View Full Version : Power supply inductor query.

06-08-2012, 12:00 AM
The 24VDC 800 mA power supply I'm using to power up the contactor and motor speed control relay coils came with this device in the output lead. I assume it's an inductor? Back when I was freakin' out over the perceived contactor coil current draw issue I removed it thinking it may have had some effect on the problem. That problem of course turned out to be ignance on my part.:rolleyes:

Now that I have the P.S. installed & working nicely, I found the inductor on the bench as I was cleaning up. The P.S. originally powered a document scanner. Is the inductor needed in the P.S.'s new role as a simple power source for the 2 coils?


06-08-2012, 12:14 AM
That looks like a common mode choke used to filter out electromagnetic interference (EMI) generated by a) the device being powered and/or b) the power supply itself. Since it looks like it was probably closer to the device end, it was probably intended to server purpose b) primarily. However, if the power supply is a switching supply it can generate lots of interference that can affect radios and other devices so it could be worthwhile to reinstall it as close to the power supply as possible.

06-08-2012, 12:34 AM
It is usually a toroidal ferrite ring, used for HF suppression.

06-08-2012, 01:18 AM
Whitis and MaxHeadRoom are both correct. What they have not spelled out but that you may care about, is that switching power supplies operate in the many-kilohertz range or even higher.

A switching power supply definitely can create interference in nearby electronics, but the ferrite suppressor will have no measurable effect on the device you are powering, since it effectively absorbs or damps out only high frequencies. The further it is from the source of the high frequency (the power supply) the more of your power cord is available to act as an antenna and radiate interference. If you're curious and have time to kill, you might put a small radio near the power cord before and after you re-install the suppressor.

Virtually all power supplies for electronic equipment these days are switched mode supplies - they can have efficiencies up in the 90%s, much much better than older linear designs. It's also possible that it was intended to zap interference generated by the scanner, since the digital circuitry in that would also run at high frequencies.

06-08-2012, 01:24 AM
Is the inductor needed in the P.S.'s new role as a simple power source for the 2 coils?

To actualy awnser your question:

No. Its not needed and it won't change anything if its installed or not in this application.

Paul Alciatore
06-08-2012, 03:13 AM
I have to disagree about it not being needed. It may not have much of an effect on the equipment being powered, but if your supply is radiating RF, that may have an effect on anything in the area including any radios and TVs. That includes those of your neighbors.

I would strongly recommend that you reinstall it.

06-08-2012, 06:35 AM
Don't bother splicing the original ferrite back in. An old PC power supply will yield a ring core that you can pass a couple of turns through.

J Tiers
06-08-2012, 09:18 AM
Those common mode devices are really NOT a huge deal.....

They are pretty much a "part of last resort", which cannot do more than knock off the last couple of dB of noise. They are not "magic", they operate on known laws* and can do only so much.

As was mentioned, if they are close to the END of the wire, they are likely more for the powered device than the power supply itself, since teh entire length of wire is already available as an antenna.

I would not get too upset about it. If you are using the supply in a different application than the original one, the entire EMI situation is changed, and you have no idea if the "problem' is changed or even affected by the inductor core. The core, if it did any real good, was "designed-in" for the original case, with no necessary applicability to whatever you are doing now. NOW, it could make things better, OR WORSE.

Do what you want... it won't affect the basic operation of the SMPS.

* Details: What the core does is to add an RF impedance in series.... somewhat inductive, and also generally quite resistive (the ferrite is made to be "lossy"). They are rated for their RF resistance vs frequency.

They MUST be put on at a point where there is substantial CURRENT flowing at the undesired frequencies.... all they can do is reduce current by raising impedance. Voltage is not affected.

But, in order to do any attenuating of the "signal", they have to have a "shunt impedance' after them that DRAWS that current. Just putting one on a piece of wire may not have ANY effect, because there is no "termination" on the wire at the frequencies of interest.

Put at the base of a quarter-wave antenna, sure..... but that is for a SINGLE frequency.... EMI is typically a wide spectrum of harmonics. Your wire isn't a good antenna for all frequencies, and the location of maximum current changes with frequency.

06-08-2012, 11:22 AM
Thanks all for the education.:) Since it's such a low power device, I'm not too concerned about wrecking the neighbors' talk radio listening experience. When I installed it I cut off the extra AC power cord and the DC output leads so maybe that'll reduce the antenna effect.

My concern was if it was removed would it affect the P.S.'s operation or reliability...sounds like not.

However, I hadn't thought about RFI maybe affecting the nearby CNC electronic components, ie: B.O.B., spindle speed sensor board, stepper controller, etc. Should have that stuff running later this weekend and will see if anything funny happens. I s'pect the KB DC motor controller puts out its share of noise too.

J Tiers
06-08-2012, 11:41 PM
Very unlikely that it would be detectable...... probably was put on to get a few db of "margin" way down at the limit of the allowable noise...., you'd need a good "quasi-peak averaging" detector on a spectrum analyzer to find the noise.