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Old 9th January 2009, 07:51 PM   #1
mr.duck is offline mr.duck  United Kingdom
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Default Building a class II PSU (double insulated, no safety earth)

After some googling I came across an interesting thread in another forum. Someone asks if the following transformer is suitable for making a class II PSU... http://www.rapidonline.com/productin...moduleno=72692 And the answer was YES. Here is the original thread... http://www.electro-tech-online.com/c...er-safety.html

So I think it is possible to make a DIY class II PSU. My plan would be to use that transformer in a plastic enclosure like a regular AC/AC converter. If I fuse both secondaries then that would stop the transformer from being overloaded and going into meltdown. Then any single failure such as complete failure of primary insulation would not pose any immediate danger. Just like a real class II device. The only potential problem I could think of was if metal screws were used to mount the transformer and connectors to the case then that might be unsafe if they conduct mains electricity to outside of the PSU.


Of course it won't be a real class II PSU because it won't be tested/certified as class II. But will it be safe? (assuming I can do it right)
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Old 10th January 2009, 09:14 AM   #2
MondyT is offline MondyT  United Kingdom
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Hi Mr.Duck!

The transformer is fine although the standards quoted are a practically obsolete. BS3535 and EN60742 have been largely replaced by harmonized standard EN61558 and it’s many relevant parts. Grim has it nailed in your other thread.

Double insulated equipment cases are usually glued or if screws are used, they will be of a type that cannot be undone with a normal screw driver (Torx etc). If the screws heads are exposed on the outside of the case they will be in channels or tubes that cannot be accessed by the “Standard test finger” and in places so that all the relevant creepage and clearance distances are assured on the inside of the case.

Fusing the txfrmr secondary windings is wise as you discussed, but you should not forgo primary fusing as secondary fuses will not help should the transformer primary turn to turn insulation breakdown, a condition that is not un-common in very small transformers. The majority of commercial Class II equipment use transformers with inbuilt thermal fuses in the primary winding. Any fuses in plugs (UK) should not be relied on here

If a small PSU is required then cases like this would help...

http://uk.farnell.com/multicomp/ps2f...lack/dp/645771

These are designed for the job (glued together for example) and could possibly have moulded mounts for a standard size small EI transformer. All the relevant creepage and clearances will still be down to you however.

If you are designing a PSU for your own use only (not for use by the general public or children for example), my advice would be to leave Class II construction to the commercial world and concentrate on a safe Class I construction with a PE facility. You will still need double insulated transformers. This type of power supply is generally better for audio use anyway as electrical noise etc is more easily controlled. Even with this construction, you must consider all the relevant protection in the form of fusing etc and ensure that your earth connections can handle at least 2 times the design current of the PSU and the resistance does not exceed 0.1 Ohm with all metal work bonded securely to this PE.

Cheers
Ray
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Old 10th January 2009, 10:03 AM   #3
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by MondyT
and ensure that your earth connections can handle at least 2 times the design current of the PSU and the resistance does not exceed 0.1 Ohm with all metal work bonded securely to this PE.
I don't think this advice is onerous enough.

Let's set the supply impedance to 0r3.
Now calculate the voltage on the case if the supply half of the mains circuit has 2/3 of the impedance i.e. 0r2.
Now add up the return (Protective Earth) and the 0r1 of the internal circuitry.
The voltage on the exposed conductive parts will be 240 * 0.2 / [0.2+0.1+0.1]=120Vac. I don't want to touch that waiting for the fuse to blow.

Does the regulation require that the maximum fault voltage (on the exposed conductive parts) is limited to 50Vac?

BTW,
using those 0r1 and 0r3 values gives a peak fault current of >800A.
The fuse is going to blow pretty quickly but will the earth wire rated to twice the design current survive long enough?
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Old 10th January 2009, 12:49 PM   #4
MondyT is offline MondyT  United Kingdom
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Hi Andy

I assumed the PSU to be a portable device or one with a plug and mains lead on it (I am not sure why!)

Here is the part of EN61558-1 dealing with protective earth requirements of a Class I device...

QUOTE

24.4 The connection between the protective earth terminal and parts required to be connected
thereto shall be of low resistance.

Compliance is checked by the following test.

A current derived from an a.c. source, having a no-load voltage not exceeding 12 V and equal to 1.5 times the rated input current or to 25 A, whichever is greater, is passed for 1 min between the protective earth terminal and each of the accessible conductive parts in turn.

NOTE 1 Rated input current is determined as the quotient of the rated output by the rated supply voltage, or for polyphase transformers, by n times the rated supply voltage, n being the number of phases.

