What is the science behind a dummy load?

Flugufrelsarinn · Sep 20, 2013

How does a dummy load represent a laser diode?

To my knowledge all I need to make a dummy load is a bunch of rectifier diodes in series with a 1 ohm resister. What does this do to simulate a laser?

SKeeZ · Sep 20, 2013

NO4H99 said:
A laser diode is in essence a resistor. It converts potential energy of the electric circuit into kinetic energy in the form of light. Because you don't want to kill your diode testing your circuit, you use a resistor in its place. It simulates the resistance caused by your LD.

A laser diode is not necessarily a resistor just because it takes energy and expends it as light. light also does not have kinetic energy becuase it has no mass... the resistor in a dummy circuit is used to pull down the current and expend it as heat instead of light and heat (i may be wrong but i don't think so). So, if we want to be more specific, a laser diode is a light emmiting diode that converts current to light. Be a little more careful what you say, just as friendly advice. its not cool to misinform people.

ryansoh3 · Sep 20, 2013

I'd just like to add that the resistor has a sole purpose of making current easier to measure via volt-meter because with an one-ohm resistor, 1A = 1V.

NO4H99 · Sep 20, 2013

Fair enough. All LEDs convert current to light, though.

Also light does have kinetic energy according to special relativity and Planck. The full planck constant actually states: "mass^2 = [(Energy^2)/c^4] - p^2/c^2" - which allows a mass-less particle such as a photon to have kinetic energy & momentum in space-time.

EDIT: Negative rep for improper science?? really??
Please google kinetic energy of mass-less photons, planck's constant, or kinetic energy of light.

It is quite possible to have kinetic energy without mass. Take your laser for example. That beam of light contains no mass, but is measured in watts (which is a measurement for power). Since power is measured in Joules (or your chosen energy unit) per second, it must have energy.

After you are done, please take back the negative rep.

Thanks

SKeeZ · Sep 20, 2013

I appologize, i was under the understanding that massless particles can have momentum but not kintetic energy because kinetic energy is movement of massive particles. these have velocity but not mass. KE and Momentum are two different things. also the neg rep was for a number of answers that were not accurate and/or not helpful, not just on this thread.

NO4H99 · Sep 20, 2013

Its all good.

Your last post was completely factual, in the newtonian sense Energy (E=F*delta(X) & F=m*a, requiring mass for Energy). Special Relativity, however, described how to deal with situations that deviated from Newtons theories (usually dealing with extremely small or massive particles, or particles moving near the speed of light - like this situation). In almost any other situation, Newton's laws would work perfectly (or so close to perfectly that the difference was negligible.)

SKeeZ · Sep 20, 2013

fair enough, I guess i just assume newtons laws apply all the time... I'd fix the neg rep but i can't.

djQUAN · Sep 20, 2013

NO4H99 said:
A laser diode is in essence a resistor.

It's actually not that simple. A resistor is a linear device. In which the current is in linear proportion to the applied voltage across it.

The diode junction in a laser diode is not. The voltage across it is almost constant (not perfect due to dynamic resistance) or varies just a small amount depending on the current flowing across it and even with the junction temperature.

Since laser diodes (and LEDs) have high forward voltages, we just use an appropriate number of silicon diodes to emulate or just reach the ballpark Vf of the diode in the final application. The resistor is there to make current measurement easier.

NO4H99 · Sep 20, 2013

djQUAN said:
It's actually not that simple. A resistor is a linear device. In which the current is in linear proportion to the applied voltage across it.

The diode junction in a laser diode is not. The voltage across it is almost constant (not perfect due to dynamic resistance) or varies just a small amount depending on the current flowing across it and even with the junction temperature.

Since laser diodes (and LEDs) have high forward voltages, we just use an appropriate number of silicon diodes to emulate or just reach the ballpark Vf of the diode in the final application. The resistor is there to make current measurement easier.

Correct

, just trying to simplify.

