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To be precise, standard SMPTE 40 specifies: "The recordings shall be made so that the audio records will reproduce properly at 96 perforations (18 in [457mm]) per second, which is 24 frames per second".

Most people use 90 feet per minute or 18 inches per second as the nominal transport speed of 35mm prints during projection. But if the distance between perforations is exactly the aim of 0.1870 inches, the actual transport speed for 24 fps is 89.76 feet per minute or 17.952 inches per second.

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Is there any risk of damaging the capacitor by discharging it through a direct short? Perhaps it would be preferable to discharge it through a large power resistor, or even through a 100-watt tungsten lamp, to more slowly dissipate the stored energy?

Nonetheless, that capacitor could pack a mighty "wallop" if the current goes through your body!

Large capacitors are usually cylindrical, like a soda can. They'll be well marked with values in microfarads and volts, such as "20,000 uF 25 V." In a typical power supply, the power transformer is connected to the diodes, which are in turn connected to the capacitors.

The correct way to discharge a capacitor is by using a resistor, NOT by shorting across the capacitor. When you short across the capacitor, not only will you create a spark, but you run a good chance of damaging the capacitor or another component. You can actually melt the tip of a screwdriver by using it to (intentionally or accidently) short out a large capacitor.

Safely discharging a capacitor through a resistor is easy and convenient. I use a 10K ohm 2 watt resistor with alligator clips soldered to each lead. I clip the resistor across the leads of my voltmeter, then use the voltmeter, set to measure DC voltage, to measure the voltage across any large capacitors in the circuit. When I encounter an undischarged capacitor, the voltage on the voltmeter shoots up, then quickly drops off to under one volt as the capacitor discharges.

In a power supply circuit with multiple capacitors, the smoothing capacitors usually wired together, so discharging one will usually discharge them all.

NOTE: Be especially careful when servicing tube circuits, as they utilize MUCH higher voltages than transistor circuits, and are much more dangerous. In a typical tube power supply, the power transformer will be directly connected to one tube, which will in turn be connected to the capacitor(s). When checking a capacitor in a tube power supply, use the voltmeter alone to check the voltage first. If the voltage is over 100 volts, use a 100K ohm capacitor instead of a 10K ohm capacitor.

Power supplies are supposed to contain a protective component called a "bleeder resistor" that essentially acts as a small power leak, so that when you turn the equipment off, any stored energy is bled off over the course of a minute or two. However, if the bleeder resistor fails, or is not present, the power supply can still contain dangerous voltages even after being powered down for weeks. This is especially true of tube circuits.

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The most common cause is a hole burned right through a shutter blade by someone opening the dowser and changeover with the motor off. This can happen if a motor, belt, or gear fails and there is no film motion failsafe. It should be rather easy to see a holed shutter though...
Anyway, good luck

Standard safety equipment for discharging capacitors in the broadcasting industry is with a grounding probe suppled and physically attached to the transmitter main frame. (so it does not get lost) It is nothing more than a grounded metal rod with a wooden handle. This probe has to be hung up on a special interlock switch so the switch closes after the probe has been used to discharge capacitors.

When the doors and panels are replaced after servicing, all these switches on the doors, panels, HV probes, etc. will complete the safety interlock so the transmitter can be fired up.

The discharge probes are similar to what we used with some RADAR sets I serviced while in the navy.

Screwdrivers or alligator clips were used for the boob tubes you have sittin' in your front rooms that you use to watch the cheap VHS movies you rented from Blocksucker Video.

In solid state transmitters, many of the capacitors are even larger in electrical value than what you find in the xenon power supply. The discharge probes do not have any series resistance in them whatsoever. They were just a "dead short".

Oh, by the way - how long is a short circuit?

Paul

If there is excessive current ripple from the power supply, you won't necessarily see the 60Hz flicker in the arc with the motor stopped, but the 12Hz flicker "beat frequency" between the running shutter and current ripple WILL be obvious. So I wouldn't rule out excessive current ripple just because it can't be seen without the shutter running.

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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Some of those cap really store a lot of energy and can seriously injure you and possibly kill you not to mention damaging other components and maybe cause a fire!

A xenon lamp is NOT a short or zero ohms (what? impedance? ...AC?) across the power supply. If it were, there would be no light (or heat) out. A typical 7kw lamp runs about 165 amps at about 42 volts. In order to develop the 42 volts across the lamp, it must have resistance. Thus the power supply "sees" a load around 0.25 ohms. A 15kW lamp would see less than 0.1 ohms at 38 volts and 400 amps. These are small but very important "real" numbers and why humongus size, low-resistance wire must be used for the DC to the lamp.

The "Shorting Stick" or automatic shorting bar that is dropped across the HV caps of high-power transmitters or similar high-energy equipment is not for the normal discharge of the capacitor's stored energy. But rather it is assumed that the caps are already discharged. The stick and other devices are exclusively for safety and extra security for the technician when working on the equipment in the event the normal bleeder resistor or circuit fails. It is ALWAYS prudent to short out big caps before working on high-power equipment. Some caps will hold their high-voltage energy for days and longer...including xenon switching power supplies and DC ignitors. Some caps will even regain a charge after being shorted out. And, yes, the equipment can be damaged by a direct short across the cap. Most high-energy capacitors have a safety blow-out plug that will safely vent the internal gases/liquids in the event of excess heat or pressure build up as from a short.

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I'm still of the opinion that it is wiser to discharge the capacitors through a resistive load than a direct short. I agree, once the caps are discharged, a shorting jumper is a good idea for operator safety. A "Lockout-Tagout" policy on turning off all power must be followed too.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion