Tutorial No 12V clamp meter to test current loads? No problem...

You are using a big PSUs, jrk's or Monster Moto's and very big motors. You don't know how much current your components will use for working with and you have no clamp meter on tap.

Solution: You can use car fuses instead. If you want to know how much load your motors need, you have to blow a 12V car fuse . You have to open with a low A fuse, let's say 10A.

If the fuse will survive, your system needs less than 10A. If the fuse blows, replace it by a stronger fuse, let's say 15 A etc.

The first fuse which is not blowing gives you the needed information. It is a little bit laborious but 100% effective.

Happy testing!

trying highest fuses first, prevent you to blow all your fuses

only the first "too low" fuse will blow

lol you are in the right but I like them blowing, fuses and h-bridges

Haha or just go on ebay and buy one for 5 quid !!

Blowing fuses? Huh?
@Pit , @RacingMat , @SilentChill
http://www.altronics.com.au/p/q0470-100a-1.0-milliohm-0.meter-shunt/ , use a shunt.
Just wire one of these across it and stick a multimeter on the lines to see how much current it draws
The shunt helps give a reading thats low enough for a multimeter to read so you get an idea what current is flowing.
Thats more acurate.

Or just use a clamp meter for 5 quid

@bsft : of course, you're right
(@Pit likes blowing things... I didn't dare to stop him)

I bought this shunt @3$ on Aliexpress
http://www.aliexpress.com/item/15A-75mV-Measure-DC-Current-Shunt-Resistor-for-Ammeter/503343325.html

Could shunts blow as well? If so I will give a try...

@Pit, anything will blow up if you push it hard enough, including me
A shunt can handle 100 amps, I think it will be safe to use on one of the lines at least. It them converts that current to 100Ma I think so a multimeter can read it safely

I can check volt and current by electrocuting myself, but not very accurate though...

@bsft : Not want to be the wise @ss on this one but "A shunt can handle 100 amps, I think it will be safe to use on one of the lines at least. It them converts that current to 100Ma I think so a multimeter can read it safely" is not correct. Shunts come in different Amp ratings and enables us to measure voltage across this resistor and with the use of Ohms law (U=IxR so I=U/R) we can determine the current that flows through the shunt resistor. If it is linear like 100A = 100mV it is easier to determine what the current is/was going through the shunt resistor. To determine the size you need to know the nominal current of the consumer (motor) and by rule of thumb multiply that by 5 (roughly). When you have a digital multimeter with a high/low memory function you can record the peak currents during operation. Or as mentioned before use a clamp-on meter....
@Pit : Yes shunts can be destroyed too if they are too small.... so a 25 Amp shunt in your 750W circuit will start glowing like a dim filament until it goes "t*ts up"....

To determine what type of fuse is required you need to know the maximum allowable current going through the smallest diameter wire in that circuit. If the design is done correctly the wire gauge should be ample to supply the consumer i.e. if your motor is a 12Vdc/320W then the current (theoretically) be (P=UxI so I=P/U) 320/12= 27A... and a BIT more for running under load and poor efficiency. Your wire diameter is 6mm2 (=49 Amp. full load) then a 40 Amp slow fuse will do the job. Why slow? because a motor is an inductive load and has a high inrush current for a few milliseconds so it would not pop at first start. Car fuses/holders are a good alternatives and cheaper to replace then a toasted consumer or worse your simrig.....

Also remember that the size of wire ensures low resistance so better supply of those so much needed electrons to the motor. It is like breathing through a pipe, which is more relaxed when the diameter is large enough to supply you of the air....

For wiring sizes there are plenty sites of which this one: http://www.elecsa.co.uk/Technical-Library/Wiring-Regulations/Inspection-and-Testing/Current-Carrying-Capacity-of-Cables.aspx is very informative. If you know all this stuff no harm done, if you don't then maybe you do now.....

But wont you risk blowing other stuff besides the fuse in this case? Like boards and such?

Thanks for the info, I was wondering how to find what kind of fuse I would need, I guess I have to blow a few to find out. BTW, anyone else used their tongues to test 9v batteries when you were kids?

Concerning the 12Vdc and up DC driving circuitry, we will have; the H-bridge motor drive limitations are known through the specifications but most boards have their own build in protection circuits, of which over current is one of them; the motor limitations have a known appetite as it states on the label or again on the specifications; and the wiring & terminal limitations due to isolation material and wire size and ampere rating, which can be found on the various internet sites. The weakest link is determining the fuze size and type and if you look at the three components; the motor is often the beefiest; the motor drive has its own protection circuitry baked in their electronics; the wiring & terminals are often variables due to what is available and the most dangerous of them all due to the fire hazard it causes. If a wire or terminal is not rated for its (ab)use it can create heat causing the isolation to deteriorate (melt), which can lead to a short circuit that results in the smallest diameter wire to (hopefully) go up in smoke. If your fuse is properly selected this should be the smallest wire in the circuit else your power supply (switching or battery) will keep feeding the short till you have a failing (switching) power supply or fire place in the shape of a simrig..... either or a 50 cents worth fuse and some brain work can prevent months of collecting and tinkering on your pride and joy going up in some and worse your house or garage.

Concerning 110 - 240 Vac, just read and ask someone that is experienced in handling this form of power and the bits that come with it as this will drive you out of the gene pool if not followed as stipulated by the guide lines of your country.