Why 47-Ohms and Why 5-Watts

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jimh
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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Why 47-Ohms and Why 5-Watts

Postby jimh » Thu May 14, 2020 10:55 am

In E-TEC rigging when there is no conventional analogue TRIM GAUGE in use, the TRIM SENDER must be supplied with current by some other means.

If an E-TEC engine is rigged with an I-COMMAND WIRING HARNESS, Evinrude has already provided a method to supply current by incorporating a 47-Ohm 5-Watt resistor into the wiring harness

If is also common that and E-TEC engine might be installed with an MWS Wiring harness. The MWS Harness is shown below in Figure 1.

MWS-HarnessAnnotated.png
Fig.1. The Evinrude MWS wiring harness, from a diagram by Evinrude, but with annotations to identify the circuit functions.
MWS-HarnessAnnotated.png (106.92 KiB) Viewed 6100 times


One method to supply current is to install a 47-Ohm 5-Watt resistor into that wiring harness. This modification can be easily made by using the existing loose wiring conductors in the MWS WIRING HARNESS as shown below in Figure 2 by connecting the external resistor at terminals marked A and B:

MWS-Harness.png
Fig. 2. To adapt an MWS wiring harness to supply current to the TRIM SENDER, connect a 47-Ohm 5-Watt resistor between callout A and B as shown above.
MWS-Harness.png (27.44 KiB) Viewed 6112 times


To explain why a 47-Ohm resistor is used, I deduced the basis for the substitution of a 47-Ohm resistor to replace the usual analog trim gauge in an article

Outboard Engine TRIM Circuit Details
viewtopic.php?f=10&t=2777

That article explains how the analogue trim gauge circuitry worked, and how the internal and external resistance of the gauge created the equivalent of a 47-Ohm resistance.

As mentioned in my article

Tank Sender and Trim Sender Resistance
viewtopic.php?f=9&t=3043

the typical EVINRUDE trim sender has a range of about 10 to 88-Ohms. The resistance is the lowest when the engine is at full-up tilt position. Knowing the range of the trim sender resistance is important in order to know the power than may be necessary to dissipate in the external 47-Ohm resistor.

The necessary power dissipation rating for the external 47-Ohm resistor can be deduced from the circuitry that exists when the 47-Ohm resistor is supplying current to the TRIM SENDER. The voltage in the circuit divides between the 47-Ohm resistor and the resistance of the TRIM SENDER. To simplify the analysis, here we assume that the TRIM SENDER can have a minimum value of 0-Ohms.

When the TRIM SENDER is at zero-Ohms, then all the voltage in the the circuit will be across the 47-Ohm resistor. The voltage will be the battery voltage. If the engine is running and the battery is being charged, the voltage should not be more than 14.5-Volts.

Power (P) is related to voltage (E) and resistance (R) according to Ohm's Law as

    P = E^2/R

For

    E = 14.5
    R = 47

the power dissipated in the resistor will be

    P = 14.5^2/47
    P = 4.5 Watts

Use of a 5-Watt resistor should give adequate power dissipation, as the maximum power dissipation occurs only when the engine is at full tilt-up position and there is power applied to the TRIM circuit. This condition is unlikely to have a long duration.


The power rating of the resistor is mentioned in my article at

Using NMEA-2000 Instruments with the Evinrude MWS Wiring Harness
viewtopic.php?t=2768

and a 5-Watt resistor is suggested.


The power rating of the resistor is also mentioned three times in my article at

Advanced E-TEC Rigging
http://continuouswave.com/whaler/refere ... anced.html

First in the section under the sub-heading

"NMEA-2000 Gauges with I-Command Harness"

And also mentioned a second and third time in the section under the sub-heading

"NMEA-2000 Gauges with System Check or MWS Harness"

A 5-Watt resistor is suggested.

When the RADIO SHACK company was in business, I used to give a URL link to their webpage showing how to order a 47-Ohm resistor from them of suitable power dissipation, which I think was actually a 10-Watt rated resistor. Unfortunately, RADIO SHACK has gone out of business.

The power rating of the 47-Ohm resistor is also mentioned in my article

TRIM Gauge with E-TEC 90
viewtopic.php?t=2997

and shown in an illustration therein. A 5-Watt resistor is suggested

To be more clear, the power dissipation of the resistor should be at least 5-Watts, and there is no harm in using a higher power dissipation resistor, should, for example, a 10-Watt resistor be more easily obtained.

I don't have any color photographs of how someone has installed a 47-Ohm resistor when they were modifying a MWS Rigging Harness. The method of connecting the existing wiring to the resistor should be done in a suitable manner to provide insulation to all wiring.

A 5-Watt resistor that is dissipating its rated 5-Watts of heat will become significantly warmer than the ambient air surrounding the resistor. To predict exactly how warm the resistor will become is a complex calculation. The resistor will try to shed heat into the surrounding air, so the surface area of the resistor itself, the materials used to make the resistor, the ability of air to circulate around the resistor, the ambient air temperature, and the mounting axis of the resistor (horizontal or vertical) must be considered. Some data about temperature rise of a resistor may be given on a resistor's data sheet from the manufacturer.

A more general rule can be used to predict the temperature rise: the higher the power dissipation rating of a resistor, the lower the temperature rise will be for a given voltage drop across the resistor. In other words, to keep the resistor from getting too hot, use a larger power dissipation rating.

We now consider what the average voltage drop across the resistor might be. Returning to my article on trim sender resistance, the typical value for a Evinrude trim sender at its mid-point is noted as 44-Ohms. Let us assume that that this is a reasonable basis for an average value of the TRIM SENDER resistance.

If we simplify this value to 47-Ohms, then we can say that only half the applied voltage of 14.5-Volts will appear across the external 47-Ohm resistor. The power dissipation in the 47-Ohm resistor will then be:

    P = 7.25^2/47 = 1.1-Watt

On this basis, the temperature rise of a 5-Watt resistor above ambient should be reasonably small. The electrical designers of the Evinrude E-TEC and the I-COMMAND WIRING HARNESS would have made similar calculations to decide of the power dissipation necessary and the resistor value necessary to to replace an analog trim gauge that would have normally been in the circuit.

Below is a link to a 47-Ohm 10-Watt resistor with cooling fins and a small heat sink. Since an individual owner only needs to buy one of these, the $3 cost of this resistor will not be a concern.

https://www.mouser.com/ProductDetail/ARCOL-Ohmite/HS10-47R-F?qs=sGAEpiMZZMtlubZbdhIBIEidPY%252Bx%2FLaaBHyIAZ%2Fz2Hs%3D

See the specification sheet:

https://www.mouser.com/datasheet/2/303/acl_hs10-1890310.pdf

Note that if the resistor is NOT thermally bonded to an additional head sink, the rating decreases to 5-Watts. Simple wire lead resistors of 47-Ohm and 5-Watt rating can be found for less than a dollar in single-lot quantity.

jimh
Posts: 11673
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Why 47-Ohms and Why 5-Watts

Postby jimh » Sat Apr 02, 2022 10:18 am

jimh wrote:If an E-TEC engine is rigged with an I-COMMAND WIRING HARNESS, Evinrude has already provided a method to supply current by incorporating a 47-Ohm 5-Watt resistor into the wiring harness.

Regarding the general location of where the 47-Ohm 5-Watt resistor physically can be found in the Evinrude OEM I-Command Wiring Harness, there is an anecdotal report from a person that has located the resistor in the cable: the resistor is concealed under the sheathing that protects the cable and is located approximately midway between the ends of the cable assembly.

Cf.: https://www.etecownersgroup.com/post/show_single_post?pid=1332006398&postcount=32&forum=136484