DIY: Fix Timing Issues in Kerde Timer Mod
In the previous DIY article, DIY: Turn a Kerde Timer into a Real Clock, we showed how to modify the Kerde timer using the 8025T chip. In this follow-up, we'll continue with that modification.
If you've done the mod as explained before and noticed that while the timer was accurate at first, it gradually started to fall behind—say, by 2 minutes and 43 seconds—then you're not alone. So, how do we fix it? If you're curious, keep reading.
In this situation, you'll need to open up the timer again and measure the actual working current. If you find it's around 0.6mA, chances are the button battery is already drained and only putting out 1.9V or so.
One possible fix is to tap the 8025T's power from the mainboard's relay power using a zener diode voltage divider. But be very careful—never work on a live circuit. Otherwise, you might accidentally set off a mini firework show while soldering, which could even trip your power socket.
And if so, here's the result:
The 8025T's "ECG" is now a flat line. The working current spiked to 1.7mA, and the chip is basically fried.
Here's what a healthy "ECG" looks like for comparison:
Friendly Reminders:
1. Never solder on live circuits, especially on capacitor-drop power supplies like this one—it's extremely dangerous.
2. If you're using dual battery power, always check the actual working current. Sometimes everything looks fine, but the current isn't. For example, the 8025T has a no-load current of around 1.5μA, but once the battery starts to drain, timing errors can grow significantly. If the battery voltage drops below the chip’s working threshold, the inaccuracy can be even worse than the 8025AC.
3. Why the mini firework? The design here uses a hot ground. The live wire goes straight into the diode bridge, while the neutral line is dropped through a resistor-capacitor combo before also entering the bridge. So if you try tapping power while it's plugged in, current may flow through a diode in the bridge and straight to your soldering iron—boom. Always be careful when working with electricity.
The End
