Strobe Lights & Inverters

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Photography equipment has evolved over the years allowing for smaller, faster and more powerful equipment. The drawback to this maturation and leapfrogging of new technologies is that many manufactures don’t publish all the necessary data allowing users to make sound decisions.

The Strobe Light Problem

Professional photographers, for example, often travel to remote locations to film their subjects and thus require the ability to recharge and operate their equipment off-grid. Obviously, they want to be able to minimize bulk and weight whenever possible while maximizing the benefits of these new technologies. However, like most of us, they forget to update their power supplies accordingly.

Newer strobe lights (flashes) use high power capacitors to store energy that is used instantaneously to produce the flash. After the flash, these capacitors are fully discharged and need to be charged. These capacitors will draw maximum power (current in amps, A) immediately when charging commences. For the first 1/10th of a second (or less in some cases) the capacitor will draw 3 times its effective watt rating for the flash. For example a flash with a “true” power rating of 300 Watt-sec will have an effective power draw of around 900 W or 7.5 A at 120 VAC during the first 100 milliseconds.

Why Is This Significant?

  1. If the cable connecting the inverter to the batteries is not sized correctly to handle the maximum current or distance between the two components, then a very large drop in voltage will occur. The recommended size of cable for a 300Ws flash is AWG #10, #8, #6, #4, and #2 for 1ft, 2ft, 3ft, 4ft, and 7ft respectively.
  2. Many inverters will emit an audible alarm when the DC input voltage drops to around 10.5 volts, but will not shut down until voltage reaches 10 V. They automatically recover once DC voltage reaches around 11.5 V.
  3. In practical terms let’s use a Samlex PST-30S-12A Inverter designed to deliver maximum continuous power of 300 W (2.5 A at 120 VAC) and surge power of 500 W (4.17 A at 120 VAC) for less than 1 second. The starting power of 900 W from the 300 Ws flash is required for only milliseconds as noted above. That short burst of power is too fast for the overload protection circuit in the inverter to be triggered and hence cannot shut down during the overload condition. This exposure will overload the components of the inverter and cause premature failure. THE SOLUTION: use a 600 watt inverter (PST-60S-12A) that has at least 1000 W surge protection.
  4. Some smaller inverters have cigarette lighter adaptors and can be used in vehicles. The average load these receptacles can bare is 12 A and they usually use AWG #12 wire. Since the wiring is thin, this same flash will produce excessive voltage drop during the first 100 milliseconds of the recharge cycle. It will also cause excessive stress in the power transistors and in the magnetic components producing abnormal noise.

In short, you need to pay attention to the effective maximum power draw your device will demand. The published draw rate may be misleading and may cause premature equipment failure.

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