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Lm350 variable Power supply problem
Lm350 variable Power supply problem
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Old 19th June 2019, 04:29 AM   #1
Anirvan is offline Anirvan
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Join Date: Apr 2019
Exclamation Lm350 variable Power supply problem

Hi everyone!!

I recently built a variable Power supply using Lm350 for doing experiment at home. I used a 18-0-18 2Ampere transformer for this so that I can add a +/- 12 Volts dual supply circuit in future. But the problem with this supply is that when I checked the output of the supply with an AC line tester I found that the tester neon is glowing at both Positive and negative output of the supply. Also there's a leakage current at the metal cabinet I used as enclosure for the power supply. I connected a wire with the cabinet to the Earthing line of my electric supply to see if it resolves the issue. But I saw that the leakage current of the metal cabinet is successfully grounded with this method but the AC leakage at the output of regulator ic still exists making the neon of ac line tester glow. So please tell me what could be the reason behind this and how to fix it ?
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Old 19th June 2019, 10:38 PM   #2
MAAC0 is offline MAAC0  Portugal
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Join Date: May 2010
First of all. That twisted white/green AC power wire isn't safe and without a rubber grommet in the input hole of the case You are asking for trouble. As You have a metallic case use a grounded AC lead.
As what the neon concerns … 18+18*1.4=50V can light a neon bulb.
Disconnect the transformer wires and test for insulation between the primary and the secondary. Test also between the transformer case.
Is the regulator Tab insulated from the case ?


What Voltage do You get at the output ?
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Old 19th June 2019, 11:45 PM   #3
FauxFrench is offline FauxFrench  France
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Location: The mountains, calm and quiet.
MAACO has given you many good tips.
As I understand it, you have safety earth available in your laboratory. When you connect this safety earth to the cabinet, the glow-lamp no longer turns on when you touch the cabinet with the safety tester. But, when you touch "-" or "+" at the output of the power supply, the safety tester still turn the glow-lamp on. Right?
Such voltage testers are very simple and good for telling you if there may be a danger. But, they do not tell you if there is sufficient current available to pose a problem. The glow-lamps turn on without (hardly) any current passing the lamp.
The safety tester is intended to be used on the primary wires for which it will turn on when you touch the phase wire and perhaps also (less) when you touch the neutral wire (depending on the current and distance to the network grounding point). When you move to the secondary side of a transformer, there is a well defined voltage difference between the two secondary winding terminals. But, referred to earth, the whole secondary winding is floating potential wise. The potential may be many hundreds of volts. It is a voltage without current capability (like static electricity) but the safety tester turns the glow-lamp on because the glow-lamp needs no current. The cabinet is grounded to safety earth such that no potential difference exists to earth (no glow-lamp reaction). If you do not want the secondary terminals to make the safety tester turn-on the glow-lamp, you just tie (connect) the "-" terminal to safety earth as well.
Such connection of the secondary winding to safety earth may cause loop problems and often the secondary winding is just left floating. Leaving the secondary winding floating with respect to (safety) earth does not pose a safety problem because there is no current available.

Last edited by FauxFrench; 19th June 2019 at 11:57 PM.
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