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eHam Forums => RFI / EMI => Topic started by: KJ4DMO on February 28, 2023, 03:12:28 PM

Title: Cheap Switching Power Supply - Reducing Noise
Post by: KJ4DMO on February 28, 2023, 03:12:28 PM
Hello all,

Reading a lot of the threads about power supplies, but cannot seem to find feedback on my idea....

I have purchased a couple cheap switching 12V/30A power supplies off Amazon (China). Here is an example: https://www.amazon.com/gp/product/B01IHNE746/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 (https://www.amazon.com/gp/product/B01IHNE746/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1)

I understand these may be very noisy. I cannot test that theory on my IC7300, because they are the only supplies I currently have. But I'm wondering if I could pre-emptively reduce any RFI by doing the following:
Has anyone tried this, or have any thoughts on if this would reduce or eliminate any RFI from the power supply?

I always like tinkering with stuff like this...

Thanks!
Steve KJ4DMO
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: WA3SKN on February 28, 2023, 03:27:03 PM
First determine if there is a problem.  You will want a benchmark to determine if anything you do will be effective.

-Mike.
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: K6AER on February 28, 2023, 05:38:53 PM
.01 MFd, 200 volt caps from all outside connection to frame ground will take care of most of the RF hash.

You can buy them from DigiKey.
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: LA9XNA on February 28, 2023, 09:34:28 PM
To compare the noise lavels from your PSU you can use a low amperage linear supply 3A is plenty or just a car battery.
Because the testing can be done on RX you will only need the standby current for the rig.
For your rig a 1,5A would be more than plenty.

For filtering you could look into toroide chokes on primary and secondary side of the PSU and also maybe a line filter from a discarded PC ATX-PSU.

You might be lucky to be able to get the noise down to a sensible level, but might be better off by looking for a used linear power supply bought at a estate sale or swap meeting.
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: KT4WO on March 01, 2023, 02:40:12 AM
"eliminate any RFI from the power supply?"

Junk these and buy a RS-35 or larger. No noise.
BTW- the Samlex supply's are better but still suck at
160 and 80 meters.

I tried a few of those cheap supply's, one actually burst into
flames!! (at 12amps!! and was 30amp supply)  YMMV   
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: KJ4DMO on March 01, 2023, 05:04:10 AM
.01 MFd, 200 volt caps from all outside connection to frame ground will take care of most of the RF hash.

Thank you. What do you mean by "outside connection"? Do you mean both the positive and negative DC outputs? Also on the AC inputs? I simply solder the cap from every lead in and out to frame ground?

Thanks!
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: KJ4DMO on March 01, 2023, 05:07:45 AM
Junk these and buy a RS-35 or larger. No noise.

I tried a few of those cheap supply's, one actually burst into
flames!! (at 12amps!! and was 30amp supply)  YMMV

The $250 price tag for the RS-35 is what is inhibiting me from this at the moment, hence trying to get away with something cheaper. I have been concerned about the quality and risk of a fire, which is partly why I'm thinking of putting these in an ammo can with a some ventilation. It could contain the burn if something failed.
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: W5RG on March 01, 2023, 11:44:08 AM
I have used these P/S on K3's and many other without any problems or extra noise..maybe I have been lucky but the 20A and the 30A have both worked well. The only problem is they do not have a switch for off/on.. Bob
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: WA3SKN on March 02, 2023, 08:13:44 AM
You might want to measure the voltage also.  Most computer supplies measure out at 12.0 or 12.1 volts, while most amateur radios require 13.8v +/- 15%... very close to the minimum needed.

-Mike.
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: KJ4DMO on March 02, 2023, 09:41:33 AM
You might want to measure the voltage also.  Most computer supplies measure out at 12.0 or 12.1 volts, while most amateur radios require 13.8v +/- 15%... very close to the minimum needed.

These are adjustable. I adjusted them under load to 13.8v for the IC7300 which is 13.8 V DC ±15% (11.73v to 15.87v).
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: KH6AQ on March 06, 2023, 04:43:10 AM
.01 MFd, 200 volt caps from all outside connection to frame ground will take care of most of the RF hash.

