Trolley questions

Al Stangenberger · **Joined:** Mon Aug 23, 2004 12:59 pm **Posts:** 650

robertmacdowell wrote:

OK, now imagine all is the same, but the headlight has lost electrical contact with the bracket. The voltmeter will read WHAT? ________

A fresh intact coat of Awl-Grip paint on a headlight bracket is enough to create this condition.

The first time we hung a headlight on a newly-painted bracket on SN 1005 it did not light up because of this.

DCLamont · **Joined:** Wed Aug 01, 2012 5:07 pm **Posts:** 15

Quote:

In your diagram, you see two points where the "red" resistor attaches to the black lines. Suppose you attached a voltmeter at those two points. The headlamp is on, and securely on the bracket. We agree the voltmeter would read zero, or darn close, eh?

OK, now imagine all is the same, but the headlight has lost electrical contact with the bracket. The voltmeter will read WHAT? _______

1. The red resistor is me ....soaking wet and firmly grounded the voltage across me is 32 VDC and the current flow is 30 milliamps
2. The red resistor is me ....dry and 100,000 ohms resistance, the voltage is 32 VDC and the current flow thru me is .3 milliamps

I can change the current flow thru the headlight resistor but I cannot change the voltage drop ...until the thing heats up enough to increase the resistance.

robertmacdowell · **Posted:** Sun Sep 16, 2012 11:50 am

/facepalm no, no, no, no, no... No magic 32v voltage regulating resistors...

OK. Let's try this a different way. Forget the red resistor. Forget the voltmeter.

Normal circuit. The headlamp is 32 volts and 32 watts, to keep the math simple. Dropping resistor is correctly chosen for 600v trolley voltage.

The curator wants to fill a museum display case, so he takes the original headlamp bulb out. He replaces it with a commonplace 25 watt bulb made for 120VAC.

What happens and why?

RCD · **Posted:** Tue Sep 18, 2012 9:51 pm

I asked Mr. Bartlett, President of the Shelburne Falls trolley Museum, what he thinks. He said there was a potability to experience deadly amounts of current.

Jdelhaye · **Posted:** Wed Sep 19, 2012 2:59 am

My high-school electronics classes may be 30 years in the past, but I doubt Ohm's law has changed in the meantime...

This is a voltage dropping resistor, NOT a voltage dividing resistor network

When the circuit is opened, the supply voltage (600 volts in this case) is the same as the voltage differential across the open points in the circuit.

Mr Macdowell, and those who agree with him, are correct.

I would strongly suggest that those who disagree should NOT attempt to prove him wrong by attempting this in person, as this community as a whole does not need to lose any valued workers.

Jeff

filmteknik · **Posted:** Wed Sep 19, 2012 9:45 am

This is all rather astonishing. I know safety wasn't as big a concern back in the day as now and we didn't then live in a nanny state but to think you can be killed just by picking up the headlight while power is on is stunning. Or, if the headlight goes out because it's not making a good ground via the bracket, you better not touch it to try to improve the contact because as long as the light is out, the case is at 600V.

This isn't 20/20 hindsight; they knew all this too. And yet it was too much trouble to make a 2 pole plug? Shocking!

Steve

Randall Hicks · **Joined:** Wed Aug 25, 2004 9:44 am **Posts:** 154

Hilarious, isn't it? I might add that another danger is this: in many cases, such as the North Shore cars, the headlight is hung on the train door. You don't want to rely on the hinges to carry the current, so there's a ground strap from the frame to the door. But if this strap breaks or becomes disconnected, you could possibly get a shock just by opening the door while the headlight is hung on it. So be sure the circuit is turned off at the cabinet before opening the door.