Electronics > Beginners
Odd output from Elenco XP-15K Power Supply
tomherrick:
Bought the kit and put it together yesterday. First step afterward was to check the VDC output. When the pot was turned all the way up to the 15V mark I got -0.53V. Yup, negative. When I turn it all the way back to zero Volts it reads -0.65. Voltage across P2/P1 reads 17VAC; same reading across P2/P3. When I unplug the unit, the LED slowly dims to off and the multimeter value dropped from -0.65 to -0.32 over the course of a minute or so and continues to drop very slowly.
I've been over and over the instructions and rechecked the following: polarity of the diodes, caps and LED; output connections from the transformer to P1, P2 and P3; ensured that the regulator was oriented properly. I went through the test procedure and shorted across the output; the unit did not turn off.
The error is likely mine and I keep looking past it when I QC my own work... Suggestions???
Thanks.
tpowell1830:
Hi Tom, welcome to the forum.
Take some very crisp pictures of the board from both sides. Also, a shot of the schematic and post them in another post so that we can see your work and the schematic. You have a size limit for your jpg's (I can't remember what size), but the uploader will complain if they are too big.
tomherrick:
Here are photos of the circuit board and the schematic.
T3sl4co1l:
Easy enough to inline as well. Click the attachment, right-click the image, copy link. Paste into an img tag:
Hah, looks like it's drawn in Multisim of all things.
Tim
Zero999:
As posted in the other thread:
--- Quote from: rstofer on January 21, 2019, 02:16:00 am ---Diodes D5 and D6 make up a poor man's zener diode and the voltage between ground and the point where the diode connects to R4 should read -1.4, give or take. Black wire of you meter goes on ground. This negative voltage is what allows the regulator to get down to 0V.
I don't know the AC voltage between the blue and yellow wires of the transformer but about 1.4 times that value should be measured at pin 3 of the LM317 as a DC voltage.
Pin 1 of the LM317 should move between some negative value, like -1.4V, up to some positive value, nearly 15V, but I haven't calculated the exact range.
--- End quote ---
Yes, this is a classic way of getting near 0V from the LM317, which can normally only go down to 1.25V.
It could be a problem with the potentiometer.
--- Quote from: tomherrick on January 20, 2019, 10:22:44 pm ---Bought the kit and put it together yesterday. First step afterward was to check the VDC output. When the pot was turned all the way up to the 15V mark I got -0.53V. Yup, negative. When I turn it all the way back to zero Volts it reads -0.65. Voltage across P2/P1 reads 17VAC; same reading across P2/P3. When I unplug the unit, the LED slowly dims to off and the multimeter value dropped from -0.65 to -0.32 over the course of a minute or so and continues to drop very slowly.
I've been over and over the instructions and rechecked the following: polarity of the diodes, caps and LED; output connections from the transformer to P1, P2 and P3; ensured that the regulator was oriented properly. I went through the test procedure and shorted across the output; the unit did not turn off.
The error is likely mine and I keep looking past it when I QC my own work... Suggestions???
Thanks.
--- End quote ---
What are the voltages from pin 3 to 0V and pin 1 to 0V?
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