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How to measure DC power supply earth leakage


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Measure what!?! I hear you exclaim!

Well sit down and let me tell you a story..

I was testing my QHY camera away from my usual power supply and used a standard 12v adapter to power it, on it comes, filter wheel whirring.

I accidentally brushed my face past one of the screws which attaches the camera to the imaging train and ZAP!! a mild electric shock! I'm thinking oh no... camera's busted... but it worked fine.

So after a bit of searching, it turns out that some power adapters will leak AC into the ground (earth). To see if this is happening to yours without getting a shock (although it's only a shock if you're not expecting it).

You will need a multimeter and well yourself.

Switch you multimeter to 20v AC and place the ground wire on the outside of the round 5.5mm pin and then the red against any conductive object (I used my thumb which worked fine... although I would recommend against it.) like a radiator pipe (these are grounded).

If there is current leak you will see the voltage jump up on the display. Unfortunately  there's not much you can do about it once you have identified a leak but buy a new power supply that doesn't.

That's all for today folks!

Wyvern

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Interesting! I had just that experience last week when I touched the tripod leg of my HEQ5-Pro. A bit like when you lick the contacts of a PP3 to see if it's dead (does everyone not do that??).

Should we be earthing our mounts? Is it dangerous? I assume not, if it is at 12V/13.8V.

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@padraic-m It's actually quite a lot more than that, because it's AC leakage (I assume from the transformer). It was topping out at about 60v AC although the currents involved are absolutely miniscule, probably single figure milliamps or less.

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Posted by: @padraic-m

Interesting! I had just that experience last week when I touched the tripod leg of my HEQ5-Pro. A bit like when you lick the contacts of a PP3 to see if it's dead (does everyone not do that??).

Should we be earthing our mounts? Is it dangerous? I assume not, if it is at 12V/13.8V.

If the PSU specs label shows two squares, (i.e. one inside the other), it means it is double-insulated, so you should be OK.

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If you have a power supply that has a 3 pin mains plug, there may be a deliberate connection inside the supply from output negative to mains earth.

It is also possible for there to be a deliberate earth screen built into the transformer to stop strange effects due to leakages in the transformer (a big subject).

With a supply that has a 2 pin mains plug, you have to rely on there being minimal leakage through the supply.

While the tingles won't harm you, sensitive electronics is another matter. A good policy (in my opinion anyway) is to earth the mount and measure the earthing situation in any mains power supplies. That way you won't get any tingles from an electrically 'floating' mount, camera, etc.

Just to add a bit more. Some single board computers (used as clever mount controls) have the supply negative connected to their metal box, or (USB etc) cable braids. Then laptops and conventional monitors can have supply negaqtive connected to earth/metal parts.

It gets quite complicated. While individual components meet safety requirements and good construction practice, when they are all put together there can be unexpected results.

On the other side of the coin. If you shuffle on a dry floor with insulating footwear, you give the handset, camera, etc a big shock. Static discharges like this can cause a lot of expensive damage. Dry nights in the UK are rare. however, if you trialling and assembling indoors.....

HTH, David.

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I have my observatory equipment earthed with a 4ft copper earth spike in the ground outside the building in damp ground not far from a minor water course.  A 10sqmm household rated earth cable connects first to the mount and other local equipment then onto the power distribution system in the warm room.  This takes an isolated PSU (double insulated) of 13.8v (Maplin) connecting to a 45AH car battery as backup and the main low voltage power distribution box containing V/A digital meter on the input and Ammeters on each output with switches and fuses in the +ve rails.  The -ve side is connected to the big earth cable. 

The mains power that comes in from the house goes into a garage type consumer unit with leakage trips and power and lighting overcurrent trips.  Lighting and power to the sockets is wired from this unit.

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  • 1 year later...

Ideally all your circuits should be connected before power is applied, this should reduce glitching caused by floating - ve.

 

 

One gotcha to be aware of. A lot of the mosfet pwm circuts on amazon/ebay switch the - ve rail. If you use them for controlling a dew heater don't use a metal box to mount the sockets. It won't damage anything but your heater strips will always be at full power. 

 

I used to get a tingle through my nose when trying to look through the viewfinder of my old 350d DSLR powered by an ac adapter. The ac adapter was cheap but fine. On a plastic  bodied camera that means the damp is allowing current to flow. Very unerving in the dark. I temporarily earthed my mount and the problem went away. A proper earth strap should be used to bond the negative to ground, ideally at the power supply. (You shouldn't rely on a chain of dc connectors to provide your earth path.) 

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