Fuse selection

Post by **woo** » Mon Jun 13, 2005 1:59 am

shawn,

You wouldn't use any of the ranges in the photo because they measure resistance. Send a photo of the whole multimeter and we'll see what you have got.

Post by **Shawn** » Mon Jun 13, 2005 2:53 am

I thought I was looking for resistance of componants?
here it is in all it's quality!

Post by **woo** » Mon Jun 13, 2005 1:32 pm

Shawn,

You are looking at measuring the Amperage of your component. Assuming you were only provided with two leads with the Meter, the RED lead goes into the socket marked '10A DC'. The black lead will go into the black socket, presumably marked 'COM' or similar.

Please be aware that when measuring Amperage mistakes can prove costly. First of all the live wire (Negative if your car is the original Positive Earth, or Positive if the polarity has been changed to Negative Earth to run an Alternator or Radio, etc) needs to be broken. Then the Ammeter is connected between the two wires. This makes the Ammeter part of the cars wiring circuit.

Say your car is the original Positive Earth and you were checking a headlight. You would (1) Locate the Bullet Connector that connects the headlight to the Wiring Harnes. (2) Turn your Ammeter to the Black 10A position. (3) Unplug the bullet connector and connect the Black Lead to the Wiring Harness connector and the Red Lead to the wire that leads to the headlight. BE VERY CAREFUL THAT THE BLACK LEAD METAL PROBE (WHICH IN THIS CASE IS LIVE) IS NOT ALLOWED TO TOUCH THE CAR BODY OR ANY OTHER METAL. THIS WOULD CAUSE A SHORT CIRCUIT DIRECTLY TO EARTH WHICH COULD DAMAGE YOUR WIRING HARNESS, ETC. (4) Turn on your headlight and read the meter. The display will show the Amps being used. Don't let the current flow through the Meter for more than 10 seconds, preferably a lot less. (5) Turn off the Headlight. (6) Disconnect the Ammeter. (7) Reconnect the Bullet Connector.

If you are in any doubt about doing this I would advise you not to. I don't want to insult you at all

but this type of measurement must be done with the greatest care to avoid damage to the Car or the Meter. I would advise you to get the hang of how the connections work by checking the Amperage of a Bulb in a Torch, or something similar, before you start on the Car.

Post by **lowedb** » Mon Jun 13, 2005 2:05 pm

There's a few issues here.

You can use resistance to work out current and power for some things, but not many on a car. For example the resistance of a bulb when cold is much lower than when on, so the resistance you measure would give you the wrong rating. Motors have a resistance, but also a 'back EMF' which has the same effect as a resistance in opposing current flow. The back EMF depends on the speed of the motor and is zero when stationary.

Both of these lead on to an important principle, inrush current. When bulbs are cold, or motors just switched on, the initial surge of current is much higher than the rated current. If a fuse is only rated for the running current, it most likely will open during inrush, perhaps after a few thousand operations or maybe more quickly.

Normally when choosing a rating for a fuse, the current requirement and inrush are considered. A cable is selected that will provide this, with acceptable volt drop with the cable as long as it needs to be. The fuse is selected to cope with all connected loads and deal with inrush. The highest resistance branch of the circuit is then checked to make sure it will open the fuse within a short enough time to prevent the cable being damaged. With small cables, the resistance of the cable can be high enough when a long length is used to prevent sufficient current flowing to blow the fuse.

Post by **brixtonmorris** » Mon Jun 13, 2005 9:45 pm

woo!
thats a very usefull list youve made there