help me to find transformer secondary mA.

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Briez

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I am using single phase step down transformer of voltage in primary 230ac to voltage on secondary 18v ac. But I don't know what is its mA rating on secondary side. Also help me to calculate mA rating for digital circuit which contains 3 led, 3 led seven segments, 1 microcontroller, 1 30A 12v relay. I want to drive this circuit using step down transformer.

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Transformer is rated for VA and bridge DC out will be RMS to Peak voltage converted or 1.4x higher with no load and a 18V transformer with no load due to transfer impedance will also supply a higher AC voltage with no load. You ought to be using a 12Vac XFMR not 18V, or possibly even less for better efficiency driving LEDs design at 5V.
 

Briez said:
But I don't know what is its mA rating on secondary side.
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Compare its dimensions with those in a catalog. This will give you an idea.

Heat is the prime indicator. Draw increasing amounts of current from it. Wait several minutes after each increase. When it becomes too hot for you to leave your fingers touching it, that is the max mA.

For a quick test, read its voltage at no load, then draw as much current as it takes for its voltage to drop 5 percent. I can't say that will tell you its max safe current, but you can compare its reading with a transformer whose max is known.

Also help me to calculate mA rating for digital circuit which contains 3 led, 3 led seven segments, 1 microcontroller, 1 30A 12v relay. I want to drive this circuit using step down transformer.
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This sounds as though it will take a few hundred mA. Maybe 1/2 A, depending on how much current your relay coil needs to operate. Also on whether you multiplex the digit displays.
 

I presume you are using a small µC (e.g. PIC) with not more than 20 or 30 mA supply current and a linear voltage regulator for the 5V supply. Relays and suggested 7-Segment display current can be read from the datasheets. I guess you end up with 100 to 200 mA DC at 12 V. Putting in bridge rectifier current form factor of typical 1.6, you need 160 to 320 secondary current, 2 to 4 VA transformer rating for 12V secondary voltage.
 

Laminated Steel-core transformers come in various ratings of Watt/Kg depending on how thin and quality of cold-rolled grain-oriented steel.

But I suspect you want to measure the rating.

If you have a voltage and current meter in your DMM and a few resistors like a small 12V halogen bulb you can record the V vs I on a graph or table and see interpolate where it drops to 16.0Vac THis is where the designed VA rating voltage is given. So if 0mA is 18V, 500mA@17V then expect 1000mA at 16V or 16VA using a resistive load.

Unfortunately LED's are not like resistive loads unless you only consider the ESR and added series Rs to limit current after a full wave bridge. But close enough for government work.

Ohm's law prevails.
 

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