Electronics > Beginners
Odd output from Elenco XP-15K Power Supply
tomherrick:
It appears that on this pot the "wiper" is in position 1 as shown in the linked diagram. When I put a DMM lead on 1 and 2 or 1 and 3 I got the same kind of variable readings.
ArthurDent:
The pot ends are the outside terminals and the center terminal is the wiper as shown in the attached diagram. If you measure the resistance between the outside terminals when the pot isn't connected to the board, the resistance will be always be 2K no matter how the shaft or knob is turned. However, the way this pot is wired in the circuit it is as a two terminal variable resistor so when it is on the board the resistance between the middle and bottom terminal (on the schematic) will vary from zero to 2K as the shaft and knob is turned. The middle and top terminal are wired together and the resistance between those two terminals will always be zero.
If you haven't cut the protruding pins off the pot, wire the 10K resistor to the outside pins.
rf+tech:
With the wiper externally connected to one end of the pot via a copper track on the circuit board, as shown in the Elenco schematic, it is easy to come to an incorrect pin identification.
Visualize the wiper rotating with the shaft, through about 300° or rotation. One should see that the simplest construction technique would have the wiper connected to the middle terminal.
For clarity, please refer to this Wikipedia entry:
https://en.wikipedia.org/wiki/Potentiometer
On the right side of the page, an interior drawing and photo show how this type of potentiometer is constructed.
For the application in this power supply, connecting the extra resistor across the two outer-most pins is the foolproof method.
Attempting to connect the resistor between the wiper and one end has a 50% chance of not working as expected - because the resistor would be shorted out by the external connection between the wiper one end.
And by the way, if the wiper were to be externally connected to the opposite end, zero Volts would be full CW and maximum output would be at full CCW.
@ArthurDent:
With the pot measuring 2.38 k and 147 Ω, the parallel resistor value ends up less than 10 k. It's a bit far from a standard value, so the OP will likely ask, and learn more by the exercise of solving for the optimum value. Please do check me on the math.
RF+ Tech
ArthurDent:
One thing that hasn't been mentioned is that your intended use of this power supply is for testing marine electronics. I randomly checked the specs on a Hummingbird ChartPlotter and that draws 615Ma. I suspect that other pieces of equipment might require a power supply with higher current capacity. The power supply you've put together is rated at 200Ma (.2A) at 15 volts so it could not be used for testing this particular chartplotter.
What would be a good idea is to check the manuals on the pieces of equipment you want to test power on your bench to find what amount of current they need and get a power supply with an output current capacity that is greater than the highest current drawn. There are some supplies that output 13.8VDC at several amps that are designed to power mobile equipment.
tomherrick:
OK, well, thanks to all for your time and assistance. I've spent too much time on this already. Apparently misread the spec as I thought it was 2A at 15VDC.
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