Smiths Graseby 3200, Technical & Service Manual

Page: 50 / 117

Smiths Medical International Ltd.
Issue 5 (August 2004)
3200 Service Manual 4 — 7
Once the switch-mode controller has started working, it obtains its power supply from a
bootstrap winding on pins 2 and 3 on the transformer. The power supply from the bootstrap
winding is rectified by diode D5, smoothed by C13 and regulated by the Zener diode D6.
The PWM signal output from pin 5 on IC1 controls the operation of Q3 causing it to cycle on
and off and regulate the flow of current from D7 through the primary winding. C1 and R1, C2
and R2, C3 and R4, C4 and R3 are all transient suppression circuits.
The voltage on pin 12 (IN1) of the secondary winding is rectified by diodes D1 and D3 (see
Figure 4.13 ). The output from D1 is then smoothed by C6 and passed to pin 4 of PL11.
The DC output from D3 is filtered by C5, C7 and L1. The link to PL11 pin 3 is protected by
a 1 amp fuse, FS1.
When the mains supply is being used there is a voltage present on pin 4 of PL11. By using
R6 and R7 as a potential divider, this voltage is used to control Q2.
When there is no mains supply, Q2 is not active and so prevents the batteries from discharging
through R8, R9, RV1 and R10.
When Q2 is activated, R8, TH1, R9, RV1 and R10 form a potential divider between the output
of D3 and 0 V. The output from this is used as the reference voltage for the adjustable
precision shunt regulator D4. Capacitor C8 is a transient suppressor.
Thermistor TH1 varies the reference voltage according to temperature, thus compensating for
battery charging characteristics.
The feedback process occurs when the voltage at the wiper of RV1 exceeds the reference
value, D4 conducts and activates the opto-coupler IC2. The current through its photo-transistor
causes a voltage to be developed across resistor R19. This voltage is applied to IC1 which
changes the mark-space ratio of the output signal on pin 5.
An over voltage situation resulting from a malfunction in the regulator is prevented by a
crowbar protection device. When the output voltage exceeds 13 V, the Zener diode D2 conducts.
This causes a voltage to be developed across R5. This voltage is used to activate the silicon
controlled rectifier Q1 which will sink enough current to result in fuse FS1 blowing.
Regulator board
PL 11 output
PL12 output
See Table 4.2 for the plug PL11 connections.
Table 4.2 PL11 connections
Pin Output
1/2 Ground
3 7 V DC when on mains or battery supply
4 7 V DC when on mains supply
PL12 provides a link to the rechargeable batteries and the link is protected by FS3 a 2 amp
fuse.