Le Tonkinois Varnish
B & D Murkin
UK main importers for
Le Tonkinois varnish
When I repaired the RAC chargers I did not have a circuit diagram and had to methodically trace out the power section. Once I had identified transistor Q2 was controlling the output MOSFETS I did not do any further investigation on the lower power section.
I was very pleased to be contacted by Phil Harris G4SPZ who owns a similar model of charger, the Ring RCB320*, which he managed to repair in 2020 by replacing the blown AR501 rectifier diodes.
Phil discovered my website and was grateful to find the full circuit diagram.
Together with Dr Phil Cadman G4JCP, they analysed the operation of the circuit to try to establish how the automatic charge control works. The following is their brief summary.
Note that there is at least one error on the circuit diagram; resistor R17 (200k) should be shown connected to GND and not to the positive line. As drawn, the circuit could never work.
Charge rate is selected by changing primary tappings on the mains transformer. The AC voltage appearing across the secondary windings is full-
The control element is a programmable shunt regulator IC1 (TL431) containing an internal precision 2.5 volt reference which is compared with the potential applied to its ‘adjust’ terminal. IC1 is normally OFF, but as the voltage across the charging battery rises, the voltage applied to IC1’s ‘adjust’ terminal from the potential divider R16-
The function of 18 volt zener diode Z1 and transistor Q1 appears to be to disable the automatic voltage-
*Note: the Ring RCB320 is a de-
Phil Harris CEng MIEE G4SPZ
Many thanks to the two Phils for supplying this information.
I am also an old radio amateur G0GUV, its great to hear from them and to know there are still some of us around who are happy working on circuits at component level. 73s