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A question about inverters from a newbie......

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mrsussex

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Hi all,

I am a photographer looking for advice on how to power a studio flash system, on location using a battery/inverter combination.

I have a studio flash pack, about 10-15 years old, with no specification label on it. It is a 240v unit with a 5amp fuse, so clearly it uses less than 1200watts. It is basically a box of pretty big capacitors.

I want to run this from a 12v battery/inverter combination (petrol generators are apparently no good for this due to the dirty (spikey) supply they give, unless you buy an expensive one.

My flash pack is also fitted with a auto sensing input voltage thingy, which means I can plug it into a 110v or 240v mains supply without needing a transformer.

My questions are:

Apparently, one of the issues is that after the flash fires and the pack draws current, the voltage from the inverter drops slightly and confuses the voltage sensor causing it to not work.

Is this true? If I bought a 1500watt inverter, would this get around the problem since it is rated higher than the 1200watt flash pack?

I also understand that it will need a pure sine wave inverter, can anybody recommend a good make that is available in the UK?

Is there likely to be a way to bypass the voltage sensor, since I will only ever use the pack on 240v?

How dangerous to my equipment is it likely to be if I plug it into an inverter as an experiment? It's very expensive kit and I obviously don't want to brick it!

The makers of my lights used to make a switched mode power supply for it, which enabled you to run it from a 12v car battery. They don't seem to be available anywhere, either new or secondhand. Is there likely to be another unit that would work?

The switched mode unit has its' own socket, presumably to bypass the voltage sensor. Could a decent electronics engineer be able to work out what this socket requires and then either buy or make a switched mode supply that will work?

As you may have guessed, I only have a basic grasp of electronics (I'm just a lowly snapper), so I'll apologise now if none of this makes sense or if my questions are stupid!

I have a deadline for this, so I am panicking a little and any advice would be gratefully received and possibly stop me having a heart attack!

Thanks all
 
The problem is that I don’t know how the voltage sensor in your flash pack actually works so I have to make some assumptions.

Yes, a larger rated inverter will not load down as much. Inverters are rated in VA or kVA (thousands of volt amperes). The difference from watts which measure DC power is the VA allows for a phase shift between voltage and current referred to as the Power Factor. A load (your flash unit) that is either using a large inductance or capacitance can have a significant PF that makes the inverter work much harder than the watts indicate. The minimum rule of thumb is 20% larger inverter than you think you will need.

For what you are using it for a rectangular output will probably work just as well as sign wave. Even when they say “pure sine wave” most inverters are going to have several % of harmonic distortion in their output. Normally only computer logic chips and communications equipment really need clean input power.

I don’t think it will be too dangerous to your equipment as long as the input voltage does not exceed the flash pack’s maximum rating. Again, I can’t be sure without evaluating a schematic of the device.

Yes, a professional with a good reputation should be able to work that out for you. The trouble is that there are people out there that are not that good so look for a friend you trust who can recommend somebody they have had experience with.
 
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