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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
I have been up to researching things and came across a circuit using the LM723 regulator. The LM723 is supposed to be more precise than the others (LM317 and such). It also allows current limiting that I can set. Now my question is, is this a safe way to run a laser diode? Will this kind of current limiting work right? Im not really sure what the LM723 does when the set current limit is reached. (Does it shut off until the current is decreased?) Could I simply set the desired current throught the diode to be a little more than what it would see? The current limiting works by measuring the voltage across a sense resistor right after a pass transistor? and then responds accordingly. Im just not postive as to WHAT exactly it does. (Shutting off would be bad, adjusting the output such that the current stays there would be good). Seems this would be an excelent way to run a laser diode? Thanks, Matt
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The LM723 doesn't shut off, it just regulates the output current. It's not particularly accurate in constant-current mode, since the sensing element is just a single transistor, whose Vbe varies with temperature and so on. But if you don't mind that, it should work as a constant current source fine.
I built a bench power supply with a LM723 years ago.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, Having some trouble...I went ahead and built the original circuit using the op-amp and transistor, except I changed the 2.5ohm sense resistor to a 0.1ohm one, and the supply voltage is from a 5volt 40amp flatpac. I cant get more than 1.2amps out of it (just using one transistor and op-amp rather than the two in the circuit above). I dont understand?
When I used a 12 volt battery, I was able to get around 2.5amps... For the volt ref, I used a 10kohm and 2kohm trimpot to make a divider...Can someone help me out? It also reaches max current out, and then slowly climbs a bit higher and is about 20mA greater after a minute or so. Im so confused...(Op-amps are new to my projects) Matt
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
mattrg2 wrote ...
When I used a 12 volt battery, I was able to get around 2.5amps... For the volt ref, I used a 10kohm and 2kohm trimpot to make a divider...Can someone help me out? It also reaches max current out, and then slowly climbs a bit higher and is about 20mA greater after a minute or so. Im so confused...(Op-amps are new to my projects) Matt
I don't know your exact circuit but I guess your transistor does not get enough base current to open it enough for the current you want. Try to use a darlington or a transistor with higher Hfe. The rise in current is probably as the transistor heats up, the Hfe usually goes up a bit.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Dr. Kilovolt wrote ...
mattrg2 wrote ...
When I used a 12 volt battery, I was able to get around 2.5amps... For the volt ref, I used a 10kohm and 2kohm trimpot to make a divider...Can someone help me out? It also reaches max current out, and then slowly climbs a bit higher and is about 20mA greater after a minute or so. Im so confused...(Op-amps are new to my projects) Matt
I don't know your exact circuit but I guess your transistor does not get enough base current to open it enough for the current you want. Try to use a darlington or a transistor with higher Hfe. The rise in current is probably as the transistor heats up, the Hfe usually goes up a bit.
Hello, Thanks for the reply. The circuit is the one found here: Could it have anything to do with the 5 volt supply not being 12volts? I will look for a better transistor....Any reccomendations on a transistor? Matt
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yeah, the TL08x series are really not well suited to running off a single 5v supply. They'll hardly deliver any current. You should probably add a -5v rail just for the op-amps, or use a rail-to-rail op-amp like the LMC660.
And yes, a Darlington would help, since the TL08x can only output about 20mA, which is not a lot to drive a power transistor. Or even a P-channel power MOSFET if you have any.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Alright, so I monkeyed around some more with the circuit to see about just finding a 12volt flatpac.(Uisng the 12volt battery again)..Current gets up past 5 amps (more than adequate), but the transistors (TIP41)get SMOKING hot, even on a largish heatsink! The current constantly climbs higher than what its set at. (Totally defeats the purpose of an op-amp). Is this from the heating of the transistors? Why are they getting so hot? Im beginning to wonder if I shouldnt just get a 5amp regulator IC and run it in constant current mode...Never had this rising current, nor the nasty heating...Its just the transistors too, everything else stays cool...I was thinking it could be the low ouput current from the op-amp, but the guy who originally designed the circuit seems to be doing alright with it.. Im getting closer! Matt
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
*bump* This is a good thread, let's see if I can tap some more knowledge off of the experts. I have a very similar diode module that has been lying around for some time, but I should put it to use eventually. I am not too worried about the diode itself, since 2V @ 5A can easily be supplied with batteries, but if I do need to run the TEC, I wonder if there is a ghetto way to do it. Do I absolutely need closed loop control, or can I just feed the TEC with its rated current, switching it on simultaneously with the diode? I am not too worried about getting 808nm from the diode right now, since I am not going to be pumping Nd:anything too soon, but I am worried about freezing or overheating the diode. I guess I will be save if I monitor the 10k NTC manually and switch off the power if anything bad happens, but how do I know at what temperature "bad" starts? And if I do need a proper TEC controller, do I need to start reading up on control theory, phase margins, loop stability and all that, or are there controller ICs around that do all that for me?
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Dr. Shark wrote ...
*bump* This is a good thread, let's see if I can tap some more knowledge off of the experts. I have a very similar diode module that has been lying around for some time, but I should put it to use eventually. I am not too worried about the diode itself, since 2V @ 5A can easily be supplied with batteries, but if I do need to run the TEC, I wonder if there is a ghetto way to do it. Do I absolutely need closed loop control, or can I just feed the TEC with its rated current, switching it on simultaneously with the diode? I am not too worried about getting 808nm from the diode right now, since I am not going to be pumping Nd:anything too soon, but I am worried about freezing or overheating the diode. I guess I will be save if I monitor the 10k NTC manually and switch off the power if anything bad happens, but how do I know at what temperature "bad" starts? And if I do need a proper TEC controller, do I need to start reading up on control theory, phase margins, loop stability and all that, or are there controller ICs around that do all that for me?
Might as well make an update I got the op-amp /pass transistor circuit to work somewhat. It uses both op-amps of the dual op-amp to drive TIP42 transistors. (one per op-amp)). My circuit seems to have a few bugs right now, but none make it non functional. 1-Current does not go from 0-whatever. It goes from 14mA to 4amps or so. (any ideas why?) 2-Needs to run off of 12 volts. (At 5 volts, current max is 2amps). 3-Current shifts slightly when run at higher amps. (Transistors heat up, causing this). The current changes slowly and by a few mA's at a time. (+/- 5mA total). Otherwise, its pretty good! Ive been using it to run my c mount laser up past 3.5amps without issue. I am going to use expresspcb to move this circuit off my breadboard, and to beef it up. Im going with two dual op-amps driving a total of 4 transistors. A 5amp volt regulator will bring down the power supply voltage enough to minimize heating, but not enough to cause available current draw to suffer. (Using a 12volt flatpac now). This ought to kill the temperature rise problem. (heating spread out). As for the temperature control...http://www.rason.org/Projects/fancont/
fancont.htm This could probably run a TEC if a larger transistor is used, no? Now the problem is fitting all this into a mini-board from expresspcb! Ive got it all excpet the temp control... Matt
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SIMPLE Laser PSU
