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Diodes in Vehicle Electronics

Spike Suppression and Polarity Diodes

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Diodes  in Vehicle Electronics 

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Diodes in Vehicle Electronics

Spike Suppression and Polarity Diodes

  

This email is Spike Suppression Diodes which weren't necessary until computer circuits began to appear in vehicles. In BC time (before computers), spike suppression wasn't necessary. Here's why.


Fig. 1

In Figure 1 above, a solenoid (or coil) is operated by a mechanical switch wired to operate the solenoid from the ground side.

 


Fig. 2

In Figure 2 above, the switch CLOSES to operate or "power up" the solenoid. Electron current flows from the negative terminal of the power source (vehicle battery or generator) and travels up through the solenoid. Electron flow enters the solenoid at the bottom and exits at the top of as it flows back to the positive terminal of the voltage source or B+. The polarity of the voltage drop across the solenoid is negative at the bottom and positive at the top. This field voltage drop can be measured with a DMM and is indicated by the arrows pointing outward.  

 

As electrons flow through the solenoid winding an electromagnetic field is created around the solenoid winding with the same polarity voltage drop. That is, positive at the top and negative at the bottom of the solenoid winding. The arrows pointing outward indicate the expanding movement of the electromagnetic field.

 

As long as the switch remains CLOSED the electron current continues to flow and sustains the energy of the electromagnetic at a specific level which moves and holds the solenoid's plunger (not shown) creating the movement to accomplish a task.  

 
Fig. 3

In Figure 3 above, the switch OPENS to de-energize or "power down" the solenoid. Electron current from the power source stops flowing through the coil. The electromagnetic field suddenly collapses as indicated by the arrows pointing inward. The action of the collapsing electromagnetic field is kindly referred to as an "energy dump" as stored electrical energy is released back into the circuit.

 

During an energy dump the voltage drop is reversed across the coil being driven by the lines of force moving inward this time. Electrons pour out of the top terminal of the solenoid, now negative, and travel around the circuit through the power source to supply electrons to the highly positive voltage at the bottom of the solenoid.

 

All the stored energy in the electromagnetic field immediately dumps back into the circuit.with significant electrical force that causes arcing across the switch contacts that have just begun to OPEN. 

 

In one of our electronic classes we demonstrated with a lab scope how a 30 ohm solenoid operated at 13.0V creates an energy dump with a voltage spike that rises to about 140 volts. The time duration of the energy dump only lasts for a few milliseconds but the voltage spike amplitude produced is sufficient to create an arc across the opening switch contacts. Notice in Figure 3 a small arc effect is shown as the contacts OPEN. The only negative impact of the energy dump is a little arcing at the switch contacts. That is not problem for a switch with heavy duty contacts.

 

If the switch fails due to contact damage from the arcing it is possible to prevent the arcing by controlling the energy dump A diode is connected across the solenoid terminals as shown below in Figure 4. REMEMBER, electron flow is against the arrow in the diode symbol.  


Fig. 4 

  

The collapsing electromagnetic field causes the voltage drop across the solenoid to reverse its polarity during the energy dump. That is the same polarity voltage that allows electrons to pass through a diode. The spike suppression diode turns ON during the energy dump and electrons pour out of the top of the solenoid. They see a low resistance path to the opposite side of the solenoid (now positive) through the diode.

  

Energy dump electrons flow through the diode to satisfy the need for electrons at the bottom of the solenoid. There is no energy dump electron flow the long way around through the power source and there is no arcing across the switch contacts when they OPEN. The energy dump is safely contained at the solenoid terminals and the switch contacts do not experience arcing. In BC days (before computers) the arcing at the switch contacts was dealt with by using big switch contacts to survive arcing for a long time.  

 

However, with computer control of solenoids the energy dump must be suppressed with spike suppression diodes. Mechanical switches to control solenoids are replaced with solid-state switches called Transistor Drivers mounted inside on-board control units. Transistor drivers cannot withstand the powerful energy dumps that would pass through the transistor. The only option is using spike suppression diodes to protect the transistor driver from the energy dump..

 

Below in Figure 5 there are 2 spike suppression diodes to suppress the energy dump that occurs when a solenoid is turned OFF.  

  

Fig 5

 

Spike Suppression Diode #1 is placed across the starter solenoid. This diode was added to surpass the energy dump when the starter solenoid powers down after the engine begins to run. When this energy dump is not suppressed there could be problems with the PCM loosing adaptive memory from the energy dump running the long way around the circuit. The result was cold engine driveability problems that did not clear up until the PCM had enough drive time to relearn adaptive strategies. The problem reappeared with the next crank when another energy dump occurred.   

 

Spike Suppression Diode #2 is inside the relay and wired across the relay coil. It protects the transistor driver from a very destructive energy dump when the driver stops electron current trough the relay coil. One or two energy dumps is all it takes to kill the transistor if the spike suppression diode inside the relay becomes OPEN. Some relays use a small value resistor instead of a diode to control the energy dump.

 

Diode #3 is a special purpose diode. It has nothing to do with spike voltage suppression because it is not connected across a solenoid or relay winding. The diode is inside the PCM and connected in series with the B+ voltage applied to PCM circuits. This diode is a polarity sensing diode and only allows electron flow through PCM circuits when the correct voltage polarity is applied to the vehicle. That is when the positive battery cable is positive and the negative cable is negative or normal polarity. The polarity diode allows electron current through PCM circuits. Anytime reverse voltage is applied to a vehicle such as when jumper cables are crossed in an attempt to jump start a vehicle, the polarity diode prevents electron current through PCM circuits and saves the PCM from damage.  

 

You are probably thinking ..... "How do I check a spike suppression diode or a polarity diode?" That becomes a major topic of discussion covered in a few lessons of the 60 Lesson Home Study Vehicle Electronics Course. It is too big a topic to handle in these short emails.

 

Till next time, 

  

Vince Fischelli  

Director of Training

Veejer Enterprises

Web site: www.veejer.com

Email: vince@veejer.com  

Phone: 972.276.9642

Fax:  972.276.8122 

 

 

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Posted: Mar 18, 2016,
Categories: Fire Mechanics,
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