EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Abayomi on August 12, 2016, 11:20:54 pm
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Any help please. I was building a power supply circuit that required 30v 1A transformer but i did not stumbled on the transformer so i bought 2A 15-0-15 transformer ,strangely at the rectifier stage I measured the output at the smoothing capacitor it is 52.1v. Funny enough I connected a resistor in series with a zener diode of 30v across the 1000uF still got the same result
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This all seems correct- the capacitor will charge to the peak voltage from bridge (~ 1.414 x 30V)
You added the zener and resistor and measured the voltage across the capacitor as being the same as before.
The 30 volts can be measured across the zener. The current available from the voltage regulated circuit will be limited by the resistor value and the voltage difference- as the drawn current increases, the current through the zener will decrease until no current flows through the zener and the voltage will no longer be regulated and will start to fall.
You have two output voltages to use- one of 50V DC high current and the other of 30v DC low current.
Regards,
BT
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The "30V" transformer must produce 38V RMS for the rectified and filtered DC to be 52V.
If the transformer is a cheap tiny Chinese one rated for only 100mA then it will probably produce 38VAC or more with no load. The 30V transformer rated at 2A will probably produce 31.5V with no load.
Maybe your mains voltage is much too high?
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Thanks Audioguru- I forgot to point that out to Abayomi.
The transformer rated voltage will be when the transformer is supplying the rated current. If no current is being delivered then the voltage will be higher- how much higher depends on the transformer physical construction which is beyond the scope of this thread (text book required !)
BT
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In the old days we used to get the output down to near the RMS voltage (but still highly load dependent) using inductors and capacitors to filter the 100/120Hz. Nowadays I suppose you use an active device as a filter/regulator.
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Thanks Audioguru- I forgot to point that out to Abayomi.
The transformer rated voltage will be when the transformer is supplying the rated current. If no current is being delivered then the voltage will be higher- how much higher depends on the transformer physical construction which is beyond the scope of this thread (text book required !)
BT
And the lower is transformer power, the higher will be the difference between voltage rating under load and not loaded voltage.