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Flexidisc sander
Charger Repair 4 - Finding the second fault
After the obvious checks for wiring connection errors, bad connections etc it was time for specialist fault finding. Either an additional component had also been damaged in the original fault or the output transistors had blown whist replacing them.
I could not obtain a circuit diagram so it was down to tracing out the main circuit which is very time consuming. The purpose of the investigation was to find just enough information to make simple diagnostic measurements. Once the circuit tracing was finished diagnosis could be started.
(In 2018 Zoltán Hódos Sr sent me a copy of the full circuit diagram. Thanks for that information.)
Click on images for better quality
The two IRF3205 power mosfet output transistors work in parallel and are controlled by transistor Q2. The data sheet for the IFR3205 shows they start conducting with the gate somewhere between 2 and 4 volts positive with respect to the drain, anything above that and they are on.
The charger was connected to a discharged battery. A fully charged battery might drop the charge rate automatically, giving confusing circuit measurements.
If transistor Q2 was conducting the voltages on the gates of the output transistors should have been pulled up to the positive supply rail, switching on the charging current. I first checked the positive voltage rail voltage on Q2 emitter, it was within a volt of the battery positive terminal.The collector to emitter voltage of Q2 measured less than 0.1 volts which would be correct for a fully conducting transistor. I also applied a short circuit from Q2 collector to emitter as a double check. As there was no charging the fault had to be further on in the output circuit.
Bypassing the fixing screw connection.
The IRF3205 metal tab is connected to its drain so tape was used to insulate the top front chassis to prevent short circuits. The pcb gets its positive voltage via the fixing screw and a temporary connection was made using leads with crocodile clips. Two in parallel were used to give a more reliable connection.
The toggle switch is open circuit during normal charging so no connections to it were needed.
The upper front chassis insulated with duct tape.
Printed circuit board connections.
Transistor Q2 location.
Pins left to right Emitter Base Collector.
The output switching circuit.
The fault localised to these components.
The faulty zener diode.
Zener diode Z2 location.
The final result
This output circuit also includes the rectifiers and the mains transformer which had been superficially checked but not at high currents.
A wire from the pcb to the toggle switch comes from the transformer centre tap. In the later charger version this connection goes via an additional 0.5 ohm resistor. The other toggle switch wire is connected to the charge meter. By temporarily rearranging that wiring I shorted out the output transistors source to drain, with the meter still in circuit. Those changes are described on page 2 Transistor bypass test.
When the charger was switched on it started charging which eliminated the rectifiers and transformer, localising the fault to the components shown inside the green ring on the circuit diagram.
That left just the two output transistors together with a few passive components as the problem.
I guessed the most likely cause would be the output transistors damaged by static during replacement.
After restoring the temporary wiring re-
Test circuit for the IRF3205 Power Mosfet.
The test circuit component values are not critical.
The variable resistor can be any value between 100 ohm and 10k.
The led and 1k resistor could be replaced by a voltmeter
The led should be off for gate voltmeter readings below 2 volts
The led should be on for gate voltmeter readings above 4 volts
I made up a simple circuit to test them using components from my spares box.
Unexpectedly they both tested good.
With the transistors out of the circuit board and out of contention that left 5 resistors, a Zener diode and a capacitor plus the printed circuit tracks.
The copper tracks were undamaged and the 1k and the two 47 ohm resistors tested ok. The two 10k resistors were effectively in parallel so a meter across either them should have read about 5k but it read much lower, approximately 1k varying a bit each time I measured.
The resistors did not look cooked so much more likely to be the capacitor gone leaky. Wrong, with one leg of the capacitor unsoldered still the same result.
Unsoldering the zener diode Z2 gave the correct 5k reading. When I tested the zener after it had been heated during unsoldering the resistance across it had dropped to about 150 ohms in each direction. Even at the higher resistance it had been shorting out the drive to the two output transistors. I replaced it with a 1.3w BZV85C 15volt zener diode, Maplin part no QF57M. Ebay is an alternative source.
As both the zener and the capacitor were fairly close to the heatsink which must have got very hot during the original damage I considered replacing the capacitor at the same time.
The capacitor is a 0.1uF 50v electrolytic rated at 105 deg C. Maplin only stocks the 85 deg C version so I tested the old one for capacitance and leakage before putting it back. Later note: They are listed on Ebay 2014.
Then it was back to re-
Any of the components in that part of the circuit ringed in green plus transistor Q2 are vulnerable to damage.
The final question was why had the chargers blown up.
When I investigated further the answer was surprising, details are on the final page.
Both chargers failed again, 2014 update on next page.