# AC Hum

• posted

Group:

I have a cordless telephone. The base unit is powered by a wall-wart whose original spec was input 240Vac (frequency unspecified but as it was bought and operated in the UK it was de facto 50Hz) and output 9Vac/300mA.

When I moved to the US I replaced the original wall-wart with a wall-wart who spec was input 120Vac/60Hz and output 9Vac/300mA.

When I use the phone, I hear a hum. I assume that this is a AC hum - is this reasonable?

Assuming that the hum is a AC hum. I assume that the phone circuitry was optimized to minimize a 50Hz hum. Are there any simple solutions for removing / reducing the 60Hz hum?

Cheers

Den

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inside your phone there will be some sort of rectifier possibly followed by a voltage regulator. assuming there is no defect in you phone it is possible that the voltage is too low to be properly regulated. in this case the hum you hear is 120 Hz ripple of a full wave power supply.

it could also be that the filter capacitor cap in the power supply has opened up

it is also a possibility that the phone line itself has a problem. are other phones on the same line clean?

>
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All of the other phones in the house (corded and cordless are fine), however, all of the handsets coupled to this specific coredless phone exhibit the problem. For this reason, I'm assuming it's an issue with the base unit? What to do?

D

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Some power supply units have " on board " regulation, others just supply an unregulated supply.

If your phone had a regulated supply and you swapped it for an unregulated type this could be causing the problem...

to test the supply, ( 1 ) measure the DC output voltage with a resistor of around 50 ohms across it, this should be around the 9V mark. ( 2 ) measure the AC voltage.. this is the ripple on the supply, it should be negligible for a regulated supply. ( 3 ) measure the voltage with a light load ( say a 50,000 ohm resister ) , if the DC voltage is significantly larger than in ( 1 ) it's an unregulated supply.

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It's a 9V AC output not DC.

D

unregulated

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as I do not know your skill level in electronic servicing nor value of the equipment I would hesitate before recommending anything more then the following:

1: take it to a qualified technician

2: try a different wall wart (but sill the same value)

3: place unit in a shoebox on the top shelf in you closet. obtain new unit at the Super Duper Wonder Bargain Made in Korea Store.

going beyond these steps i feel would create a possable fire hazard.

an additional thought.. as the unit is from the UK is it FCC type accepted? are you transmitting in an allowable band for the US?

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Both of mine work on DC.... Are you sure your original supply was AC...

Jonathan

9Vac/300mA.
• posted

OK

So get ready to hit me!

The base unit was plugged into a surge protector. Take the surge protector away and the hum disappears!

Cheers

D
• posted

| So get ready to hit me! | | The base unit was plugged into a surge protector. Take the surge protector | away and the hum disappears!

Could be a bad ground in the surge protector? Or maybe a spike leakage generating harmonics of 120 Hz?

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i'd suspect a high resistance path causing low voltage and thence poor ripple regulation.

wow first time i have ever used thence in a sentence. i hope my syntax is right

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Yup, *very* definitely AC.

D
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It happens that if the original was an AC wall wart, that you can then use a DC supply instead and it will work just fine. Another point with an AC supply is that the original current rating is a minimum, and a supply rated at any higher current will work just as well.

The reverse is not true, and if the original was a DC supply, then you must use a DC supply. Likewise with a DC supply you want to use one that is the same current rating or not too much higher (20-30% higher would be about the limit). The problem is that an underloaded DC supply will have a higher voltage. So a

9VDC 500 milliamp supply that is replaced with a 9VDC supply rated at 1.2A is probably going to put more like 12VDC, or even higher, on the equipment.
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Does that work for all applications, that a AC source can be replaced with a DC source. I assume not if there is a motor or a frequency dependant issue?

D
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Nope. It works for wall warts that power electronic equipment, period.

The "AC Supply" is just a simple transformer. It necessarily must connect to a rectifier to provide DC to the electronics.

A "DC Supply" has the rectifier in the wall wart right along with the transformer.

The rectifier in an equipment designed for such an AC supply doesn't really care if it is fed alternating polarity voltage, or just one steady polarity. And when that is the case, a DC supply will work just as well as an AC supply.

On the other hand, if the "AC source" (as opposed to an "AC Supply") feeds a transformer, a motor, or anything else in the equipment, then a DC supply absolutely will not work.

Hence, modems and wireless telephone sets are examples of equipment that if designed for an AC input, will work with DC too. While something like a FAX machine might or might not, and you don't want to find out by experimenting...

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OK, so now getting well off-topic, but to continue my education.

I have an battery powered inflator for an air-bed. The unit has a socket for a power supply from a wall wart that is market 6V. The wall-wart that came with the unit was specified as 7.5VDC/700mA. Does this mean that I could replace the wall wart with a 6V unit outputing the same power i.e.

7.5V * 0.7A = 5.25W; 5.25W @ 6V = 0.875A

Why would the wall-wart be apparently overvoltaged for the unit, or is this what you mean by the 20%-30 rule of thumb range (I had assumed that you meant current not voltage)?

Cheers

D
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That would probably work, and would merely be a little slower. The motor would run with less voltage, drawing less current and would probably pump slower.

The fact that 6V is very close to 7.5V means it would probably work. The underloaded 6V power supply would probably supply a little more than 6V too, so it wouldn't be a whole 1.5V difference. If you used a 6V supply rated at 1.4A, it might actually put out 7.5VDC at half its rated current draw (it might not too, and it would vary from one model power supply to another).

On the other end, if you used something like a 4.5V supply, it might not even have enough poop to keep the motor running, and might burn things up.

The only way to know for sure is to put the power supply on the item, and measure the voltage and the current to see what you get. A short test won't harm anything.

The 20-30% was in relation to the current rating. If you replaced the original 7.5V supply rated at 0.7A with one that was also 7.5V but rated at 2A, it is very likely that with only a 0.7A load it would have a voltage significantly higher than

7.5V.

The voltage of an AC supply, which is just being stepped down by a transformer, will have pretty good regulation under varying loads. The only change will depend on the resistance of the coils. The windings will likely burn out before the voltage is dropped too very much.

But a DC supply is a *very* different beastie. Some basic designs have better voltage regulation than others, plus of course there is the possibility that a supply is equipped specifically with a regulator circuit to maintain some range of voltages. You can expect that if you pay \$50 for a power supply it will likely have better voltage regulation than one you buy for \$2.95! Or perhaps the right way to say that is to note that the \$50 model may or may not have a regulator circuit, but the \$2.95 model almost certainly does not.

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Floyd

Thanks for taking the time to explain. I appreciate it.

Cheers

Den

message

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There are two kinds of wall warts, regulated and unregulated. A regulated power supply will put out a constant voltage up to and including its rated current limit (beyond that it may droop or it may shut down or it may self-destruct). An unregulated DC supply usually has a transformer and a diode or two. It may also have a capacitor to filter the hum and present an almost DC voltage for some current draw. The manufacturer may have measured the output of the wall wart they supplied and found that at the current the unit draws the voltage was close enough to the 6V required to be usable -- and it was cheaper than a 6V regulated supply wall wart.

Norm

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