Mains voltage spikes

Discussion in 'General Electronics Chat' started by Shagas, Sep 2, 2013.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Hello ,

    I've moved to university dormitories a month ago as I will be starting studies soon and a few days ago I got myself a temporary 1 meter tall refrigerator for
    7 bucks from some guys who were leaving the dorms.

    Of course this is an old , probably more than 10 (maybe 15) year old fridge and i've noticed some odd things happening when the fridge relay turns off.

    Whenever the fridge relay turns off I get a spike in my Studio monitor speakers , just a slight pop from the tweeters , similar to when i turn them on .

    Can someone explain what is happening?
    Is the fridge that old that the back-emf from the relay causes a spike in the mains voltage?
    Is this dangerous for my electrical equipment? (pc , monitor , speakers etc)
    I'm thinking maybe the back-emf diode on the relay is at fault or something like that ...

    Also , do older relays (or even new ones) give out bursts of EMI when they engage the circuit?


    Thanks in advance for the help
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    AC relays are just as prone to producing BEMF as a DC relay, but you cannot use a Diode on AC for obvious reasons.
    In this case you need a R/C snubber, resistor and capacitor in series across the coil or solenoid.
    Typically .22μf with a 100Ω in series.
    Max.
     
    Last edited: Sep 2, 2013
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  3. Shagas

    Thread Starter Active Member

    May 13, 2013
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    So is it possible that this Snubber went old and is not doing its job right?
     
  4. #12

    Expert

    Nov 30, 2010
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    Old refrigerators don't have snubbers. Don't expect one. You are tasked with finding a way to suppress the electrical noise. There are many ways. Personally, I expect the problem is in the compressor current suddenly being shut off. That would be arcing across the relay points, not the relay coil.
     
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  5. MaxHeadRoom

    Expert

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    You could put the snubber across the relay contacts, may need to up the values a tad.
    If the only task of the relay is to switch the compressor, it may be possible to replace it with an SSR, these are zero voltage switching?
    Max.
     
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  6. #12

    Expert

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    Good idea. Be sure the SSR is actually labeled as, "zero crossing".
     
  7. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Hmm I'm not sure I understand . So does this mean that the Mains voltage is crappy that is drops under compressor load and when the compressor disengages then the voltage spikes up?

    Arcing would explain EMI but not the voltage spike am I right?
     
  8. Shagas

    Thread Starter Active Member

    May 13, 2013
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    So I'd have to replace the relay with something like this ?
    http://www.gme.cz/ssr-rele-cosmo-ksd210ac3-p635-024

    It says it handles both DC and AC .What is the zero crossing ?
    What specs would I have to look for in the SSR?
    I guess they would have to match with the mechanical relay activation voltage
     
  9. #12

    Expert

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    1) The compressor is an inductive load that uses about 2 or 3 amps and the relay, if it has one, is an inductive load that uses a tenth or a twentieth of an amp. Therefore, opening the contacts that supply the compressor is suddenly stopping twenty times as much current as opening the circuit that supplies the relay coil. Stopping the compressor will therefore cause 20 times as much electrical noise as opening a relay coil circuit.

    In fact, I don't think a small refrigerator even has a compressor relay, so you must be looking for noise in the compressor circuit.

    2) I am assuming that the switching noise is what you are calling a voltage spike. I don't believe the power line suddenly raises to a significantly higher RMS voltage. I believe the noise is doing all the interference.

    Adding a zero crossing, solid state relay, is a wonderful thing to do for protecting the bi-metal thermostat...if the refrigerator even has a bi-metal thermostat. Please examine the culprit and find out how it is built. Old refrigerators can be very simple. It might have a small relay built right on the pins of the compressor, but that is a "start" relay. It opens a few seconds after the compressor starts, not when the compressor stops.
     
    Last edited: Sep 2, 2013
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  10. MaxHeadRoom

    Expert

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    The popular ones in N.A. are made by Opto22, but I see you are in Prague so probably not a good source?
    If you Google Opto22 I/O modules however, you will see what kind of product this is and also a description of zero crossing switch which means that It turns on and Off when the power sine wave traverses through the zero point.
    You need the suitable voltage for input and output, also depending on whether the coil is AC or DC and the voltage.
    This type with a suitable rating ebay 190892342508
    Max.
     
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  11. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Well thanks alot for the info guys . As always I learned something when making a thread , but in this case I was looking to see if there was a cheap and quick solution without spending much money .
    I spent 7 bucks for the fridge and I'm only going to use it for a month/two maybe three .
    Having said that .. I'm probably going to take a look in the fridge tmrw or the day after to check it out.

    So you think that it's just the bimetallic strip thats switches the compressor on and off?
    It is a 1 meter fridge.

    My main concern is if these spikes when the compressor switches off can affect my equipment (pc ,speakers etc).
     
  12. #12

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    I don't think so. Your speakers are picking up a radiated pulse, not a power line surge. The "pop" noise says the interference is very quick. Any machine connected to the mains is either to stupid to care, like a toaster, or it has input capacitors with a much longer time constant than, "pop".
     
  13. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Thanks ,

    So what you are saying is that there is a burst of EMI which causes the pop in the speakers right?
     
  14. #12

    Expert

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    Yes. It can travel through the air and get amplified by an audio amplifier or it can travel in the house wiring and sneak through the power supply (to the speakers) by capacitive coupling. The same thing happens at my house when I turn the ceiling fan off, but it only shows up in the speakers, not the air conditioner or the porch lights.
     
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  15. MaxHeadRoom

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    Although it is usually best to suppress interference at the source. It is also possible to put a R/L/C suppressor network on the AC input to the receiving device, your amp may already have on built in, but if not this should help.
    PC power supplies have these built in also.
    Max.
     
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  16. ian field

    Distinguished Member

    Oct 27, 2012
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    A metal oxide varistor might help, you could initially try just across the relay coil - otherwise on the mains input.

    Its well worth checking the local discount stores for multiway outlet strips with MOV surge protection, its a worthwhile precaution to also have one for the computer desk as well as one for each noisy electrical appliance.

    It might not stop the speakers popping, but it should protect your computer from the spikes.

    Likewise it won't save your kit from a direct hit lightning strike - but probably one a couple of streets away.
     
  17. Shagas

    Thread Starter Active Member

    May 13, 2013
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    I thought MOVs were in-built to all non-dirt cheap electronics which use mains. ( I watch too many EEV blog teardowns ) .
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    The flats where I live have communal bins - I rescue any electronic gear I spot before it gets buried and harvest components.

    Maybe less than 30% of appliances have an MOV - and that's not only the cheap stuff.

    I always look for an MOV.

    If you're really paranoid about spikes/surges - back up the MOV with a sidac and fuse. MOVs take a hit every time they clamp an "event" and slowly degrade, sometimes they completely disintegrate.

    With sidac backup; as the MOV degrades the spikes can get big enough to trip the sidac and blow the fuse - then you know its time to replace the MOV.
     
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