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-wave rectified by the four AR501 power diodes. Initially, when a 12 volt battery is connected, potential divider R10 and R24 cause operational amplifier B in the dual op-amp IC2 (LM358) to turn ON and the orange “charging” LED is lit. Once the mains is applied, the voltage across the battery terminals is monitored by the potential divider formed by 1% resistors R16, R4 and rheostat VR1, less the 0.7 volts dropped by forward biased diode D4. VR1 enables the cut-off voltage to be adjusted, over a range from 13.2 volts to a maximum of 15.4 volts. With VR1 in its mid-way position, presumably pre-set at the factory, charging will be cut-off when the battery terminal voltage reaches 14.3 volts.
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-R4-VR1 rises proportionally. As soon as the DC potential, smoothed by E2, exceeds 2.5 volts at this point, the TL431 rapidly changes from non-conducting to conducting. This turns Q3 OFF which turns Q2 OFF, removing the positive drive from power MOSFETs M1 and M2 and cutting off the charge current. At the same time, IC1’s cathode grounds the base of Q5, turning Q5 ON and applying a positive potential to pins 3 and 6 of IC2. This drives amplifier B OFF and simultaneously drives amplifier A ON, extinguishing the orange “charging” LED and lighting the green “fully charged” LED. Amplifier A turns Q6 ON, Q4 also turns on and, via R15 (220k), raises the positive potential applied to the ‘adjust’ terminal of IC1 to around 2.7 volts, latching it firmly into the ON state. IC1 remains latched on, and further charging is inhibited, until the latch is reset by switching off the mains supply and disconnecting the 12 volt battery.
The function of 18 volt zener diode Z1 and transistor Q1 appears to be to disable the automatic voltage-sensing circuit when 24 volt batteries are being charged; Q1 is driven into saturation with a 24 volt battery connected, and the ‘adjust’ terminal of IC1 will be held at virtually zero by the collector of Q1. It can only be assumed that the transformer secondary voltage in 24 volt mode has been chosen to limit the current into a fully-charged 24 volt battery to a safe trickle level. The purpose of 15 volt zener diode Z2 is to limit the gate-source voltage applied to the MOSFETs to below the devices’ maximum permitted value of +20 volts. The purpose of the circuit comprising E6, R29, the TVS diode Z4, resistor R30 and the opto-coupler U1 (PC815) appears to be transient suppression, designed to prevent the charge-termination circuit being spuriously triggered by arcing or inductive back-EMFs from the connected vehicle battery. The purpose of Z3, R28 and Q7 appears to be to limit the supply voltage to the LM358 to around 17.4 volts when charging 24 volt batteries, maintaining the orange ‘charging’ LED at a similar brightness to when charging a 12 volt battery.
*Note: the Ring RCB320 is a de-rated version of the RECB322 but appears very similar internally, the main differences being that the RCB320 only charges 12 volt batteries, has only one power MOSFET and two AR501 rectifier diodes, and needs no toggle switch to bypass the control circuit when used in engine start mode due to the reduced output current available from the mains transformer fitted. In addition, all other components associated with 24 volt charging are omitted.
