Keeping DC away from AC?

[alphadog]

The AC signals in my PCB are 50HZ, and can reach up to 16A.
The rest of the AC signals are uA.

I also have a few DC circuits in my PCB, one coduct 0.2A, and the rest conduct low current (uA - mA).

Today an engineer has told me that it is important to separate between the DC signals and the AC signals.

I know that its important to keep any signal (AC or DC) from high-frequency signal, to prevent coupling capacitance, but is it really necessary to separate between low frequency AC signals (50-60Hz) and DC signals?

Thanks.

 

[Ubergeek63]

usually not

 

[alphadog]

Thanks,
Could you give me an example please for when it is?

 

[RadioRon]

To be precise, we don't refer to DC as a signal since the word signal implies information content.

DC on circuit boards can suffer from AC coupling, but there are rules-of-thumb that every experienced designer uses to add filtering to the DC wires so that any AC that is accidentally coupled into the DC is automatically filtered away. The most common kind of filter to do this is the "bypass capacitor".

The amount of separation that you need between AC and DC depends on the amount of current in the AC. It is very important that you said the AC can be many amps (like 16) as this is a relatively high amount compared to most hobbyist's experience. The coupling between the copper traces is mostly inductive at these frequencies and inductive coupling is a function of current flow. The closer the traces are, the more inductive coupling. The more that one trace follows alongside another, in parallel, the more is the inductive coupling. If one trace crosses another at right angles, the inductive coupling is very low, so there is little risk compared to when the traces are side by side for some distance.

 

[Hero999]

It depends on what voltage AC and DC you're talking about and whether the separation is required for functional or safety purposes.

If the DC is SELV (Separated Extra Low Voltage, a DC supply <120VDC and separated from the mains supply) and the AC voltage is not SELV (for AC this is above 50V) i.e. mains then you need a clearance of at least 10mm is normally required to satisfy the safety requirements for class 2 insulation.

The 10mm clearance is required because we can expect 2kV spikes on the mains and a large distance is required to prevent an arc tracking across the PCB.

Another way to provide protection is to run a track connected to PE (Protective Earth) between the mains and SELV side. This will divery the high voltage pulse to earth before it reaches the DC side. Of course the appliance is no longer considered to be class 2, it's now class 3 as it needs an earth wire.

If the DC voltage is not SELV or the AC voltage is SELV then separation isn't too critical, 1mm is normally enough to provide adequate separation.

 

[Sceadwian]

Could the engineer have been talking about simple mains isolation? For example decoupling the diode bridge of an AC rectifier so you don't have a DC power supply coupled to the mains? I think the original poster needs to be a little more clear cause that kind of mistake can kill technicians that think they're working on safe DC voltages.