Measuring current/power draw from digital cameras and flashes - Page 1
Forum section: electronics / projects

author: nightfire
author_rank: Frequent Contributor
timestamp: July 08, 2022, 07:54:35 am
content: For some comparison projects I want to measure the usage and strain a digital camera or external flash puts on its power source.This also in context of capacity of genuine and 3rd-party suppliers, and especially with flashes to determine the difference in time a flash could recharge from certain kind of batteries to fill its capacitors again.So we are talking mostly Lithium batteries with those cameras around the dual cell=8V mark, rarely 12V (triple cell with higher end systems)With flashlights like Nikon SB-800, we are talking 4 AA batteries like classic Alkaline or NimH cells like eneloop, and in addition to that there is a port for an external battery pack.Current is around 500mA peak with cameras, with flashes I have no idea...The sheer capacity and discharge characteristics I could measure with my electronic load (rebadged Korad KEL102). So far, so good.The more interesting thing is the load profile a digital camera has when utilizing the battery, where the standby power consumption is quite different from taking a picture and processing it internally.What would be the best way to measure the power taken? I could exchange the battery with a stub power adapter intended for usage with an external power supply, like the Fuji CP-W126 or Nikon EP-5a- but where to put my access to my measuring equipment?Putting a good multimeter in line, or should I rather go and measure via some sort of shunt, maybe measuring some piece of the used cabling for resistance and then place my voltage probes on them?Accuracy initially is ok to be within 5%, to get some first idea how the signal/usage scenario is shaped.For comparision scenarios I could also use my entry-level scope (Rigol DS1102Z-E), including some adaptors from BNC to 4mm I recently acquired.In case of the flashes, I probably would use some dummy 4*AA-adapters to the batteries I would put on externally, or deal with an external "High-Voltage" power pack. Here I could put in my multimeter(s) to get some idea of the current drawn.Questions here:- Has anyone done similar things before and could share some experience? (Does not need to be digital cams, lots of other stuff behaves the same...)- Is it possible to do some kind of shunt like putting a precision resistor in the line and measure the voltage drop over it- and how to measure the correct resistance of the resistor? Because I do not want the voltage drop to be too significant, I would have to use small values...

author: magic
author_rank: Super Contributor
timestamp: July 08, 2022, 09:01:58 am
content: 0.1Ω (100mV/1A) may be low enough for the camera to tolerate (set the PSU to the voltage of a fully charged battery) and high enough for its voltage drop to register on a scope (set the probes to 1X mode if available or connect straight to the BNC). You can buy such resistors everywhere. If the PSU has grounded output, be sure to connect the sense resistor and the scope's grounding clip to the right PSU terminal, usually the negative.With lower resistance you may have trouble to see anything on the scope if the current isn't high enough. Then an amplifier could come in handy, something like Dave's µCurrent project.For steady state DC consumption, a DMM in 10A range will give you some idea. Low ranges (<1A) usually have too much internal resistance / burden voltage to wok. 2A ranges are uncommon.I recall testing some point and shoot many years ago and I would say that peak consumption may approach or exceed 1A. This is not based on an exact measurement, but the fact that it easily turned off due to burden voltage.

author: nightfire
author_rank: Frequent Contributor
timestamp: July 08, 2022, 03:42:29 pm
content: Ok, I see that I will have to take into account the burden voltage of the circuit also. Luckily most of the cameras also allow for power supplies being connected and therefor are accepting more than the battery voltage as input voltage, so I have some headroom for initial experiments.Burden: My old Fluke 8020A has a 2A range with 700mV/2000mA ;-)And my U1272A mentions in the specsheet that there is a 3A range (of the 10A the input allows) which should do something like "burden voltage < 0.1 V/0.01 Ω" according to specs- I am curious...I might probably take some ceramic resistor and solder some 4mm jacks on it to get it in between , OR in case of a dummy battery I might go for measuring the resistance of the whole cable length from the external jacks with a 200mA load to get some accurate results (appliance tester)-and then simply measure the voltage drop over these and then calculating the current/power drawn.