Why does the waveform on my scope look like this?

Thread Starter

belektrik

Joined Jan 11, 2020
8
I have a simple circuit 110VAC through a step down transformer to 37 volts; That has 10,000uF capacitor across it so the unloaded output voltage shows on my scope as 52 volts; The green line.

I have a 2SA1302 transistor that has it's emitter connected to the positive voltage rail, the collector connected to a 4.7K Ohm resistor then a blue led then ground.
11243504879858.jpg

The base of the transistor has a 220 Ohm resistor.

As you can see from the images below just the basic setup and yellow waveform has a 900mV ripple.

As I place my finger on the base resistor; the LED starts to light up but the ripple only gets worse.

The third image has 10V pk-pk ripple. Why is that when all i'm powering is a simple LED?
scope_0.pngscope_2.pngscope_1.png
 

DickCappels

Joined Aug 21, 2008
6,086
I think there is a good chance that what you are seeing is 50 Hz that your body is picking up from the AC mains. I am not sure of how your scope is connected so I cannot tell for sure.

Across the base and emitter or from the base to ground? Is anything grounded?
 

Reloadron

Joined Jan 15, 2015
4,883
I have a simple circuit 110VAC through a step down transformer to 37 volts; That has 10,000uF capacitor across it so the unloaded output voltage shows on my scope as 52 volts; The green line.
Rectification? I see no mention of any rectification?
That has 10,000uF capacitor across it so the unloaded output voltage shows on my scope as 52 volts; The green line.
Looks like you are DC coupling the scope. Try AC coupling and look at the waveform reducing the vertical gain.

Ron
 

Thread Starter

belektrik

Joined Jan 11, 2020
8
OK, we toss in base for gate. So is there rectification?

Ron
Yes, 110 is stepped down to 36 volts, that is passed into a rectifier IC. After that there's 10,000uF capacitor.

The step down transformer's output is labelled 37v but after the capacitor the scope shows 52 volts; this is most likely since it's unloaded.
 

Reloadron

Joined Jan 15, 2015
4,883
Yes, 110 is stepped down to 36 volts, that is passed into a rectifier IC. After that there's 10,000uF capacitor.

The step down transformer's output is labelled 37v but after the capacitor the scope shows 52 volts; this is most likely since it's unloaded.
That's normal. You have 37 VAC RMS on your transformer secondary. Following rectification that would be 37 VDC less the forward voltage drop of the rectifiers. Next the capacitor will charge to the Epk (Voltage peak) which is going to be about 1.414 * 37 and I see 52 Volts there. I would expect to see considerably less ripple with a properly rated 10,000 uF cap in there as a filter.

Ron
 
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