Electronics
PlasmaQuant MS Series
186
The 36 kHz clock time output is a saw tooth waveform that provides the input at U5
pin 1. The active low signal triggers the analog switch at a 36 kHz rate, placing 12 V
pulses at U5 pin 3. Each clock pulse triggers U6 pin 1, a dual decade binary coded
decimal (BCD) counter.
U5 pin 6 changes state for every 10 clock pulses on U5 pin 1. The 3.6 kHz output at pin
6 clocks U5 pin 9, generating a 1.8 kHz square wave at U5 pin 11 and a 360 Hz pulse
waveform at pin 14.
U5 pin 14 sets an active high at U4 pin 4 disabling the output of pulse-width
modulator U4 for 20 clock cycles (550 µs).
During the last 10 clock pulses of this period (275 µs), U6 pin 11 is set high which sets
U5 pin 9 low. The active low at U5 pin 9 closes the analog switch across pins 10 E 11.
The closed switch provides a sample of the motor back EMF voltage to the sample and
hold capacitor C49. This voltage across C49 is buffered by U7 pins 5-7 and used as the
speed feedback signal until the next sampling period.
Current limiting protection for the DC motor and gearbox is provided by sensing the
voltage across R44 which is compared to the 0.35 V reference at U8 pin 5. When the
sensed voltage at U8 pin 6 is greater than 0.35 V pin 7 goes negative, forward biasing
D31 and decreasing the voltage at U8 pin 2.
Figure 10-8 - Back EMF sensing circuitry
The cooled spray chamber uses a solid-state Peltier heat pump to cool the spray
chamber. The Peltier cooler requires a drive of 0-4 A at a maximum of 16 V. The
current to the Peltier device is controlled by a voltage output from a DAC on the
Control PCB. A voltage of 0V from the DAC is zero current to the Peltier. A voltage of
5v from the DAC results in a current of 4 A (maximum) to the Peltier. The Peltier has a
separate enable line for switching the control off and on. A switching regulator is used
Peltier cooling supply