IIRC a resistor has a current directly proportional to the voltage (as expressed by Ohm's Law). The current existing in a diode, however, depends on the number of electrons with enough charge needed to break the diode's "energy barrier" (which is specific to that diode, depending on its material, etc.). When the potential difference (∂Voltage) increases, the energy needed to cross the p-n junction is decreased. As a result, the overall current is exponentially increased.

However, outside factors (such as higher ambient temperatures which increase the amplitude of vibrating atoms or the changing amount of dopants in the semiconducting substance) affect the thresh-hold of energy needed to pass the "junction". ...Which is both the reason why a dummy load cannot be 100% accurate, as well as one of the primary differences between a resistor and diode.

Cyparagon · Sep 24, 2013

NO4H99 said:
A laser diode is in essence a resistor. It converts potential energy of the electric circuit into kinetic energy in the form of light.

1) Laser diodes are not like resistors at all.
2) Resistors do not convert from potential energy.
3) Resistors do not convert energy to kinetic energy.
4) Resistors do not convert energy to light.
5) Laser diodes do not convert from potential energy.
6) Laser diodes do not convert energy to kinetic energy.
7) Electric circuits do not have kinetic energy.
8) Photons themselves may technically have kinetic energy, but light is electromagnetic energy.

This was just a terrible explanation, and you should be ashamed of yourself.

NO4H99 said:
...electrons with enough charge..

9) Electrons cannot change their charge.

NO4H99 said:
the thresh-hold of energy needed to pass the "junction".

10) Voltage is needed to "pass the junction," not energy.

Fiddy · Sep 24, 2013

Cyparagon said:
1) Laser diodes are not like resistors at all.
2) Resistors do not convert from potential energy.
3) Resistors do not convert energy to kinetic energy.
4) Resistors do not convert energy to light.
5) Laser diodes do not convert from potential energy.
6) Laser diodes do not convert energy to kinetic energy.
7) Electric circuits do not have kinetic energy.
8) Photons themselves may technically have kinetic energy, but light is electromagnetic energy.

This was just a terrible explanation, and you should be ashamed of yourself.

9) Electrons cannot change their charge.

10) Voltage is needed to "pass the junction," not energy.

Word Cyparagon!

NO4H99, It seems you have no idea, please dont try and teach newbies wrong information.

My input:

The rectifier diodes = simulating the Vf or forward voltage drop of the diode.

The 1 ohm resistor = simple ohms law, if you apply 1V to a 1ohm resistor you will draw 1 amp. If you apply 50 volts to a 1 ohm resistor, you will draw 50 amps.

so in that circuit, V = I hence why we can measure the current with a voltmeter across the 1 ohm resistor.

ryansoh3 · Sep 24, 2013

I've always wondered why people used 1 Ohm resistors instead of a ammeter. :thinking:

Wouldn't the resistor drop an additional 1.8-2.3V depending on the current, and don't the resistors have relatively poor tolerances than ammeters?

Fiddy · Sep 24, 2013

No not that much.

If the resistor selected by the user is too small power wise, the heat being disipated by the resistor will cause an increase in resistance, hence resistors have a positive temperature co-efficenet.

A increase in resistance will mess up the voltage reading and hence display a different current than what you are try to measure.

ammeters use resistors to measure current too

ryansoh3 · Sep 24, 2013

Hmm... is there any benefit to using a resistor instead of an ammeter?

I do realize that ammeters also have current shunts but don't most DMM's double as ammeters?

Fiddy · Sep 24, 2013

Yes, you dont have to put the instrument in series with the load becuase your using measuring the voltage across the resistor to see the current value.

Yes most DMM's do.

If you use a DMM and put it in series with the load and have it on the wrong setting, some boost (not sure about buck, deffinatley not linear) drivers may blow up if they see no load conditions.

Using a resistor is safer for your driver and conveinent.

ryansoh3 · Sep 24, 2013

I see, that makes sense. Thanks for your explanation. :beer:

What is the science behind a dummy load?

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