Background
I have successfully used these electronic mains timers for over 10 hears, purchasing the first one in earlyl 2010. I used them extensively in the greenhouse to control grow lights and heating, particularly during the seed germination and growing season. In the last couple of years some of the timers started to fail. I knew that they contained rechargeable NiMH batteries to maintain the time and programs so it was highly likely that they had reached the end of their useful lives. I had opened the timers when i first got them to see how they were constructed and knew that the mains side of the circuitry was quite simple. During use over the years i had noticed that the got warmer than expected even when in standby with the device to be controlled was switched off. At that time i had opened one of the timers and discovered that the zener diode was very hot to the touch and that the component side of the board around it was discoloured due to the long term heating effect.
While repairing and refurbishing these devices i discovered that two main problems had to be resolved.
1. The 1.2V, 40mAh rechargeable NiMH battery had to be replaced to power the digital controller and program memory section.
2. The zener diode providing voltage regulation was running at or beyond its design parameters resulting in failure and damage to the mains circuit board.
While repairing and refurbishing these devices i discovered that two main problems had to be resolved.
1. The 1.2V, 40mAh rechargeable NiMH battery had to be replaced to power the digital controller and program memory section.
2. The zener diode providing voltage regulation was running at or beyond its design parameters resulting in failure and damage to the mains circuit board.
This is how the five timers i have appeared after the summer 2020 growing season.
The screens were blank and no matter how long they were connected to the mains, the batteries did not recharge nor did the displays reappear.
I was pretty certain that the NiMH batteries inside had reached the end of their useful lives.
The timers are easy to open requiring the removal of a couple of crosshead screws and releasing some plastic clips.
In this photograph the case has been opened and the rechargeable battery voltage is being measured.
As the display is showing, the voltage is just over 0.6V but it should be 1.2V.
I ordered a couple of replacement batteries and waited a few days for them to be delivered.
This picture shows the old battery on the left, removed from the circuit and the new replacement battery on the right before being fitted to the circuit board.
The battery is rated at 1.2V 40mAh and uses the NiMH chemistry.
This picture shows the original battery fitted to the AC mains circuit board before it was replaced.
The zener diode that generates the 56V DC voltage to control the relay is to the right of the electrolytic capacitor and is an orange/black colour, it is the middle component of the five to the immediate right of the electrolytic capacitor. It is just possible to see the slight brown discolouration of the circuit board around the zener diode caused by the excessive heat.
The timer was tested before the battery was replaced to measure the voltage of the zener diode as the markings were not visible.
The temperature of the zener was so high that it was untouchable which would explain why some of the zener diodes in the timers appeared to have failed!
The zener diode that generates the 56V DC voltage to control the relay is to the right of the electrolytic capacitor and is an orange/black colour, it is the middle component of the five to the immediate right of the electrolytic capacitor. It is just possible to see the slight brown discolouration of the circuit board around the zener diode caused by the excessive heat.
The timer was tested before the battery was replaced to measure the voltage of the zener diode as the markings were not visible.
The temperature of the zener was so high that it was untouchable which would explain why some of the zener diodes in the timers appeared to have failed!
This is a picture of another one of the timers i have where the battery needs to be replaced and the zener diode has failed. The zener diode is the third of the black and silver components to the right of the blue relay with the silver part of it's body pointing to the top of the picture. This picture shows the moost clearly how badly discloured the circuit board has become due to the excessive heating of the zener. This device spent almost all of its life very close to its maximum junction operating temperature temperature of 125 degress centigrade. The underside of the circuit board is also discoloured. Thankfully one end of the zener diode is soldered to a large copper pad which helped to dissipate the heat away from the component which i think is accidental as the other end is soldered to a narrow track which would not help the reduce the heating.
In this picture the rechargebale battery has been replaced. When the battery tags were soldered the LCD display immediately started to operate.
The failed zener diode with a rating of 56V 1.0Watt was replaced with a higher rating of 56V 5Watt. It was mounted off the circuit board surface to avoid having to over stress the leads to fit the circuit board holes.
This also allows the discolouration of the circuit board to be more easily seen beneath the zener diode caused by over heating of the original device.
The timer was plugged into the mains for several hours and the temperature of the replacement zener diode ws low enough to allow it to be easily touched which was not the case with the original 1Watt zener!
The failed zener diode with a rating of 56V 1.0Watt was replaced with a higher rating of 56V 5Watt. It was mounted off the circuit board surface to avoid having to over stress the leads to fit the circuit board holes.
This also allows the discolouration of the circuit board to be more easily seen beneath the zener diode caused by over heating of the original device.
The timer was plugged into the mains for several hours and the temperature of the replacement zener diode ws low enough to allow it to be easily touched which was not the case with the original 1Watt zener!
This is one of the repaired timers. The battery has been replaced and the 56V zener diode has also been replaced.
The timer was tested by setting up various on/off cycles using a radio as an AC Mains load.
After the testing was completed the timer was put back to use controlling theheated sand bed heater in the greenhouse.
For a few pounds and a few hours of effort the timers have been repaired rather than going for recycling or into landfill.
I find it both sad and shocking that for just a few extra pence/cents a higher power rated zener diode could have been used to prevent the heat problems and the failure of the component. It seems ironic in these Environmentally conscious days that a component designed to use energy in an efficient manner by timing its use has been designed with a 'self destruct' component that results in the discarding of dozens of other perfectly good working parts. That also goes for the installation of a non replaceable rechargeable battery. Of course there is one very major benefit to the manufacturer....they do not need to supply spare parts and can sell you another brand new model a few years down the line!
The timer was tested by setting up various on/off cycles using a radio as an AC Mains load.
After the testing was completed the timer was put back to use controlling theheated sand bed heater in the greenhouse.
For a few pounds and a few hours of effort the timers have been repaired rather than going for recycling or into landfill.
I find it both sad and shocking that for just a few extra pence/cents a higher power rated zener diode could have been used to prevent the heat problems and the failure of the component. It seems ironic in these Environmentally conscious days that a component designed to use energy in an efficient manner by timing its use has been designed with a 'self destruct' component that results in the discarding of dozens of other perfectly good working parts. That also goes for the installation of a non replaceable rechargeable battery. Of course there is one very major benefit to the manufacturer....they do not need to supply spare parts and can sell you another brand new model a few years down the line!