi Simon,

    I assume you have check the Data sheet for some guidance but just in case here is a link to it:

     

    http://datasheet.octopart.com/LT3501EFE%23PBF-Linear-Technology-datasheet-17021123.pdf

     

    Some problem in the voltage dividers tied to FB-1 and FB-2 could have this effect but I have never worked with this circuit. Check out the sample circuit in the Data Sheet as it may be clearer than the schematic you have above though the resistor values may not be the same as they are showing a different set of regulated voltages.

     

    John

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  • mcb1
    0 mcb1 over 8 years ago

    In the datasheet for the converter, the diodes across SW1 and SW2 are Schottky diodes

    image

     

    https://www.vishay.com/docs/89071/b360a.pdf

     

    In your circuit you have indicated they are 3A 40v 500mV, so the first question would be are they B360A or an equivalent.

     

    I agree with the previous two respondants that the current is too low to run a RPi, and the filter capacitor value too small.

     

     

    mark

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  • michaelkellett
    0 michaelkellett over 8 years ago in reply to mcb1

    Definitely check those diodes and increase C9.

     

    Do you have a picture  - this part will be very fussy about layout.

     

    MK

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