Products
Rewards 
from HOLOOLY

We are determined to provide the latest solutions related to all subjects FREE of charge!

Please sign up to our reward program to support us in return and take advantage of the incredible listed offers.

Enjoy Limited offers, deals & Discounts by signing up to Holooly Rewards Program

HOLOOLY 
BUSINESS MANAGER

Advertise your business, and reach millions of students around the world.

HOLOOLY 
TABLES

All the data tables that you may search for.

HOLOOLY 
ARABIA

For Arabic Users, find a teacher/tutor in your City or country in the Middle East.

HOLOOLY 
TEXTBOOKS

Find the Source, Textbook, Solution Manual that you are looking for in 1 click.

HOLOOLY 
HELP DESK

Need Help? We got you covered.

Chapter 32

Q. 32.6

What does the circuit of Fig. 32-28a do if a coil current of 30 mA or more closes the relay contacts?

Step-by-Step

Verified Solution

The E-MOSFET is being used to turn a relay on and off. Since the relay coil has a resistance of 500 \Omega , the saturation current is

 

I_{D(sat)}=\frac{24 V}{500\Omega }=48 mA

Assume this is less than the I_{D(on)} of the MOSFET that has an ON resistance of only 10 \Omega .
Figure 32-28b shows the equivalent circuit for high V_{GS}. The current through the relay coil is approximately 48 mA, more than enough to close the relay. When the relay is closed, the contact circuit looks like Fig. 32-28c. Therefore, the final load current is 8 A (120 V divided by 15 \Omega ).

In Figure 32-28a, an input voltage of only +2.5 V and almost zero input current control a load voltage of 120 Vac and a load current of 8 A. A circuit like this is useful with remote control. The input voltage could be a signal that has been transmitted a long distance through copper wire, fiber-optic cable, or outer space.

Note: The diode across the relay coil is used to protect the transistor from the large voltage spike generated by the coil when the drain current cuts off. The rapidly dropping current creates a huge voltage spike that can damage the transistor. The polarity of this induced voltage is such that it causes the diode to conduct, momentarily clamping the coil voltage to 0.7 V. Some E-MOSFETs have an internal protection diode from source to drain.