Earthed 12V Mains adaptor

[riccardo]

Do mains adaptors with the DC ground connected to earth exist? / Where could I buy one?

I am making a device which is powered from a 12V, 1A mains adaptor. The device has a high voltage DC module in it which outputs about 20kV. The way it is used, the high voltage is discharged into the air, and therefore doesn't directly return to the DC GND.

This causes a problem where a typical isolated mains adaptor will become oppositely charged until I can hear little discharges occurring inside it. My workaround for this is just earthing the DC GND rail via an ESD plug (internal 1M resistor). This works fine except it for the inconvenience of having two separate plugs.

Does anyone know of a PSU with this integrated, or a mains adaptor that could be disassembled so I can make the modification internally? (Most seems to be glued together or have a plastic earth pin)

 

[alec_t]

You could use the ground connection available in a multi-socket adaptor and just plug the 12V adaptor into one of the sockets.

 

[summitville]

I do not think it is legal to use the Outlet's Ground Wire as one of the Current Carrying Conductors of your High Voltage circuit.

 

[ronsimpson]

Ground Wire as one of the Current Carrying Conductors

On the power line side there is (line, line, ground) or (line, neutral, ground).
On the isolated side there is 12-V and 12-return. The 12-V gets turned into 20kv at 300uA. (assuming 50% efficiency) Probably less current.
The 300uA for the high voltage is returning to the 12-return. (12V ground) I do not see it running through the green wire.
Because the 12 volt supply is isolated it can float off to some voltage. Probably a 1 meg resistor to green wire ground will keep it some where near 0 volts. I think only a couple of u amps will keep it near ground.

Years ago I talked with CSA, UL, and VDE about running current on green wire. "on current" So I asked about 1uA. They said that 1uA=0. So I asked about 10uA and they said 10uA=0. Then they said 100uA=0A. After much talking they decided that "no current" is a current level so low that you can not get shocked by it. I have battled this many times and often the "0 current" is 5mA or 2mA, it seems to change every time.

Bottom line is that the high voltage current is not flowing in the green wire. The current wants to flow from 12kv to 12-return.

 

[riccardo]

OK, yest it WANTS to flow back to 12V return, however the nature of electrostatic fields means this may not happen.
BTW, its UK mains sockets.

Lets say I point this HV output towards a metallic object, or just up into the air while its creating a corona discharge. Ions will fly off and eventually ground themselves on something nearby. Now the place they find ground, may have no return path to the 12V return of the PSU. Therefore as this process continues, the 12V supply gets increasingly charged to an opposite voltage to the HV output up to the point something will breakdown and it will forcibly find a return path to earth. I can even hear this happening inside the adaptor as little sparks must be tracking over something, presumably reaching the mains neutral wire.

I want to connect the 12V return rail (which is also HV return) to earth via a 1M resistor as it allows these 'lost' charges to trickle back up to the supply and keep all the low voltage electronics at ground level. It's not really being used to carry current as such, but is to prevent it from becoming electrostatically charged.

 

[Nigel Goodwin]

You don't need an earth - you just need a discharge path to 'earth'.

As you presumably know, there are Class I and Class II devices, with Class I been earthed, and Class II not.

If your supply was Class I you have an earth pin, and could connect that to earth your output.

For Class II there is no earth (and an earth isn't allowed - as it would fail testing) - but your situation applies to pretty well every TV and set-top box available. The solution is simple and used in most products, you have suitable rated components (high voltage resistors with a capacitor in parallel) across the primary/secondary barrier. These discharge any static build up on the secondary via the primary to the live/neutral of the mains.

However, the cheap and simple plug type PSU's may not include these components, or may only include capacitors which won't discharge static well without the resistor.

Can you build a mains PSU in your unit, rather than using an external plug?.

 

[JimB]

Try this:

Farnell part number 2524402
£15.08 each.
Sadly they are on back order at the moment.

Look at an extract from the data sheet:

JimB

 

[riccardo]

Thank you.

I think the VEC40 PSU isn't suitable, negative is to ground, not earth. it has 1500V isolation between input (where earth is) and output (where ground is). Please correct me if I am misinterpreting that.

The resistor path to neutral sounds like what I need. I had a look for Class I 12 V PSU and found this one. However it seems to show insulation resistance from input or output to frame ground is 100M. It also states "-V not connected to AC FG".

It this 100M suitable for the discharge path since the current is tiny?

Now I suppose I could disassemble this and add the resistor path to neutral, but this seems dangerous since an improperly wired outlet could then potentially put live mains onto the device earth via the resistor). Connecting a resistor internally to the earth pin might seem to be a suitable way to do it. What do you think?

 

[Nigel Goodwin]

Thank you.

I think the VEC40 PSU isn't suitable, negative is to ground, not earth. it has 1500V isolation between input (where earth is) and output (where ground is). Please correct me if I am misinterpreting that.

I rather suspect you are - they wouldn't mention it unless it was connected to earth/ground. The input ground isolation is because you don't connect live or neutral to earth/ground/chassis inside the unit. When it's plugged in to the mains the neutral is obviously connected to earth at the sub-station.

Here's the primary side of a PSU from an old analogue satellite receiver, the components we're discussing are at the bottom, R40 and C21, it's a classic class II design for static discharge.

 

[riccardo]

Thank you. RS has stock so I will order and give that a try!