Electronics > Beginners
LM317AHVT and Heatsink
mike_mike:
I tried to modify the schematic in order to have a minimum load current of about 10mA.
I changed the 430R resistor into a 120R resistor. I used the same potentiometer (10K). I connected a 3.3K resistor in parallel with the potentiometer to have a variable output voltage from 1.25V to about 26V.
I tried the schematic and it works.
Are those modifications correct ?
mikerj:
--- Quote from: mike_mike on July 30, 2018, 01:58:43 pm ---I changed the 430R resistor into a 120R resistor. I used the same potentiometer (10K). I connected a 3.3K resistor in parallel with the potentiometer to have a variable output voltage from 1.25V to about 26V.
I tried the schematic and it works.
--- End quote ---
Having a relatively low value fixed resistor in parallel with the variable resistor gives very non-linear behaviour of the voltage control e.g. for a standard 270 degree potentiometer the first 90 degrees would change the voltage by about 17v, but the last 90 degrees would only change the voltage by about 3v.
This may be acceptable (or even desired) behaviour, but if you want more linear behaviour use a lower value potentiometer and a higher value fixed resistor to set the maximum voltage (e.g. 2k7 pot, 27k resistor)
mike_mike:
Can this modification (from my last message) lead to unwanted power supply behavior, such as oscillations ?
mikerj:
--- Quote from: mike_mike on July 30, 2018, 03:15:50 pm ---Can this modification (from my last message) lead to unwanted power supply behavior, such as oscillations ?
--- End quote ---
No, the modifications should not affect stability. Reducing the upper feedback resistor to 120 ohms should improve voltage regulation with small loads.
JS:
I don't know about the 1.2Ω resistor, seems a tad high, if you lower that value more current will go by the transistor and less for the regulator to take. I don't know your current requirements, at 3A (taking 0.65V for VBE) the transistor will be taking 2.6A and the reg 0.4A. If you lower it to 0.47Ω the transistor will take 2.8A and the reg 0.2A...
JS
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