Introduction
This short blog details the repair of an HPM main-powered digital switch timer, model D817SLIM. The fault symptoms are a blank LCD which does not restore even after the timer is plugged back into the mains.
 |
| HPM D817SLIM Mains Digital Switch Timer |
Opening the Digital Timer
Disclaimer: Repair of any mains-rated electrical equipment should be conducted by qualified personnel. The information provided in this blog is for reference purposes.
The mains-powered timer (unit) had not been powered for several years and the LCD that displays the time and timer functionality was blank. Connecting the unit to the mains did not restore the operation of the LCD.
To open the unit, there are two plastic screws on the rear of the unit. The screws appear to be a Y-wing or a variant of that screw. A 2.4 mm wide flat blade was found fit the head pattern and the screws were removed. The two-piece plastic case uses internal clips so the same screwdriver can be used to unclip the case halves. There are circuit boards on either side of the plastic cases.
 |
| Internals of HPM D817SLIM Mains Digital Switch Timer |
Dead Battery
Upon opening the case, a small Ni-MH battery with corrosion or leakage was visible on the circuit board. The part was identified as a Troily 1.2 V 80 mAh Ni-MH battery.
 |
| Troily 1.2 V Ni-MH Battery |
Instead of replacing the battery, it was decided to use a standard electrolytic capacitor. If this unit was to be used in an area with frequent and long interruption to mains power, a battery replacement would be advised instead of the capacitor.
To remove the circuit board with the battery, two small Philips screws were removed.
 |
| Removed Troily Battery from HPM Circuit Board |
Battery Replacement
The battery was then unsoldered and replaced with a 1000 uF 25 V electrolytic capacitor. Since the operating voltage of the mains timer circuit is less than 5 V, a 6.3 V capacitor with a larger capacity could likely be used. Testing showed that with a 1000 uF electrolytic capacitor, the switch timer remained powered for approximately 6 minutes after a mains failure.
 | |
|
Other options to repair the unit, such as replacing the Ni-MH battery with a similar type or using a supercapacitor may be possible.
