Measuring current draw from the battery of my portable speaker? - Page 1
Forum section: electronics / beginners

author: muki16
author_rank: Newbie
timestamp: June 13, 2015, 10:24:18 am
content: Hi,I've got a newbie question:I recently built myself a portable speaker consisting of an old wooden ammunition box, a little class-t amp, a PA-speaker and a 12V sealed lead-acid(gel?!) battery (see photo attached). As far as I can tell, everything works perfectly and sound quality is pretty good.I would like to measure the current draw from the battery at different volume levels to calculate battery life, but that does not really work(yes I know, dumbass question, but I really don't have much electronics knowledge). I' ve read that I have to connect the multimeter in series with the battery. So I disconnected the cable from the negative terminal of the battery and put the multimeter in between (in DC mA setting), but nothing happens and the amp does not power up.Can anyone help and tell me how to connect the meter properly?Thx in advance.

author: rolycat
author_rank: Super Contributor
timestamp: June 13, 2015, 10:32:28 am
content: Have you checked the fuse in your multimeter?

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 13, 2015, 10:34:35 am
content: Try the A input of the meter. Maybe the (switch on) current is too high and you've blown the fuse in the meter.

author: muki16
author_rank: Newbie
timestamp: June 13, 2015, 11:11:39 am
content: Ok, that really seems to be the problem. I got a second hand Fluke 83 from ebay which was supposed to be "fully working", but when I measure the fuses in Ohms-mode there is no beep (btw, why do voltage measurements still work with both fuses blown?).Seems I will have to get new fuses first (the two fuses [see picture] cost almost half as much as I paid for the meter without the holster and they seem to be hard to find in Germany). Can I buy other/cheaper ones? Are there standard sizes for these?So many questions.... Thank you all for helping.

author: Shock
author_rank: Super Contributor
timestamp: June 13, 2015, 11:21:02 am
content: You could have easily blown it yourself if you're not careful by measuring voltage on the current settings.Generally it pays to do some manual calculations beforehand, the power amp may have current draw and max power labelled on the outside (not to be confused with output wattage) and is also probably fuse protected, along with your supply voltage (which you can measure if unsure), these will give you indicators on how much current you're expecting. Then you would use an appropriate range, pays to start on the higher ranges when as a "just in case".

author: muki16
author_rank: Newbie
timestamp: June 13, 2015, 11:35:03 am
content: I didn't measure voltage on the amps setting, but these fuses seem to have blown before I got the meter and I did some DC voltage measurements with it in the last weeks. So wanted to know how it technically works that there is no fuse (or no intact fuse) needed for voltage measurements.But anyway, I did of course do some calculations and I'm pretty sure that the amp does not draw much more than 0,8A with one channel active at the 8Ohm speaker, but nevertheless I would like to measure it. I want to know how the current draw is affected by turning down the volume, I want to know how much the bluetooth module draws and I want to know whether there is a difference (with respect to current draw) between adjusting the volume on my phone and directly on the amp.So I will have to get new fuses first, but the damn things cost almost 30 € including shipping

author: Wytnucls
author_rank: Super Contributor
timestamp: June 13, 2015, 11:40:41 am
content: Using a multimeter requires some discipline:Always start current measurements on the 10A range and always move the test lead back to the voltage jackaftercircuit disconnection. Only move to mA/uA ranges if you need more resolution, being aware of rising burden voltage.Disconnect the test leads from the circuit,beforeyou switch your meter off.Bite the bullet and buy the proper HRC fuses for your Fluke meter; you may need their protection one day.Competent use of the meter means you won't blow those fuses ever again.The fuses only protect the A/mA/uA ranges and nothing else in the meter.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 13, 2015, 11:41:11 am
content: Quote from: muki16 on June 13, 2015, 11:35:03 amA bit of thin wire across the fuse terminals should get the job done.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 13, 2015, 11:49:15 am
content: Quote from: muki16 on June 13, 2015, 11:35:03 amThere is practically no current flowing trough the meter when measuring voltage, the meter has a very high impedance (10M Ohm).So a fuse would not do anything and would never blow, a fuse is a current limiting device. However, there are other protections on the voltage input of the meter to prevent overvoltage.QuoteI hope you are smart enough to see he's kidding

