Tracker-2800-2800S.pdf - 第73页

73 6. Place the red test probe to th e SCR's anode lead and th e black test probe to the SCR's cathode le ad. 7. Observe the SCR's anode-gate si gnature. Gate-Anode Gate-Cathode Anode-Cathode Figure 4-31 .…

MRD3056 With No Light MRD3056 With Light

Figure 4-29. Signatures of a NPN C-E Junction Phototransistor at 15V and 1K.

The phototransistor's signature is similar to a diode's signature in reverse breakdown mode when not

activated by light and as a short signature when activated by a bright external light.

SCRs and TRIACs

A SCR and TRIAC are semiconductor components that are used in switching applications. A SCR

(silicon controlled rectifier) is used for DC switching circuits. A TRIAC is used for AC switching

circuits. This section will demonstrate how to dynamically test these components.

Silicon Controlled Rectifiers (SCR’s)

The SCR is a switching semiconductor device that conducts positive current only. Its symbol and

equivalent circuit can be seen below. When the gate (G), is at the same voltage level as the cathode

(K), the SCR acts like an open. When the gate (G) is forced more positive than the cathode (K),

positive current flows between the anode (A) and the cathode (K).

Figure 4-30. Diagram of a Silicon Controlled Rectifier.

Do the following to display the analog signature of a SCR:

1. Select the 20V, 10K and 60Hz.

2. Place or clip the red test probe from the Tracker 2800's Channel A jack to gate lead (G) of the

component.

3. Observe the gate-anode signature of the SCR.

4. Move the black test probe from the SCR's anode lead to cathode lead (K) of the component.

5. Observe the gate-cathode signature of the SCR.

6. Place the red test probe to the SCR's anode lead and the black test probe to the SCR's cathode lead.

7. Observe the SCR's anode-gate signature.

Gate-Anode Gate-Cathode Anode-Cathode

Figure 4-31. Signatures of a SCR - C106B Type at 20V and 10K.

8. Connect the Tracker 2800 DC Voltage Source to the Gate. Increase the DC Voltage Source level

while observing the anode-cathode signature.

Figure 4-32. Testing a SCR using the DC Voltage Source

Review

Solid state photosensitive switch components are turned on by light.

SCRs and TRIACs are solid state switches that are turned on by a control input pin called a gate. SCRs

conduct current in one direction while TRIACs conduct current in both directions.

Applications

The SCR and TRIAC can also be a problem to troubleshoot. They may be used to switch large

currents. Quite often these components are susceptible to degradation and eventual failure. The Tracker

can easily show these failure.

Using the arrows buttons to

increase/decrease the DC voltage level

SECTION 5 TESTING INTEGRATED CIRCUITS

5-1. DIGITAL INTEGRATED CIRCUITS

Digital integrated circuit (IC) chips are made from transistors on a common substrate. Their analog

signatures are typically variations of the discrete diode and transistor signatures. Most logic ICs,

contain multiple circuits in one chip. These chips can have pins from 14 to over 200, although quite

often many pins share quite similar signatures. This can make troubleshooting easier by giving us an

easy-to-find signature to use as a comparison. In this section, it is important to understand how the

Tracker 2800 and ASA respond to these circuits.

Integrated Circuit Failures

A functioning IC may stop working for a number of reasons. Some of the most common causes of IC

failures are:

 EOS - Electrical Over Stress. The IC’s maximum electrical specifications have been exceeded. This

condition may result in the IC developing internal shorts and opens.

 ESD - Electrostatic Discharge. Repeated exposure may cause internal resistance to develop in the

IC junctions. This internal resistance may vary from 5 k to 25 k with a typical value of 20 k. ESD

exposure can cause internal flaws such as resistance, opens and shorts.

 Dendrites - A process flaw, that results in particles growing between conductors on a substrate

causing shorts.

 Ionic - Contamination introduced at the time of manufacturing that contamination develops into

leakage between substrate channels. This causes 5 k to 25 k of resistance.

 Purple Plague - Destructive interaction between gold and aluminum metal layers. Junction

connections become very brittle and may cause internal opens.

 Corrosion or Electromigration - Another process flaw in which aluminum metallization causes

pinholes, corrosion and resistance. This will create opens and resistance.