Diodes for spikes?

[RudiGreyling]

Hi Guys,

Looking at Van?s electrical drawings, there is mention of Diodes for spikes.
How does a Diode arrest spikes? What type and rating are these diodes???

Thanks,
Rudi

[penguin]

The Aeroelectric connection presents a good explaination for most of this stuff. Can't remember exactly what's going on, but the diodes aren't critical, I think they stop the contact points burning in the long term.

Pete

[gasman]

Contactor coils are inductive by design, and when switched off, they generate a voltage spike that can be hundereds of volts in amplitude. This can erode the contacts of the master switch.

On your next order to Vans, just order two and install them as per instructions that come with them.

[Captain_John]

Rudi,

There is some deep electrical theory in the reason for the diodes. If you want to do some googling on the topic, it is called "back EMF" or "counter EMF".

Here is the Wiki on it.

Lemme know if you have questions.

CJ

[cytoxin]

there are clamping diodes, despiking diodes and current limiting resistors. diodes are often used in conjunction with resistors. not only do they help save your contacts they also prevent the high voltage spike from going through any component on that circuit. if youve ever turned off the a/c in your car and heard a snap in the radio there is chance your clamping diode across the a/c coil is bad. there are diodes and current limiting resistors on each injector circuit.these are however not across the leads but only on one lead. because they would tend to slow down the injecor time turning off. if vans says install them and you are going to have delicate electronics then you would be well advised to install them.
basically they are installed in reverse polarity across the leads of coils to allow the votage spike to dissipate without going back into the system..on time critical circuits they wont be accross both leads but only one one to ground. then used with a resistor to prevent overcurrent as when diodes fail they usually fail shorted and resistors usually fail open. YMMV

[rv8eh]

Another stab at it:

When you energize a DC coil (such as a coil on a relay), you are storing energy in a magnetic circuit (building and maintaining the magnetic field that pulls in the relay).

When you open the coil circuit to shut off the relay, the energy stored in the magnetic field (as it collapses) tries to keep the current flowing by building the voltage across the open circuit (usually fails at this). The net result is that when you open the circuit, a voltage opposite in polarity is briefly generated, and this voltage can be large enough to cause insulation damage, or arcing and degradation of the switch you opened.

When you put a reversed diode across the dc coil, it allows the reverse polarity (sometimes called "back EMF", as mentioned in an earlier post) current to flow, and due to the properties of a silicon diode, limits the reverse voltage to approximately 0.7 V.

PLEASE MAKE SURE THE DIODE IS INSTALLED CORRECTLY, OR YOU WILL LET THE SMOKE OUT, AND NEED A REPLACEMENT DIODE.

With the diode installed, the coil field will collapse more slowly than for an open circuit, and the main contacts on the relay will probably open more slowly. This might or might not be significant to the operation or service life of the main contacts, depending upon the particular relay and the operating conditions. Cytoxin sounds like he's done some engineering around these kinds of decisions.

I have seen the results of leaving off the diode on a very large coil (conveyor tramp magnet), at 125VDC and a few amps. It left a very vivid impression on everyone who heard the bang. That memory makes it easy for me to remember to use the diodes on smaller coils too.

[lucky333]

These are scope shots of the spikes generated by the master and starter contactors when switched off. Note the 100 volt/div scale of the scope. Each of these generate several hundred volts worth of transients.

Diodes are a must across any DC coil to protect not only the switch but sensitive electronics on the bus. Cheap protection.

I have seen the results of leaving off the diode on a very large coil (conveyor tramp magnet), at 125VDC and a few amps. It left a very vivid impression on everyone who heard the bang. That memory makes it easy for me to remember to use the diodes on smaller coils too.

Howard is spot on. And it doesn't take a big coil, either. That -400V shown in the lower trace is pretty close to the 500V limit that you see on lots of low cost meters. I've seen more than one get smoked measuring these 'low voltage' circuits on even 'little' contactors like we use..

John

[Rick S.]

Check out this site and go to snap jacks (bi-directional zener transient voltage suppressors) ....you can also order them cheaper at Newarkone electronics but these are pretty easy to order from here unless you have some experience searching electronics catalogs. Perihelion has some nice stuff and good info on the site.

http://www.periheliondesign.com/

[RudiGreyling]

HI Guys,

Thanks for the feedback, good stuff!

Now can anybody tell me what Type and Rating so I can go buy at my local Electronic shop in South Africa, instead of importing from the USA?

Regards
Rudi

[lrfrey]

Yup Rudi, lot's of neat theory! Anyway, any diode of 50V @ 3A or larger will be fine. These are very cheap and widely available. Less than a buck US.

Larry