[Zumba]

I need to make a DC power supply for a 120v electromagnetic chuck. The chuck draws a peak current of 0.87A.

How important is the smoothing capacitor for such an application? What would the max V ripple be with no capacitor, and how would that effect the chuck? I've got a couple 30A, 250v bridge rectifiers in my drawer and am hoping I can just use one them without having to buy a cap and wait for it to ship to my door. Thanks.

[Al_The_Man]

Many DC clutches, brakes and chucks are operated without smoothing.
The ripple is 100%, but 120Hz.
Due to inductance, There is a natural delay on drop-out so you should not see any visible effect by using unfiltered DC.
Al.

[gar]

Zumba:

The current is what determines the chuck clamping force. I suspect that you have a fairly large inductance. My guess is that dropout time may be 100 MS or longer based on an electromagnetic clutch I have used.

Full wave rectified 60 Hz is 8.3 MS/half cycle. Thus, as an off hand guess I suspect you may have less than 10% ripple in the current.

I believe most standard Fluke digital meters in both current and voltage modes filter out the DC component in AC position. Note: these meters will not read the peak-to-peak ripple, but an approximation of the RMS value. Maybe multiply the RMS value by 4 for an over estimate of peak-to-peak ripple. If it were a sine wave, which it is not, then the peak is 1.414 ( 1/0.707 ) times RMS, and peak to peak is double that. So my 4 times is probably quite an over estimate.

First, put a Fluke in DC current mode and check that you have the expected DC average current. Then just switch to AC position to check AC ripple. Probably turn your current off before switching ranges.

The DC removal I have verified on my Fluke model 27 in current mode. A Simpson 260 or 270 does not remove the DC component in AC position. You have to use the Output jack, but I do not think this works for current.

The average value of a full wave rectified sine wave is 0.636 of the peak while the RMS value is 0.707 of peak. For 120 input to a bridge rectifier the average DC output voltage is 0.636 of 120/0.707 = 107.95 V. Thus, 108/coil resistance = expected DC average current in Amperes. The RMS/AVE ratio is 1.112 .

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