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Efficiency of adjustable 3 terminal current regulator?
Efficiency of adjustable 3 terminal current regulator?
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Old 30th July 2018, 02:30 AM   #1
leadbelly is offline leadbelly  Canada
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Default Efficiency of adjustable 3 terminal current regulator?

I may be dense but I don't understand how to calculate the voltage and current drop when an adjustable 3 terminal regulator is used as a current regulator, e.g. single resistor on an LM317 or 338.

If it's there in the datasheet or easily calculated from other datasheet parameters please just say so.

If you want application background, it's not a difficult service, just a 1A or 2A current regulator on a car electrical system, so as low a voltage drop as possible would be preferred.
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Old 30th July 2018, 09:11 AM   #2
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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Here is a data sheet and to calculate the power dissipated in the device, use Kirchhoff's law.
http://www.ti.com/lit/ds/symlink/lm317.pdf
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Old 30th July 2018, 11:24 AM   #3
FauxFrench is offline FauxFrench  France
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For linear regulators, calculation of the voltage drop is easy. The "current drop" (current going straight to ground before the regulator output) is normally little compared to the output current, except for really low-power applications.

The series voltage drop is the difference between the input- and output-voltage. The series power loss is this voltage drop times the output current. If you then compare that to the output voltage and current (output power) you can calculate efficiency.

For efficiency and heating reasons, you want the voltage drop across the series regulator to be as little as possible. But, the voltage drop must not (at any moment) go below the minimum voltage needed to keep the regulator active (non-saturated), even temporarily when the input voltage sags due to loading or voltage ripple. If the voltage regulator saturates, you no longer have a regulated output.
On top of that, you have a maximum voltage the regulator can stand.

Last edited by FauxFrench; 30th July 2018 at 11:29 AM.
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Old 30th July 2018, 11:38 AM   #4
brig001 is offline brig001  United Kingdom
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If you want low dropout, I don't think the LM317 is the right device. The regulator itself needs 3V input to output, and the reference voltage = voltage across the sensing resistor is 1.25V. Total dropout = 4.25V. The LT3080 http://www.analog.com/media/en/techn...ets/3080fc.pdf would allow you to halve that, or make one yourself and you could get easily below 1V

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Old 30th July 2018, 12:10 PM   #5
FauxFrench is offline FauxFrench  France
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Absolutely right, as long as you ensure that the input voltage at no moment in time sags to a level where the (lower) series-drop of the low-dropout regulator is not respected. The voltage ripple on the buffer capacitors (just before the regulator) may easily be several volts (with high loading) such that the difference in using an LM317 or a low-drop regulator becomes less pertinent.
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Old 30th July 2018, 04:12 PM   #6
brig001 is offline brig001  United Kingdom
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Good point normally, but this is on a car electrical system, so guessing it’s 12VDC and nice and smooth
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Old 30th July 2018, 04:33 PM   #7
DF96 is offline DF96  England
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A current regulator will drop whatever voltage is needed to maintain the required current in the load, if it is able to do so. Hence a current regulator does not 'have' an efficiency; the whole circuit has an efficiency.

Low voltage drop in a current regulator probably means that the current regulator is not doing very much and so is probably unnecessary.

Do you actually want a current regulator, or a current limiter? These are not the same thing.
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Old 30th July 2018, 06:41 PM   #8
leadbelly is offline leadbelly  Canada
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Indeed, what is really needed I guess is a low dropout current limiter. Got an example?

I have at the back of my mind the commercial battery isolators used in dual battery auto systems, which range in capacity from 100mA to 50A, and AFAIK are made of single current limiting diodes, but I do not know if the components are readily available, nor are the best choice necessarily for DIY.
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Old 30th July 2018, 08:54 PM   #9
Khron is offline Khron  Finland
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It might not be a bad idea to perhaps provide some details about the actual application you'd need this for
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Old 31st July 2018, 03:17 AM   #10
PRR is offline PRR  United States
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Quote:
Originally Posted by leadbelly View Post
...battery isolators used in dual battery auto systems, which range in capacity from 100mA to 50A, and AFAIK are made of single current limiting diodes....
No. They are two plain one-way diodes. Goes from the one alternator to two batteries, engine and 'house'. You can drain the house battery flat and the engine battery is not depleted at all. The numbers are the maximum expected current. (Any current-limiting, if any, is in the alternator system.)
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