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Old 3rd December 2002, 11:59 PM   #1
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Default transient voltage suppressor

I've been using an amplifier based on LM3875. One problem with it is that it makes a very annoying "POP' noise whenever I turn the lights on/off. I guess this is due to a so-called "spike voltage" from the AC mains. I've checked some textbooks on basic electronics and found that TVS (Transient Voltage Suppressor) could be used to kill this kind of spikes. Now, I'm wondering if any of the DIYers here tried this. If so, please let me know the result. This POP sound is killing me. Thanks.

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Old 4th December 2002, 12:39 AM   #2
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For AC transients I am not a fan of TVS Diodes (seen to many of them blown). I would use something like the Panasonic ZNR type (check DigiKey) or a MOV type.
Ultimate protection system would be:
Line IN
a gas discharge tube
a low value resistor
ZNR or Mov
line out

(for DC, best bet is TVS diode)
Note: TVS diodes fail to a short ckt - not nice on AC line!
ZNR & MOV fail open ckt.
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Old 4th December 2002, 02:07 AM   #3
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What you really need is a snubber across the light switch contacts. A 47r 1w resistor and a 1uF 250 VAC X cap *ac rated* in series should help a lot. Something similar across the mains input of your amp would be good too. *Important* use an X type AC rated cap that has UL, CSA and other safety agency approvals. Put both the cap and resistor inside a small metal box just to be 10000% sure of fire safety.
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Old 4th December 2002, 02:18 AM   #4
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Question?

Is the pop really due to mains voltage, or due to RF spike and amp leads acting as antenna?

-Dan
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Old 4th December 2002, 02:58 AM   #5
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Thanks, crown300.
I checked digikey for Panasonic ZNR (type D). I was able to understand what MAV (Maximum Allowable Voltage) means. As I measured, the AC mains is 120V, and hence MAV has to be higher than 120V. Right? Will 130 or 140V do? Then, what should I choose for Vc (Varistor Voltage) and Max. Clamp Voltage @ Test Current? Sorry for a dumb question and thanks for your help. BTW, I've lived in Raleigh, which is close from Pittsboro, for one and a half year. It was a pretty nice place to live. :-)

Thanks, Circlotron.
It seems opening the switch box of the light will be a little bit difficult for me because I'm living in an apartment. :-) But, thanks for your valuable advice. I'll file your recommendation for my future reference.

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Old 4th December 2002, 03:55 AM   #6
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Quote:
Originally posted by JAZZ2250
Thanks, crown300.
I checked digikey for Panasonic ZNR (type D). I was able to understand what MAV (Maximum Allowable Voltage) means. As I measured, the AC mains is 120V, and hence MAV has to be higher than 120V. Right? Will 130 or 140V do?

JAZZ2250
120V is the RMS value of the voltage. It is something like 170V peak. Do not use anything less than about 200V or risk shorting out the AC lines. I think an input filter ( you can buy these as a module) would be a good choice instead.

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Old 4th December 2002, 05:12 AM   #7
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So, 120VAC at mains is RMS value and MAV is not? Confusing. 120VAC RMS = 120*1.414 = 170V, and Vpp = 2*170V = 340V. Shouldn't I choose MAV = 350V or higher? Again, confusing.
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Old 4th December 2002, 12:51 PM   #8
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Quote:
Originally posted by JAZZ2250
So, 120VAC at mains is RMS value and MAV is not? Confusing. 120VAC RMS = 120*1.414 = 170V, and Vpp = 2*170V = 340V. Shouldn't I choose MAV = 350V or higher? Again, confusing.
170Volts to neutral, is worst case. Yes, it is 340Vp-p. Neutral is ground potential, and the hot goes from -170 to +170V. The maximum potential will be 170 volts peak, but the polarity will change.

-Dan
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Old 5th December 2002, 04:04 AM   #9
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OK, then, another dumb question. Am I going to connect two ZNRs (wih MAV rated at 200V) btwn "neutral and hot" and "neutral and ground"? Or, one rated at higher than 340V btwn ground and hot (nothing connected to neutral)?
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Old 5th December 2002, 04:23 AM   #10
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I believe the traditional setup is to use 330V MOVs between:
Hot and Neutral
Hot and Ground
Neutral and Ground

or just between Hot and Neutral

Check this out though since I am not an expert.

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