Author Topic: Power supply power factor  (Read 746 times)

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Offline raptor1956Topic starter

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Power supply power factor
« on: June 08, 2020, 11:08:51 pm »
I just picked up the new DJI Mavic Air 2 drone and since I plan on charging my batteries in my car using an inverter I wanted to know what the power draw of it is.  So, I plugged the little power brick for the drone charger into a Kill-A-Watt and saw that the charge power was around 36W, a bit less than I guessed, but the VA was over 70VA making the power factor pretty bad at not much more than 0.5.  That seems really low, what might be the reason?


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Offline NiHaoMike

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Re: Power supply power factor
« Reply #1 on: June 09, 2020, 12:23:51 am »
Not much point having PFC for small loads. See what voltage the power adapter is and you might be able to find something like a laptop adapter for a more elegant solution than an inverter, or even power the charger from a larger battery out in the field.
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Offline themadhippy

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Re: Power supply power factor
« Reply #2 on: June 09, 2020, 12:40:53 am »
Quote
but the VA was over 70VA making the power factor pretty bad at not much more than 0.5.  That seems really low, what might be the reason?
more than likely there using a capacitor dropper  instead of a more expensive transformer
 

Offline srb1954

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Re: Power supply power factor
« Reply #3 on: June 09, 2020, 12:44:53 am »
A power factor of 0.5 is fairly typical for a simple power supply without power factor correction.
 

Offline raptor1956Topic starter

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Re: Power supply power factor
« Reply #4 on: June 09, 2020, 04:57:39 am »
I didn't expect the PF to be over 0.8 but I wouldn't have expected it to be under 0.6 either.  Now, add in the losses with the inverter and to get, say, 40W out I'll need to source right at 100W or 8A -- within the range of the cars outlet.


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Offline Kleinstein

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Re: Power supply power factor
« Reply #5 on: June 09, 2020, 06:00:21 am »
The cheap meters may have some error in reading the power factor, if there are sharp spikes in the current (like with a rectifier and capacitor). Many cheap inverters don't produce a nice sine. So the power factor may be different with that waveform. This can range from works well to not working at all. Many such charger say one should not run them on inverters because there is a chance to cause trouble.

It depends on the inverter how well it can handle a load with low power factor. Delivering 80 VA does not mean the inverter has to draw > 80 W from the source.

If the charger uses an external power supply (e.g. SMPS to some 5-24 V), the more logical way would be a direct way DC to DC with not 115 V 60 Hz in between. There may be suitable ones for laptops or similar.
 
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