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Common ground

Karthik rajagopal

May 9, 2016
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Hi all,
I started working on the circuit for my lab bench power supply . I am using a 12-0-12 v center tap transformer for this. I took 24v from the transformer ,rectified it and used it to power external loads . Then I used the 12v part to power the cooling fan.I thought of including my arduino (to add some current sensing features ) on the 12v side and so gave a common ground between the upper and lower bridge rectifiers. On powering it the capacitor on the 12v side blew up as the voltage across it was 24v. How did that common ground increase the voltage on the 12v side to 24v? Should I in the first place consider them as two individually powered circuits for me to provide them a reference point ?
Plz explain.
I have also attached the circuit diagram.

Thanks in advance .
 

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Bluejets

Oct 5, 2014
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Show how you did your common ground and other items.
i.e. show complete circuit.
 

duke37

Jan 9, 2011
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You have created a short due to the centre tap connection.
You can get what you want (I think) by using a bridge rectifier on the transformer outer connections. This will give 24V rectified. The transformer centre tap will give a mid point voltage.
 

Karthik rajagopal

May 9, 2016
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Show how you did your common ground and other items.
i.e. show complete circuit.
Thank you for the reply. I didn't even proceed further with the circuit, I faced problem with this initial step itself and hence didn't want to proceed till I get a proper idea on what I was doing .
 

Karthik rajagopal

May 9, 2016
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yeah

that link between the negative rails on the output shouldn't be there
You have created a short due to the centre tap connection.
You can get what you want (I think) by using a bridge rectifier on the transformer outer connections. This will give 24V rectified. The transformer centre tap will give a mid point voltage.
Thank you for the reply.
How does the center tap connection cause a short circuit? I removed the common ground and now it works without a problem .But I am still not clear on the flow of current in my initial common grounded setup .
 

Karthik rajagopal

May 9, 2016
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replace the blocks labeled "bridge rectifier" by the equivalent diode circuit and trace the connections.
Tried tracing them....Actually did that method first . It confused me even more than the 2nd one. Plz help me figure out what's actually happening in the circuit.
 

Harald Kapp

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Nov 17, 2011
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See how you short circuit e.g.L2 for the polarity in this example:
upload_2020-3-24_8-7-2.png

You can use your circuit if the transformer does not have a center tap (as shown in your circuit) but two separate output windings. Then connect each winding to one bridge rectifier and make the common connection on the DC side only.
 

Karthik rajagopal

May 9, 2016
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See how you short circuit e.g.L2 for the polarity in this example:
View attachment 47473

You can use your circuit if the transformer does not have a center tap (as shown in your circuit) but two separate output windings. Then connect each winding to one bridge rectifier and make the common connection on the DC side only.
Now I am able to get it. Thank you for the clear explanation.
 

Karthik rajagopal

May 9, 2016
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Is there any way to isolate both the ground terminals?
I am not able to get any readings in my arduino without giving a common reference point .
How to proceed with this?
 

Harald Kapp

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This is one way, as I stated, if the center tap is not connected at the transformer:
upload_2020-3-24_18-3-56.png

Or this way if the center tap is connected at the transformer:
upload_2020-3-24_18-5-8.png
 

Frankchie

Nov 14, 2017
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See how you short circuit e.g.L2 for the polarity in this example:
View attachment 47473

You can use your circuit if the transformer does not have a center tap (as shown in your circuit) but two separate output windings. Then connect each winding to one bridge rectifier and make the common connection on the DC side only.

Harold,
I don't see a short circuit relative to L2 or anywhere else for that matter. I think D4 is reverse biased relative to L2.

Further, I don't see how that short circuit would blow up the cap that the op stated.

I'm wondering if the op has inadvertently created a voltage doubling circuit that blew up the cap.

I'm a little rusty on this stuff, so hopefully this is not a stupid question.
Thanks,
Frank
 

Karthik rajagopal

May 9, 2016
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Harold,
I'm wondering if the op has inadvertently created a voltage doubling circuit that blew up the cap.

Initially my 16 v cap blew up which I subsequently replaced with a 50 v cap. After measuring the voltage across the cap(after replacement) instead of the 12 v, I read 24 v which was the same voltage across the capacitor on the 24 v side. As far as my understanding , voltage doubling can't occur as both caps get charged simultaneously and no summing of voltages occur.

I don't see a short circuit relative to L2 or anywhere else for that matter. I think D4 is reverse biased relative to L2.
Yes, you are right. I missed to notice this.
Now what could be the possible cause for this raise in voltage? Is there a way we can simulate these using software?
 

Frankchie

Nov 14, 2017
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As far as my understanding , voltage doubling can't occur as both caps get charged simultaneously and no summing of voltages occur.

Now, upon closer examination, I think the instantaneous voltage across the 12v cap is 24 volts. That is, in Harald's diagram D1 and D7 conduct on the same 1/2 cycle effectively putting 24v across your 12v cap.

Edit: Actually it's more than 24v If you consider peak voltages.

Frank
 
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