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Topic: LED photostrobe Alpha test (Read 3 times) previous topic - next topic

focalist

#10
Apr 16, 2012, 09:39 pm Last Edit: Apr 16, 2012, 09:48 pm by focalist Reason: 1
Hmm.. I was uncertain about that feedback loop, as there would of course be current from the other regulators.  If there is a diode or transistor in the output stage preventing current from coming from the other regulators, it will work I would think.  I even have some diodes that can handle the power level, though I still wonder if they would be unnecessary or just added drain.

Hmm.. Gonna need a test load too.... Maybe an incandescent bulb, 12v.. Close enough for government work... I may have a 55 watt headlight bulb spare to use as a nice load....

TheMOSFETs arrived in the mail today, in all honesty now that I have read about them, I am excited to give them a try.  No more Darlingtons for me on heavy loads... so much better at carrying current.. And so much cleaner and faster switching.. Guess it was time for me to move out of the 1940s...  makes it clear how far my formal electronics training went... basic theory, and I wasn't paying attention anyway.  On the other hand, I am extremely proficient in home firefighting now  :smiley-mr-green:
When the testing is complete there will be... cake.

focalist

#11
Apr 18, 2012, 08:29 pm Last Edit: Apr 18, 2012, 08:30 pm by focalist Reason: 1
http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm138&reg=en&fileType=pdf

Okay, so I have looked over the specs on typical adjustable regulators, and it still looks like a reasonable way to provide current limiting.  Since I have LM317T in hand, I want to make three of the current regulator circuits from the spec sheet (1.2 ohm between output and adjust makes for 1a regulator).

In thinking about this, and getting ready to wire up a test circuit, I was looking at the schematic in spec sheet for running the LM317 in tandem as a VOLTAGE regulator.. and it shows the setting resistor "shared" between the adjust connections on the regulators.. This makes sense to me, I think, as the resistor output voltage remains the same, even if the current is spread across the multiple terminals.. And as it only cares about that voltage, "sharing" the resistor ensures a single feedback loop to the regulators, keeping them balanced.

Given this is the case, does the same design consideration apply if I were connecting multiple LM317 current sources?  I am thinking that a single voltage split three ways will be better than three sorta close voltages...
When the testing is complete there will be... cake.

Udo Klein

Quote
Darlingtons for me on heavy loads... so much better at carrying current.. And so much cleaner and faster switching.. Guess it was time for me to move out of the 1940s...  makes it clear how far my formal electronics training went... basic theory, and I wasn't paying attention anyway.


This makes me wonder if you are driving the mosfets properly. Do you account for the gate capacitance?
Check out my experiments http://blog.blinkenlight.net

focalist

#13
Apr 18, 2012, 09:51 pm Last Edit: Apr 18, 2012, 10:06 pm by focalist Reason: 1
How so?  I thought  darlingtons are inherently slower, in fact I was concerned if they were getting fully ON before going off again.  I thought that MOSFETS are inherently better until you are into MHz switching rates,
Isn't the gate charge dissipated when the pin is taken low?  I haven't even added a gate to source high value resistor yet, which I understand is typically how the gate charge is bled off.

Even without any additional components, no gate resistor, it seems to work great.. Though the gate resistor is a better idea so as to protect the AVR from the current draw of charging the gate. I have read conflicting rcommendations regarding the use of gate resistance, and even use of the gate to source resistance.  

Even if there is a charging delay, that only shortens the length of the emitted pulse from the LED, I would only have to compensate by adding that time interval to the turnoff delay.  If I have read properly, these should be able to switch in the tens of nanoseconds in their normal use.

Am I missing something?
When the testing is complete there will be... cake.

jwatte


Quote
Darlingtons for me on heavy loads... so much better at carrying current.. And so much cleaner and faster switching.. Guess it was time for me to move out of the 1940s...  makes it clear how far my formal electronics training went... basic theory, and I wasn't paying attention anyway.


This makes me wonder if you are driving the mosfets properly. Do you account for the gate capacitance?


I think that quote is out of context. The words right before that quote were "no more Darlingtons ..." and the "so much better at carrying current" sentence references MOSFETs, not Darlingtons.

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