Nuts and Volts - May 2014

(at 120 Hz) that brake control is either none or a lot.

The challenges are that the kickback is positive, it needs to be kept under 200V, and there is a lot of energy behind the kick from this motor. I ended up with 100 µF in the first place because a large ( nonelectrolytic) 10 µF capacitor did very little to the splash, which almost immediately would go beyond the 250V rating of the oscilloscope. In my parts bin are some large diodes such as DTV32, STPR1020CT ( 2), STPR2045CT ( 2), TYN058, and others, if that helps. I've read a little about snubber circuits, but would like some technical advice.

#1 If I assume that RLY1 is not connected to short out the power supply and is shown de-energized, when RLY1 is energized, the motor connections will be reversed and the motor (acting as a generator) will try to drive the positive rail of the supply more positive. C4 will soak up some of the reverse current and when Q1 is turned on, it will limit the voltage to about 12 volts (1 ohm x 12 amps). Input to the power supply has to be turned off at this time.

There are two problems: During the transition time of RLY1 when the motor is not connected to anything, the voltage will shoot to the moon, possibly damaging the motor, and the delay time in PCB17 may not turn on Q1 in time to limit the voltage below 200 volts. One solution is to connect a 12 amp diode and series resistor across the motor to limit the voltage to 200 volts (R = 200V/12A = 16 ohms). A 75 watt light bulb would no doubt work as the resistor.

Russell Kincaid Milford, NH

#2 Quickly braking an electric motor requires a circuit that will quickly dissipate quite a bit of energy. Texas

Instruments has produced a paper, "The Art of Stopping a Motor," available at http://e2e.ti.com/blogs_/ b/motordrivecontrol/archive/2013/ 10/18/the-art-of-stopping-a-motor. aspx. TI also has motor control dev kits that offer braking circuits and techniques that might help in Tsidqah's circuit. The kit documentation usually includes code and circuits. The Microchip Technology application note, AN-905, "Brushed DC Motor Fundamentals," offers a way to provide a shunt to ground that brakes a motor: http://ww1.microchip.com/down loads/en/AppNotes/00905B.pdf. I suggest the designer replace the 4PDT relay with a MOSFET H-bridge circuit that offers more flexibility for various braking applications. Also, real time voltage and current measurements taken during braking with a load resistance should help determine the characteristics needed in a braking load and switching circuits. As for a testing load, consider a non-electronic clothes iron or a toaster oven.

Jon Titus Herriman, UT

#3 I'm not sure what you are doing with the 4PDT relay as shown, so maybe it is a drawing error. The way it is drawn, the motor feed and motor are both shorted! For motor reversal, you only need a 2PDT relay. If you are paralleling contacts for increased current, that's a bad idea since there is no guarantee the contacts will open or close in unison. So, in effect, one contact will still carry the load. Paralleling for redundency is also not good because if a contact welds itself, bad things can happen when the parallel contact switches.

I assume — since there is no logic flow provided — that the TRIG signal goes low before the EBRK signal goes high (and what about some protection for the input to the opto-isolator U6?). Likely, the spike is exceeding the LED PRV rating. I would think clamping diode(s) would help with the CEMF and perhaps an R/C, as well. Hope this helps!

Len Powell Finksburg, MD

[#3142 - March 2014] Capacitor Forming

I pulled some excellent quality electrolytic capacitors from the power supply of an amplifier. The capacitors are rated at 40 VDC but were used in a 10V circuit. I wanted to use the capacitors in a 24 VDC circuit, but I was told that the capacitors "formed" at 10V and wouldn't work at 24 VDC, regardless of the original rating. Is this true?

#1 True, aluminum capacitors will deform with years of operation at reduced voltage (or no voltage). The typical aluminum capacitor will have a leakage current that increases with capacitance and voltage. A 40 µF 40V cap would have leakage in the order of 2 µA.

To test your capacitor, connect it through a 10K resistor to 24 volts (or 40 volts if it is available); if the voltage rises to 24 volts or more, it is okay. If not, leave it connected until the voltage does rise above 24 volts. You can use a smaller resistor to speed up the process, but don't let the cap get hotter than 50 degrees C.

Russell Kincaid Milford, NH

#2 Short answer... maybe.

You can often reform electrolytic capacitors by slowly ramping the voltage up — or above — your expected voltage. Keep a current meter in series and measure the charge current (after you have reached your top voltage). It should be in the microamp to low milliamp range for a good capacitor. If it's higher than a milliamp or two, you have a cap that