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Topic: Voltage regulators (Read 2 times) previous topic - next topic

Gadget999

i have a 7805



i have connected

20v to the input
gnd to gnd
I expect to see 5v on the output

except i see 20v (near enough)

do I need to put capacitors around the regulator input and output ?

like this





CrossRoads

Yes. Your drawing says Transformer - is that correct?
If so you need rectifier also to convert the AC into rough DC, the caps and regulator then make smooth DC for the circuit.
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Jack Christensen

Yes, capacitors are required, but their absence doesn't explain to me why the output isn't pretty close to 5V. Technically speaking, a transformer doesn't supply positive and negative, but AC. What exactly is the "transformer" being used here? DC needs to be supplied to the circuit as illustrated.
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Gadget999

#3
Oct 27, 2011, 10:25 pm Last Edit: Oct 27, 2011, 10:27 pm by Gadget999 Reason: 1
i lifted the drawing from another web page

the supply to the regulator is 20v DC (not ac)

i am very confused :-S

CrossRoads

Add the caps and see if that helps.
Polarized electrolytic 3.3uF, 4.7uF, 10uF along those lines on the input.
1uF on the output.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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