use of a multimeter

[Steve]

How do i measure current being drawn from my new car stereo amplifier, using my bog standard digital auto-ranging multimeter? I expect it to be about 30-50 amps, and naturally didn't want to connect the multimeter's puny wires across this (since an ammeter has to be connected in series)

Also, to measure voltage across, say, a lampholder terminals, do i just turn the empty lamp on and probe the 2 terminals with range on AC volts? It just doesn't feel right!

 

[andy]

How do i measure current being drawn from my new car stereo amplifier, using my bog standard digital auto-ranging multimeter? I expect it to be about 30-50 amps, and naturally didn't want to connect the multimeter's puny wires across this (since an ammeter has to be connected in series)

Also, to measure voltage across, say, a lampholder terminals, do i just turn the empty lamp on and probe the 2 terminals with range on AC volts? It just doesn't feel right!

most only go upto 10A. check with meter

and for voltage, yes

 

[Qedelec]

To measure current you will need to incorporate a shunt to take most of the flow. Refer to your Ohms law to work out the value.

Measuring across a empty bulb holder will give the supply voltage under no-load conditions.
Insert the lamp and you will have the supplied voltage under load conditions.

Question! Why do you need to know?

 

[Steve]

well i have a new energy saving fluorescent lamp in the landing light, the old one was 11w, this one is 20w, only thing is, when its switched off, one end of the tube flickers faintly every few seconds so i was thinking there must be voltage across the contacts. Is this capacitance effects due to 2-way switching and length of cable? I remember something like this a while ago on here. I was just wondering what voltage was present with the switches off. (no its not on any dimmers either, in case anyone thought this was related to my recent dimming fluorescents post)

and i was wondering how long i could run my new amplifier with the engine off before running my 40AH car battery flat!

 

[ban-all-sheds]

Never seen a general purpose multimeter go beyond 10A, and even then you're limited to a few seconds.

Buy this instead:

http://cgi.ebay.co.uk/TWIN-METER-TE...555200201QQcategoryZ25411QQrdZ1QQcmdZViewItem

STANDARD DISCLAIMER - I am not selling this item, nobody I know is selling this item, I have no financial or other interest in whether, or for how much, this item sells.

 

[plugwash]

the other way is to make a shunt

get a few meters of cable of suitable rating measure its resistance and then measure the voltage accross it whilst running the car hi-fi equipment through it

this does however require good low resistance measurement which your average multimeter may have trouble with.

 

[olly_k]

crafty you will only measure a sort of 'average' current with most meters anyway and if no music then it will be quiescent current not what you want I suspect!
Like most people said you would need to create a shunt and be able to measure it's resistance fairly accurately and measure voltage accross that with your speakers booming away and car engine swithed on (to minimise voltage drop and hence measure correct current drawn) then leave your meter, which I assume has a min / max / avg function for a few minutes then if you don't get neighbours complaining before this you will have your answer
Do you want to choose a suitable fuse and power cables for your new amp or is this a learning process?

 

[Hotwire]

Some info on resitors, you probably know but

1ohm resistor = voltage drop of 1volt per amp

.1ohm resistor = voltage drop of 100mv per amp

.01ohm resistor = voltage drop of 10mv per amp

I would suggest going for the later as heat will start to become a problem as will the voltage drop. If you took your maximum figure of 50amp you would have a v drop of 500mv with the .01ohm res and it would need to be capable of dissipating 25watt.

Low ohmic value resitors are available from Farnell electronics, thats were i bought a .01ohm about 4yr,s ago. They used to do free delivery then, dont know if they still do.

 

[Steve]

The amp is up and running, i have the cable and it is on a 60A fuse on the cable near the battery (amp itself has 2no 30A fuses built in). It sounds sweeeet too. Not that i'm the sort of person who drives round estates playing music loud for the sake of it. I'm more of a music connesseiur! I think rather than arse about with the suggestions on here about shunts etc, i'll just carry a pair of jump leads everywhere with me and see how it goes!

 

[olly_k]

I think rather than a**e about with the suggestions on here about shunts etc, i'll just carry a pair of jump leads everywhere with me and see how it goes!

The simple answer is to keep the car engine ticking over whilst running the sub... you could also put a switch between stereo and amp inhibit line if the sub has one???

LOL If you think you are a music connesseiur check out the new speaker systems to be installed in Audi A8's sometime this autumn

 

[megawatt]

BAS wrote:

STANDARD DISCLAIMER - I am not selling this item, nobody I know is selling this item, I have no financial or other interest in whether, or for how much, this item sells

.

Can we assume you're not actually advertising this item then

I'm off b4 the mod gets me

 

[Steve]

yes, the amp has a remote power line, the aerial feed from the head unit. But i want the amp on all the time the stereo is on, here is why:

the bass output from the head unit has to be on minimum to stop the sub exploding because it is that powerful! And if i have a switch for the amp, i would need to turn the bass back up on the head unit.

Then again, there is a bass control etc on the amplifier, i will have a play with my screwdriver, in case it was set at maximum when it left the factory I had my headlights on on the way to work this morning, and didn't notice any tell-tale dimming of the lights on heavy bass notes (pretty loud), so my alternator and battery must be in pretty good shape. I guess i've got nowt to worry about! (yet )