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I couldn't wait any longer, I needed to figure out why many RPL lasers will run at 100% duty cycle and full
output power while with others the "diode protection circuit" will activate and shutdown the diode after only
2-3 minutes.I was supplied with both electrical and mechanical schematics from the manufacturer that showed me for the most part how the laser works electrically, except for the area I was most interested in; for this part
of the schematics, there was only a "black box" outline so to speak. With input and output, but no details of
what's inside.
So I spent this past Sunday afternoon dissecting an RPL I have that had a short 40 second duty cycle. I was able
to gain some access to this part of the circuit without disturbing the critically aligned diode and optical
assembly as this section can be removed all in one piece. This "diode protection circuit" is mounted on a
seperate circuit board that is about 5mm x 20mm in size, has 2 ICs, a couple of resistors and a few caps, the
circuit board was inside of a section of shrunken heat shrink tubing to both protect it and make it more
difficult to easily access it. The next step was to spend lots of time looking up the components as they were
not common. I then made a schematic for this circuit board by removing the ICs and using my microscope and DVM
to verify the traces.
The most interesting thing I found is that a "diode protection circuit" is not it's primary function. The main
function of this circuit is battery protection. As you know the RPL uses a rechargeable lithium-ion cell, also
as you may have read in the news recently, these batteries can be quite dangerous if they are over-charged,
over-discharged or discharged too quickly. This is not a problem if "protected cells/batteries" are used, but
there is no guarantee that someone won't at some point use an unprotected cell, in fact the ones I use are
unprotected. Without some way to detect a battery low or over-current state,a laser that uses a lithium-ion cell
and is left on until the battery really goes dead, may end up with more of a surprise than they ever expected
from their laser. This circuit is basically the same as what you would find in a lithium-ion battery charger. It
senses the voltage drop of the cell as well as current. When it senses an over-discharge or over-current state
it will turn off the current by way of a pair of very low impedance MOSFET switches. It's these MOSFET switches
that give the secondary function of "diode protection". MOSFETS are very sensitive to ESD or voltage spikes, so
all MOSFET ICs have protection diodes across them to channel any spikes to ground, this also protects the diode
from these same spikes or ESD that can easily damage a laser diode. My guess is that this circuit was based on
older cells which could only supply an 1-2 Amps of current for a short time, but that's not the case with the
high capacity cells we use. The 2400-2600mAhr cells available now can supply 3-4 amps for nearly their full
discharge cycle.
This is getting long, so I'll add more in the next few days. I'm going to finish this up this weekend and post
pictures. I have run several experiments on different modifications to this circuit to make it more in line with
the current capacity of 2400-2600mAhr cells and intend to do more this weekend and find the most effective
circuit changes. by just changing the circuit so that the MOSFETS would always be turned on, allowed the RPL to
run for 25 minutes at full 275mW output before I turned it off, a big improvement in duty cycle. This would be a
fix if protected cells were to always be used, but I can't count on that. So I will work on a change to the
circuit so that the battery is still protected from over-discharge but 100% duty cycle is allowed.
This makes much more sense as the 2.5W IR diode in the RPL is only running at ~75% of it current rating when the
RPL is using it's maximum current of 2.0 Amps, it should not be shutting down at 75% of it's rating due to
overheat and it's not, it shutting down because the battery protection circuit is being set to start it's
shutdown timer at too low of current for the capacity of todays rechargeable lithium-ion cells.
I'll post more in the within the next week as well pictures and rework instructions for any RPL who are handy
with a good soldering station and whose laser is out of warranty and want to do this fix.
More to come
output power while with others the "diode protection circuit" will activate and shutdown the diode after only
2-3 minutes.I was supplied with both electrical and mechanical schematics from the manufacturer that showed me for the most part how the laser works electrically, except for the area I was most interested in; for this part
of the schematics, there was only a "black box" outline so to speak. With input and output, but no details of
what's inside.
So I spent this past Sunday afternoon dissecting an RPL I have that had a short 40 second duty cycle. I was able
to gain some access to this part of the circuit without disturbing the critically aligned diode and optical
assembly as this section can be removed all in one piece. This "diode protection circuit" is mounted on a
seperate circuit board that is about 5mm x 20mm in size, has 2 ICs, a couple of resistors and a few caps, the
circuit board was inside of a section of shrunken heat shrink tubing to both protect it and make it more
difficult to easily access it. The next step was to spend lots of time looking up the components as they were
not common. I then made a schematic for this circuit board by removing the ICs and using my microscope and DVM
to verify the traces.
The most interesting thing I found is that a "diode protection circuit" is not it's primary function. The main
function of this circuit is battery protection. As you know the RPL uses a rechargeable lithium-ion cell, also
as you may have read in the news recently, these batteries can be quite dangerous if they are over-charged,
over-discharged or discharged too quickly. This is not a problem if "protected cells/batteries" are used, but
there is no guarantee that someone won't at some point use an unprotected cell, in fact the ones I use are
unprotected. Without some way to detect a battery low or over-current state,a laser that uses a lithium-ion cell
and is left on until the battery really goes dead, may end up with more of a surprise than they ever expected
from their laser. This circuit is basically the same as what you would find in a lithium-ion battery charger. It
senses the voltage drop of the cell as well as current. When it senses an over-discharge or over-current state
it will turn off the current by way of a pair of very low impedance MOSFET switches. It's these MOSFET switches
that give the secondary function of "diode protection". MOSFETS are very sensitive to ESD or voltage spikes, so
all MOSFET ICs have protection diodes across them to channel any spikes to ground, this also protects the diode
from these same spikes or ESD that can easily damage a laser diode. My guess is that this circuit was based on
older cells which could only supply an 1-2 Amps of current for a short time, but that's not the case with the
high capacity cells we use. The 2400-2600mAhr cells available now can supply 3-4 amps for nearly their full
discharge cycle.
This is getting long, so I'll add more in the next few days. I'm going to finish this up this weekend and post
pictures. I have run several experiments on different modifications to this circuit to make it more in line with
the current capacity of 2400-2600mAhr cells and intend to do more this weekend and find the most effective
circuit changes. by just changing the circuit so that the MOSFETS would always be turned on, allowed the RPL to
run for 25 minutes at full 275mW output before I turned it off, a big improvement in duty cycle. This would be a
fix if protected cells were to always be used, but I can't count on that. So I will work on a change to the
circuit so that the battery is still protected from over-discharge but 100% duty cycle is allowed.
This makes much more sense as the 2.5W IR diode in the RPL is only running at ~75% of it current rating when the
RPL is using it's maximum current of 2.0 Amps, it should not be shutting down at 75% of it's rating due to
overheat and it's not, it shutting down because the battery protection circuit is being set to start it's
shutdown timer at too low of current for the capacity of todays rechargeable lithium-ion cells.
I'll post more in the within the next week as well pictures and rework instructions for any RPL who are handy
with a good soldering station and whose laser is out of warranty and want to do this fix.
More to come
), but am still curious.