AC to DC converter half voltage  - Page 1
Forum section: electronics / repair

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 06, 2023, 06:38:46 pm
content: 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

author: srb1954
author_rank: Super Contributor
timestamp: June 06, 2023, 07:16:28 pm
content: Quote from: sdancer75 on June 06, 2023, 06:38:46 pmWhat 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.

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 07, 2023, 05:35:48 am
content: 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 17VReally 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?

author: J-R
author_rank: Super Contributor
timestamp: June 07, 2023, 06:32:51 am
content: 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.

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 07, 2023, 08:02:37 am
content: Quote from: J-R on June 07, 2023, 06:32:51 amThanks,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,

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 07, 2023, 01:57:57 pm
content: Here the backside of the DC converter PCB

author: wasedadoc
author_rank: Super Contributor
timestamp: June 07, 2023, 05:44:25 pm
content: 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.

author: J-R
author_rank: Super Contributor
timestamp: June 07, 2023, 07:34:34 pm
content: 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?

author: wasedadoc
author_rank: Super Contributor
timestamp: June 07, 2023, 10:02:42 pm
content: 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.

author: sdancer75
author_rank: Frequent Contributor
timestamp: June 08, 2023, 05:55:57 am
content: Quote from: J-R on June 07, 2023, 07:34:34 pm1) The DC Voltage when the battery is connected, is a little bit higher ~8V2) 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.