Electronics Basics: Reading current?

[enigmahack]

So I'm not into the electrical engineering side of things, I'm more of a software dev/testing sort... however with lasers I've been seeing a LOT of technical people floating around that seem to know their stuff. (I don't know enough to say yes or no, but since other people that also seem to know their stuff agree, I have to conclude that they both know their stuff or it's an elaborate hoax and I'm not real. But I digress)

Anyway... I recently repaired a laser pointer that I had previously thought to have been dead, but because of where the diode came from and the power source I'm using, I know the diode is capable of being pushed MUCH harder than it is. (Which I want to do)

So, without getting too crazy, I'm curious about some of the common things folks do when putting together their home-made/custom lasers... Like measuring current draw - I have a digitial multimeter, and I know how to measure resistance and how to measure voltage... But the other stuff escapes me.

Does anyone know of any tutorials that exist that will help me get started in the basics of electrical "stuff" for the lack of a better term?

 

[Elektrotechniker]

Great thread. I'm in the same situation as you, enigmahack

 

[Kevlar]

To measure current draw you put your DMM in amps mode then use your DMM instead of a tailcap.

 

[enigmahack]

Ahhhhh.... This is fantastic, thanks Kevlar.

I guess my question then is: WHY does that work. It's nice to know how, but I would really like to understand the basics behind why that measures current. I have a pretty basic idea of what current, voltage, resistance, and amps are, but it would be nice to be able to understand them a little more to be able to come up with this stuff on my own.

Teaching a man to fish vs. teaching him to make a fishing rod ;-)

 

[Eudaimonium]

Ahhhhh.... This is fantastic, thanks Kevlar.

I guess my question then is: WHY does that work. It's nice to know how, but I would really like to understand the basics behind why that measures current. I have a pretty basic idea of what current, voltage, resistance, and amps are, but it would be nice to be able to understand them a little more to be able to come up with this stuff on my own.

Teaching a man to fish vs. teaching him to make a fishing rod ;-)

To understand that, you've gotta understand how this whole electricity stuff works.

You see, there are two values - voltage, and current.

Voltage is actually a difference in electric "potencial".
For example, you've got two points in space.
One point is at 0 volts potencial.
Another is at 5 V potencial.

If you connect those two points, physically, with electric conductor (copper, for instance), those two different points are trying to "equalize" in the energy potential. Which means, one with more energy (in our example, 5V one) will transfer energy to lower potential one, until they are equalised.

This energy transfer is called current

That's just about as much there is.

Now, measuring voltage is done in parralel to components - one probe of DMM measures one point, another one another point - DMM shows you the potential difference between those points.

Measuring current through - is done in *series*.
Which means : ---[component]--[DMM]--[Component]--

Now, if you want to measure the current draw of your laser, you should place your DMM in series with battery:
[Battery]--[DMM]---[Host]
Where host is just metal actual host of the laser, since that metal is connected to diode and further some components.

The current will flow through the DMM, where it will get measured - that's why it needs to be in series, not parralel.

 

[enigmahack]

Wow.
Mind blowing, yet one of the better explanations I've heard. I totally have to read up more on all of this stuff...

Thanks!

 

[Coherent Light]

That will give you the current draw the driver is pulling, and not what the LD is getting.

 

[DrSid]

If you measure current, the current must flow through the DM. In current measuring mode, DM has very low resistance (close to zero). You can measure current only between points where the current already flows in the circuit, and you must break the circuit to insert the measuring device. Think of current meter as simple piece of wire.

If you measure voltage, you can measure voltage between any two points of the circuit. Voltage meter has very high resistance (close to infinite). You don't have to break anything. You can think about voltage meter as piece of insulator.

Just make sure you don't have DM in current mode when you actually want to measure voltage. In best case it will burn DM's fuse. In worst case it will burn the circuit you tried to measure.

 

[Eudaimonium]

That will give you the current draw the driver is pulling, and not what the LD is getting.

In the case of linear drivers - like LM317 LM1117, and all those drivers these green lasers use, it's the same current since they are one-side drivers.

It's DC/DC converters, or boost drivers, that have different currents at input and output.

 

[Coherent Light]

^^ I should have clarified My statement. many are using boost drivers for the 405/445, and that was My intended target...

Thanks Eudaimonium.:beer:

 

[Eudaimonium]

It's all cool, it's great that you brought it up. Now people can read that it's different with DC/DC drivers, and not confuse stuff here.

 

[Elektrotechniker]

In the case of linear drivers - like LM317 LM1117, and all those drivers these green lasers use, it's the same current since they are one-side drivers.

It's DC/DC converters, or boost drivers, that have different currents at input and output.

And in case of DC/DC converters such as the flexdrive, how do you measure the current draw of the LD ? Or there is no point asking this, since we preset the current with a test load?

 

[DrSid]

You use test load for for presetting because you don't want to kill the diode. But for measurement alone, you can simply insert current meter between diode and driver.

 

[Eudaimonium]

You use test load for for presetting because you don't want to kill the diode. But for measurement alone, you can simply insert current meter between diode and driver.

Doing that is guaranteed to kill your diode/driver/both.

You only need one milisecond or less of open circuit , which happens a lot if you just hold probes with your hands, to blow the main IC of the driver, and charge up the capacitor, which in turn destroys your laser diode as soon as the connection is re-established.

DO NOT do this. Measuring the current is done when you are presetting the driver with test load. If you need to measure current later, you need to disassemble everything and then use test load to measure current again.

 

[Elektrotechniker]

Doing that is guaranteed to kill your diode/driver/both.

You only need one milisecond or less of open circuit , which happens a lot if you just hold probes with your hands, to blow the main IC of the driver, and charge up the capacitor, which in turn destroys your laser diode as soon as the connection is re-established.

DO NOT do this. Measuring the current is done when you are presetting the driver with test load. If you need to measure current later, you need to disassemble everything and then use test load to measure current again.

Thanks.

 

[DrSid]

Doing that is guaranteed to kill your diode/driver/both.

You only need one milisecond or less of open circuit , which happens a lot if you just hold probes with your hands, to blow the main IC of the driver, and charge up the capacitor, which in turn destroys your laser diode as soon as the connection is re-established.

DO NOT do this. Measuring the current is done when you are presetting the driver with test load. If you need to measure current later, you need to disassemble everything and then use test load to measure current again.

I did not say anything about holding something in your hands. Some drivers can be used with open circuit, some can't, and capacitors can complicate everything, but that all are different (though relevant) problems.

Imagine you want to measure current-voltage graph of the diode. That can't be done with test load, you need actual diode.