The voltage drop between the protective earth terminal and the accessible conductive part is measured and the resistance calculated from the current and this voltage drop.

In no case shall the resistance exceed 0.1R

In case of doubt, after 1 min, the test shall be conducted until steady state conditions are established.

NOTE 2 Care is taken that the contact resistance between the tip of the measuring probe and the conductive part
under test does not affect the test results.

NOTE 3 The resistance of the supply flexible cable or cord, if used for convenience in the test, is not included in the resistance measurement.

NOTE 4 The cores of IP00 transformers are considered to be not accessible.

END of QUOTE

Portable Appliance Tests for Earth bonding are based on this standard. Granted, EN61558 is a standard dealing with transformers alone, but most specific standards related to samll power supplies can be distilled back to EN61558

Cheers
Ray
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Old 10th January 2009, 02:27 PM   #5
mr.duck is offline mr.duck  United Kingdom
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Yes I knew about the thermal fuses in class II devices. But was unable to find any suitable transformer so I gave up on the idea until I found that thread.

Yes that plastic case is exactly what I had in mind. But any plasic case should be ok if I can glue it together securly? How about a case that fits together like this one... http://www.rapidonline.com/Tools-Fas...nt-cases/77366? You can see the screws will not be exposed on the inside of the case because of the plastic channels / tubes that they thread in to.

Ok I will make sure to fuse the primary as well to be on the safe side.

Also I could not bother with input and output connectors. Instead have flying leads along with somthing like this... http://www.rapidonline.com/Cables-Co...f-bushes/62449 or one of these... http://www.rapidonline.com/Cables-Co...p-bushes/62451.

I spent some time searching for ready made class II AC/AC converters. But the best I could fine was only 12V 3A type. I would idealy need 2x 18-25V and at 2A+ for each secondary. Being able to make my own would be ideal.
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Old 10th January 2009, 02:55 PM   #6
Did it Himself
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How about a different approach. Use an AC-DC plugtop converter which is class-II to start with, the modern switch mode ones are very compact. Then simply put additional filtering and regulation inside your equipment case.
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Old 10th January 2009, 06:02 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by MondyT
.... the following test.

A current derived from an a.c. source, having a no-load voltage not exceeding 12 V and equal to 1.5 times the rated input current or to 25 A, whichever is greater, is passed for 1 min between the protective earth terminal and each of the accessible conductive parts in turn.

NOTE 1 Rated input current is determined as the quotient of the rated output by the rated supply voltage,
Please explain what the value of the rated current is.

If the appliance is supplied from a BS1362 socket and the plug top is fitted with a 13A fuse.
Is the current referred in the test calculation 13A?
or if the mains transformer is rated at 500VA
Is the current referred to in the test calculation 2.08A?
Since the appliance is operating on the UK mains system the rated supply voltage is 240Vac.
What is the quotient? It appears to be either 500AmpsVolts or 3120AmpsVolts.
Now apply the 1.5 factor and the test current (from a <=12V emf source) is either 750A or 4680A.
Passing this through a <=0r1 will give a voltage drop. But it is obvious to me that the actual resistance that is being measured during this test is far lower than 0r1.
What have I done wrong with my interpretation?
I am no expert in this regulations thing, but I seem to recall that there was/is a 50V limit. Is that in 16th Edition?
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Old 12th January 2009, 08:07 AM   #8
MondyT is offline MondyT  United Kingdom
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Hi Andy

The rated current will be rated current of device. For a 500VA device, you correctly stated the rated current as 500/240 = 2.08A and so you would test the earth connection at *at least* 3.13A

Cheers
Ray
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Old 12th January 2009, 08:47 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by MondyT
Hi Andy

The rated current will be rated current of device. For a 500VA device, you correctly stated the rated current as 500/240 = 2.08A and so you would test the earth connection at *at least* 3.13A

Cheers
Ray
what happened to the quotient? This brings the test current up to 750A.
Now pass that through the earth circuit resistance from a <12V emf.
By necessity the earth circuit resistance must be <0r16 total for both the [supply + the amp earth circuit].
This will result in something less than 0r014 for the test resistance.
It is likely to be much less giving test voltage readings well under 10V.
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Old 12th January 2009, 10:13 AM   #10
MondyT is offline MondyT  United Kingdom
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Hi Andy

I am not sure what you mean here?

"the quotient of the rated output by the rated supply voltage" just means the rated output (or rated power) *divided* by the rated voltage.

A quotient is a result obtained by dividing one quantity by another

Cheers
Ray
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