[MOVED] Why a tester shows glow in rectified DC output

[Ahmmed Razu]

I have have converted 220V AC to 12V DC by transformer, rectifier bridge and filter (Full bridge). My output is ok but there is one question my mind. That is "when i touches the tester to the DC output it glows".
why the tester glows in DC output?

 

[WimRFP]

Re: Why a tester shows glow in rectified DC output

There will be some capacitance between the primary and secondary windings of your transformer. Depending on the design of the transformer, and where you connect Live (L) and Neutral (N), there may be sufficient capacitive current/voltage from the mains towards the secondary to ignite the tester.

Just to make sure there is no faillure, check your transformer insulation. Are you sure you don't use an autotransformer where the secondary is connected with the primary?

 

[Ahmmed Razu]

Re: Why a tester shows glow in rectified DC output

If I do not use transformer, what will happen if i directly convert 220V AC to 220V DC.
Does the tester glow continuously if i connect it to the DC terminal?

 

[WimRFP]

Re: Why a tester shows glow in rectified DC output

Do you have a (spice) simulator? Put your problem into it and try to explain what you see.

Where you have "plug" you apply a 311Vp sine source between the wires. The lower wire in the plug you connect to the ground (node 0). Use 1N4007 as rectifiers. Use (for example) 1 kOhm for the load. Put a voltmeter between "tester" and ground and you will see your waveform (it looks like half wave rectified, so it has an AC component). The peak peak ac component is likely more then enough to overcome the breakdown voltage of your neon tester.

Because of the AC component, and the half wave waveform itself, the neon tube will flicker.

 

[Ahmmed Razu]

Re: Why a tester shows glow in rectified DC output

The tester still glows instead of adding smoothing filter. If the breakdown voltage of neon gas is exceeded in case of 311V then why it shows glow in 12V rectified DC , in that case the breakdown voltage is not exceeded. All power system (AC) and power supplies (DC) contains transformer, then does it mean that all rectified DC power supplies output (AC-DC) will lead to glow a tester?

If not then whats the difference? Does your reply produces same result in case of laminated sheet steel and fer-rite core transformer?

 

[WimRFP]

Re: Why a tester shows glow in rectified DC output

Did you make sure your transformer is fine (see posting #2)?

You should draw a capacitor between your primary and secondary winding of the transformer (posting #1), that may help you to understand what happens. The whole secondary circuit (also the wire containing your GND) has AC voltage on it. That can be above 100V, however the current is limited by the capacitance between primary and secondary. If you connect GND to the safety ground (PE), then the tester will not glow anymore. When you put the tester on "DC" and with your other hand you touch GND (posting #1), the tester will not glow.

50 Hz transformers do not have a ferrite core as that would result in a very large transformer (saturation flux density of laminated steel is around 4 .. 5 times higher then ferrite). Ferrite core transformers you see in switched mode power supplies.

 

[tanmay.dash12]

Re: Why a tester shows glow in rectified DC output

hello the output obtained is 12v (1 AMP current) due to this the tester glows...when after getting o/p from bridge rectifier the output should be fed into ics 7805 and 7812
so that a suitable and perfect filtered output of 12v and 5 volt is obtained .
7805 stands for 78 for (+) and 05 for (5v)
similarly 78 for(+) and 12(for 12v) ...