Author Topic: Dodgy switch mode power supply ?  (Read 8867 times)

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Offline adam1213

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Dodgy switch mode power supply ?
« on: January 24, 2010, 12:50:46 pm »
I have a powertech switch mode power supply (MP-3130) from Jaycar that keeps giving me shocks when ever I touch either wire from it and my computers serial port ground. Has anyone else noticed similar issues with this plug pack? (I might have damaged the plug pack through varies things (eg linking it up to a relay such that when the relay goes on the plug pack is shorted / power is lost when the relay opens causing this process to repeat - resulting in continual shorting / spikes from coil))

AC voltage when touching either terminal (without touching any thing else) aprox 35v
AC voltage between it and my serial port ground: 85v - current:149 uA


Images below show the signal between one of the wires from the plug pack and ground on the serial port from my computer.
(In order to reduce the voltage to get the signal to fit on the scope I put a 910k resistor between the probes. (I only have a 1x probe atm and the scope doesn't appear to want to go past 10v/div))
« Last Edit: January 24, 2010, 12:56:22 pm by adam1213 »
 

Online jimmc

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Re: Dodgy switch mode power supply ?
« Reply #1 on: January 24, 2010, 09:19:14 pm »
I assume that you are using 240v 50 Hz mains.

As part of the interference suppression network in the power supply there are usually two capacitors (typically 2.2nF) connected between the mains input and the output (often the negative side) i.e. one from L and one from N. (There is no earth pin to connect the capacitors to.)
These capacitors, Class Y, are designed never to fail short circuit.
This means that the output floats at about 120v AC (half the supply), this is not dangerous to humans (although it can kill some semiconductors) as the current will be limited to about 165uA maximum by the reactance of the capacitors.

Your measurements are consistent with this model so I think your power supply is OK.

Linear power supplies (the heavy ones with a 50Hz transformer) don't generate the same interference and do not require the mains filtering. Their leakage is much less.

It always surprises me that this effect is seldom mentioned, maybe Dave could mention it in a blog.

Jim
 

Offline Zero999

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Re: Dodgy switch mode power supply ?
« Reply #2 on: January 25, 2010, 01:45:41 pm »
Don't worry, the shock won't do you any harm, the current is far too low.

One thing that can be a problem is that if you've got many SMPSes with the 0V rails connected together, the currents can add together, although you'd need many to get a harmful shock, the risk to electronic components is greater. This could be happening with your PC's SMPs but I suspect that the PC's 0V rail is connected to earth ground so it won't be a problem.

If it's a problem, try connecting the 0V to earth ground with a 100nF capacitor or powering the supply from an isolation transformer. The capacitor will bypass the current to earth reducing the voltage to a safe level (both for you and semiconductors) and an isolation transformer will isolate the incoming mains supply from earth. You shouldn't need to do both, either the isolation transformer or capacitor will do, depending on which is more convenient.

Note that transformer's VA rating might need be much higher than that of the SMPS's power rating because will have a poor power factor unless it has active PFC.
 

Offline adam1213

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Re: Dodgy switch mode power supply ?
« Reply #3 on: January 25, 2010, 02:00:04 pm »
I assume that you are using 240v 50 Hz mains.
That is correct

Don't worry, the shock won't do you any harm, the current is far too low.

The shock is still annoying - I have a pic programmer than needs the connector to be disconnect and the power plug to be be moved from the programmer to the board with the PIC which makes it easy to accidently get shocked by the transformer.
 

Offline Simon

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Re: Dodgy switch mode power supply ?
« Reply #4 on: January 28, 2010, 08:08:59 pm »
why have you all missed the obvious ? adam do you have an earth in you house ? I had a problem where I was getting a shock off my computer and I found that that particular socket had no earth, on fitting and earth it was solved. the filter circuits in most stuff today have that configuration others mentioned, the centre should be earthed and so will become 0 but if left floating will be at 120 V ! I would think that the symetrical layout of the filter is so that it does not matter which way round the unit is plugged in, for example in italy (and i think the rest of europe except the UK) you can put the plug in any way round you want making the neutral and live change at random
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Offline Zero999

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Re: Dodgy switch mode power supply ?
« Reply #5 on: January 28, 2010, 10:22:41 pm »
That could be true but I suspect that it isn't the PC that's the problem, it's the other power supply which is probably double insulated.
 

