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12 V power supply.... Is this normal?

adityarainmaker

Sep 21, 2013
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I was trying to build a conventional bridge rectifier and bought a 1.5W, 15 0 15 transformer with a 220V input. When I checked the outputs of the transformer, it measured V rms across the positive and center tap at 17V, across the negative and center tap at 17 V and across the positive and negative at 34V, which makes sense to me since the input is around 252 V AC. When I connect a diode on the positive coil and measure the DC output across the diode and the center tap it shows 8V, which is also intuitive and matches. The problem starts when I put the multimeter on AC mode and see that there is an AC voltage of 17 V across the diode output and the center tap. Seemed most counter-intuitive to me!! It is like with the same connections, it shows 8V DC and 17 V AC! Any suggestions why this is happening? I have got similar outputs 1N 5408, 1N 4148, 1N 4007, 1N 4002 and BA 159! Please help.
 

duke37

Jan 9, 2011
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The transformer outputs AC so there is no such thing as a positive output.

The measurement of a pulsed waveform with a DC and AC component will depend on the way the meter works. An oscilloscope would be a better way of measuring this.
 

adityarainmaker

Sep 21, 2013
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The transformer outputs AC so there is no such thing as a positive output.

The measurement of a pulsed waveform with a DC and AC component will depend on the way the meter works. An oscilloscope would be a better way of measuring this.

By positive, I meant the end that gave a positive DC output with respect to the center tap. Would you expect an AC voltage also on the same multi meter?
 

davenn

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By positive, I meant the end that gave a positive DC output with respect to the center tap. Would you expect an AC voltage also on the same multi meter?

Again... a transformer doesnt have a positive or negative DC output, transformers ONLY have AC outputs and it cycles 50 or 60 (depending on your country) times a second

but back to the voltage readings ...

a 15 - 0 - 15 VAC transformer is quite likely to have at least 17 - 0 - 17 VAC out with no load. It will drop under load

cheers
Dave
 
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Fish4Fun

So long, and Thanks for all the Fish!
Aug 27, 2013
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hehe, and if you measure the DC output voltage following a bridge rectifier with a capacitor across it you will measure 17 * 2^1/2 = ~24V (depending on the rectifier voltage drop and capacitor leakage current) across the center tap and either leg, or 34V * 2^1/2 = 48V (same note). The reason for this is AC is typically measured in RMS (root-mean-squared). With no load your DC voltage will be the peak AC voltage - diode drop and leakage across the capacitor.

Fish
 

adityarainmaker

Sep 21, 2013
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I drew a schematic.... Can you help me figure out why there is a AC output in one configuration and not in the other. Thanks!! Really appreciate your guidance!!
 

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adityarainmaker

Sep 21, 2013
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Can you help me with this one.... ?? I guess I am a novice running into troubles with every step I take! I have ignored the AC readings hoping that it must be something with the way the multimeter works. Can you help me figure out the fault with this one that I have created using the same transformer.... Need a 5V output... with L7812 I am getting a voltage of 11.5 V...
 

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davenn

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Can you help me with this one.... ?? I guess I am a novice running into troubles with every step I take! I have ignored the AC readings hoping that it must be something with the way the multimeter works. Can you help me figure out the fault with this one that I have created using the same transformer.... Need a 5V output... with L7812 I am getting a voltage of 11.5 V...

so why is your circuit showing a 5V regulator ... confirm what you really want please

apart from that this needs to be sorted out.....
OK the 1000uF capacitor is connected incorrectly ... it should be between the input to the regulator chip and the 0V rail
there should also be a 0.1uF in parallel with it.
the 0.1uF on the output of the chip should be between the output and 0V rail

there's going to be ~ 40V DC into the 7805, this is in excess of its ratings
if you only need 5V out ( using the 5V regulator) then use only 1/2 the transformer .... the centre tap and only one of the windings. its going to be up ~ 22VDC out after the rectifier and capacitor .... its going to be a bit high for a 5V reg and you are going to be dropping some ~ 18V through the reg ... its going to get VERY HOT

again, before we go further...

1) what regulator are you really using ??
2) what voltage do your really want out ??

Dave
 
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Fish4Fun

So long, and Thanks for all the Fish!
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adityarainmaker,

Honestly, why not just order a 5V power supply? I don't mean any disrespect, but you are obviously in over your head and intent on needing a 5V power supply, you should be able to purchase one ready to go for < $5. You almost certainly have one laying around your house in the form of an old cell-phone charger....I just don't see where this is going. If you are attempting to learn about electronics then you need to do a lot more reading, if you just need a 5V power supply then the cheapest/easiest path is to buy one or re-purpose one. Starting with a 30V 1.5VA (specified 1.5W) transformer to build a 5V power supply is silly. The rating specified implies a maximum current of only 50mA; I am sorry, I just can't see this build achieving anything. Scrap what you got, chalk this up as a loss and buy/salvage what you need and go forward.

Fish
 

adityarainmaker

Sep 21, 2013
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so why is your circuit showing a 5V regulator ... confirm what you really want please

apart from that this needs to be sorted out.....
OK the 1000uF capacitor is connected incorrectly ... it should be between the inpuut to the regulator chip and the 0V rail
there should also be a 0.1uF in parallel with it.
the 0.1uF on the output of the chip should be between the output and 0V rail

there's going to be ~ 40V DC into the 7805, this is in excess of its ratings
if you only need 5V out ( using the 5V regulator) then use only 1/2 the transformer .... the centre tap and only one of the windings. its going to be up ~ 22VDC out after the rectifier and capacitor .... its going to be a bit high for a 5V reg and you are going to be dropping some ~ 18V through the reg ... its going to get VERY HOT

again, before we go further...

1) what regulator are you really using ??
2) what voltage do your really want out ??

Dave

I want to power a small Laser Diode (only one) rated at 5V, ~100 mW. As you can see, I have used the center tap and only one side of the winding. Incidentally, my bridge rectifier output reads 16V wrt to the the center tap. Would you suggest, I go in for a 8 0 8 or a 10 0 10 transformer. They cost about a dollar so that should not be a problem. I already have a 12 0 12 transformer rated at 1W. Thanks a lot for your help!!
 

davenn

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I have used the center tap and only one side of the winding.

no, your post #8 showed you using the full transformer

OK now that you have told us what you want to power, you should be aware that a laser diode requires a very specific driver circuit.

do some googling on laser diode drivers there's lots of info available

Dave
 
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