Hello, this might be answered several times already, but I can seem to pick correct keywords to find an answer :]
1) I need a linearly-regulated PSU of 9V@2A. Using three 7809 in parallel with load resistors to regulate.
2) Picked a transformer with VA of 40 for the job (2x12V@1.67A, in parallel = 12V@3.34A), expected to be more than OK.
3) Able to to get only ~1A of clean supply, at 1.5A it already is way overloaded (regulation of, traffo gets really warm, voltage before regulator drops to 9.5V).
Seems the transformer, although seems more than adequate for the job, fails to deliver required current (9Vx2A = 18W required, 40W rated. Fails even at 13W). My question is - did I missed something in selecting the required transformer or should I suspect it's quality and stated ratings?
My calculations:
- I need 9V@2A and will use 2x4700=9400 uF smoothing caps.
- This will give me 2.13V ripple voltage (2A / (2*50Hz*0.0094F) )
- And all-time required peak voltage of 15.63V ( 9V (output) + 2.5V(regulator dropout) + 2.13V (ripple) + 2V (diode drop) )
- This translates into 11.08V@2A requirements for transformer and I stepped it up to 12V@3.34A
Just to check it with the other calculation method:
- At 9V@2A draw, voltage before regulator should be 13.75V, more than enough for 7809 to work correctly ( 12*1.41V (secondary rms) - 2V (diode drop) - 1.05V (Vripple/2) ).
1) I need a linearly-regulated PSU of 9V@2A. Using three 7809 in parallel with load resistors to regulate.
2) Picked a transformer with VA of 40 for the job (2x12V@1.67A, in parallel = 12V@3.34A), expected to be more than OK.
3) Able to to get only ~1A of clean supply, at 1.5A it already is way overloaded (regulation of, traffo gets really warm, voltage before regulator drops to 9.5V).
Seems the transformer, although seems more than adequate for the job, fails to deliver required current (9Vx2A = 18W required, 40W rated. Fails even at 13W). My question is - did I missed something in selecting the required transformer or should I suspect it's quality and stated ratings?
My calculations:
- I need 9V@2A and will use 2x4700=9400 uF smoothing caps.
- This will give me 2.13V ripple voltage (2A / (2*50Hz*0.0094F) )
- And all-time required peak voltage of 15.63V ( 9V (output) + 2.5V(regulator dropout) + 2.13V (ripple) + 2V (diode drop) )
- This translates into 11.08V@2A requirements for transformer and I stepped it up to 12V@3.34A
Just to check it with the other calculation method:
- At 9V@2A draw, voltage before regulator should be 13.75V, more than enough for 7809 to work correctly ( 12*1.41V (secondary rms) - 2V (diode drop) - 1.05V (Vripple/2) ).
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