[HELP] Bonding Transformer Center Tap to Earth

Thread Starter

muno

Joined May 17, 2022
38
Hello, Im building a linear power supply using a torroidal transformer which has 230V primary(single phase, 2 wires) and a 28V-0V-28V on secondary. I will be setting up the transformer using full wave rectifier center tapped. Considering that the center tap is a current carrying conductor(based on my design) and earth conductor will only carry current in case there is ground fault, is it safe to bond the transformer center tap to earth? If not then how am i going to create an earthing conductor or path for my power supply project? Thanks.
 

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AlbertHall

Joined Jun 4, 2014
12,072
As long as you have no path to earth connected to the output of the supply then there will be no current in the earth wire. The current in the centre tap will only flow to the output of the supply.
 

LowQCab

Joined Nov 6, 2012
2,171
The Question is ......
Why do You want to connect your Secondary to "Earth" ?
"Earth", and a "Common-Ground", or "Chassis-Ground", are not the same thing.

What does the rest of the Circuit look like ?
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Ian0

Joined Aug 7, 2020
5,527
I think it’s clearer from your original diagram. There is a voltage between your LOAD- terminal and your 0V terminal which will force current to flow between those two points, completing the circuit from your transformer secondary.
There is no voltage between your 0V terminal and your E terminal, so no current will flow.
However, if your transformer fails and there is A short between the primary and the chassis, then there will be a voltage between 0V and E so current will flow to earth, instead of continuing through the transformer and back to the neutral.
The MISSING current from the neutral will be detected by an Earth Leakage Trip/Residual Current Circuit Breaker/Ground Fault Interruptor or whatever you would like to call it, which will disconnect.
 

dl324

Joined Mar 30, 2015
14,700
Considering that the center tap is a current carrying conductor(based on my design) and earth conductor will only carry current in case there is ground fault, is it safe to bond the transformer center tap to earth? If the power supply is in a metal enclosure, you should connect earth ground to the chassis, but not the transformer center tap.
Why would you want to do that? The transformer provides isolation from line voltage and earth ground.
If not then how am i going to create an earthing conductor or path for my power supply project?
For cases where a circuit requires earth ground, you connect circuit ground to earth ground. I can't remember the last time I needed or wanted to do that.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
Generally the only time where you earth ground a transformer secondary is when the system has a transformer that isolates the 120vac for general control system purposes.
In this case, you are allowed to earth GND one of the secondary conductors in order to set up a local earthed Neutral.
 

Thread Starter

muno

Joined May 17, 2022
38
so bonding the secondary center tap to earth gnd is useless? Should i seperate the earth then? The purpose of this power supply project is to charge laptop computers and diagnose laptop motherboards.
 

Ian0

Joined Aug 7, 2020
5,527
For your application, and if your transformer doesn't have an earthed inter-winding screen, then I would recommend a resistor between 0V and Earth, to prevent any capacitive coupling between primary and secondary charging up and making the output float at half mains supply (albeit at high impedance).
If this happens it can make a spark on connection if there is another path to earth, through an oscilloscope, for instance, and it can damage high impedance inputs. A resistor (about 1k) will prevent this from happening.
 

LowQCab

Joined Nov 6, 2012
2,171
Grounding any part of a Transformer derived Power-Supply normally doesn't provide any benefits,
but in the case where You may be working on bare Circuit-Boards with sensitive Chips,
or may be using other Static preventative equipment,
Grounding the Secondary may be a good idea for extra Static protection.

"" ......... I've seen linear bench power supply on the web which has 3 terminals output(+, -, and GND), ..........."

Why do You think this might be an advantage when working on a Computer ?
A Negative-Voltage, ( relative to Ground ), in the wrong place, can cause instant "Blue-Smoke".

All You would have to do is use a Bridge-Rectifier instead of the 2 Diodes You have now,
then just Ground the Secondary-Center-Tap, and provide an extra Terminal to connect to .
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Thread Starter

muno

Joined May 17, 2022
38
Grounding any part of a Transformer derived Power-Supply normally doesn't provide any benefits,
but in the case where You may be working on bare Circuit-Boards with sensitive Chips,
or may be using other Static preventative equipment,
Grounding the Secondary may be a good idea for extra Static protection.

