Introduction
Bought a digital time switch from Q0010 last week for about SGD $12/= to replace my present mechanical time switch which has been used to control a 3-litres Jojirushi water air-pot that draws about 3.0 -3.5 amperes. The digital time switch has no other name on it except “Timer” on its cover. It is made in Differences between Digital and Mechanical Time Switches
The main difference is in the construction.
The mechanical time switches on the left have gears and wheels that are driven by an electric motor synchronised to the electrical mains. These wheels rotate a time dial that can be programmed to activate a level that will turn on or off a micro-switch to control the electricity supplied to the socket outlet.
The digital time switches on the right have no gears, wheels and moving parts. The timing is driven by a digital clock much like an ordinary quartz alarm clocks except in this case, instead of activating a mechanical switch that turn-on the alarm circuitry, the digital time switches have electrical relays that control the electricity supplied to the socket outlet.
Dismantling the Digital Time Switch
It is quite easy to open the casing of the time switch by first finding the two “catches”, one at each end of the time switch as shown, then use a small screw driver to carefully pry open the casing which will “pop up” from its base readily.
The Electronics and Components
The digital time switch has 3 components
1) the switch socket outlet assembly
2) the supply unit and the output relay
3) the time clock and display screen
Description of the Circuits
The supply for the electronics is obtained through a simple transformerless power supply unit that consisted of a 400 volt supply capacitor, a power limiting 1 ohm resistor and a full wave rectifying bridge. The power supply unit is supplying a 24 volts dc supply to run the relay circuits as well as 1.2 volts to charge the Nickel Metal Hydride (Ni-MH) battery and also to run the time clock and display unit as shown in the attached.
The time clock and the programming electronics are hidden behind the display screen’s circuit board
The relay has a name plate rating of 16 ampere @ 240 volts. The unit has a spare battery chamber which is not used, probably left over from the older design that used AAA rechargeable batteries.
The Rating of the Time Switch
Although the name plate of the time switch has a rating of 13 amperes, the driving electronic has relay rating of 16 amperes. This, however, does not mean the time switch can be used for appliances with current rating of 13 amperes so long as it is less than 16 amperes. This is because most of the appliances have inherited electromotive force (emf) during starting up or switching on. For example, appliances with electric motor can induce current sometimes as high as 12 times the rating of appliances for as long as 10 seconds during starting up. That back emf induced could easily damage the time switch if there are frequent start-ups even though the rating of the appliance is less than 3 amperes. As a rule of thumb, it is not advisable to load more than 4 amperes for most appliances with or without motor start ups.
Heating up of the Time Switch
Time switch will heat up although its electrical components consume little or no electricity. This is because there are always resistances in the socket outlet's connection and the wiring. A typical resistance of 0.3 ohms in the connection circuit will give up about 2.7 watts of heat if the current is 3 amperes. Try touching an electric light bulb of about 3 watts, one would always find it warm to touch.
Comparing to Mechanical Time Switch
This digital time switch was running cooler than the mechanical time switch probably for 2 reasons
1) The mechanical one consumes slightly more power;
2) The mechanical one is a bit old giving rise to higher contact resistance.Programming
The programming is quite straightforward. The unit has 8 buttons in 2 lines. The top ones (3 to 7) are for adjusting and setting the weeks, hours, minutes and an R/Recall button to cancel or recall the program. The bottom ones are for setting of clocks and do the programming with an extra button for manual/auto turning on and off the time switch.
As usual, the manual coming along with the unit is written probably by a technical guy who assumed everyone if fully versed with function of the time switch. It is only with trial and error can one started to understand what was written in the manual. Fortunately, nothing will destroy the time switch if one is playing around with the buttons without the unit connecting to any power mains.
The Life Span of Battery
All rechargeable battery has a certain life. For Ni-MH battery, it is not more than 5 years; after which, one would have to replace it. The unit is supplied with a button or coin type of battery which is often seen in laptop/desktop computers for the CMOS circuits. The battery is not readily from the neighbourhood store. One might be able to get them from Sim Lim Tower
Delivery
The track and trace list of Q0010 shows that the unit was prepared for shipment on 31 March and reached the delivery partner by 1 April who delivered it on 3rdApril. The cost of delivery is another SGD$4/= or about 30% of the unit cost.
Conclusion
This time switch is a worthwhile investment. It works very well compare to the mechanical time switch with flexibility of weekly programming which a mechanical time switch cannot offer. The only snag is that it came with a button or cell Ni-MH battery which is not only expensive to be replaced, it is also not readily available. The alternative is to replace the whole unit or convert the battery to the AAA type; fortunately, this problem only comes after 5 years.
Manuals
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Manuals
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