# About PCB layout need help.

Joined Nov 21, 2018
899
Hi all.
A newbie is learning PCB layout. got something confused:

A: The VR1 = 15.3 mean VD2 = 17.1-15.3 = 1.8

B: measured VD2 = 0 ?

C: VR2-R3 = 17.3

D: VR2 = 0 ?

E: VR2 = 17.3, that mean VLDR2 = 0?

F: VR2 = 17.4, mean VrV1 = 0?

G: D1 not on no matter R1’s value:

H: D1 on when short R1.

I: don't whats wrong, every thing different from the youtube, picture I shown the voltage from LM317T doesn't change, why? he used B125C1000, which not work for me, so I used just a BRIDGE.

how can be so?
Thanks

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#### DickCappels

Joined Aug 21, 2008
10,226

The bridge and C1 form a peak detector for the incoming sine wave which is 13.0 volts RMS the peak voltage of a sine wave is SQRT(2) times the RMS value, which comes to 18.4 volts peak. The bridge drops about 1 volt, which is why the output of the bridge shows only 17.1 volts. The voltage across R1 is about 2 volts lower than the output of the bridge because the LED drops almost 2 volts.

Joined Nov 21, 2018
899
thanks.
The question is can't measure out "the LED drops almost 2 volts", the meter shown '0' in picture B, why?

#### eetech00

Joined Jun 8, 2013
4,004
thanks.
The question is can't measure out "the LED drops almost 2 volts", the meter shown '0' in picture B, why?
Are you using an LED that is simulating an analog LED model? or an animation?
You can also measure the voltage across R1 (referenced to ground), then do the math.

Joined Nov 21, 2018
899
Are you using an LED that is simulating an analog LED model? or an animation?
You can also measure the voltage across R1 (referenced to ground), then do the math.
Thanks.
I used LED-BIGY from Proteus library and do an animation, the LED only ON when short to GND as picture H, otherwise even R1 = 1, it is not ON like picture G, (I did set R1=1).

do you mean the Vmeter can only measure from GND to some point? the picture D doesn't measured neither.
but E and F measured OK.

Last edited:

#### eetech00

Joined Jun 8, 2013
4,004
Thanks.
I used LED-BIGY from Proteus library and do an animation, the LED only ON when short to GND as picture H, otherwise even R1 =
The voltage drop across a real LED varies with the current passing thru it. This behavior would be reflected in a LED diode spice model. Animated LED's in proteus use proteus animation language and are usually intended to provide an on/off visual representation only, not specific electrical behavior.