Author Topic: How to check a 5V phone charger for good DC?  (Read 659 times)

Offline vtsteam

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How to check a 5V phone charger for good DC?
« on: March 30, 2022, 02:28:47 PM »
I've got a bunch of 5V  phone chargers from old cheap flip phones. I'd like to use one for my DRO project, but it needs to be regulated and fairly clean DC, not a charger which depends on filtering/regulation in the phone itself to clean up the power. I know some chargers are good DC and some not. This is one that does not just have a USB socket, but includes a cord to the phone. The legend on the charger does say 5.0V DC ... but is it good DC?

How can I tell without an O-scope to look at the output for noise? I do have a DVM. And/or is it possible also to hook up a headphone and just have a listen? If so I imagine I'd need a fairly high value resistor in series?

Or, any other ideas....?
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline awemawson

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Re: How to check a 5V phone charger for good DC?
« Reply #1 on: March 30, 2022, 02:32:41 PM »
Stick an appropriate load resistor across it and measure the AC ripple via a capacitor with your multimeter
Andrew Mawson
East Sussex

Offline vtsteam

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Re: How to check a 5V phone charger for good DC?
« Reply #2 on: March 30, 2022, 02:35:17 PM »
Thanks Andrew, but what value appropriate load resistor, capacitor, and what value am I looking for on the multimeter reading?

The output is rated at 550 ma btw.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline awemawson

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Re: How to check a 5V phone charger for good DC?
« Reply #3 on: March 30, 2022, 04:24:21 PM »
Ohms Law is your friend.

5 volts @ 550 mW so about 50 ohm should suit. (47 nearest preferred value).

Large electrolytic - actual value unimportant but say 100 microfarad at 15 volts working. It just needs to be low impedance at 50  60 Hz.

Ripple voltage should be approaching zero but probably a few 10's of millivolts - hundreds being to much.

Andrew Mawson
East Sussex

Offline vtsteam

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Re: How to check a 5V phone charger for good DC?
« Reply #4 on: March 30, 2022, 05:38:59 PM »
Thanks Andrew. Ohm's law is well understood here, but doesn't attempt to set a value for "a load" since there's still the practical question of how much of a load? Looks like you're saying a load at half the charger's max rated current is what you recommend, so I'll go with that. And thanks for the cap and acceptable V values.  :beer:
« Last Edit: March 30, 2022, 06:32:49 PM by vtsteam »
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

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Re: How to check a 5V phone charger for good DC?
« Reply #5 on: March 30, 2022, 05:59:37 PM »
Should be pointed out for anyone else reading and attempting this that the resistor has to be rated at least a couple of watts. Not sure I have anything like that, might have to get creative.....
« Last Edit: March 30, 2022, 09:45:28 PM by vtsteam »
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

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Re: How to check a 5V phone charger for good DC?
« Reply #6 on: March 30, 2022, 06:29:55 PM »
I found an old 120V 20W appliance bulb salvaged from a microwave oven, and it happened to test out at 57 ohms. Of course resistance changes when powered and heated up, but in this case the voltage is so low that it doesn't light. I checked the current at 5 volts and it was 250 ma. About half of full load.

I had a 2200ufd cap, so I used that, and my meter showed on AC about 11 mV and 4.9 V DC, so I'm thinking it's good to use.

Thanks Andrew.  :beer:
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline awemawson

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Re: How to check a 5V phone charger for good DC?
« Reply #7 on: March 31, 2022, 03:02:31 AM »
Glad to help
Andrew Mawson
East Sussex