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Old 12-19-2007, 04:51 PM #1
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Default 2 DDL drivers, 2 different results

Hey all, the first DDL driver I made works great, witht the 4 diodes and 1 ohm resistor as a dummy load, it can go from ~2 mV - ~412 mV and turning he pot adjusts really smooth.

Now the 2nd on I made the same way, using the same parts (except the pot, but still a 100 ohm). The 2nd circuit with the dummy load gets up to about 135 mV max, and down to ~1.8 mV at the lowest. I tested it out with my Senkat diode and a 1 ohm resistor in series and it manages to get ~176 mA max(the pot is reading about 1.7 ohm). I followed Rog's test thread, and re-traced the connections to everything and it seems correct. What could possibly be wrong? A bad lm317? The pot seems to be working, as it meters from about 1 ohm- ~97 ohm. I'm not sure what to test at this point.


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Old 12-19-2007, 05:12 PM #2
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Default Re: 2 DDL drivers, 2 different results

Have you checked for 1.25volts across the sense resistors to see if the LM137 stays in regulation?

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Old 12-19-2007, 06:12 PM #3
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Default Re: 2 DDL drivers, 2 different results

Quote:
Originally Posted by rog8811
Have you checked for 1.25volts across the sense resistors to see if the LM137 stays in regulation?

Regards rog8811
WhenI metered that, it showed 1.16-1.17 volts, i figued that was close enough, but maybe not?
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Old 12-19-2007, 06:46 PM #4
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Default Re: 2 DDL drivers, 2 different results

Quote:
Originally Posted by desslok
[quote author=rog8811 link=1198086708/0#1 date=1198087926]Have you checked for 1.25volts across the sense resistors to see if the LM137 stays in regulation?

Regards rog8811
WhenI metered that, it showed 1.16-1.17 volts, i figued that was close enough, but maybe not?
[/quote]

Your input voltage is a tad too low.
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Old 12-20-2007, 04:38 PM #5
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Default Re: 2 DDL drivers, 2 different results

Quote:
Originally Posted by desslok
[quote author=rog8811 link=1198086708/0#1 date=1198087926]Have you checked for 1.25volts across the sense resistors to see if the LM137 stays in regulation?

Regards rog8811
WhenI metered that, it showed 1.16-1.17 volts, i figued that was close enough, but maybe not?
[/quote]

Yep, a bit too low. But 1.24V is also a bit too low. The LM317 tries to keep the sense voltage exactly at 1.25V, any different and you aren't letting it regulate properly.
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