Ohm's Law Question: AC vs DC?

[Wharfcreek]

I'm looking at an HV winding on a power transformer that's putting out roughly 400VAC. The PT is rated for about 65ma on the HV winding. I'd like to load it with a resistor at about 40 ma just to do some testing. If I do an 'Ohm's Law' calculation using voltage and current to determine resistor value, it occurred to me that Ohm's Law may be based on DC, and I had AC in the transformer outputs. SO....... I'm just curious if there's any kind of formula that's applicable to using Ohm's Law when dealing with AC voltages? WC

 

[PakProtector]

If the 400VAC is an RMS measurement( with sine wave peaks root2 times this ), then Ohm's Law is appropriate for use on that basis. V_rms=i_rms*R. Since the R is constant, the i will of course be varying...because the voltage is as well. this is The *SHORT* story. Some googling will tell you much...
cheers,
Douglas

 

[Decibel_116]

What are you trying to test if I may ask? You would need a fairly high wattage resistor to connect directly across 400 VAC. Not sure what this accomplishes other than creating heat.

 

[BinaryMike]

Be careful when estimating actual transformer loading. If the original rating is for DC output current, rather than AC winding current, then rectifier structure should have been specified. If it wasn't, and you have a center-tapped winding, then it's reasonably safe to assume FWCT with capacitor input filter. See this doc from Hammond. Stancor specifies more conservative numbers for similar cases, e.g. 20% greater secondary current for the FWCT with cap input case. PSUD software can help you dial this in.

 

[PakProtector]

And as Mike pointed out, the filter matters. An L-C filter will get the highest current delivery, albeit at a lower voltage( about .9x the RMS input ). Same amount of power, but since it is at a lower voltage, and the current is delivered relatively continuously, the amperage is higher.
cheers,
Douglas

 

[Retrovert]

To augment with a few additional advantages to what Douglas pointed out, an LC filter has a wider conduction angle, so (1) the load on the transformer and rectifier are constant, which reduces transformer heating, lowers rectifier stress, substantially reduces both the capacitor stress (slower charging rate over longer time) and the Ohmic heating (goes as the square of the current), and does not as readily stimulate transformer ringing via the square-wave current draw.

All good things.

With modern higher mains voltage the voltage reduction from LC will likely will not matter for most applications.

 

[Tom Bavis]

Center-tapped windings are usually rated for the DC current after a rectifier and cap-input filter. This value is pretty close to the RMS current in the winding (so with a 65 mA DC load, there's about 65 mA in each half of the winding). With a bridge rectifer, RMS current in winding will be about 1.5 times the DC current, with a voltage doubler, about 3 times the DC current. (All are cap input) You should probably simulate or test if you want to run near the full rating.

If a transformer is rated for choke-input use, it'll be marked as such (or the catalog rating will show it). Usually these will be higher voltage and current ratings.

 

[kward]

The very definition of RMS is that it causes the same effective level of heat dissipation across a fixed resistive load as does its equivalent DC counterpart. Thus for example 100 VAC RMS causes the same dissipation through a load that consists of only a fixed resistor as will 100 VDC. So ohms law absolutely applies in both cases under these conditions.

It gets more involved under phase lead/lag situations between voltage and current such as exist with reactive loads. You could google "power factor" if you wanted to learn more about that.