Direct Drive/Measuring in Test Load - Tips

Hello all.

I've been doing a lot of research lately and I found out a few things.

Say you're building another death ray that needs a fairly high amount of voltage. Or current. Or both. Well, the main problem that happens with high voltage projects, and/or direct drive builds is the resistance that tends to build up in thinner wires. Wikipedia has a nice store of info for various resistances across many wire gauges (AWG). So in short, the wider the wire, the LESS resistance it has. And with different metals, the resistance could vary as well.

American wire gauge - Wikipedia, the free encyclopedia

Well, typically there is a slight problem, as people don't know their exact wire gauge, as the insulation around it could vary... and a whole lot of other factors. People also tend use the resistance function on DMMs when they are trying to measure resistance. This is wrong for a reason. With the wires and internal PCBs also in the measuring circuit, the resistance reading is unreliable and not very accurate at all, in fact the resistance is higher than the actual resistance in the wire you are measuring. Not even top-of-the-line DMMs can measure the resistance accurately.

However, this should be known to all people who are doing a direct drive or something similar that uses a lot of current and needs a very low resistance. The correct way to measure the resistance across a piece of wire or just any plain resistor without the interference of the cables is called four wire resistance measurement. This method uses voltage measurement to bypass the resistance in the wires. And also, its almost the same when us laser-using people set drivers with test loads. What happens is typically, a current of exactly 1 ampere is pushed though a wire or unknown resistor value. Then a DMM measures the voltage across the unknown resistor. be sure to note that you want to measure wires at the very tips so that you know the resistance of the entire piece of wire, since wire is like a resistor... in a way. This works for a very, very simple reason.

Ohms law.

V = IR.
Where V is voltage, I is current, and R is resistance. For all of you who don't understand this, please do not use lasers in any way, fashion, or form (I have a cousin who keeps playing with my lasers with his stupid foolishness). Rearranging the formula gives us

V / I = R.

Since the current is exactly 1 ampere, it can be eliminated from the equation.
Where V = R. The voltage value is equal to the resistance, whether it be 1 volt, 1 millivolt, 1 microvolt, which translates into 1 ohm, 1 milliohm, 1 microhm, and so forth.

If your behind is too large to even read all that, you should really consider doing the research for yourself. Here's a great video by afrotech. He makes other videos on electricity that can REALLY help you in building lasers. He also includes a bit of humor in his videos for a nice laugh. Also with subtitles.

4 wire (Kelvin) resistance measurement tutorial - YouTube

Thats it for that, and have fun with building new death rays for all us laser-lovers to see... phatlights tooo.

___

For the next part for all you new people, using the above method is one thing, but also its a useful device to measure current... once you rearrange everything up.

The above equation V = IR is ohm's law. Use it a lot.

Typically, you will need rectifier diodes (usually a few 1N4001 s) to simulate your diode's forward voltage drop, and a 1 ohm resistor. Since the current is the unknown and the resistor value is constant, you can find out the current easily by, once again rearranging ohm's law. This time...

V / R = I

And since R is 1 ohm, V = I ... where it may be 1 volt, 10 volts 100 volts whatever, and translated to 1 amp, 10 amps or even 100 amps, which is terribly unsafe for anyone working with lasers.

Thats it, now enjoy your new test load tips, lasers, death rays and other various sorts of light emitting sources... or just anything at all.


Iso

Thanks for that.... I see you have done a lot of
research...

The Video is very informative to measure in the
mOhm ranges...

As to test Loads that nominally use 1 Ohm resistors..
have you seen this from 12/30/2008.....

http://laserpointerforums.com/f44/current-28278.html#post474315

or these.....



http://laserpointerforums.com/f39/wtb-test-load-aka-dummy-load-pcb-22821.html

http://laserpointerforums.com/f39/fs-selectable-test-load-up-3a-63450.htm

http://laserpointerforums.com/f64/fp-super-3-amp-test-load-62826.html

http://laserpointerforums.com/f39/3a-test-loads-jib-now-stock-65885.html


Laser Diode Tests loads and how to use them have been
around for quite a while....:beer:


Jerry

Measuring like displayed above is actually pretty much the same as the 4 wire method for measuring resistance, but the other way around: You have a resistor of know value and measure the voltage across it to get the current.

This approach is very accurate as the input resistance of the multimeter is typically very high and does not interfere with the measurement. Accuracy is limited by the tolerance of the resistor... and though most are rated 5% tolerance, typical deviation from nominal is in the order of 1%.

Isoleucine said:

... Not even top-of-the-line DMMs can measure the resistance accurately .... The correct way to measure the resistance across a piece of wire or just any plain resistor without the interference of the cables is called four wire resistance measurement. ...

Click to expand...

Top-of-the-line DMM's all have 4 wire resistance measurement. My Keithley 199 DMM's have a 1mOhm resolution and 4 wire ohms measurement, usefull to determine the exact resistance of sense resistors.

Isoleucine said:

people don't know their exact wire gauge, as the insulation around it could vary...

Click to expand...

Wire strippers and caliper :beer:

It's not as precise if the wire is stranded, but almost all wire gauge is a multiple of 2, so you can round. That silicon 26 awg is common. 4 inches of 26 awg is ~13mΩ. So it only wastes 40mW of heat (0.5% loss of power) at 1.8A

Awesome thread! I need to understand this in depth sometime. Really helpful just with one look

To address all of you without quotes...

This thread is just on how to do 4 wire measurement. And yes, I've read about a lot about testloads, seen most if not all the pictures by ROG8811, read all the old threads, and seen most of the videos that LPF members make... and abused the search bar to the point where my broswer crashed once.

Sometimes, just doing the 4 wire measurement may just bypass all of the caliper measuring and other stuff. But yes, measuring with a caliper is useful if you are terribly unskilled at using a DMM, yet you can do math well. And lastly, the DMMs that I deal with are NOT reference DMMs. Just handheld ones made by various companies. Those reference DMMs are super-out-of-my-price range and scope. Pardon that.