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Old 4th January 2006, 09:11 AM   #1
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Default Do I need to isolate the mains for switching power supply and traditional transformer

Do I need to isolate the mains for switching power supply and traditional transformer?

I am building a power supply unit for a circuit that needs both analog and traditional power supply for an audio system. There are two switching power supply units, one transformer and three power regulating units that are going to be put in one aluminum case. There is an additional iron box to house the two switching power supply units and seal them from other components. The aluminum case will have one IEC socket and from there, leads to switching power supply units and a traditional transformer. My questions: -

Since switching power supplies and transformer and drawing current from one IEC socket source, do I need to do some sort of isolation between this two type of current? How?

Thank You

PS - I am also posting this in the Power Supply Design forum.
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Old 4th January 2006, 09:18 AM   #2
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Old 4th January 2006, 09:19 AM   #3
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I'm not sure what you mean about 'analogue' and 'traditional' power supplies. Basically, you can connect a switching PSU and a non-switching (normal transformer & cap) PSU to the same IEC socket without issue.
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Old 4th January 2006, 10:15 AM   #4
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Oh error in my first sentence. It should read: -

I am building a power supply unit for a circuit that needs both SWITCHING and traditional power supply for an audio system.

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Old 4th January 2006, 10:32 AM   #5
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Quote:
Originally posted by Aaron323
Oh error in my first sentence. It should read: -

I am building a power supply unit for a circuit that needs both SWITCHING and traditional power supply for an audio system.


Think of it this way. Suppose you decide to run the supplies from different IEC sockets. In the end, both will be supplied from the same mains run anyway. So, you may as well directly take them from the same socket .

Another issue (possibly that is what you mean) is that switching power supplies sometimes generate hf noise that propagates back to the mains, and that can possibly come back to your 'traditional' (a better name would be 'linear') power supply.
A good switcher should have little of this, and the design of the linear supply should take care of any mains noise anyway.

If both supplies are in one box keep then apart if possible to avoid field coupling.

If you linear supply is to power low power circuitry, take a standard EI transformer, NOT a toroid, because toroids are so wideband that they readily transform mains noise to the secondary as well.

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Old 4th January 2006, 02:41 PM   #6
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I'm just curious: Why 50Hz and linear supplies are being employed when you have SMPS capabilities? It would be cheaper, smaller and lighter weight to extend the SMPS to handle the additional outputs.
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Old 5th January 2006, 08:07 AM   #7
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I think I have not make my discription clear.

If there is adverse effect by connecting the mains of two supply types together, I'll plug the two switching power supply directly to the wall mains and the transformer power supply will be plugged to an isolation transformer before connecting to the wall mains to avoid noises. In this respect, I will need to have two IEC sockets.

The modification is to a preamp that I am using two DACT CT-102 as supply for the L and R channel. There is also a digital circuit in the preamp to control the overall gain that I am using a traditional 5V power supply. DACT CT-102 is using a switching power supply to convert main voltage to 48VDC before going through the regulation circuit therefore I have this question in mind. DACT CT-102 is not able to provide an isolate 5V for the digital circuit and that's why I am using a separate source.

Hope this clarifies.
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Old 5th January 2006, 08:07 AM   #8
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Offline - I am not arroncake.net man.

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Old 5th January 2006, 10:46 AM   #9
Eva is offline Eva  Spain
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But any offline SMPS should provide isolation, and the 50Hz PSU also provides isolation. There should be no problem, no additional isolation transformers are required. Also, at the power levels required by a DAC, EMI is of little concern if the SMPS is designed properly, altough you may consider placing additional common mode filter cores at the outputs of all PSUs to improve decoupling at HF. Anyway, if there were mains conducted EMI issues, a standard isolation transformer wouldn't solve it.
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