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Constant Current Driver Design (Also, i'm back :) )

paul1598419

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I did not say one couldn't measure current with a scope, just that it is not as easy. Anyone who has spent time using one knows they measure voltage. You have to get creative to turn that into current. That was my ONLY point.
 



RedCowboy

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I have seen people use LED drivers ( flashlight drivers ) to drive laser diodes and they seem to work, likely just need to be modified to suit the forward voltage that the diode needs.


What do you mean by a 60 mA ripple? There is no easy way to measure current with a scope. When talking about ripple, it is always in terms of a voltage.
You use a resistor and measure across it, same difference.

---edit---
If you look inside a volt meter you will see a thick piece of copper wire, that's a shunt that the meter measures across to give you the current reading by looking at the voltage drop, very tiny drops even.
 
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paul1598419

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I have seen people use LED drivers ( flashlight drivers ) to drive laser diodes and they seem to work, likely just need to be modified to suit the forward voltage that the diode needs.




You use a resistor and measure across it, same difference.

---edit---
If you look inside a volt meter you will see a thick piece of copper wire, that's a shunt that the meter measures across to give you the current reading by looking at the voltage drop, very tiny drops even.
If you wish to measure the current in some leg of a circuit using a scope you need to cut the leg and insert a resistor in series with it. This is not always easy as it can change the load in that part of the circuit. And if the resistor isn't a 1 ohm you will also have to use math to make sense of it. This can be the case when the leg in question has a low impedance so the resistor used is very small. All of these things are doable. I never said they weren't. It is just more complicated than how we use a scope in most circumstances.


Several years ago I tried to use an LED driver to drive some LPC836 laser diodes. After losing 3 or 4 of them I quit trying. It is certainly possible to use one and not have the laser diode die immediately, but probably in circumstances where the diode is under driven. It is certainly a fact that these drivers are much noisier than laser diode drivers.
 
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paul1598419

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Yes, I have a clamp meter and am very aware of them. Try using one on a trace on a PCB. Can't be done....or at least not easily.
 

Alaskan

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That link is misleading, they list current probes for an O'scope, but then don't show them and below that advertisement, show clamp on meters.
 

RedCowboy

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Refresh and look at the digikey link, you can use a shunt for PCB you don't have to use a loop, it's just another option however for watching a driver you can easily clamp a wire and see what it's doing.

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When I don't solder my driver directly to the LD pins I like to check my actual current the LD is getting using an inductive pickup, try it and see if you set your driver where you thought with that bunch of rectifiers and resistor, you may be surprised as the resistance on the output side of your driver can change what it will output, may not be the same as when you solder your diode pins straight into the driver's output.
 
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paul1598419

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Those clamp probes are for automotive use. When you want to look at how cylinders are firing you can use a clamp probe as a normal scope probe won't work. They are actually converting the current to a usable voltage so it is accessible.
 

diachi

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This is not always easy as it can change the load in that part of the circuit.
Current sources already have a shunt resistor. Otherwise they wouldn't be able to regulate the current very well, would they?
 

Cyparagon

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If you wish to measure the current in some leg of a circuit using a scope you need to cut the leg and insert a resistor in series with it.
No! as I've already pointed out, Constant current drivers already have a resistor to sense the current as part of the feedback loop.

if the resistor isn't a 1 ohm you will also have to use math to make sense of it.
Correct! as I've already pointed out, use ohm's law. It's one of the first things people learn in electronics. If you don't think ohm's law is easy, circuit design isn't for you.

Try using one on a trace on a PCB. Can't be done...
Leads are often ran to the laser diode anyway. Soldering the diode pins directly to the PCB isn't necessary. Besides, you've lost the scope of this thread entirely. OP wants to design a driver. Using an oscilloscope for measuring ripple current is not only easy by comparison, it is also NECESSARY for proper troubleshooting and verification.
 

paul1598419

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Yes, if one is looking at a current source, which has a sense resistor in it, then you have your resistor already there. But, this discussion went off to how easy it is to measure circuit current anywhere and that was also what I was trying to show that this is not the case. It is funny how some of you take my words out of context or omit the rest of the sentence to make your point. Yes, in a CC driver it would be fairly easy to use a current probe with a scope to measure the ripple current that it produces, especially when you are measuring at the diode, but that is not always the case if you are working in some other type of circuit. the lower the current is that you are trying to measure the more difficult it becomes to get an accurate reading in AC with a scope. These are not all of the problems that can arise from trying to use a current probe with a scope in a circuit.
 

DTR

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Here is something interesting I found the other day when digging around the net from Osram. It has a lot more info than standard datasheets as well as a section with detailed info on suggest ways for designing laser drivers when using their diodes. Goes over what they consider needed features of a laser driver, Info for both linear and switching types with lists of components and links to suppliers. See section C.
 

paul1598419

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Though there isn't any new information in this article, it does have a collection of many necessary considerations about laser diodes in general and drivers for them in particular. It would be a great read for anyone making a first attempt at building a laser diode driver. Thanks, Jordan.
 
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