How to isolate ground

Thread Starter

mayurrane180

Joined Jan 22, 2021
2
I am connecting 2 different circuits together. both circuits need 12VDC supply. So I am using a bridge rectifier to power them up. As they have common ground, they are not working as planned. I want to separate ground for both circuits. but at a same time I need only one AC power plug in my project.
Can anyone suggest me what to do?
 

crutschow

Joined Mar 14, 2008
27,171
What is the input to the bridge rectifier and what is the output filter capacitor?
Post a schematic.

You could use a transformer with two isolated output windings and two bridge rectifiers.

Can the two circuits work without a common ground?
 

djsfantasi

Joined Apr 11, 2010
7,689
Can you draw a picture of how they are hooked up and take a picture of it? Then use “attach files” and share that picture with us.

What do you mean, they aren’t working? It’s hard to solve a problem without knowing what it is. I’d provide details of what the two circuits do.

On another note, since they both require 12V, I would think you could use just one power supply. Maybe.
 

Thread Starter

mayurrane180

Joined Jan 22, 2021
2
What is the input to the bridge rectifier and what is the output filter capacitor?
Post a schematic.

You could use a transformer with two isolated output windings and two bridge rectifiers.

Can the two circuits work without a common ground?
Input of the bridge rectifier is 110V AC. O/P of filter capacitor is 12V DC.

Yes both circuits work without a common ground.
 

andrewmm

Joined Feb 25, 2011
1,461
you state at the beginning you want to split the two circuits grounds because its not working

Can you elaborate on why thy are not working as planed ?

I have feeling that you are trying to fix a problem that might not be the one you think you have and want to clarify this first ,
 

BobaMosfet

Joined Jul 1, 2009
1,767
I am connecting 2 different circuits together. both circuits need 12VDC supply. So I am using a bridge rectifier to power them up. As they have common ground, they are not working as planned. I want to separate ground for both circuits. but at a same time I need only one AC power plug in my project.
Can anyone suggest me what to do?
While your description is pretty good, we need a schematic- doesn't have to be professional or anything, just even a hand-drawn schematic would be great.

You should not isolate ground between 2 circuits without good reason. Normally, ground works as a reference for both, so the voltages relative to each other are on the same scale, from the same reference point. If you isolate ground, voltages between could be any amount of volts in difference from one another.

You do not connect AC ground with DC ground. If this is what you've done, that would explain the weirdness. AC ground will couple an AC signal onto your DC and it won't like it. You do need to isolate grounds between AC and DC.
 

MaxHeadRoom

Joined Jul 18, 2013
23,084
Generally if two separate circuits do not have a Power common, the some means of coupling the two circuits are required, often done with a Opto isolator etc.
Max.
 

MisterBill2

Joined Jan 23, 2018
8,723
If two circuits,each using 12 volts DC power, can not function correctly, then yo need to provide two separate 12 volt power supplies.
But Now I am asking a question: What are the two devices that do not work correctly when both connected to the same 12 volts common?

Without more information all of us are just wasting time guessing.
So more information is needed so that a useful answer can be provided.
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
5,966
Just to follow up on my posting of the full wave bridge rectifier (BR) there's forward voltage (Vf) drops across each diode. With a positive going sine wave two diodes are active. Each having a typical Vf of 0.6V; so 12V - 1.2V = 10.8V. Just saying that in case someone wants to argue the output DC voltage.

If you filter that DC voltage you will get 1.414 times the voltage. 10.8 x 1.414 = 15.3V. To further argue the output, the input of 110VAC can vary as well. The transformer exact output can vary. So all voltages are referenced in relative terms. Actual voltage will always vary.
 

MisterBill2

Joined Jan 23, 2018
8,723
And we get no response yet from the TS. I wonder if it is a positive ground car radio and a negative ground booster amplifier. THAT would be funny.
 

MisterBill2

Joined Jan 23, 2018
8,723
The statement about only wanting one AC power plug really makes me wonder. A couple years back I got a great deal on a nice car stereo receiver/deck all built into a nice box with good speakers, because it did not work. The problem was that a single 12 volt 200ma wall wart was not providing enough power.
We may have a similar situation here, but all of the details were not mentioned. In fact, almost NONE of the details are given.
 
The link in post #16 is interesting and useful, but does not seem to relate to the TS complaint.
Well, it's education. A few new people think you get 12 VDC from 12 AC. bridge rectifers generally give you a positive and negative voltage, but they don't have to.

Even got duped with the fact that a 3A AC secondary means you get much less than 3A when rectified. I built a stere amlifier where the power supply was specified 70 Vac ct at 3A. I doubled it, and it was still wrong. The "fine print" said electrostatic speakers which doesn't need the current. It had 30A transistors in the output. Even if the current was 3A, that is 3^2*9 or 72 Watts. It's like 0.62*3 for the current. I had the transformer custom made. 4x35V at 3A ea.

You have a current limitation and a voltage limitation, say 90V p-p. You should probably design the power supply for a 6 ohm load.

in the design of an AC-DC linear power supply, the peak AC voltage gets captured. The capacitor in the filter, "fills" in the voltage and it averages the current, so the output current is considerably less.
 

MisterBill2

Joined Jan 23, 2018
8,723
One more thing, is that since the TS mentioned not wanting more than one plug, I am guessing that the power supply in use is a wall wart. Or a switcher supply , even more likely, as a switcher would have a bridge rectifier getting 120 volts.
So now what we need to know is the current rating of that power supply and the current draw of each of the things. Quite possibly unknown values, or not understood values.
 
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