Electronics > Repair

AC to DC converter half voltage

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sdancer75:
The power supply output of a portable vacuum cleaner is AC 17V-200ma (input 220v).

This, is connected directly with a DC converter PCB circuit (photo attached ) which is supposed to output at least 13v DC to charge a battery pack of 12v. Instead, the DC output is ~8V. Everything in this little PCB board seems to work fine (diodes, resistors and amplifying transistor).

So the questions are.

1) Does this little PCB do a full bridge rectifying with only two diodes (the 3rd is connected to led I suppose) ?

2) Why the output is only 8v? Is that ok? How does it supposed to charge a battery pack of 12v?

Thanks

srb1954:

--- Quote from: sdancer75 on June 06, 2023, 06:38:46 pm ---The power supply output of a portable vacuum cleaner is AC 17V-200ma (input 220v).

This, is connected directly with a DC converter PCB circuit (photo attached ) which is supposed to output at least 13v DC to charge a battery pack of 12v. Instead, the DC output is ~8V. Everything in this little PCB board seems to work fine (diodes, resistors and amplifying transistor).

So the questions are.

1) Does this little PCB do a full bridge rectifying with only two diodes (the 3rd is connected to led I suppose) ?

2) Why the output is only 8v? Is that ok? How does it supposed to charge a battery pack of 12v?

Thanks

--- End quote ---
What are measuring the voltage with?

An ordinary DMM on the DC measurement range will indicate the average voltage of the waveform, which will be 2/PI = 0.637 times the peak voltage for a rectified sine wave. If your DMM indicates 8V average the peak voltage will be around 12.6V, which is just barely sufficient to charge a 12V battery.

sdancer75:
The DC output of this AC-to-DC converter is for sure ~8V. The output is connected directly to the batteries. I tested every component on the PCB and I found it ok. Also, I measured the output of the AC power supply (it has only a transformer, ie linear psu) which is also ok i.e 17V

Really it surprises me the source of the problem. Also, can two diodes rectify the full AC signal? I expected a four diodes rectifier pattern here, but I found two + one.

What else can I do to conclude the source of the problem?

J-R:
Please post a photo of the other side of the PCB so we can see the circuit?
A single diode is all that is technically needed.  This blocks the AC waveform when it is negative so you only get a positive wave.
Can you probe different areas of the circuit with an oscilloscope instead of a DMM?
Are you measuring the 8V DC with the battery connected?  If the battery is bad then the voltage may not be able to go higher, for example if a cell is shorted.

sdancer75:

--- Quote from: J-R on June 07, 2023, 06:32:51 am ---Please post a photo of the other side of the PCB so we can see the circuit?
A single diode is all that is technically needed.  This blocks the AC waveform when it is negative so you only get a positive wave.
Can you probe different areas of the circuit with an oscilloscope instead of a DMM?
Are you measuring the 8V DC with the battery connected?  If the battery is bad then the voltage may not be able to go higher, for example if a cell is shorted.

--- End quote ---

Thanks,

As soon as I return home, I will post a photo from the backside. No, I measure 8VDC with the battery pack disconnected.

I will let you know, meanwhile, I attach some extra photos from the frontside.

Regards,