Discussion in 'General Electronics Chat' started by Nathan Hale, Feb 21, 2015.

1. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
Hi folks! hope all is well.

I opened up my wife's broken CD player / radio a few days ago.
It is the exact one as shown in this link below.
http://www.amazon.com/GPX-BC232R-CD-PLAYER-BOOMBOX/dp/B00T0GFTN8/ref=sr_1_3?ie=UTF8&qid=1424578507&sr=8-3&keywords=GPX BC232R

The sticker on the back of the device says that it needs 6 "C" batteries of 1.5 volts each. well that would mean this thing takes 9 volts to run. BUT...
When i opened up the radio and connected my Oscilloscope to the wires that are on the other end of the transformer it is showing a peak voltage of about 17.8 volts!

When i took out the O scope probes and stuck a multimeter that measures AC voltage, it said that there were 12.45 volts!!!Here is the exact kind of multimeter i used.
http://www.amazon.com/gp/product/B000EVYGZA/ref=oh_aui_detailpage_o05_s02?ie=UTF8&psc=1

a) Why was my o scope showing a peak voltage of 17.8 volts? shouldn't it have said 9 volts?
b) why was the hand held meter showing 12.45 volts? shouldn't it have said 9 volts also?
c) is the voltage needed to run the device different if it is running on DC or AC?

Thank you.

Jan 15, 2015
963
232
a) Why was my o scope showing a peak voltage of 17.8 volts?
b) why was the hand held meter showing 12.45 volts?
c) is the voltage different if it is running on DC or AC?

With the scope you are seeing the AC peak which is 1.414 times the RMS value which is the DC value. 12.45 * 1.414 = 17.6. Note in your image the Vrms value is 12.8 volts.
Possibly it has a regulator in there or just runs fine on 12 VDC or the 9 VDC the batteries provide. Either way, I do not see a problem at this point. Makes sense to me anyway.

Ron

3. ### WBahn Moderator

Mar 31, 2012
17,757
4,800
The 17.8V is the peak amplitude of a sinewave.

The 12.45V is the RMS voltage of that sinewave. The RMS (root-mean-square) is the "effective" voltage in terms of the ability of the voltage waveform to deliver power to a resistive load. For a sinewave, the relationship is Vpeak = sqrt(2)·Vrms.

The point that you are measuring at is the input point for AC power. That almost certain then gets rectified and regulated down to something below 9V. The 9V from the battery probably also gets regulated down to that same lower voltage.

4. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
Thank you! Learned something new today. But...Why would the guy who designed the radio put a transformer that is spitting out 12. 5 V instead of the required 9V? Don't you think he would have worried about the extra voltage busting something?

5. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
Aha! That answers my question. thank you.

6. ### WBahn Moderator

Mar 31, 2012
17,757
4,800
That input voltage is unregulated and the voltage output of walwart-type supplies tends to be quite a bit higher when unloaded than when loaded. The circuit needs a stable voltage and the regulating circuitry needs some "overhead" voltage in order to accomplish that. The regulator is designed to tolerate a pretty significant variation in input voltage and still work properly. That's not to say that you WANT the input voltage to be too much higher than the minimum that the regulator circuit needs because, if it is a linear regulator, that just means more power that has to be dissipated. In today's world of SMPS (switched-mode power supplies) this is less of an issue.

Jan 15, 2015
963
232
Also, just as a side note, the DMM you are using is a good general purpose meter. I doubt it is a true RMS responding meter but rather an average responding RMS indicating meter. So while the meter does not respond to the RMS value it should do OK with a good sine wave as far as what it indicates.

Yeah, I figure the voltage from the transformer is rectified and there will be a small drop and later maybe regulated.

What exactly is the problem with the baby boom box. I actually have a similar looking one.

Ron

Jan 15, 2015
963
232
The similar looking unit I have laying here is an old Sony. The one I have does not use a wall wart but has a standard 120 VAC power cord. I assumed the original poster's box also ran on mains power sans a wall wart? Wall wart in the box.

Ron

9. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
mine doesnt have a wal wart. it has a standard 120 vac power cord.the problem was the CD player.it wouldn't work when u stuck in a cd and hit play.