Electronics > Beginners
LM317AHVT and Heatsink
MarkF:
It's NOT a matter of one or two pots.
It's the 3 resistors you have in parallel with the pots. Get rid of them!
Edit- Removed bad info
MarkF:
I just noticed that you have the resistors on the emitters.
They should be on the collectors for PNP transistors.
Remember the outputs are being summed together and
you want to compensate for small variances of each transistor.
You always want the resistors on the output side.
And REMOVE R3. If you can't set R6 to meet the minimum load, then add a constant current source (not a resistor)
--- Quote from: mike_mike on November 22, 2018, 07:39:41 pm ---If I switch from the first schematic (with dual pots) to the second one (with 2k pot), what would change in the functionality of the power supply ?
--- End quote ---
None with corrected two pot circuit.
- You would design for the single pot to get your desired voltage range.
- Then you could just use a single 3/4 turn pot
- Or you could add two 3/4 turn pots with the smaller value being around 10% of the bigger depending on how fine of a control you want
- Or you could use a single 10-turn pot
macboy:
--- Quote from: MarkF on November 23, 2018, 05:10:36 am ---I just noticed that you have the resistors on the emitters.
They should be on the collectors for PNP transistors.
Remember the outputs are being summed together and
you want to compensate for small variances of each transistor.
You always want the resistors on the output side.
--- End quote ---
No, it is definitely correct to have the resistors on the emitter side!
They act as "emitter degeneration resistors" in order to effect the current sharing. As more current flows through one of these resistors, the emitter (and therefore, base) voltage is affected in the way which reduces base current, creating a negative feedback. This enforces tight current sharing between the transistors, as changing Vbe by just a few millivolts can dramatically affect collector current. This scheme is not as effective for MOSFETs because of the much looser relationship between gate voltage and drain current.
mike_mike:
Thank you for the replies.
1. I have a 2K/2W potentiometer with the part number WXD3540. In the datatsheet it specifies "Independent Linearity". This means that the potentiometer have linear characteristics ?
I don't see any other information about the linearity in datasheet.
2. I made an accidentally short circuit between C and E at one of the transistors, with the multimeter probe, while I was taking some measurements and while was connected a 6-7A load at the output. After the short circuit I tested the power supply and it worked good. If the power supply works good, is there any recommendation for replacing the transistors or other components ?
MarkF:
If it works? - Don't fix it.
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