Bosch Relays, Diodes and Resistors - Team Camaro Tech
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post #1 of 13 (permalink) Old Jul 17th, 11, 10:57 PM Thread Starter
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Bosch Relays, Diodes and Resistors

Hi guys -

I understand the concept of voltage spike suppression with a relay, but don't really know what equipment it might damage in my car. I have a couple of Bosch relays wired into my car - one for the starter and one for the electric RS headlight doors, wired from the headlight switch. Both of these relays have an internal resistor across posts 85 & 86 for suppression purposes.

My research tells me that a diode does a better job than a resistor.

My questions:

- If I add a diode to the socket will it be compatible with the internal resistor?

- Is the resistor alone "good enough"? I don't have any stereo equipment. The electonics I might be concerned about are the gauges, the tach and the MSD E-curve distributor.

Thanks for any insight.


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post #2 of 13 (permalink) Old Jul 18th, 11, 02:21 AM
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Re: Bosch Relays, Diodes and Resistors

A quick search found this...

Resistor: When a voltage is applied across a resistor, an electric field is established. This electric field "pushes" the charge carriers through the resistor. This "push" gives the charge carriers a "drift velocity" in the direction from high potential energy to low potential energy. As the voltage increases, the drift velocity increases. Since the amount of current flowing through a resistor is directly proportional to the drift velocity, the current is directly proportional to the voltage, which produces the electric field, which produces the drift velocity. This is the origin of Ohm's Law.


Diode: However, in a diode, the number of charge carriers is dependent on the number of electrons that have enough energy to move up an energy hill and across the p-n junction, producing current flow through the diode. The size of this hill, or energy barrier, is dependent on the amount and type of dopants in the semiconductor material of which the diode is made. As a voltage is applied (in the forward bias), the size of the hill is decreased, so more electrons have the energy needed to cross the p-n junction producing current flow. The number of electrons with the energy needed to move up the hill and across the junction increases exponentially as the voltage increases. Thus, the current increases exponentially as the voltage increases.

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post #3 of 13 (permalink) Old Jul 18th, 11, 04:14 AM
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Re: Bosch Relays, Diodes and Resistors

A zener diode is best, in the ranges of 1/8-1/2 amp, 16V-18V (pick a voltage level) and attach across the coil with the arrow pointing away from ground/return or the white line on the supply side.

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post #4 of 13 (permalink) Old Jul 18th, 11, 04:39 AM
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Re: Bosch Relays, Diodes and Resistors

Everett is right about the zener. Relays are typically only a problem when they actuate, the making and breaking of the contacts is "noisy.".

Most likely to be impacted is a pop on the radio.

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post #5 of 13 (permalink) Old Jul 18th, 11, 05:12 AM
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Re: Bosch Relays, Diodes and Resistors

It's not the contacts that you are suppressing, it's the coil. When the coil is de-energized it generates inductive kick, same as you get with your ignition coil.


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post #6 of 13 (permalink) Old Jul 18th, 11, 06:47 PM
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Re: Bosch Relays, Diodes and Resistors

sorry for the hijack, but in laymans (english for idiots like myself) terms, what diode should be placed across which terminals on a bosch type relay, given the posted info?
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post #7 of 13 (permalink) Old Jul 18th, 11, 07:17 PM
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Re: Bosch Relays, Diodes and Resistors

Quote:
Originally Posted by onovakind67 View Post
It's not the contacts that you are suppressing, it's the coil. When the coil is de-energized it generates inductive kick, same as you get with your ignition coil.

So is 1 pos and 3 neg or 1 neg and 3 pos.

That is what Derek wants to know.

I believe 1 is pos and 3 is neg.

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post #8 of 13 (permalink) Old Jul 18th, 11, 08:27 PM
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Re: Bosch Relays, Diodes and Resistors

The diode across the relay coil you have pictured above, is to suppress ac line transients from falsely energizing the relay (re: relay chatter).

That is typical for an RF circuit to shunt a coil as pictured, as RF is harder to control.

Relays and the function of them will have no adverse effects on gauges, tachs and MSD's.

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post #9 of 13 (permalink) Old Jul 18th, 11, 11:19 PM
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Re: Bosch Relays, Diodes and Resistors

The diode serves to suppress the inductive kick when you de-energize the coil. In the diagram, 1 would be the +DCV and 3 is ground.

Diode snubbers
Main article: Flyback diode

When the current flowing is DC, a simple rectifier diode is often employed as another form of snubber. The snubber diode is wired in parallel with an inductive load (such as a relay coil or electric motor). The diode is installed so that it does not conduct under normal conditions. When current to the inductive load is rapidly interrupted, a large voltage spike would be produced in the reverse direction (as the inductor attempts to keep current flowing in the circuit). This spike is known as an "inductive kick". Placing the snubber diode in inverse parallel with the inductive load allows the current from the inductor to flow through the diode rather than through the switching element, dissipating the energy stored in the inductive load over the series resistance of the inductor and the (usually much smaller) resistance of the diode (over-voltage protection). One disadvantage of using a simple rectifier diode as a snubber is that the diode allows current to continue flowing, which may cause the relay to remain actuated for slightly longer; some circuit designs must account for this delay in the dropping-out of the relay. This delay often leads to greatly decreased life of the relay contacts due to arcing.


http://en.wikipedia.org/wiki/Snubber

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post #10 of 13 (permalink) Old Jul 19th, 11, 09:35 AM Thread Starter
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Re: Bosch Relays, Diodes and Resistors

Interesting.....and thanks!

I'm assuming the GM starter solenoid produces this same spike, as does the horn relay, and that the GM engineers thought it wasn't a problem? Am I worried about nothing?


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post #11 of 13 (permalink) Old Jul 19th, 11, 09:41 AM
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Re: Bosch Relays, Diodes and Resistors

Yes, it does. But, accessories are off during the start position.
By the time the ign/acc voltage comes into play, spike is gone.

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post #12 of 13 (permalink) Old Jul 22nd, 11, 09:39 AM
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Re: Bosch Relays, Diodes and Resistors

Quote:
Originally Posted by onovakind67 View Post
It's not the contacts that you are suppressing, it's the coil. When the coil is de-energized it generates inductive kick, same as you get with your ignition coil.
Actually both. The coil generates the most noise, but we've had to turn on contacts soft with a parallel FET to pass radiated emissions in the past, depending on the spec.

Thanks for the correction though, I'm the mechanical guy of our group and all this invisible electric hocus pocus stuff, well, you just can't see it.

All that said I wouldn't worry about it too much...

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post #13 of 13 (permalink) Old Jul 22nd, 11, 05:30 PM
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Re: Bosch Relays, Diodes and Resistors

The contacts are in a totally separate circuit from the coil, hence the diode only snubs the coil. You may employ other devices to reduce contact arcing, but it will be another device.

Here is some interesting data from Arc Suppression Technologies:

http://arcsuppressiontechnologies.co...ubberMyth.aspx

http://arcsuppressiontechnologies.co...20Snubbers.pdf

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