DIY Laser Torch

[roSSco]

Lift one leg out of the breadboard and connect the meter between the lifted leg of the diode and where the leg of the diode would go if it was still in the circuit.

 

[knimrod]

1. Disconnect the positive lead from the battery.
2. Connect the positive lead of the Amp meter to the battery positive lead.
3. Connect the negative lead of the Amp meter to the positive power input of your circuit.
4. Measure current.

The amp meter essentially takes the place of the wire going from the positive lead of the battery to the positive power input of your circuit.

 

[roSSco]

or you could remove either of the purple jumpers going to the LED and replace it with your meter set to measure current.

 

[roSSco]

1. Disconnect the positive lead from the battery.
2. Connect the positive lead of the Amp meter to the battery positive lead.
3. Connect the negative lead of the Amp meter to the positive power input of your circuit.
4. Measure current.

The amp meter essentially takes the place of the wire going from the positive lead of the battery to the positive power input of your circuit.

That will tell total circuit current, not current to the LED.

 

[knimrod]

That will tell total circuit current, not current to the LED.

Umm.. Sorry.  No.  The current in a series circuit is the same everywhere.  

 

[roSSco]

[quote author=roSSco link=1186966870/210#212 date=1199932330]That will tell total circuit current, not current to the LED.

Umm.. Sorry. No. The current in a series circuit is the same everywhere. [/quote]

Look at his circuit. Is it a series circuit?

No it is not.

 

[knimrod]

Look at his circuit.  Is it a series circuit?  

No it is not.

Yep.. There is only one path for the electrons to flow.

See "Kirchhoff's Laws"...

 

[roSSco]

[quote author=roSSco link=1186966870/210#214 date=1199932993]
Look at his circuit. Is it a series circuit?

No it is not.

Yep.. There is only one path for the electrons to flow.

See "Kirchhoff's Laws"...
[/quote]

 

[troop231]

1. Disconnect the positive lead from the battery.
2. Connect the positive lead of the Amp meter to the battery positive lead.
3. Connect the negative lead of the Amp meter to the positive power input of your circuit.
4. Measure current.

The amp meter essentially takes the place of the wire going from the positive lead of the battery to the positive power input of your circuit.

Ok, I understand now! Thanks a bunch! I'll have to try this out tomorrow though.

 

[roSSco]

[quote author=knimrod link=1186966870/210#210 date=1199932181]1. Disconnect the positive lead from the battery.
2. Connect the positive lead of the Amp meter to the battery positive lead.
3. Connect the negative lead of the Amp meter to the positive power input of your circuit.
4. Measure current.

The amp meter essentially takes the place of the wire going from the positive lead of the battery to the positive power input of your circuit.

Ok, I understand now! Thanks a bunch! I'll have to try this out tomorrow though.[/quote]

Those are not the correct instructions. Try my method.

 

[troop231]

[quote author=troop231 link=1186966870/210#217 date=1199933791][quote author=knimrod link=1186966870/210#210 date=1199932181]1. Disconnect the positive lead from the battery.
2. Connect the positive lead of the Amp meter to the battery positive lead.
3. Connect the negative lead of the Amp meter to the positive power input of your circuit.
4. Measure current.

The amp meter essentially takes the place of the wire going from the positive lead of the battery to the positive power input of your circuit.

Ok, I understand now! Thanks a bunch! I'll have to try this out tomorrow though.[/quote]

Those are not the correct instructions. Try my method.
[/quote]

Explain in some detail why they would be incorrect?

 

[roSSco]

Current splits in a parallel circuit, which is what you have. If you measure at the input you will measure the TOTAL CURRENT CONSUMED BY THE CIRCUIT, NOT THE INDIVIDUAL COMPONENTS.

 

[Gazoo]

[quote author=troop231 link=1186966870/210#217 date=1199933791][quote author=knimrod link=1186966870/210#210 date=1199932181]1. Disconnect the positive lead from the battery.
2. Connect the positive lead of the Amp meter to the battery positive lead.
3. Connect the negative lead of the Amp meter to the positive power input of your circuit.
4. Measure current.

The amp meter essentially takes the place of the wire going from the positive lead of the battery to the positive power input of your circuit.

Ok, I understand now! Thanks a bunch! I'll have to try this out tomorrow though.[/quote]

Those are not the correct instructions. Try my method.
[/quote]

Knimrod's instructions are spot on and I suggest we all get in the habit of measuring current this way if using the LM317.

It is too risky to measure current between the laser diode and the output...one slip and it's bye bye diode. kirchoff's law does confirm the current will be the same, and if you goggle it you will understand.

There are exceptions...for example using a boost circuit you would see a higher current between the power supply and circuit, than you would see between the output and the diode.

Another example would be using the 7135 with a 33 ohm resistor in parallel with the diode. You would get a reading of 350ma's going to the circuit, but if you measure between the laser diode and the output you would see 250ma's since the 33 ma resistor is absorbing 100ma's of current. But if you were powering a diode with the full 350ma's the 7135 puts out, you would see 350ma's at the input and output.

With the 317 it doesn't matter, the current will be the same whether measured at the input or output.

 

[roSSco]

Why would you want to use a $400.00 Fluke to estimate the current going to your diodes? The only way to get the actual current (in the circuit shown) is my method.

 

[roSSco]

I guess close counts in horse shoes, hand grenades and now, laser diodes.

 

[knimrod]

It's not an estimate, it's an empirical determination of the actual current and a practical demonstration of a fundamental rule of circuit theory taught in basic electronics.  

Tell ya what roSSco, why don't you do the test yourself and report back here???  Measure the current on the input side and then measure at the laser diode side.

You will find that the Kirchhoff's laws hold true in practice as well as theory.  If you are using the traditional LM317 current regulator circuit, it is indeed a series circuit in that there is only one path for the electrons to flow...

Specifically, Kirchhoff's first law law states that: The algebraic sum of current into any junction is zero.  Which really means: What comes in must come out. Or, The sum of current into a junction equals the sum of current out of the junction.

And as a clarification to Gazoo's comment, there are no exceptions to Kirchhoff's laws regarding a series circuit but there are different methods for current regulation that are not really series circuits.  And he is absolutely right that there is little risk when measuring the current on the input side as compared to the risk of breaking and making the connection on the laser diode leads.