AC to DC converter half voltage

[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]

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

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]

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.

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,

[sdancer75]

Here the backside of the DC converter PCB

[wasedadoc]

The diode immediately behnd the barrel jack is a half wave rectifier and there is no reservoir capacitor.  So a meter showing around 7 Volts is to be expected.

[J-R]

What is the DC voltage displayed on the DMM when the battery is also connected?
I would suspect a bad battery at this point.  Is it SLA/AGM or perhaps NiMH?

[wasedadoc]

Yes the voltage when the battery is connected will be informative.  Even though the meter is showing a little more more 7 Volts when the battery is not connected (as in the photo) the positive peaks will be about 23 Volts.  When a 12 Volt battery is connected some of the waveform will be above the battery voltage and charging current will flow into the battery during those periods.

[sdancer75]

What is the DC voltage displayed on the DMM when the battery is also connected?
I would suspect a bad battery at this point.  Is it SLA/AGM or perhaps NiMH?

1) The DC Voltage when the battery is connected, is a little bit higher ~8V
2) The battery pack is  18650 Li-ion with Lithium Battery Charger Protection Board 18650 BMS (photo attached)


PS : How to know if a battery is bad? Currently, they are all discharged.