"" ......... I've seen linear bench power supply on the web which has 3 terminals output(+, -, and GND), ..........."

Why do You think this might be an advantage when working on a Computer ?
A Negative-Voltage, ( relative to Ground ), in the wrong place, can cause instant "Blue-Smoke".

All You would have to do is use a Bridge-Rectifier instead of the 2 Diodes You have now,
then just Ground the Secondary-Center-Tap, and provide an extra Terminal to connect to .
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im using only 2 diodes because i want to extract more current from the secondary winding , heres the link about my transformer torroidal transformer secondary current

and yes, i will be using this linear power supply on bare boards, mostly to detect short circuit compoments like smd caps, mosfets... etc.
 

LowQCab

Joined Nov 6, 2012
2,171
"" im using only 2 diodes because i want to extract more current from the secondary winding , ""

The Transformer is limited by the Wattage that it can handle without over-heating.
This is rated in "VA", or Volts times Amps, You can trade one for the other,
as long as You don't exceed the maximum Wattage-Rating of the Core.
You don't get more Amperage for free.
If You get increased Amperage, it must be at the expense of a lower output Voltage.
You can't get around the simple Math problem.

With a Toroidal-Transformer, it's easy to re-Wind the Secondary to suit your needs.
If You are not getting the 120-Watts that You expect,
simply re-Wind the Secondary with ~10-Gauge Stranded-Building-Wire to eliminate
"Copper-Losses" from Secondary Windings that may be wound with marginal Wire-Size.
This will also let You fine tune the Voltage Output.

To reduce losses from the Rectifiers,
use some FERD-Diodes,
they only have as low as a ~0.3-Volt Forward-Voltage-Drop, instead of the usual ~0.7-Volt drop.
DigiKey p/n 497-19457-ND
The also have a FERD-Bridge-Rectifier, but they're rather expensive, and a bit over-kill.

You can also create multiple separate Secondary Windings on the same Core,
they will all work simultaneously, as long as the total Wattage-Load, (VA), is not exceeded.
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Thread Starter

muno

Joined May 17, 2022
38
Thanks for the great explanation. I decided not to bond the center tap to earth. Maybe i can create a seperate earthing for my project. By the way, can i use schottky diodes inplace of FER Diodes?
 

LowQCab

Joined Nov 6, 2012
2,171
You can use any Diode You like,
as long as it can handle the Voltage and Current that You expect.
This is where digging into the various Diode-Spec-Sheets may reveal a lot of unexpected surprises.
They will contain graphs that will tell You pretty-much exactly how
the Diode(s) or Bridge-Rectifier will perform in your Circuit.

The Diodes, in a low-Voltage Power-Supply, are quite often a very significant source of Power-Loss,
since they normally have a ~0.7-Volt "Forward-Voltage"-Drop.

0.7-Volts X expected Current = number of Watts lost to heating-up the Air around an Aluminum-Heat-Sink.

The usual way of reducing this loss is to use way-over-sized, and/or, Low-Forward-Voltage Diodes.

You can virtually eliminate this loss with a couple of MOSFETs and a small supporting Circuit to drive them,
but You are probably not interested in the extra complexity and cost.

Schottkey-Diodes are not "automatically" better, may of them are quite lame.
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Lo_volt

Joined Apr 3, 2014
268
As Ian0 said, and depending on the device that you are powering, you may want to put a high value (>10K ohms) from your power supply reference (what you are calling (-) on the output) to the chassis of the device. The transformer isolates everything downstream of it so that there is the possibility that your (-) can float away from the chassis potential without it.
As MaxHeadRoom said, PC power supplies are connected directly to the chassis. Depending on what other techniques the power supply designers used, that may reduce RF emissions that could interfere with other electronic devices nearby.
 
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