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That Tingly Feeling (SMPS)
That Tingly Feeling (SMPS)
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Old 1st October 2018, 12:21 PM   #1
avtech23 is offline avtech23  Australia
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That Tingly Feeling (SMPS)
Default That Tingly Feeling (SMPS)

I'm finalising one of my builds at the moment and was checking the safety of the finished amp when I realised there is AC on the chassis.

I am using an SMPS brick that has 3 pin AC in and 24v DC out.

Between Earth and SMPS Negative was a steady 85vac.

Tonight, there is 10vac and when shorted to ground via ammeter, I have 0.2mA. When plugged into the chassis and touching earth to chassis using one hand, there is definitely an unpleasant tingle. Wonder how unpleasant the 85v would have been..

Most importantly, is this normal / safe for an SMPS?
Should this PSU go in the bin or is it ok to use?

The amp is a self contained Bluetooth unit so it won't be grounded out via any inputs.

*Edit: I have 3 of this type of SMPS and I have 10vac, 9.8vac and 88vac.
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Last edited by avtech23; 1st October 2018 at 12:30 PM.
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Old 1st October 2018, 01:24 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The laptop 'brick' is usually left floating with only a class X capacitor to reference the output to what should be earth but 9 times out of 10 it goes to live or neutral or both with 2 capacitors as an RF filter.
Connect the earth to the negative side of the output through a 10R 1W resistor. That will render it safe to use.
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Old 1st October 2018, 02:08 PM   #3
DF96 is offline DF96  England
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If it was simply leakage through a X capacitor forming part of a filter then you should get similar readings for each supply. As they are all different that raises the question of whether it is a fault. Some supplies forget about all the regulations about insulation quality and creepage distance etc.

Don't use a resistor, as that gives no safety at all. It will just remove the tingle. If a short occurs the resistor explodes, then you die.
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Old 1st October 2018, 02:11 PM   #4
avtech23 is offline avtech23  Australia
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Thanks Jon.

The bricks are hermetically sealed though so not sure how to go about getting a resistor in there.

Are they in an unsafe condition as is?
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Old 1st October 2018, 02:17 PM   #5
avtech23 is offline avtech23  Australia
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Quote:
Originally Posted by DF96 View Post
Some supplies forget about all the regulations about insulation quality and creepage distance etc.
Interestingly, the power supplies came with all the right 'marks' (CE etc) but I guess they can print whatever they like at manufacture..

Quote:
Originally Posted by DF96 View Post
the resistor explodes, then you die.
I'd like to avoid that if possible!!
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Old 1st October 2018, 03:00 PM   #6
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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To comply with CE approval, the interference must be reduced using class X capacitors to 'leak' the interference to ground. Whether that be to earth or back to the mains.
The output can drift against earth without much of a problem. See Big Clive on youtube. He strips down many power supplies and can explain what happens better than I can.
YouTube
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Old 1st October 2018, 03:10 PM   #7
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by avtech23 View Post
Thanks Jon.

The bricks are hermetically sealed though so not sure how to go about getting a resistor in there.

Are they in an unsafe condition as is?
You can do as Jon says without getting inside the casing.

I will be less dramatic and expect you to live on for years.
The 0.2mA short-circuit current seems indeed to be a designed value, thus, not due to an uncontrolled fault mechanism.

In the past safety earth was not a standard. Being left between the choice of not connecting the chassis to anything or to some primary potential, the mid-point between phase and neutral was often used. The midpoint was formed with two safety capacitors of equal value. Most of us, having passed 50 years of age, know that tickling feeling. Had it been deadly, most of our younger forum members would never have been born as their parents would have been killed. With Jon's advice the SMPS's can probably be used.

An important issue is also if they are really worth re-use.
SMPS's with output voltages of 10vac, 9.8vac and 88vac (Vac from an SMPS?) do not sound like mainstream needs. With prices of modern SMPS, why bother with awkward output voltages? I believe those you have can still be used but you will not loose much by giving them to recycling.
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Old 1st October 2018, 09:40 PM   #8
avtech23 is offline avtech23  Australia
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The SMPS are all brand new 24vdc but also have the AC with on their negative output with respect to earth.

Quote:
You can do as Jon says without getting inside the casing.
Any suggestions on how I could achieve that? The input jack on the chassis is metal so the SMPS negative is immediately connected at that point.
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Old 1st October 2018, 10:09 PM   #9
SemperFi is offline SemperFi  Wake Island
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U have brand name and type? Easier if we look at data sheet.
U get different voltages at different times? Could it be the earth is not at the same potential to the live and neutral at different times? Your line being loaded differently?
Most likely the ac input and dc output is drifting a little and there shouldnt be enough leakeage to be harmful.
Any how please post datasheet or type of brick so we know what we're dealing with.
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Old 1st October 2018, 11:48 PM   #10
avtech23 is offline avtech23  Australia
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They are generic Chinese made same as this:

5.5mm x 2.5mm AC 100-240V to DC 24V 5A Switching Power Supply Adapter Transformer Sale - Banggood Mobile

Don't think there would be a data sheet available for this particular one.

Neutral and Earth should be tied to the same potential here so not sure why the difference in the voltage I measured. The 85vac was present on the first ever switch on of the units, the ~10vac on subsequent switch ons.

The output is rock solid 24vdc.
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