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Old 06-22-2010, 12:07 PM #1
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Default My first DIY driver

I'm quite happy with this for my first attempt, however when testing it using a dummy load it only delivered 150mA... I used a single 2R2ohm resistor in series with the 100ohm pot on the base of the LM317. I thought using a lower value resistor would give me a higher max. current? Anyone willing to explain this to me?



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Old 06-22-2010, 12:34 PM #2
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Wink Re: My first DIY driver

If you tried to take higher current from it, you probably just fried a bit your trimmer ..... remember that in this configuration, the trimmer always take the same current as the load, so need to hold the same power (based on the current passing through) .....

As example, supposing you try to set it at 500mA, the 1,25V that you need to drop on the assembly, gives you 1,25*0,5=0,625W (or 625mW, if you prefer) ..... where, usually, those trimmers are 100mW (the small ones) or 250mW (the ones like your one in the pic) only, in power holding .....

Better that you use a different schematic, if you want to use this system with higher currents ..... like this one, as example:



here, part of the current is took from the fixed resistor in parallel to the trimmer, and the trimmer just change from a set of diferent min and max values, that you choose from the resistors values ..... more stable, more easy to regulate, less danger of fried trimmers and, consequently, strange current variations.


And, don't reuse that trimmer, cause if it's damaged, also a new build can work bad !
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Old 06-22-2010, 12:44 PM #3
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Default Re: My first DIY driver

Thanks HIMNL9, that's exactly the type of thing I need to know. I will put together a driver from your schematic tomorrow night and report back.

ps. can I omit those 100nF ceramic caps? They don't look very important.... ?
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Old 06-22-2010, 12:48 PM #4
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Default Re: My first DIY driver

looks like a DDL driver.
i learnt this from Eudaimonium:
if you get rid off the rectifier diode& pot and just add a say 12 ohm resistor you will get 100mA.
a 2 ohm resisotr will be ~650mA
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Old 06-22-2010, 01:55 PM #5
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Default Re: My first DIY driver

Quote:
Originally Posted by jupiter8 View Post
Thanks HIMNL9, that's exactly the type of thing I need to know. I will put together a driver from your schematic tomorrow night and report back.

ps. can I omit those 100nF ceramic caps? They don't look very important.... ?
You can omit them, if you're sure that your PSU don't produce very high frequency spikes ..... they are there just for more safety (i always used this circuit for external drivers, both with LM317/117 and with LM1117, and never had problems with them) ..... just be sure to calculate the resistors for the current range that you need, keeping the values in the "safe" range for the trimmer (or just say me what you need and i pos tthe values )




@ TTerbo: this IS the so-called "DDL" driver (that is simply the only way for use these regulators as current drivers, published originally in the LM117/217/317 datasheet from the manufacturer ), just modified the current regulation section for not fry the trimmers at high currents ..... anyway, i don't know from where the name "DDL" come out, maybe the first one that posted it here had this nickname ..... and yes, if you need only a fixed current output, using a single resistor, or a parallel if there's not the right value, simplify the circuit too .....
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Old 06-22-2010, 11:00 PM #6
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Default Re: My first DIY driver

Trimmer for me, I'd like to be able to adjust things.

I'll write down your example problem for calculating the values of the resistors and work through it until I can do it myself. Cheers HIMNL9
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Old 06-23-2010, 01:55 AM #7
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Default Re: My first DIY driver

You might have blown the diode. I'd suggest keeping the series resistor to a value that results in a curreny your laser diode can actually handle... if you must use this circuit at all.

Deadal has done great work on making things easy with these circuits, but they are not ideal laser drivers by far.
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