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Author Topic: wall wart rfi?  (Read 3307 times)

Posts: 56

« on: June 29, 2007, 05:33:33 AM »

I've discovered my digital answering, caller ID telephone with a 7.5vac, 580ma wall wart is generating rfi. I've been looking for noise sources I hear on HF, but I've discovered this noise on 146.64, with an HT. It is prevalent around several other ac outlets in the house. The noise is gone when unlugging the suspected wall wart. Is there a fix for this or must I replace the wall wart, which will be a problem. The answering machine is new. Doubt if there is any warranty however.
Thanks for your help and advice.

Posts: 8911


« Reply #1 on: June 29, 2007, 06:04:45 AM »

"must I replace the wall wart, which will be a problem. The answering machine is new. Doubt if there is any warranty however. "

Warranty wouldn't help you... unless you're willing to undertake an expensive and lengthy legal battle to get conducted/radiated RFI emissions standards for consumer gear tightened up and enforced ;-)

If you wanted to replace the wall wart, you wouldn't want to replace it with another one from the same company anyway.  It's not broken, it's just poorly designed.

A lot of people have had success replacing these noisy switching power supplies with good old linear 60Hz transformer supplies...

you might be able to find one at Radio Shack, but you have to be careful not to buy another cheap switcher there.  But if you're sure it's the wall wart, not the phone, and you really want to easily dispense the problem, this is the way to go... if you happen to have a 12V @ 600mA wall wart around, you can use an LM317 regulator to drop that to 7.5V and you'll have a nice quiet supply for your phone.

I'd recommend toroidal chokes on the cord but from what you've described it sounds like just having the wall wart plugged in causes the noise all throughout the house ... so it sounds like you need a filter on the AC input side.  You could buy an RFI-filtered power strip to plug the wall wart into and see if that helped, but honestly, replacing the supply with an older junk-box wall wart with a regulator tacked on might be cheaper.

Good info on tackling RFI here:



Monkey/silicon cyborg, beeping at rocks since 1995.

Posts: 6642

« Reply #2 on: June 29, 2007, 06:06:08 AM »

Shielding and filtering are the answer. You are in luck that you have determined the culprit!
Is the noise being transmitted via the house wiring and/or low voltage wires, or is it being directly radiated? Some aluminum foil and an alligator clip to ground should shield any direct radiation.  Connecting the wart to a extension cord with a "brute force" type filter will eliminate the house wires, and a split ferrite core can be used to filter the low voltage wires.
Try these to cure the problem!



Posts: 594

« Reply #3 on: June 29, 2007, 06:37:59 AM »


May I suggest this. Try a Radio Shack filtered wall wart. They do have regulated supplies that ARE NOT filtered. I discovered you have to read the specs on the transformer, not those on the box.

Let me say this. I have a Cisco wireless print server
that generates un-godly amounts of noise on 10,12,13 and 14mHz. I tried ferite chokes but they had zero effect.

In case the choke suggestion above fails, try replacing the transformer.

J C S  

ps congrats on finding your noise source. Be greatfull that is was on your own property too!

Posts: 14491

« Reply #4 on: June 29, 2007, 08:03:17 AM »

Does the wall wart generate noise even if it is not connected to the answering machine? If so I'd say it must have a problem (perhaps with a defective over-temp cut out). If it puts out 7.5VAC then it's nothing more than a transformer - no switching regulators or anything inside.

If it only generates noise when connected to the answering machine then its probably the answering machine generating the RFI and feeding it back through the power cable. In this case some ferrite filtering on the power cable may help.

Bob  AA4PB
Garrisonville, VA

Posts: 144

« Reply #5 on: June 29, 2007, 09:29:44 AM »

I recently bought a Belkin 4-port USB hub and the wall wart that came with it was much smaller than other similar hubs I had previously purchased.  After noticing a real increase in noise in the HF thru VHF range I probed around the supply with a sniffer loop on my spectrum analyzer and found it to be absolutely filthy with noise.  This new supply was obviously a switcher while the older models came supplied with the older, heavier analog supplies that were just about clean when similarly observed.  Attaching the older supply to the new hub cleared up the problem immediately.

I told Belkin about it and returned the unit to them, receiving a identical replacement that surprisingly (or not) arrived with the older analog supply.  Now I routinely check any new device that comes into the shack with wal warts to preclude installing them only to find out they're big-time noise producers.

Dino KL0S/4

Posts: 40

« Reply #6 on: June 29, 2007, 10:10:40 AM »

A lot of the newer wall wart supplies are switchers. I have a 2 meter repeater and the antenna system is on top of a water tank. The local PD saw fit to install a part 15 video link receiver and transmitter about 10 feet from my four bay folded dipole repeater antenna. The day they turned it on I suffered a loss of weak signal reception. We could hear some harsh grunge in the receiver that just covered up local HT operators. I was transmitting it along with our normal audio.  

At first I did not know where it was coming from. No info on their equipment as to whom it belonged to. So I was able to access the AC power to their box and I turned it off. MAGIC! no more received grunge. I left it off for over a week. I placed a note on their stuff and one day the Chief of Police called me. He told me I could not turn off their "official" equipment. I told him " I can and I did!" I offered to help them find "their" trouble and we did cooperate on the fix.

It turned out to be a wall wart switcher in their box powering one of their receivers. It said "Made in China" on the case. I had them change over to an analog DC power supply at ground level and feed DC to their box. Now it is nice and quiet.

I did explain to the chief all about the FCC and part 15 devices and that they could "NOT" interfere with a licensed service. He is now happy we are not having a problem and the FCC is not involved.

Posts: 14491

« Reply #7 on: June 29, 2007, 10:30:40 AM »

... with a 7.5vac, 580ma wall wart...

This *isn't* a switching supply. It's a transformer. If it was a switching supply the output would be "vdc". The regulator (either switching or linear) is inside the answering machine.


Bob  AA4PB
Garrisonville, VA

Posts: 5688

« Reply #8 on: June 29, 2007, 01:06:52 PM »

AA4PB is correct.  

Try a clampon ferrite bead designed for suppression on the power wires, as close to the powered device plug as possible.  

(wrap the cord 10-20 times tightly around a large screwdriver blade, close to the device and listen -- if the offending noise is suppressed a little bit, go buy a ferrite for it -- quick 'n dirty test)


Posts: 0

« Reply #9 on: June 30, 2007, 07:25:26 AM »

To check to see if it actually is the wall wart or the telephone just plug the wall wart into another device, a computer fan, or just something to apply a load to replace the telephone.

IF the noise continues then its the wall wart.  IF the noise does not appear when using the alternate device as a load but ONLY when you use the telephone attached to the wal wart then you know its the telephone and NOT the wall wart.

IF its the telephone do some research int RFI resistant telephones using google.  WARNING: dont even expect Wall Mart Cheap.  

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