author: radioFlash
author_rank: Regular Contributor
timestamp: June 13, 2015, 11:59:07 am
content: Blowing the fuse on a multimeter is a pretty common mistake.* The fuse is only for the current measurement modes, so you can still measure voltage in a meter with a blow fuse.* In current measurement mode, the meter has very low impedance, on the order of miliohms. The impedance is through the current shunt, which looks like a bent piece of thick wire for the 10A mode and a large resistor for the mA/uA mode. Because it's low impedance, the meter can act as effectively a short circuit, so DON'T have the meter in current mode and connect it directly across a power supply or battery. You'll can blow the fuse, the power supply, or cause a fire, depending on what fails first. Even small batteries can deliver a surprising amount of current.* If you want to measure power consumption, you typically really need two meters--one to measure voltage, and one to measure the current.

author: muki16
author_rank: Newbie
timestamp: June 13, 2015, 11:59:11 am
content: Quote from: Wytnucls on June 13, 2015, 11:40:41 amOk, I see. Maybe I did not pay enough attention. But I am still pretty sure that the fuses were blown before I got the meter, because I never switched the meter to either one of the amps settings before and by measuring the circuit on the mA range I would have only blown the smaller fuse.I will of course get the proper HRC fuses. I decided on a used FLUKE instead of a cheap brand for safety reason mainly.But one last question:is it really (or relatively) safe (for me and the meter) if I lets say solder a piece of wire across the two ends of the blown 11A fuse and measure the circuit again on the Amps range? I mean, the whole circuit is 5A fused and it never blew the fuse, so there should not really be much risk of damaging the meter, right? I'm asking because I would like to do the measurement now and those fuses will take some days to get here.

author: muki16
author_rank: Newbie
timestamp: June 13, 2015, 12:02:12 pm
content: QuoteOk, bad question according to this answer??

author: rolycat
author_rank: Super Contributor
timestamp: June 13, 2015, 12:11:41 pm
content: Fluke multimeter fuses are available from RS Germany at reasonable prices. The 440mA fuse is 4.30 Euros and the 11A fuse is 7.90 Euros. Not a huge price to pay for safety.Since your current circuit runs off a relatively small 12V battery you should personally be fairly safe to bypass the fuse for this one measurement. You risk damaging the meter if there is a short, though. And throw the bodged 'fuse' away immediately afterwards.

author: Shock
author_rank: Super Contributor
timestamp: June 13, 2015, 12:22:35 pm
content: Bridging the fuse is not worth the risk, since the meter is secondhand you do not know the condition of it you could end up with a worse problem. Go buy the proper fuse.Even an auto fuse is better than nothing, but you don't want to get into bad habits to save a few bucks.

author: muki16
author_rank: Newbie
timestamp: June 13, 2015, 12:26:10 pm
content: OK. Thx for the info. Still 23 € including tax and shipping but definitely worth it.And ok, I will wait for the fuseAnd thx to everyone for your replies. I think I now have understood a bit better how a DMM works and how it has to be used.

author: Tom45
author_rank: Frequent Contributor
timestamp: June 13, 2015, 01:13:53 pm
content: While you are waiting for the fuses, you can measure the current draw by inserting a low resistance value shunt in series with the power lead. Then measure the voltage across that.For example, a little over 1 meter of 24 gauge copper wire will have a resistance of 0.1 ohms. If your circuit draws 1 amp, then you'll read 0.1 volts across that shunt.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2015, 01:17:33 pm
content: Quote from: radioFlash on June 13, 2015, 11:59:07 amIdeally, you need a proper power meter so the reading is still correct if the voltage drops under load. If the battery has a low enough impedance, it's pretty safe to assume the voltage remains constant.If this were an AC circuit or DC with lots of ripple then you would need a power meter (two meters wouldn't be good enough).