Thank you. What do you mean by "outside connection"? Do you mean both the positive and negative DC outputs? Also on the AC inputs? I simply solder the cap from every lead in and out to frame ground?

Thanks!

A safety concern
Caps can be freely added to the DC output lines but not to the AC input lines. If you do add caps on the AC inputs to chassis they must be Safety Rated Y-capacitors. These are required for both UL and CE certification. 250 VAC Y-caps are designed to not fail when subjected to 1500 VAC or 2121 VDC, which is a test that 100% of such certified equipment must pass. An example is the Murata DE2E3SA103MA3BT02F, 10,000 pF, 250 VAC, X1, Y2 Safety capacitor at Digikey.

     https://www.digikey.com/en/products/detail/murata-electronics/DE2E3SA103MA3BT02F/10690104

0.01 uF as suggested by K6AER is a good maximum value. Higher capacitances, especially paralleled with those already in the power supply and other equipment on the AC circuit, can draw enough current to GND to trip GFI circuit breakers.

On the Amazon page I saw no FCC authorization on the power supply photo or the specifications. So, it may or may not have a suitable internal EMI filter. But even if it does, the allowable conducted noise may be enough to cause objectionable received RF noise. If you have RF noise issues a non-invasive solution (no AC line wires are galvanically contacted by your added filter) is to add a CM (Common-mode) filter in the form of the AC line cord being wrapped through a ferrite core. For HF the AC line cord -- all conductors including GND -- can be wrapped 8 turns though one FT240-31 or FT240-43 ferrite core. These are available at Amazon.
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: W9IQ on March 06, 2023, 05:41:47 AM
If you will be using ferrite beads/toroids as part of the filtering solution, keep in mind that you generally want to pass both the source and return wires through the ferrite in order to avoid core saturation. So wind the DC plus and minus or the AC neutral and hot wires as a pair around the core (but not AC and DC on the same core!).

Mix 31 ferrite is the best ferrite for broad HF filtering while mix 43 is a close second.

- Glenn W9IQ
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: G4AON on March 12, 2023, 03:03:57 AM
Steve, to misquote a pet charity advert, a power supply is for life, not just Christmas... those Chinese power supplies are very cheap, you tend to get what you pay for.

I would have concerns with power supplies that do not include a proper over Voltage protection circuit, many switching supplies quote over Voltage protection but in most cases it isn't an additional independent circuit that you find in properly designed equipment.

Several of us built these "add on" protection units, unfortunately parts for them are currently difficult to obtain due to high demands from industry. The original article was published in our RadCom magazine several months ago complete with kits from the author. The kits are no longer offered, but the author has put the circuit, board files and parts list on his web site at: https://gm4wzg.co.uk/wp/home/projects/bob/ (https://gm4wzg.co.uk/wp/home/projects/bob/)

This page will give you some useful ideas on adding filtering to your shack, the mains filter is based on 230V UK mains but you can easily adapt it to USA rated components (higher current rating needed on 120V).
https://gm3sek.com/2019/10/11/clean-up-your-shack-2019/ (https://gm3sek.com/2019/10/11/clean-up-your-shack-2019/)

73 Dave
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: K1KIM on March 12, 2023, 08:05:30 AM
I have built 5 or 6 switching PS's from sources ranging from computer to LED transformers.

I've added voltage and ammeters using a bridge to the front of the boxes they are in along with thermistors to control the fans in some cases.

When measuring the "noise" with my O'scope I have readings of 35 to mV pp.

The larger issue with inexpensive switching supplies is an over voltage spike that can destroy your transceiver. There is no crowbar circuit in them.

I have found that the IOTA power supplies are the best quality for what they are.

That said, my best equipment runs off a linear PS with a crowbar circuit built in.

The Astron RS-35 etc was a good suggestion when you have the funds.
Title: Re: Cheap Switching Power Supply - Reducing Noise
Post by: KH6AQ on March 13, 2023, 02:49:47 PM
I use Meanwell switching powers supplies for all my industrial equipment. Quality, selection and if nothing else they mean well. They have effective EMI filters and I have never had a Meanwell fail. Amazon stocks the 12V, 30A model ($28.41) which can be increased to 13.2V using the trimmer pot.

https://www.meanwell.com/