Learning About High Power Circuits

Troubleshooting the Elec Trak E20 tractor is forcing me to learn some new things about electrical circuits.  My training was as a computer tech an that was back in the mid 80’s.  So I’m used to 5V low current circuits.  Now I’m trying to follow the logic of relays and high current circuits.  There was an optional Arc welder for this tractor that would run from the battery pack.  You have to be very careful around the high power circuits as you can short something out with your tools, and it will arc with enough current to melt metal.

Right now I’m working on the power resistor bank.  This adds some resistance to circuit feeding the motor to serve as speed control.  You are basically turning power into heat.  Therefore it is better to run the circuit with the resistors by-passed and select the speed with the transmission, which saves power.  This power resistor bank is made by wrapping a metal band around an insulated plate and tapping it two places in the wrap.  This gives you 3 different resistance values.

If you look at the pictures below you will see this metal band (about 1″ wide) and how it melted the metal away from the connection post.  Now there is no longer an electrical connection which is at least one reason why the tractor will not go.  This could have been caused by the terminal (which is really a brass bolt) being loose.  This would increase the resistance which would in turn increase the heat and cause the metal to melt away.  I’ve got some replacement metal bands coming in to try and repair this power resistor, which some in the Elec Trak community call a “Toaster”.

E20 Power Resister

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