Cleaning up the mains - noisy fridges

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Re: Cleaning up the mains - noisy fridges

Post by shufflebeat »

ef37a wrote: You could of course turn the fridge off for the duration? If like my son you work mostly in the wee smalls, nothing will spoil if the door is left shut for hours,but you will have to come up with a foolproof warning system to turn it back on before pitting.
Dave

Bog standard timer switch on the fridge before beginning recording? (Remove it for normal use if necessary).
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Re: Cleaning up the mains - noisy fridges

Post by Folderol »

shufflebeat wrote:
ef37a wrote: You could of course turn the fridge off for the duration? If like my son you work mostly in the wee smalls, nothing will spoil if the door is left shut for hours,but you will have to come up with a foolproof warning system to turn it back on before pitting.
Dave

Bog standard timer switch on the fridge before beginning recording? (Remove it for normal use if necessary).

I'd go along with this simple crude solutions are often more practical than the sophisticated stuff :)

Another note on caps across the mains. Not only are ordinary caps not suited to having their plates squeezed by real power, so will likely have a short life before going bang, but you could find yourself in the unfortunate position of making the whole system resonant at some frequency kicked out as the internal stat switches. This could make the problem much worse and could do some real damage (apart from the cap going bang again). Filter caps always have a resistor with them to protect against this (often built in) so you'd see something like 100n+470R 300VAC.

Incidentally in the old valve TV days almost all mains filter caps were rated at 1000VDC - we usually replaced them with 1500VDC.

Something that just might help at a pinch would be a varistor placed directly across the motor terminals - a bit counter-intuitive but it's the inductance of the motor that produces the spike.

One of these maybe:
http://uk.rs-online.com/web/p/varistor/7118136/

but needs to be fitted by someone who knows what they are doing!
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Re: Cleaning up the mains - noisy fridges

Post by dubbmann »

hi,

i had a friend who had this problem, we simply did the 'unplug the bugger' trick and that worked.

btw, i'm not an electrical maven but i always assumed that it was an issue with the sudden current draw causing a voltage drop, not a 'spike' per se. have i got it backwards?

cheers,

d
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Re: Cleaning up the mains - noisy fridges

Post by Folderol »

dubbmann wrote:hi,

i had a friend who had this problem, we simply did the 'unplug the bugger' trick and that worked.

btw, i'm not an electrical maven but i always assumed that it was an issue with the sudden current draw causing a voltage drop, not a 'spike' per se. have i got it backwards?

cheers,

d

Exactly - simple remedies.

As the contact makes, yes you get a current pulse, but (specifically with a motor) at switch-off you will get a high amplitude back emf, and this is usually far more significant.

P.S.
The absolute worst offenders for this are washing machines with mechanical timers :frown:
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Re: Cleaning up the mains - noisy fridges

Post by ef37a »

Yes D' you have it a bit A'uppards!

A load such as a fridge motor is not going to cause any significant drop in the local supply voltage (unless said supply in in a very poor state, but you would have other problems then)

The problem is actually quite complex but a good part of it is because switches do not work as we think, they bounce. When a switch closes the contacts will bounce away from each other by a fraction of a mm. The magnetic energy stored in the inductance of the motor will cause a high voltage across the switch and an arc will result. Now the very first radio transmitters were spark generators! The lumped inductances and capacitances around the switch will determine the frequencies most strongly radiated and dumped into the mains supply.

Even worse is the fact that the switch can bounce several times taking some mSecs to finally close.
A similar situation happens when the switch opens.

In another field, digital electronics, contact bounce was a PITA and EVERY contact had to have its own "de-bounce" circuit.

Dave.
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Re: Cleaning up the mains - noisy fridges

Post by tim_obrien »

A fridge will hold it's cold for several days as long as the door stays closed.
(I live in Hurricane Central - Florida - guess how I know...)

Just turn it off before you start your recording
and turn it back on later. Won't hurt anything....
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Re: Cleaning up the mains - noisy fridges

Post by Hugh Robjohns »

When I was a location TV sound recordist many years back it was standard practice to turn off fridges and the like if they made a lot of acoustic noise when they switched on and off.

However, after one occasion when a colleague forgot to switch it back on afterwards, and the production company received a bill for an entire freezer's worth of thawed food, we developed the technique of placing the crew's car keys in the fridge to make sure that we remembered to turn it back on again before we left!

:blush::D

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Re: Cleaning up the mains - noisy fridges

Post by Martin Walker »

Hah, that's an excellent idea Hugh :bouncy:

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Re: Cleaning up the mains - noisy fridges

Post by Folderol »

Martin Walker wrote:Hah, that's an excellent idea Hugh :bouncy:

Martin

You can always tell those who have worked at the coal face can't you? :D
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