Electronic broom maintenance
Some time ago we’ve purchased two of these so-called “electronic broom”, to be used in two different houses. As a matter of fact, they are not really electronic, as they don’t have any transistor or IC: the only electronic component is a LED… This product is sold in Brazil named as Vassoura Eletrônica (Electronic Broom), something difficult to admit. Anyhow, I prefer consider it as an electric mini-vacuum cleaner but without using vacuum. Below is the picture of this “marvel of the century”, product brand Munditek model PV-004.
An electric motor, powered by a Ni-MH 7.2 Vdc battery, drives a rotating brush, which “captures” the dust and throw it into a dust receptacle. There is a DC input connector for charging the battery from an external adapter. This external adapter is an ordinary one, of the type used in the majority of the small gadgets, as cellphones.
The problem was that one of the power supplies was missed. What initially could be a simple replacement became a reason for another maintenance work. The small label in the remaining original adapter shows the specifications:
AC/DC Adapter
Input 100-240 Vac 0.3 A max
Output 9 Vdc 200 mA
In principle, it’s simple: it was only necessary to get another ordinary adapter with the same specification, and all could be resolved. I have a box with a number of different adapters and power supplies (linear and SMPS), where I’ve found an adapter with compatible characteristics. Before applying it to the gadget, I’ve tested it: 9 Vdc was measured on the output connector. When I plugged it in the power connector in the broom, the LED on the “electronic broom” did not light up. Turning back to use the remaining adapter, the LED glowed. Then I decided to open the gadget and verify the way the battery is charged.
The first thing to do was to broken the seal. This “seal” was a piece of paper glued to the two main plastic parts of the product, as can be seen below:
The above sentences in Portuguese mean: IMPORTANT: Charge the battery for 12 hours before the first use and for 10 hours in the remaining uses. Never charge the battery for more than 12 hours. Use only the original battery charger.
A small knife (seen in the above picture in action) and eight screws later, the gadget was open, what showed a small printed circuit board with only four components: two resistors, the LED and a push-button switch with one SPDT contact. This push-button is driven by the foot through the big glossy button that can be seen in the top right of the first picture. The spring over the push-button provides mechanical operation stability to the external glossy button and to the switch.
The functioning was apparently simple, so I decided to draw the schematic diagram in order to simplify the understanding (and further the work). This decision has proven useful for the work, as explained further below. The schematic diagram is as follows:
The operation is centered in the reversible switch. In the RUN position, the battery voltage is applied to the motor, which drives the rotating brush through a small toothed belt. Of course, for this purpose the battery must be fully charged. In order to charge the battery, the switch must be changed to the CHARGE position and the adapter must be plugged to the input connector. The resistor R1 acts as a limiting resistor, determining the charging current for the battery. The voltage developed through R1 is used to light up the LED, which has its current limited by R3. The components positions are engraved on the printed circuit board, identifying the LED, the switch and the resistors R1 and R3, as well as the connections points for the power connector, battery and motor. There is no engraving, place or holes for an eventual resistor R2… Who explains?
A visual inspection revealed no solder dry joints. The soldered points were brilliant and clean.
Back to the two adapters. With the original adapter, the LED was ON, but with the adapter from my scrap box, the LED continued off, but with a voltage measured on R1, which means that there was still a charging current. At this point I decided to compare the two situations, by paying attention to the exact values of voltage and current in each case. With the original adapter the voltage on R1 was 1.8 V and with the adapter from my scrap the voltage was 1.6 V. Bingo! Although charging the battery, in the latter case the voltage developed on R1 was not enough to light up the LED. Explaining better using numbers: with the original adapter, the developed 1.8 V applied to the assembly R3 and LED caused about 0.1 V to develop on R3 with the remaining 1.7 V being enough to light up the LED, although perhaps in the beginning of its lightning curve. With the adapter from my scrap box, the 1.6 voltage did not reach the minimum necessary to light up the LED (apparently 1.7 V or above). The calculated charging current in the two cases were:
With original adapter: 1.8 V / 12 Ω = 150 mA
With the adapter from my scrap: 1.6 V / 12 Ω = 133 mA
So, in the two cases there was charging current, with a small current difference between them. The problem was that in the second case, as explained above, the LED remained off, and the user (housewife, maid or so) could be induced to error, thinking that the battery was not being charged or there was a fault.
I decide to compare the output voltages of the two adapters. Bingo again! The original adapter, although specified to 9 Vdc, measured 10.3 V, even with the load imposed by the charging of the battery. The adapter from my scrap box measured the rated 9 Vdc, but with some voltage decrease when connected to the load imposed by the charging current. Obviously the difference between the two voltages in these two situations caused the symptom described above, with the LED ON and OFF depending on the case.
