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Old 12th August 2014, 03:16 AM   #1
greg7 is offline greg7  United States
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Default SPST or DPST power switch?

I'm trying to eliminate a turn off transient ("pop") on my LM1875 amp I recently built. I used an NKK 15A/125V SPST toggle switch for the power switch. Is this switch possibly the root cause? Should I use a DPST toggle switch instead?
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Old 12th August 2014, 10:09 AM   #2
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LM1875 exhibits large turn "ON" & "OFF" thump regardless of whether you use SPDT or DPDT as a power switch. I've crudely measured the DC to be > 2.5V.

Soln.
1 Use DPDT to turn spk. connections "ON" & "OFF" (once your chipamp stabilises). If you are using single supply then put 200R/ 1W from o/p capacitor to GND proceeded by DPDT switch.

2 Use soft start / DC protection boards employing relays.
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Old 12th August 2014, 11:27 AM   #3
AndrewT is online now AndrewT  Scotland
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It is "safer" to use a double pole switch.
Electrically 1pole is identical to 2pole.
That is NOT the reason for pop/thump/crack/click/etc.

If you add speaker delay relay with instant OFF, it will prevent start up and shut down surges from reaching the speaker.
It won't stop clicks/cracks which are at the instant of opening the switch.
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Old 12th August 2014, 12:51 PM   #4
greg7 is offline greg7  United States
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Quote:
Originally Posted by AndrewT View Post
It is "safer" to use a double pole switch.
Electrically 1pole is identical to 2pole.
That is NOT the reason for pop/thump/crack/click/etc.

If you add speaker delay relay with instant OFF, it will prevent start up and shut down surges from reaching the speaker.
It won't stop clicks/cracks which are at the instant of opening the switch.
Why is it "safer" to use a double pole switch?
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Old 12th August 2014, 01:18 PM   #5
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Andrew lives in the UK and their power is balanced 230/240V. So he is switching two Hot lines not the Neutral.
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Old 12th August 2014, 03:00 PM   #6
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The ARRL advises US ham radio ops to use SPST for 117 lines. If you have a 1kW amplifier running off 220 then you would use DPST.

Practice varies -- a lot of test equipment which would be used in the US or Europe (where the trafo might be rewired) may have a DPST switch.

You can use a Mallory "Quench-Arc" across the switch. The contacts of the switch are like the plates of a capacitor and as they get close a spark may jump across the contacts.
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Old 12th August 2014, 03:46 PM   #7
AndrewT is online now AndrewT  Scotland
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AndrewT lives in the UK, correct.
The UK does not use a balanced dual Live and Neutral.
The UK uses a single Live and a single Neutral in most domestic premises. I happen to have 3phase (400Vac) but that is unusual for domestic.
The alternative is 3phase, which is generally available in commercial/industrial premises.

But for domestic: the UK requires 2pole switching for plug/socket outlets. Lighting with fixed wiring can use 1pole switching.

In countries that use non polar (reversible) plug tops: there the Live and Neutral can be swapped. It is very important that 2pole switching is used.

In countries where polar (that can't be reversed) plug tops: the Live and Neutral can be miswired (as they can be in the UK) and switching of only 1pole can leave equipment LIVE even though switched off.

2pole switching is safer in all countries, irrespective of what plug/socket type is used.
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Last edited by AndrewT; 12th August 2014 at 03:48 PM.
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Old 12th August 2014, 04:43 PM   #8
DF96 is offline DF96  England
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Switch-off pops can usually be cured by putting an X cap (maybe 10nF) across the mains transformer primary - after the switch.

SP or DP switch makes no difference to pops, but big difference to safety. Only use SP if you can absolutely guarantee no wiring faults will ever occur anywhere within your building or between that and the power company transformer.
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Old 13th August 2014, 12:27 AM   #9
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How could I make a dumb mistake like that?
Sorry
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Old 16th August 2014, 07:41 PM   #10
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Quote:
Originally Posted by AndrewT View Post
In countries where polar (that can't be reversed) plug tops: the Live and Neutral can be miswired (as they can be in the UK) and switching of only 1pole can leave equipment LIVE even though switched off.

2pole switching is safer in all countries, irrespective of what plug/socket type is used.
This is very wise advice, even if you don't have higher voltage like the UK. (220 will do more than just knock you on your rear. As all the British engineers I've worked with have said, it'll clean your clock and put you in a hole in the ground.)

I have seen terrible household and industrial miswiring, and it is not uncommon to see hot and neutral reversed, as it is something I always check. Sometimes the physical wiring so bad that I wonder if electricians are often drunk in addition to being simply incompetent and ignorant of the code.

Always best to completely isolate equipment with a DT switch or relay. Your local hardware store will sell you a tester for the price of lunch that lights up for proper wiring, hot reversed, neutral reversed, and bad ground.

I also replace standard outlets with GFIs for similar reasons. Saved some people from shocks a few times when equipment had internal short. More than once someone complained that the GFI kept tripping so it must be defective (not the load) and they wanted the GFI outlet replaced with a regular one to solve the problem as "this doesn't happen anywhere else".

I once opened up a work light that kept tripping the GFI and discovered that the stranded wiring wasn't properly twisted, and as a result a few strands were in contact with the housing so it was live. I pointed to the problem and said, next time a GFI saves your life don't blame it for malfunctioning.
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