7

I wanted to measure the current of a generic lithium ion battery charger. I set I plug my red lead in the A 10A max fused and black into COM. The charger is rated at 600 mA and my other lead plug is fused to 250 mA so obviously that wouldn't work.

So I put my leads on either side of the battery and there's a small spark. Did I break something?

Peter Mortensen
  • 1,676
  • 3
  • 17
  • 23
TheMidnightNarwhal
  • 159
  • 2
  • 2
  • 6
  • 10
    Can you draw it please? Sounds that you have connected your MM in parallel to the battery, effectively shorting it through your device getting max current the battery can give. I guess it is not working now... – Eugene Sh. Jun 07 '17 at 18:10
  • 13
    Your just shorted the battery, which is not nice, but not a disaster. And probably burned the fuse. Again, not a disaster. –  Jun 07 '17 at 18:10
  • @GregoryKornblum Given it was connected to the unfused socket in the MM, it is likely to be dead now. – Eugene Sh. Jun 07 '17 at 18:12
  • Also a possibility. Still not a disaster :) –  Jun 07 '17 at 18:13
  • 1
    @EugeneSh, OP says it was connected to the 10A fused socket. – Chris M. Jun 07 '17 at 18:13
  • 1
    Oh. Right. Then just blown the fuse. – Eugene Sh. Jun 07 '17 at 18:13
  • @TheMidnightNarwhal You blew the 10A fuse. Current has to be measured in series with the current flow, otherwise you create a short circuit. – Chris M. Jun 07 '17 at 18:18
  • @EugeneSh. [Here is a picture](http://i.imgur.com/ZBv5hwq.jpg) . Leads probed on each side of battery between. Everything still appears to be working fine. Charger led lights up. Battery works in flashlight. Multimeter turns on. – TheMidnightNarwhal Jun 07 '17 at 18:18
  • @ChrisM. What should I have done based on the picture of my last comment? – TheMidnightNarwhal Jun 07 '17 at 18:19
  • 3
    Never connect a meter in current mode directly across a power source. You are basically shorting out the source through the fuse... – Trevor_G Jun 07 '17 at 18:20
  • @Trevor But don't you need to have something powered on like, to measure it's amps? Like what should I have done to measure the amps getting to the battery or to see if the rated amps is actually true? – TheMidnightNarwhal Jun 07 '17 at 18:22
  • 1
    Current is flowing *through*, not *around*. And this is how you should measure it. And the *rated* and the *actual* current are not the same. – Eugene Sh. Jun 07 '17 at 18:23
  • 1
    @TheMidnightNarwhal *An ammeter is essentially a bare wire and thus does not restrict current flow*. You allowed max current to flow through the power source. That current is more than the device is designed for, and the fuse blew. When a battery is connected, the battery restricts the flow through the charger. You bypassed that. Always treat an ammeter like a bare wire (a delicate one at that). – Bort Jun 07 '17 at 18:29
  • You need to connect the ammeter in **series** between the battery and whatever the battery is attached to. – Trevor_G Jun 07 '17 at 18:35
  • 1
    It makes no sense to measure the amps _of_ a constant-Voltage power supply. You measure the amps drawn by some _load_ that is being powered by the supply. The load, in your case, was the fuse in your meter. In theory, a CV power supply will put out however many amps it takes to maintain its preferred output voltage across the load. In practice, there is a limit, but it sounds like the limit was high enough to blow the fuse. – Solomon Slow Jun 07 '17 at 21:24
  • 1
    P.S., Most of the power supplies you encounter (with LED driver circuits being one notable exception) will be pretty good approximations of a constant voltage power supply. A battery is a pretty good approximation, the power line coming into your house is a *very* good approximation, etc. Power supplies are designed that way because it's a lot easier to design useful electronic circuits if you can assume a constant supply voltage. – Solomon Slow Jun 07 '17 at 21:32
  • 1
    Car batteries can dump 1000 amps or more, melting a ring on your finger or a tool, causing sparks, a hydrogen gas explosion, or case rupture spewing boiling sulfuric acid. They try very hard to be constant voltage supplies, but usually fail catastrophically. Something to keep in mind. –  Jun 08 '17 at 00:45

4 Answers4

45

You placed an ammeter in parallel to a voltage supply, which created a short circuit through the meter and most likely blew the 10A fuse in the ammeter socket.

What you want to do is put the meter between the charger and the battery, and then connect the other end of the battery to the charger. Diagram will help:

schematic

simulate this circuit – Schematic created using CircuitLab

Edit: If everything still works fine, it's likely you tripped some kind of over-current protection in the charger before the fuse in the meter blew.

Chris M.
  • 1,330
  • 9
  • 11
6

You used an ammeter in parallel to a voltage supply, and not in series. This resulted in more current than the ammeter could handle. You have thus blown some fuse.

Scott Seidman
  • 29,274
  • 4
  • 44
  • 109
5

The principles behind your idea were OK. You wondered what the maximum current that the PSU could deliver. So you connected a very low resistance across it (the DMM) to draw the maximum load and to observe the measurement. It's the execution of it that's wrong, I'm afraid.

A simple linear supply, commonplace decades ago, could be shorted like you tried to. They consisted of a bridge rectifier and a smoothing capacitor. Very shortly after though, the high current would overheat the diodes and/or draw a relatively very high current from the supply and probably take out the supply fuse.

But a modern switch-mode PSU is more sophisticated than that and contains protection, such as output current limit and 'hiccup mode'. The latter will switch the PSU off if its output is low voltage while the current is high. It then waits a short while and switches on to try again. It repeats this cycle for as long as the output overload persists.

The practical way to determine your PSU's maximum output current is to connect your PSU output as (V+)->ammeter->Radj->(V-) where Radj is an adjustable load resistance i.e. a high-power rheostat. You also connect a voltmeter across (V+) and (V-). You set the rheostat to draw (say) 75 % max current, so 450 mA here, and switch the PSU on. You then decrease the rheostat resistance to draw increasing current and observe the output current as the output voltage starts to fall. You will be able to see when the voltage goes outside of its specified voltage range and therefore when the PSU can no longer provide the increase in load current.

TonyM
  • 21,742
  • 4
  • 39
  • 62
  • 1
    The OP was testing a Li-Ion battery charger, and not a PSU. Also, the problem was really shorting the battery, which usually can supply many amps. The charger, if the least bit intelligent, should have some short-circuit protection. – Ronan Paixão Jun 08 '17 at 13:14
0

If the meter still works you didn't break anything.

A lithium ion charger has both a constant voltage mode and a constant current mode. So it acts sometimes as a voltage supply and sometimes as a current supply. It decides which mode to use at a given time by monitoring the battery voltage, current, or, in some cases, the temperature (battery has a sensor built in, which the Samsung phone that caught fire apparently did not).

In either mode, connecting the ammeter in parallel with the operating charger+battery circuit will pull current from both the battery and the charger (depending on, as one poster said, its output current limit). So if you get a reading, it doesn't tell you anything specific about either the battery or the charger.

If you put the meter in series with the battery, as Chris M. said, you see how much charging current is being supplied to the battery, which may tell you about the state of the battery (low current means it's mostly charged and being sustained, high current means it's mostly discharged) but not really anything about the charger's maximum current.

Brian Lowe
  • 1