How to test mA output of Cordless Telephone wall wart charger? - Page 1
Forum section: electronics / beginners

author: bgc99
author_rank: Newbie
timestamp: August 16, 2011, 12:41:20 am
content: The wall wart has a listed output of 9V DC @ 210 mA. It connects to a cradle with 2 springy contacts that engage with the contacts on the bottom of the handset.I plan to connect the cradle terminals to the terminals on the bottom of the handset with pieces of wire with alligator clips on the ends. I'll cut the positive wire in the middle and then connect the multimedter to complete the positive circuit with the meter set to mA. Will that give a true reading of how many mA the charger is giving the phone?Thanks,BGC

author: Unknown
author_rank: Unknown
timestamp: August 16, 2011, 01:11:07 am
content: Are you sure the cradle is nothing more than direct connections from input jack to handset contacts? I'd be checking before I was hot the handset!Yes a current measurement is taken in series with the load. But what you will be measuring is the current draw of the device rather than the current capability of the wall transformer, assuming it is the correct item.To test the output capability you would measure current into increasing loads (lower resistance) while monitoring voltage and device temperature rise. The rated current is not the current delivered into a given load it is a measure of capacity. (not to be confused with capacitance)A 250ma load will draw 250ma at a given voltage when supplied by a 250ma supply or a 500ma supply. Ohms law in action!Remember also that the handset load will vary depending on state of battery charge.Perhaps it would be better to state what you wish to achieve rather than what you intend to do, that will widen the scope of possible responses offered.

author: IanB
author_rank: Super Contributor
timestamp: August 16, 2011, 02:10:27 am
content: I think the question is perfectly clear. bgc99 wants to measure the current draw of the phone handset and has described how to do so.Yes, the method described will do this, with the proviso that you must consider the burden voltage of the meter. When on the mA range the meter will insert a significant resistance into the circuit that will likely reduce the current. I would recommend you conduct the measurement with the meter on the 10 A range if it has one. The 10 A range usually has a very low resistance.(You won't do this of course, but never connect the meter directly across the wall wart terminals while set to a current range. This will short circuit the wall wart and may blow the fuse in the meter.)

author: bgc99
author_rank: Newbie
timestamp: August 16, 2011, 03:45:58 am
content: Yes, I want to know how much juice the phone is drawing during charging.I'm a novice with the multimeter and I'm afraid I did briefly touch the probes to the output terminals of the wall wart while in the current mode. (Barrel type where the center is +). Luckily there doesn't seem to be any damage done, the charger and meter are still in working order.When I do check I'll be sure to set the meter in the 10A range, thanks for the tip.BGC

author: KJ6EAD
author_rank: Contributor
timestamp: October 03, 2011, 04:02:57 am
content: The rate should change over the charge period. Smart charging systems for Li-Ion cells usually step through two or three discrete charge rates. Dave did a tutorial on that and I'd give you the link but it's too difficult to find in the video blog episode list.

author: IanB
author_rank: Super Contributor
timestamp: October 03, 2011, 04:23:27 am
content: Quote from: KJ6EAD on October 03, 2011, 04:02:57 amBut the phone almost certainly has NiMH or NiCd batteries, in common with nearly all cordless phones.Secondly "smart charging systems for Li-ion" is a bit of a tautology. All lithium ion cells are supposed to be charged according to the manufacturer's recommended protocol. There are no dumb charging systems for Li-ion, only dangerous ones.

author: KJ6EAD
author_rank: Contributor
timestamp: October 03, 2011, 08:34:37 pm
content: True. I had morphed into thinking the phone was a mobile instead of a cordless.