RELATIONSHIPS OF CORROSION RESISTANCE AND CURRENT FLOW

Or

PRACTICAL APPLICATIONS OF OHMS LAW

 

OHMS LAW:

E = I * R                      P = E * I                       E = Voltage, measured in Volts

I = E / R                       I = P / E                       I = Current, measured in Amps

R = E / I                       E = P /I                        R = Resistance, measured in Ohms

                                                                       P = Power, measured in Watts

 

Exambles of using Ohms Law and what it can tell us when troubleshooting electrical problems

 

#1       CORRODED SWITCH ON A 1 WATT COMPASS LIGHT

 

Lamp rated at 1 Watt - (P)

Switch corroded at 5 Ohms - (R)

Voltage supply at 12 Volts – (E)

 

I = P / E = 1 / 12 = .0833 Amp – (Current usage of lamp) – (I)

R = E /I = 12 / .0833 = 144.057 Ohm – (Resistance of lamp) – (R)

144.057 + 5 = 149.057 Ohm – (Total resistance of lamp and corroded switch) – (R-t)

 

NOW:

 I = E / R = 12 / 149 = .0805 Amp (Total current usage of lamp and corroded switch) – (I)

P = E * I = 12 * .081 = .972 Watt (Total power of lamp and corroded switch) – (P)

 

Lamp was rated at 1 Watt, so it is easy to see that the loss of wattage, (1.0 - .972 = .027 Watt), has created no noticeable difference in the illumination of the lamp.

 

 

#2      SAME SWITCH AS EXAMPLE #1, BUT NOW WITH A 40 WATT CABIN LIGHT

 

Lamp rated at 40 Watt – (P)

Switch corroded at 5 Ohm – (R)

Voltage supply at 12 Volts – (E)

 

I = P / E = 40 / 12 = 3.33 Amp – (Current usage of lamp) – (I)

R = E / I = 12 / 3.33 = 3.6 Ohm – (Resistance of lamp) – (R)

3.6 + 5 = 8.6 Ohm – (Total resistance of lamp and corroded switch) – (R-t)

 

 

NOW:

 I = E / R = 12 / 8.6 = 1.395 Amp (Total current usage of lamp and corroded switch) – (I)

P = E * I = 12 * 1.4 = 16.8 Watt (Total power of lamp and corroded switch) – (P)

 

Lamp was rated at 40 Watt, so it is easy to see that the loss of wattage to only 16.8 Watts has resulted in a lamp that hardly will glow.

 

And:

E = I * R = 1.4 * 5 = 7 Volts (Voltage drop over the corroded switch) – (E)

P + E * I = 7 * 1.4 = 9.8Watts(Power used at the corroded switch) – (P)

 

Almost 10 watts of wasted power producing only heat at the switch!

 

 

#3      CORRODED BATTERY CONNECTIONS ON A 3000 WATT STARTER MOTOR

 

Starter rated at 3000 Watt – (P)

Battery connection corroded at 1/100 Ohm – (R) – A Very Small Amount!

Voltage supply at 12 Volts – (E)

 

I = P / E = 3000/12 = 250 Amp (Current usage of motor) – (I)

R = E / I = 12/250 = .048 Ohm (Resistance of motor) – (R)

.048 + .01 = .058 Ohm (Total resistance of motor and corroded connections) – (R-t)

 

NOW:

 I = E / R = 12 / .058 = 207 Amp (Total current usage of motor and corroded connection) – (I)

P = E * I = 12 * 207 = 2484 Watt (Total power of motor and corroded connection) – (P)

 

And at the connection:

 

E = I * R = 207 * .01 = 2.07 Volts (Voltage drop over the corroded connection) – (E)

P + E * I = 2 * 207 = 414Watts(Power used at the corroded connection) – (P)

 

The corroded connection has reduced the power available, (Work), to 2484 Watts, and 414 of those watts are producing heat at the connection.  (2484 – 414 = 2070 Watts)   The power left for the starter motor may not be enough to start the engine and the battery connection is TOO HOT TO TOUCH!

With a connection with more resistance from corriosion, (the ones you see with all that white powder around the battery connection), really only sightly more resistance than in our example, the starter may not be able to turn at all.

 

 

THE MORAL OF THE STORY:

 

            Don’t allow corrosion to keep your circuits from working, and if a circuit quits, check for corroded connections and switches.  Look for heat.  If the circuit is not working because of high resistance caused by bad connections or corrosion, the loss of power will always be producing heat unless the connection is so bad as to completely stop current flow.  Check with your multi-meter.  Measure voltages.  Remove power from the circuit and measure the resistances.  Check the voltage across the fuse and/or switch when the power is onAny voltage across a switch indicates a problem when the switch is on.  Any voltage across a fuse when the power is on indicates a blown or corroded fuse and/or holder.

 

            I recently found the problem with a non-functioning bilge pump to be corrosion on the fuse holder caused by the skipper dropping it into the bilge and not cleaning it very will before reinstalling it.  A week later the pump no longer worked.  The skipper first replaced the pump and then called me in to find the problem.  He now has an extra used pump aboard.

 

            Keep battery connections clean and tight.  If the starter won’t or hardly, works, check the connections first.  Look for ANY heat.  A good connection has 0 Ohms and produces no heat.  Be careful however, a bad connection at the battery can get so hot as to cause sever burns to the hands.

 

            A lot of folks replace the battery first and because loosening, cleaning and retightening the connections at the battery, while replacing it, fixes the problem, they are convinced that they had a bad battery and did the right thing.  That’s where those folks who sell used batteries get their merchandise.  A large number of batteries removed from service have not failed.  There was simply a corroded or lose connection at the terminal.  Loosen, clean and retighten your battery terminals regularly.  Coat them with corrosion grease or Vaseline.  I used to run my race cars on the batteries folks insisted I change out for them, even when I tried to tell them they didn’t need a new battery.  I got free batteries for the race cars because folks were stubborn and just knew that they were correct about needing a new battery, because after I switched out their “old” battery their vehicle started just fine, (the same as it would have if I’d only just cleaned and tightened the connections.)

 

 

P/D/Lt/C  Mark S. Anderson, N                      January 1991 - Revised March 2007, July 2012