As it became clear that the problem was not with the gadget itself, the problem remaining on the choice of the appropriate adapter, I performed a cleaning inside this “marvelous” gadget and reassembled it. This reassembling operation was not easy. The mechanical design of the gadget is poor, with some improvisations, rendering it difficult to reassemble. Some parts inside just simply snaps on the plastic body (no screws), and during assembly when one piece is in place, another goes out of position. There were necessary a number of attempts in order to get it completely assembled.
The battery is shown in the picture below. It consists of 6 cells, giving a resultant rated voltage of 7.2 V. The capacity is 1.7 Ah, enough for 60 minutes of operation when full charged (at least that’s what they say, according to the label to the left).
As the original adapter continued to work properly, it was maintained. For the adapter from the scrap, it was evident that could not be used in this case. Another one should be chosen, within those commercially used. The nearest standardized voltage was a 12 Vdc output device. In this case, the calculations showed that the charging current should result in a value around 250 mA. The adapter chosen suffered a small drop in the voltage when loaded by the charging current, resulting in a charging value around 200 mA (C-rate of 0.12C), perfectly in accordance with a safe charging value for the battery. For more information about C-rate, you can visit http://all-about-lead-acid-batteries.capnfatz.com/all-about-lead-acid-batteries/lead-acid-battery-fundamentals/what-does-battery-c-rate-mean/. The information in this page is focused on lead-acid batteries, but applicable for all types of batteries.
To end, there was a need to change the plug type. In Brazil there is a specific standard of plug, according to NBR 14136 (Type N) standard, nearly similar to Europlug (Type C). The adapter chosen had NEMA-type flat blades. It was necessary to perform an adaptation, as shown in the picture below:
The adapter at left is the original one: the plug type as per Brazilian standard can be clearly seen. The adapter at the top is the chosen one, in which the flat blades of the NEMA plug can be seen. The yellow cord is the adaptation done, in order to connect it to the wall outlet. For more information about plugs, you can access https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets.
All of this caused me some hours of effort, but at least the gadget was not thrown away. Nature is thankful.
This article was prepared for you by Henrique Jorge Guimarães Ulbrich from Curitiba, Brazil. Retired electronics technician. Loves electronics, telecommunications and cars.
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Parasuraman S
June 10, 2016 at 12:00 am
Excellent! New area of service, hitherto untouched! Thanks for sharing and also the brain twisting calculations!
Henrique Jorge Guimarães Ulbrich
June 10, 2016 at 10:34 pm
Thanks, dear Parasuraman. Indeed, the calculations were very simple to do. Just a direct application of Ohm's law. Concerning to the new area, what happens is that different areas concentrate themselves today in electronics and computing (of course, this latter in not the case here).
Yogesh Panchal
June 10, 2016 at 1:50 pm
Good Job! sir.
Henrique Jorge Guimarães Ulbrich
June 10, 2016 at 10:35 pm
thanks, dear Yogesh
Gerald
June 10, 2016 at 4:34 pm
Thank you for sharing something different in your well written article Henrique. Your description of the difficult re-assembly made me smile as it feels so familiar. Today's appliances make you wonder how they had put them together in the first place. Assembly workers must have had more than two hands... maybe some robotic help!
Abraços, Gerald
Henrique Jorge Guimarães Ulbrich
June 10, 2016 at 10:40 pm
The difficulty in reassembling is consequence of the need to cheapen the products. However, I (and all of you, I suppose) do not agree when the things crosses the reasonable limits.
Beau
June 13, 2016 at 11:12 am
Very much agreed.
Paris Azis
June 13, 2016 at 2:45 pm
Yes, you are right Henrique. I also agree with that.
The problem is that these reasonable limits are crossed so frequently that they long ago seem to be not reasonable at all...
suranga bandara, Suranga Electronics
June 10, 2016 at 5:48 pm
Hi. Mr, Good job and Well done.
Henrique Jorge Guimarães Ulbrich
June 10, 2016 at 10:41 pm
Thanks, Suranga
Paris Azis
June 13, 2016 at 2:35 pm
Using Gerald's words, this was indeed something different but in any case interesting.
As regards these cheap, constant current-like, chargers using a single resistor to drop the current and then use this voltage drop across the resistor for lighting a LED indicating that charging is on, I have no special comments. It's the cheapeast method to charge a battery!Therefore, no accuracy in any charging parameter...Instability in current supply, non existent upper threshold voltage limit for cut off when the battery is charged, resulting in permanent charging, erratic visual indication about charging, the battery is charging while the LED is off, etc...All normal for a cheap device!