# Question about Uninterruptible Power Supply Toroid?

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#### samy555

##### Member
Hi
The following image shows a part of a PC power supply, Where clearly shows a torroid.

It is yellow core

My question is how can I use that torroid to make a 1.2 mH coil?
please give me the formula of calculation and in what frequecy band it can be used?
thanks

#### MikeMl

##### Well-Known Member
That is a bifilar common-mode choke in the mains supply to prevent switching transients from the power supply back-feeding onto the ac power lines. As such, it is supposed to block stuff in tens of kHz to tens of MHz range.

#### samy555

##### Member
That is a bifilar common-mode choke in the mains supply to prevent switching transients from the power supply back-feeding onto the ac power lines. As such, it is supposed to block stuff in tens of kHz to tens of MHz range.

Thank you MikeMl
You answered part of my question
Remain: how can I use that torroid to make a 1.2 mH coil?
please give me the formula of calculation

#### JimB

##### Super Moderator
To calculate the turns on a toroid, use the equation:

L = Kn^2

Where
L is the inductance
n is the number of turns
K is a constant for that type of toroid

If you don't know what thr value of K is, you are stuck.

You could calculate K by winding a known number of turns on the toroid and measuring the inductance, then you can calculate the value of K.

JimB

#### MikeMl

##### Well-Known Member
You may be able to get 1.2mH, but at very low d.c. current through the choke. How much average d.c. current will be flowing through the choke?

#### Mr RB

##### Well-Known Member
That is a bifilar common-mode choke in the mains supply to prevent switching transients from the power supply back-feeding onto the ac power lines. As such, it is supposed to block stuff in tens of kHz to tens of MHz range.

Sorry Mike but I will argue that one.

The toroid chokes are very common but always in the mains input area of the PCB, and always have two windings and 4 wires, and quite low turns.

That toroid is mounted next to the DC switching parts, schottkys etc and looks to be part of the DC current path between the switching devices and the DC filter caps. It also looks to have different thickness wires and more than 4 leads.

I'm pretty sure it's a SMPS transformer, possibly a separate DC-DC converter on the output of the PSU to make another DC voltage.

As for the OP's question the easy way is to wind some turns on it and measure, or measure the existing windings. It will be hard to try to find the core specs on such an old part and even if you did get the core specs you would probably still need to do some testing anyway.

#### samy555

##### Member
Ok thanks
I have another question:
Is it possible that those tow inductors in the picture are 1.2mH and 4mH?

Looking at the picture, I think the number of laps in each inductor ranges from 80 to 100 turns
Note: the inductors are part of a final filter in a USP unit.

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#### MikeMl

##### Well-Known Member
Those also look like bifalar chokes (or as Roman suggests, two winding transformers).

Do you have an audio oscillator? If so, wind a few turns onto your core, add a parallel capacitor (start with 100nF), and see where the parallel resonance occurs. From that, you can calculate the K factor for your core, and figure out how many turns it will take to create your desired inductance.

You never answered the question about the D.C. current that flows in the inductors. This sets the requirement on how big the core cross section area needs to be. You may be disappointed when the core saturates, and it stops filtering the pwm...

#### samy555

##### Member
OK, I want to be disappointed when the core saturates....................

please tell me, Is it possible that these two inductors be 1.2 and 4 mH?

#### MikeMl

##### Well-Known Member
... please tell me, Is it possible that these two inductors be 1.2 and 4 mH?

About as likely as winning the Powerball lottery....

#### samy555

##### Member
About as likely as winning the Powerball lottery....

looooooooooool
No
Let's talk a kind of solemnity
I live in a country that is not a sophisticated
Do not sell toroids
I have several PC power supply units
Each unit contains one or two torriods
I want my to build a usp
And not in front of me, but used those torriods
But I doubt that the two conductors in the above pic worth 1.2 and 4 mH

#### Mr RB

##### Well-Known Member
What is a USP? Did you mean PSU (Power Supply Unit)?

Those last two toroids are suppression chokes, they look very standard. That ferrite material is probably not suitable for a PSU back inductor etc as it is designed to be lossy.

The toroid from the first photo is probably a good material.

You have started well collecting toroids out of old equipment, you should be able to collect a lot. It is not hard to wind some turns on and put it in your buck circuit and see what happens. A toroid the size of that first one will have 1mH with approx 45 turns. That is a starting point, it can vary widely based on the material.

Do you have a multimeter with an inductance range? They are cheap enough.

#### samy555

##### Member
Sorry I mean UPS (Uninterruptible Power Supply); it is a 12VDC to 220VAC inverter

I have a multimeter but without inductance range
you are right, I must buy one
thank you very much

#### alec_t

##### Well-Known Member
I suggest you try looking through on-line catalogues of inductors to locate some of similar appearance and size to the ones you have. That may give you a ball-park figure for the likely inductance and current-rating.
Do you have a 'scope or any equipment for measuring inductance?

#### Mr RB

##### Well-Known Member
Sorry I mean UPS (Uninterruptible Power Supply); it is a 12VDC to 220VAC inverter
...

The toroid is not suitable for 220vAC inverter as it requires high frequency operation and all simple inverters (google "modified square wave inverter schematic") require a normal (laminated core) AC mains transformer.

Making a high frequency inverter is very difficult and requires high voltage switching IGBTs after the HV DC converter. That is NOT a beginner project!

#### WTP Pepper

##### Active Member
Ok thanks
I have another question:
Is it possible that those tow inductors in the picture are 1.2mH and 4mH?

Looking at the picture, I think the number of laps in each inductor ranges from 80 to 100 turns
Note: the inductors are part of a final filter in a USP unit.

Look at the windings. Similar gauge, similar number of turns wound symetrically on opposite sides of the toroid. They are not transformers but common mode filters.

#### unclejed613

##### Well-Known Member
"these aren't the toroids you're looking for..."

to make a UPS you will want a line transformer. you might find a toroidal power transformer inside an old powered subwoofer or something like that, but it will not be small. it will weigh several kilograms and be about 20-30cm in diameter..

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