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Old 8th October 2009, 03:00 AM   #1
star882 is offline star882  United States
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Default Y2 capacitors for mains isolation

Let's suppose there's an application that only needs a very, very tiny amount of power (less than a milliwatt) for a standby circuit (that applies to a lot of electronics). The circuit that needs the standby power is connected to an externally accessible conductive part so isolation from the mains is required for safety.

Instead of using a transformer or switching power supply (both of which would be very inefficient at that very low power level), what about a capacitive power supply using Y2 capacitors? Y2 capacitors are approved for connection between the mains (either hot or neutral) and device "ground", even if the device does not have a real ground. So connect a pair of Y2 capacitors from the mains to a bridge, then filter and regulate with a zener diode. The idea is that the capacitive reactance would let enough current through to power the standby circuit, while the Y2 capacitors themselves isolate the circuit from the mains. (Note that both the hot and neutral would have Y2 capacitors connected in series so it would remain safe even if miswired.)

If the standby circuit needs a higher current sometimes but mostly stays in a low power mode, a large capacitor can be charged to provide power for those peaks.
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Old 8th October 2009, 03:11 AM   #2
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There's no galvanic isolation from input to ouput. Y caps are approved for mains because the creapage distance meets UL requirements. If the caps (in what you want to do) short you have a safety problem.
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Old 8th October 2009, 03:22 AM   #3
star882 is offline star882  United States
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It would be no different, as far as safety is concerned, than if a Y2 capacitor were to short in a common mains to floating ground application.
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Old 8th October 2009, 11:49 AM   #4
Bakmeel is offline Bakmeel  Netherlands
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Those cases are class 2 isolated equipment with no metallic parts accessible for the user (like a brick laptop power supply). With Class 1 equipment (capacitors are then used for mains filtering) then you are not allowed to defeat ground. Should now a failure occur in the caps, then the circuit breaker or ground protection will (should) provide safety.

A capacitive power supply can sometimes help you out if you need a small milli-amp supply from the mains. Make sure you use only non-metallic parts (switches etc) for user interface and do not allow this power to pass over any galvanic isolation barrier.

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Last edited by Bakmeel; 8th October 2009 at 12:01 PM. Reason: mixed up safety classes
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Old 8th October 2009, 12:05 PM   #5
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Capacitive power supplies are a regular feature of domestic heating programmers here in the UK. Use correct fusing and observe the safety considerations mentioned above.
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Old 8th October 2009, 12:37 PM   #6
AndrewT is offline AndrewT  Scotland
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Y2 are safer to use on mains side connections than other capacitors for which you have no safety data.

Y2 do not isolate the mains side from the user side.

They are self healing if overloaded by short term transients upto the test voltage.
They do not short on total failure.
But into a high impedance load they will pass near mains voltage to the user side.

If you can build the WHOLE circuit inside an fully insulating box and there are NO CONNECTIONS to the outside world then you may be able to satisfy your safety criteria.
But be careful. It takes just one wire to the outside world with mains voltage on it to be potentially fatal.
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Old 8th October 2009, 01:18 PM   #7
star882 is offline star882  United States
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So all the devices I have seen with Y2 capacitors from mains to floating device ground are not designed correctly?
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Old 8th October 2009, 02:09 PM   #8
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Quote:
Originally Posted by star882 View Post
So all the devices I have seen with Y2 capacitors from mains to floating device ground are not designed correctly?
Do you have a schematic?
A floating GND is not a gnd and considered HOT.
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Old 8th October 2009, 02:42 PM   #9
AndrewT is offline AndrewT  Scotland
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Originally Posted by star882 View Post
So all the devices I have seen with Y2 capacitors from mains to floating device ground are not designed correctly?
where does this come from?
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Old 8th October 2009, 03:03 PM   #10
star882 is offline star882  United States
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Quote:
Originally Posted by AndrewT View Post
where does this come from?
Open up a switching power supply and it will probably have a Y2 capacitor or two from the mains side to the floating ground side, even if the floating ground is not connected to real ground and is accessible from outside. Even switching wall warts often have those.
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