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4hv.org :: Forums :: General Science and Electronics
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Using a LDO regulator as a CC source...

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Inducktion
Fri Dec 30 2011, 09:19PM
Inducktion Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Link2

I have the adjustable version of this.... I'm trying to figure out how to make it a CC source, but it doesn't seem to be working. With 5 ohms of resistance from Vout to Vadjust, I get .5 amps of current flow with 4 volts in... With 9 volts, it jumps up to 1 amp, which doesn't make sense. It doesn't seem to be regulating, at all, with all of the current flowing straight in from Vin to Vout through the resistors.

Is it just not possible to make CC sources with LDO regulators?
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Pinky's Brain
Fri Dec 30 2011, 09:39PM
Pinky's Brain Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
Silly question ... the 5 ohm is in series with the load right?
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Inducktion
Fri Dec 30 2011, 10:12PM
Inducktion Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
I put the output wire from Vadjust to the load. So, yes, the resistors are in series with the load, since it goes Vout -> 5 ohms -> Vadjust -> load
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klugesmith
Fri Dec 30 2011, 11:03PM
klugesmith Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Uh, how are you thinking the circuit is supposed to work?
The regulator IC does its best to maintain 1.25 V between its Vadj and Gnd pins.
Assuming you want 0.25 A, the 5 ohm current sense resistor belongs between those pins.

The traditional place to put the regulated-current load (a jumper wire would qualify) is between the Vout and Vadj pins.

There are two other places where you could put a load instead of a wire:
1) between the Gnd pin and the negative terminal of the power source.
2) between the positive terminal of the power source and the Vin pin.
They each have the advantage of one node in common with the power source.
And the disadvantage of carrying the regulator's GND-pin current (a few mA),
in addition to the regulated current in the sense resistor.
If you wanted to use a fixed-voltage regulator with no Vadj pin,
you would have to put the load in one (or both) of those locations.

Note that there are LDO regulators whose reference voltage is 0.5 V instead of 1.25 V.
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Inducktion
Fri Dec 30 2011, 11:57PM
Inducktion Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Thanks Kludge. I finally got it working; The laser diode is happy with 250 mA off current, and is lasing perfectly. No difference in intensity between 9 volts and 4.5 volts.
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Inducktion
Sat Dec 31 2011, 04:56AM
Inducktion Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
I do apologize for the double post;

However,

Link2

Does it drop 1.75 volts as rhd suggests? The drop out voltage of the regulator itself is .5 volts which is really low, but the Vref of the regulator is 1.25 volts. The diode has a voltage drop as well though.
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