Offline adam1213

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Re: Dodgy switch mode power supply ?
« Reply #6 on: January 28, 2010, 10:36:10 pm »
adam do you have an earth in you house

I have earth in my house however the plug pack does not have an earth pin. Adding an earth pin by an external power cable (even one where the plug pack then plugs into the back of it would make the plug pack a bit bigger / harder to take in and out than i want)
 

Offline Zero999

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Re: Dodgy switch mode power supply ?
« Reply #7 on: January 28, 2010, 11:55:07 pm »
It doesn't have an earth because it is double insulated.

Unfortunately I don't think there's a nice solution to your problem, an isolation transformer would work, as mentioned above but it's probably too big and heavy and would defeat the advantage of having a lightweight switch mode power supply.
 

Offline Simon

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Re: Dodgy switch mode power supply ?
« Reply #8 on: January 29, 2010, 12:05:58 am »
if it has one of those filters it needs an earth, or any metal case cannot be earth (or even pseudo earthed), personally I don't beleive in double insulation and no earth, my chainsaw has no earth absolutely crazy !
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Offline Zero999

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Re: Dodgy switch mode power supply ?
« Reply #9 on: January 29, 2010, 06:12:03 pm »
There's nothing wrong with double insulation, it's better than single insulation which requires an earth that has potential fail without warning leaving the device unsafe.

Your chainsaw might have a metal blade but the case is probably plastic and the gears are probably all made of insulating nylon or a composite material. The plug should have an RCD which will trip if you cut through the cable.

Who said anything about a metal case?

The PC is the only device with a metal case, as far as I'm aware the supply has a plastic case.

The filter doesn't need an earth because the mains side is earthing the RFI on the secondary side - I'll post a schematic of a typical filter if you like?

 

Offline Simon

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Re: Dodgy switch mode power supply ?
« Reply #10 on: January 29, 2010, 07:17:43 pm »
yea i know how the filter works but surely it was designed to have the earth connected ?, in the case of my chainsaw: say I lay it down with the chain on the cable and cut into it, as the chain bar is not earthed it does not trip the RCD (assuming there even is one!) I then touch the chain bar which is live: I'm dead,

now if the chain bar was earthed as soon as the chain goes through the wire and becomes live and the RCD or main fuse blows. the concept of double insulation is that no metal part is connected to a wire, but in the case of any cutting machine this is a farce in my oppinion.

I'm not sure what the cogs are made of bearing in mind that it is 1.7 KW and some saws are 2.5 KW this is equivalent to a 35 cc engine or greater, just because its electric does not mean its not powerful, the only advantage of petrol machinery is portability and lack of wires
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Offline Zero999

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Re: Dodgy switch mode power supply ?
« Reply #11 on: January 29, 2010, 10:11:41 pm »
I agree with you that it would be better if the chain saw were earthed even if the electrical standards don't say so.

No an SMPs EMI filter doesn't need an earth connection at all. I don't think you do know how a double insulated SMPS filter works.

A Y1 rated capacitor is connected from the primary to secondary side. It is specially designed never to fail closed circuit and is impulse rated to at least 8kV. The capacitor is required to divert any high frequency AC, capacitively coupled by the transformer to the the mains. A high voltage, high value resistor is often also connected in parallel with the capacitor to leak any static electricity and prevent it damaging the capacitor; this is more common on TV sets.

The capacitor is a very low value so its impedance at 50Hz, is high enough to limit the current to a safe level.

A capacitor is connected between the mains conductors to short circuit any RF. A common mode choke is also sometimes used

Here's a PDF which explains the different sorts of RF suppression capacitors.
http://www.evoxrifa.com/technote_pdf/rfi_fact.pdf
 


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