Solutions for noisy switchmode PS? - Page 1
Forum section: electronics / beginners

author: nzo
author_rank: Regular Contributor
timestamp: March 25, 2012, 08:30:46 am
content: Hi allI bought a small 230vAC to 12v DC switchmode PS from DealExtreme to power a small strip of LEDs for a bench light. I noticed today, while tuning the ham bands, how noisy this PS is. It's like a broadband hash noise. Is there a simple way to nuke this noise without turning the light off?Thanks

author: Rerouter
author_rank: Super Contributor
timestamp: March 25, 2012, 08:40:58 am
content: well i am going to assume that its spitting out RF and noise on your mains right? the rf is pretty easy to handle, i am guessing its a plastic case, if so, grab some tin foil and coat the inside of the case and something ontop of it to prevent it shorting out internally, if it has a ground brough in, tie the sheild to it,if its on the mains, then if it has wires, loop some ferrite beads on them, and hope,

author: SeanB
author_rank: Super Contributor
timestamp: March 25, 2012, 10:02:15 am
content: Shielded box, extra ferrite beads and chokes on both input and output leads, as well as remove the capacitor across the isolation barrier and connect the secondary side to mains earth. Should solve all the problems.

author: T4P
author_rank: Super Contributor
timestamp: March 25, 2012, 10:23:44 am
content: Ferrite chokes are merely just to filter the power line .The best and cheapest way is to use alu-foil and a little bit of tape to stop it from shorting to the foil .

author: SeanB
author_rank: Super Contributor
timestamp: March 25, 2012, 10:48:14 am
content: Best probably will be a linear supply, no noise at any amateur bands at all, with probably only double the loss of the SMPS at low power.

author: T4P
author_rank: Super Contributor
timestamp: March 25, 2012, 12:41:06 pm
content: Quote from: SeanB on March 25, 2012, 10:48:14 amAnd a whole lot larger and heavier .Just shielding will solve the problems of noise .

author: amyk
author_rank: Super Contributor
timestamp: March 25, 2012, 01:09:47 pm
content: Quote from: SeanB on March 25, 2012, 10:02:15 amI thought that was there toreduceEMI.

author: Unknown
author_rank: Unknown
timestamp: March 25, 2012, 01:10:57 pm
content: Quote from: Dave.S on March 25, 2012, 12:41:06 pmAssuming the noise is radiated, and not conducted.

author: T4P
author_rank: Super Contributor
timestamp: March 25, 2012, 02:13:53 pm
content: Quote from: alm on March 25, 2012, 01:10:57 pmRadiated noise , shielding conducts just that .

author: jahonen
author_rank: Super Contributor
timestamp: March 25, 2012, 02:31:24 pm
content: I'm almost sure that simple aluminum foil tricks won't work, the shield is not simply effective. Shield must be perfectly conductive with no seams anywhere to be effective. Even if the shield would be effective, it won't remove noise which is conducted through the shield by the leads and then radiated from them. Aluminum foil does absolutely nothing to that. I have seen copper taped things in EMC lab and in most cases they have been just a waste of material. Dominant source of the radiation is usually not the PCB but the wires connected to it.More effective approach would be to determine the source of the noise on the PCB and eliminate it. Unfortunately, that is quite time consuming and requires experience, and is usually very tedious process. Cheap chinese SMPSs are surely not the best when it comes to EMC.I'd suggest to try to find better power supply EMC-wise, or if some wasted heat in linear power supply is not too bad, to use that.Regards,Janne

author: ejeffrey
author_rank: Super Contributor
timestamp: March 25, 2012, 07:05:45 pm
content: Quote from: SeanB on March 25, 2012, 10:02:15 amThis.Quote from: DaveSIt filters both directions. In this case, the chokes keep the RF from escaping on the power line and turning it into an antenna. At these low frequencies your PCB is too small to be an effective radiator. It needs to be coupled to some long wires to get the signal out. Even at higher frequency you need the chokes because you shield the box.QuoteIndiscriminate use of aluminum foil almost never helps, and can quite easily make things worse. Aluminum foil (or better yet copper tape) can be an effective tool for e.g., covering seams or small holes in a shielded enclosure, but just wrapping it around something doesn't do any good at all. Your shield must be connected to the circuit ground with a good low impedance RF connection. It also must be connected to the shields of any cables coming in or out of the enclosure.

author: nzo
author_rank: Regular Contributor
timestamp: March 26, 2012, 01:34:45 am
content: Thanks all for your ideas and thoughts. I'll follow your advice and transfer the innards of the light's plastic base into a metal box and filter the ins and outs and see what happens.I'm not sure but it sounds like the "better" SMPS have RFI suppression built-in?

author: vk6zgo
author_rank: Super Contributor
timestamp: March 26, 2012, 02:12:52 am
content: If you have a power supply for your Ham rig,why don't you use it to drive the LEDs as well,& ditch the small switchmode?If the big one is quiet enough for your radio it should be good for the light.

author: sonicj
author_rank: Frequent Contributor
timestamp: March 26, 2012, 08:30:00 am
content: Quote from: nzo on March 26, 2012, 01:34:45 amsome even have a offset control that allow you to manually tweak the switching frequency if it interferes with other equipment.i agree with vk6zgo. run the leds on your big supply.-sj

author: nzo
author_rank: Regular Contributor
timestamp: March 26, 2012, 01:33:36 pm
content: There are several PCs and various other bits of equipment in the same space as the ham stuff. For this reason I run the receiver off a large 12v battery to isolate it from PC RFI.