Car Relay Driving Circuit Techniques

These two projects (car relay driving circuit techniques) both address the same problem but in very different ways.

The problem that both designs address is where multiple relays are used in an enclosed space specifically inside a car relay box control panel.  When driving relays the current in the armature causes heating and with a large number of relays in a small space can cause unacceptable heat increase.

The key point about the relays is that they have both a pick up or turn on current and a holding current (the current required to keep the relay closed) which is lower than the pick up current.

In normal designs it may not be critical to use only the holding current but to reduce heating this is the most important action to be undertaken.

The usual way of driving relays is to individually drive the armature using a transistor connected to the armature and from there to the power supply without worrying about holding current.  

To create a low holding current driver you would need to reduce the transistor current flowing after having ensured that the pick up current has had effect i.e. to drive it long enough so that the relay closes.  One method of doing this is to use two control pins from a microcontroller one for the pickup current and one for the holding current to drive the transistor base current at different levels.  

This would however mean using a large amount of control pins (30 for 15 relays) which would infrequently change state and would waste the resources of a microcontroller.  

The designs below both automatically adjust themselves to the current requirements made on the armature and both alter that current when a new relay is activated.  In addition they both revert to low current holding mode and this is all down without any microcontroller in sight i.e. very elegant solutions.

Additionally they allow multiple relays to be added into the circuit with out altering the basic design of the system i.e. You don't have to add a new drive circuit for each relay added.

The first one takes a process and control approach where a logic circuits and fixed timers are used to current boost the relay driver current using an impulse drive.

Multiple Relay Drive Control Circuit Design 1

The second one takes an analogue circuit design approach where a current sensing resistor and opamp  is used to create a RC timed pulse that boosts the current drive for a short time.

Multiple Relay Drive Control Circuit Design 2

Click here for more project ideas.

New! Comments

Have your say about what you just read! Leave me a comment in the box below.

Learn to program in C

in easy steps.

Essential & EASY:

PIC C Programming Course:


Claim Your: Useful

"Arduino Software Guide"

   Right Now...

Privacy Policy | Contact | About Me

Site Map | Terms of Use

Recent Articles

  1. Digital I/O Expansion - How to increase your microcontroller Input and Output pins.

    Digital I/O expansion. Several diferent serial design techniques with different tradeoffs to increase the number of input or output pins in your microcontroller design.

    Read more

  2. Simple techniques for debugging microcontroller hardware.

    Simple techniques for debugging microcontroller hardware.

    Read more

  3. How to Use the MAX7219 to drive an 8x8 LED display Matrix on the Arduino.

    The Essential Guide to the MAX7219; What it is and how you can easily use one in any of your projects.

    Read more

Custom Search

Sign up for MicroZine:
''The'' Microcontroller Newsletter

Enter your first Name and primary email address in the form below:

And receive absolutely FREE a full project for:

"Measuring Analogue Voltages
Without An ADC"

(Using only one pin).

Instant Download:
You Can
Get It Right Now

Warning: This project could be Removed
at any time.  

It will NOT be
available indefinitely SO

To avoid
disappointment get it:


Don't worry -- your e-mail address is totally secure. I promise to use it only to send you MicroZine
Remember this is a project with full description and fully debugged C Source code - and it's not available from the main website.

You can only get it through this newsletter.

To get exclusive access Enter your first name Name and primary email address Now in the form above.:

But wait !

There's more...

You'll receive more
free and exclusive reports as well as site information and site product updates.

Scroll up to the form above and sign up NOW.  Don't forget it's FREE and if you don't like it you can unsubscribe at any time.

Click Here Now to use the form above to get your Valuable information absolutely free.

Social Bookmarking
Click & Add:
add to BlinkBlink
add to
add to DiggDigg
add to FurlFurl
add to GoogleGoogle
add to SimpySimpy
add to SpurlSpurl
Bookmark at TechnoratiTechnorati
add to YahooY! MyWeb
Readers comments

"I wanted to thank
you so so so much
for all the information
you have provided in
your site it's


- Ranish Pottath

"This site really is
the best and my favorite.
I find here many useful
projects and tips."

- Milan


Learn PIC C Now
Wondering how to program your next project using C and need a great start?

"Awesome site,
very, very easy and nice
to navigate!"

- Matt

"I am a newbie to PIC
and I wanted to say
 how great your
site has been for me."

- Dave


Learn Microcontrollers
"Interested in

Sign up for The
Free 7 day guide:


"Your site is a great
and perfect work.

- Suresh


"I couldn't find the correct
words to define
yourweb site.

Very useful, uncovered,
honest and clear.

Thanks so much for
your time and works.

- Anon