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Please help with power supply
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WattsThat:
This is looking like a classic x-y problem https://xyproblem.info/

Yes you need an ac voltage for a buck converter. Got it. But what is buck convertor going to be used for inside of an existing instrument? You’re telling us how the story ends and then asking us for the plot. Then we have to ask you dozens of questions to understand what you’re attempting to accomplish.

This process (getting technical help on the internet) would work much better by you starting at the beginning with explaining what you what to do and why you want to do it rather than asking a question for which there is no simple, obvious answer.
samson:
The project is I have a frequency counter with a 10mhz crystal, I have put in a heater in for the crystal which needs 9volts dc. I use the buck converter to tap into the transformer rather than taking it from the main board. Now the frequency counter is stable.
coromonadalix:
the best thing would have been to add an small other psu in the unit,  not taking power on any section of your main instrument


you may over time consume more current then normally provided and you may have problems in the future in your main unit


it was not a good job ...  even if you say it work
samson:
Not possible to put a psu in as there is no room. Thought of this first.
Kean:
A buck converter to power an OCXO?  I would have used an LM7809 powered off the 11V AC tap (rectified and filtered of course).  Avoids additional switching noise, and likely there is little lost in a linear regulator as the heater should reach equilibrium after a short time and not draw too much power.  Assuming it is properly temperature controlled...

A separate transformer would have been best, but I understand about the lack of space.  As we have no idea of the design specs for that transformer or the rest of the circuit, I suggest you keep an eye on the transformer temperature to make sure it isn't overloaded under worst case conditions (cold environment).

If you have an AC power meter, you could measure input power with and without the additional circuitry connected, again try to simulate worst conditions (A/C on full blast?).  I would think an extra 10-20% is probably fine as that would be likely covered in the design margin, but much more could be a problem.
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