Digital multimeter diode gear circuit principle diagram

The principle of the digital gear circuit is as follows:

The gear circuit is shown in Figure 1. It is an expansion of the 200mv basic meter. The +2.8v internal reference voltage is drawn from the "v+" terminal (IC1 pin) and passes through resistors R17, R16 and RT to provide a test current to the diode under test VDX. Before the diode under test is connected, the voltages at points a and b of the voltage divider circuit are

va= ((rl4+r15)/(r17+ri6+rt+rl4+r15))v+= ((274+30.1)／(1+0.47+0.5+274+30.1))× 2.8= 2.782 v

vb= (r15／(r17+r16+rt+r14+r15)) v+=[30.1／(1+0.47+0 .5+274+30.1)]×2.8= 0.275v

集成电路7106当前的输八电压为v in=vb=0.275v=275m v 。由于该值超出了基本表电压量程200mv ，所以显示屏读数应为溢出状态(显示 1”)。当被测二极曾vdx接入电路之后， a点电压由2.782v被箝位到二极管的正向压降vf(硅管为

0.7v, germanium tube is about 0.3v), and at this time the input voltage of the integrated circuit 7106 becomes

vin= (r15／(r14+r15)) vf= [30.1／(274+ 30．1)]vf≈ 0.1 vf

It can be seen that at the input end of the integrated circuit, Vf is attenuated by 10 times, which is equivalent to expanding the 200mv basic meter to the 2v range, and directly displaying the forward voltage drop Vf of the diode under test on the display. During this period, the test current through the diode under test is

if(silicon tube)≈(v+-vf)/(r17+ ri6+rt)= (2 8-0.7)/(1+0.47+0.5)=1.066ma.

if(germanium tube)≈(v+-vf)/(r17+r16+rt)=(2.8-0.3)/(1+0.47+0.5)=1.296ma.

Obviously, the test current of the diode range is at least one order of magnitude larger than that of the resistance range, and the ratio can basically meet the test and judgment requirements of most semiconductor diodes and their pn junction unidirectional conductivity. The reference voltage Vref is supplied by the internal reference voltage source (+2.8V) of the integrated circuit through the voltage divider of RI8, RL9, RP3, R20 and R48. The voltage adjustment range of the adjustable resistor RP3 is 9.51-10.73mV. By properly adjusting the sliding arm position of RP3, the reference voltage Vref can be obtained.

Voltage Vref = 100.0mV. C8, R29, and R30 form a high-frequency filter circuit at the reference voltage input.

Q1, Q2, RT and RI6 form the test port overvoltage protection circuit. RT is a thermistor PTC with a positive temperature coefficient. After the collector junctions of the transistors QI and Q2 are short-circuited, the two are connected in series with opposite polarity. The voltage regulation principle of the emitter junction is used to suppress the high voltage intrusion of the test port under abnormal conditions. For example, if the diode gear is used to measure the mains voltage (AC220V), the current path is RT--RL6--Q1--Q2. Q2 is forward-conducting at this moment, and Q1 immediately breaks down in the reverse direction, which plays a role in limiting protection. At the same time, RT quickly heats up due to the large current flowing through it, and its resistance value will increase sharply with the increase in body temperature. Since the current flowing through Q1 and Q2 is limited, the safety of Q1 and Q2 is protected.