Help selecting a capacitor to fix power drop out issue - Page 1
Forum section: electronics / beginners

author: RickAbraham
author_rank: Newbie
timestamp: September 23, 2024, 06:11:43 am
content: Hi guys,I have a 10.1 inch LCD screen in my car, it runs off 5V and approx 900mA draw.I turn the screen on by sending power to it and turn it off my cutting power to it.When the screen is powered on and I use other functions in the car that have a high amp draw such as the starter motor and windscreen wiper motor it causesthe screen to shut off for a second or two before it comes back on.Must be very sensitive to slight voltage drop.I am thinking about wiring in a capacitor to stabilize the power feed to the screen to prevent the cutouts, but I do not know what size capacitor I require.When I power the screen off I do not want it staying on for any longer than approx 15 seconds running off the capacitor.Can I please have help in selecting an appropriately sized capacitor for my project.

author: Gyro
author_rank: Super Contributor
timestamp: September 23, 2024, 08:52:56 am
content: Welcome to the forum.Unfortunately it is not that simple. A capacitor big enough to support a 900mA current draw for 15 secondswithout significant voltage droopis going to be too high a value to consider. There is also the issue of how the 5V power supply would react to having to charge a capacitor that big at turn-on (when it would look like an effective short circuit).You don't say what is generating your 5V supply but it sounds like it is unstable when faced by the hostile environment of a car electrical system, which is full of noise, spikes and voltage variations. It isn't too surprising that the screen drops when the starter is operated because that draws several hundred amps and can cause a significant voltage drop depending on the condition of the battery. Windscreen wipers indicate that the power supply is susceptible to electrical noise rather than voltage drop - unless there is a grounding problem somewhere.This is a case of finding the cause rather than trying to apply a, very large, sticking plaster.Edit: Sorry, I misread your post slightly. You say that you don't want the screen to run for longer than 15 seconds on the capacitor, but that implies that youdowant it to run for something in the same order of magnitude, so the above comments still apply.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 23, 2024, 09:03:08 am
content: Bit weird that your 5V supply would be crapping out... I assume you are running a converter of sorts to go from 12V -> 5V? Even if cranking the engine runs your battery down to 9V, that should be plenty for most converters to keep working... Running the windscreen wiper should cause even less problems.I'd say look at the converter before anything else.

author: nali
author_rank: Frequent Contributor
timestamp: September 23, 2024, 09:11:27 am
content: Agree with the above. I can understand the starter motor cutting out the screen, especially as cars often disconnect the AUX supply when starting, but the windscreen wipers? You need to look at your 5V supply and cabling.Also, is the screen just a screen - i.e. is it a self-contained Android unit or is it displaying content from something else (in which case that "something else" may be cutting out not the screen)Even if you do find a magic multi-Farad capacitor solution don't forget you need to manage charging the thing... you can't just stick it on a switched ignition supply, it'll just pop fuses every time you switch on.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 23, 2024, 11:42:05 am
content: Can you actually measure the supply whilst this is happening?

author: PGPG
author_rank: Frequent Contributor
timestamp: September 23, 2024, 12:54:55 pm
content: Quote from: RickAbraham on September 23, 2024, 06:11:43 amI have used 3.5" LCD display consuming about 200mA from 3.3V supply. In LCD baclight was made by step-up converter generating about 30V.From this 3.3V supply also 50mA current pulses were taken and LCD backlight blinking was clearly visible.These pulses made about 10mV fluctuations at 3.3V supply what was enough to make visible effect.I write it just to give the information that LCD backlight can be very sensitive to small voltage fluctuations.