iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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Leakage current of an iMac power supply

A switch mode power supply (by design) leaks current to the ground. The typical PC/iMac leaks about 3.5mA.

I have a an iMac 27" (late 2009 model) that leaks to the ground 5mA (I measured this), this is very high and would suggest faulty capacitors or another problem in the power supply.

My question is: should I try to replace the power supply?

An additional problem is that I have an office that was built in a residential apartment with a differential line breaker of 30mA (by regulation this is the rating in the EU and this cannot be changed). Since we have 7 iMacs in the office, the iMac 27" is making the differential line breaker trip. I tried to connect the iMac 27" to a plug without grounding and the result is an extremely unpleasant feeling of electricity when you touch the case.

Any idea on how can i solve this?

Answered! View the answer I have this problem too

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Hi Dan,

Thank you for your answer. I would like to understand better. Normally you would implement the soft switch with several power FET and optocouplers. What do you mean exactly by "load of the soft switch".


I think you'll need to review Apple's power system design. Apple does not use optical isolation in the design.


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The current trickle you are seeing is not the switch mode power supply directly, but the load of the soft switch Apple uses in their iMac's. While a different power supply might be lower I don't think this is outside of Apples specs.

Here in the states we have surge suppressors that have a sense outlet (Master) so the vampire leakage of the the transformers don't waste power APC Surge Protection with master switching.

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One solution for grounding is to separate the power line for your computers and run them isolated from rest of the house. Now that you mentioned, i have imac retinas in my office and i think as I am building a new house, i will run my office wires separately, thank you for sharing the issue.

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@Rao Sahab - The word 'Ground' has many meanings here. Please don't misinterpret the meaning in use here.

I would strongly recommend you follow proper building codes in your country. Typically, each building can have unique power services. It's not recommend having separate power service (different breaker/fuse panels) between rooms or outlets within a room as you can create a more harmful condition! Which is what Luc was having an issue with.

We use the term 'Star Wired' were all of the lines going to each outlet are tied to a common point at the breaker/fuse panel. The common ground line from the panel is then tied to the grounding of the building as well as the power lines ground.

Now what I do recommend is make sure the wiring within the building is good shape using a line tester as well as make sure the buildings grounding is in good shape as over time it can degrade. Lastly, get surge suppressors and/or UPS units for your computer and the other devices it uses (printer, cable modem, etc...). You may want to ask your power provider if they offer a line monitoring service to look for dropouts and surges over a good 48 hrs or more. If they do I would recommend you borrow one so you can get an idea how good or bad your power is.


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Luca Vicini will be eternally grateful.
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