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For a lot of timer applications, you don't need an accurate digital timer, but just an electromechanical model. With the TW-01 from Chinese Kerde, you can set a time between one minute and thirty minutes and you can switch 230 V and 10 A. |
Introduction to the TW-01
Delivery
The TW-01 is delivered by several Chinese Internet shops for a price around € 5.50. The device is delivered in a nice and sturdy box and has the specifications below:
- Switching voltage: 230 V max.
- Switching current: 10 A max.
- Minimum timer setting: 1 minute
- Maximum timer setting: 30 minutes
- Dimensions front plate: 85 mm x 85 mm
- Installation depth: 32 mm
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| The countdown timer TW-01 and its packaging. (© 2018 Jos Verstraten) |
In the picture below we have combined the front and back of this timer. On the front, next to the large button for setting the time, you will see an indicator light. This is a neon bulb, which only works if you supply the timer with the 230 V of the mains voltage. Behind the front plate, which you can click out of the body with a screwdriver, there are two mounting holes at a distance of 60 mm. Two 25 mm long screws are included. On the back there is a sturdy four-pole screw connector for connecting the two wires from the power supply and the two wires that go to the load. The two black wires are the neon bulb connection wires.
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| The front and rear of the TW-01 electromechanical timer. (© 2018 Jos Verstraten) |
The timer consists of a spring that you tighten by turning the knob to a certain position. After releasing the knob, the spring relaxes. This will cause a gear to turn. This rotation is transferred by means of a gearbox with a very long delay to a cam disc, which closes the switch and opens it again. The shafts of the gears are not mounted on bearings, but rotate in recesses in the transparent housing of the device.
As shown in the picture below, the electrical switch is single-pole. The cam disk and the switch cam are designed so that the switch switches off quickly after the set time has elapsed with a clearly audible click. While relaxing the spring, you will hear a clearly audible rattling sound, the indicator light is in fact superfluous.
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| The mechanical part of the countdown timer. (© 2018 Jos Verstraten) |
The TW-01 countdown timer tested
Timer accuracy
Because it is quite difficult to set the dial very precisely at a certain time, we have conducted a statistical test. Several times we set the timer to 5, 10, 15, 20, 25 and 30 minutes and with an electronic timer we measured the exact time that the switch of the TW-01 was closed. The table below summarizes the average results of these measurements. The tested timer appears to work shorter than intended. Moreover, the percentage deviation increases as you set a longer time.
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| The average time setting accuracy on the TW-01. (© 2018 Jos Verstraten) |
Switch contact resistance
The switch can switch 10 A according to the specifications. An indication of the quality of the switch is the contact resistance of the switch, of course when it is closed. This is easy to measure by passing a known current through the switch and measuring the voltage across the switch, see the measurement setup below. Over the switch contacts, a DC voltage of 15.3 mV was measured at a current of 2 A, which corresponds to a contact resistance of 7.65 mΩ. An absolutely excellent value!
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| The measurement setup for determining the contact resistance. (© 2018 Jos Verstraten) |
Our conclusion
For non-critical timer applications, the TW-01 is an excellent and inexpensive alternative to more expensive digital timers. Because the time mechanism is completely independent of the switch, you can use the TW-01 for switching the 230 V mains voltage as well as for switching 12 V or 24 V DC voltages, such as for switching ultraviolet LEDs in an exposure box.
220V 10A 30Min Countdown-Timer
