# Candy Questions in Tech 14



## ship (Jun 24, 2005)

An old theater of mine had a stage manager's Prompt podium that has a (SP/DT or DP/SP?) switch. In other words, it goes On/Off/On. Have the switch in one position and the podium lamp at 15w is a 15w lamp. Have the switch in the lower position and it's half the intensity. Given this is prime 1920's technology, how did they do this? It was also a metal podium, should I have been concerned if I spilled a glass of water?

Given there was no ghost load lamps as below, what might be otherwise used to make this system work?



The following is a possible way: (Fill in the blanks after study.)

Short Circuiting Method of Dimming lights: Switch #A is the master Switch. Switch #B shorts group C and gives xxx to group #D. If switch #xxxxxx is open, Group C&D xxx in xxxx at 50% voltage and are thus dimmed. Any percentage of dimming or intensity is possible given the variations available in numbers of lamps and wattage between groups.
Note: (not shown) Diagram Shorts Group C Sending 110v. To hot & Neutral wires in the circuit, what is effect this will have on the lamps, switch or wires? For all intensive purposes, the neutral should defiantly be treated as xxxx for grounding and insulating purposes. Thus lamp bases with xxxxx, or grounding to metal conduit should not be used. However, given power flows thru the path of least resistance, Lamps and metal equipment in general should be xxxx in Group #C.


Do your best to post a diagram of how this all works.


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## Mayhem (Oct 3, 2005)

OK – the xxxx versus xxxxx versus xxx confused me, so I have ignored that component of this question and looked at it based upon the merits of the switch, power, lamp and purpose.

On-Off-On switches come with either 3 terminals (SPDT) or 6 terminals (DPDT) and I am wondering if you can remember which of these it was?

If it was a DPDT switch a small transformer could be used to drop the voltage to the lamp. 

A DPDT switch (shown below) is essentially 2 switches side by side, so they can control two seperate components of the circuit (in this case - both the active and the neutral feeds (provided it is rated to do so)).




The Active and Neutral wires feeding the lamp would be connected to the centre poles on the switch. Thus connecting the lamp to both the full voltage and the reduced voltage feeds, as determined by the switch position.

The Active and Neutral from the transformer would go to the bottom two poles of the switch (so that when you toggle the switch to the top 'on' position the lamp will be dimmed).

The other two poles on the switch (top) would receive the Active and Neutral feeds (before the transformer). Thus, providing full voltage to the lamp when the switch is in the bottom 'on' position.



(Note that these switch positions are based on my personal preference which it to toggle a switch down to turn something on - therefore, down would be 100% for me.)

For a DPST switch, it will require more thought but I am thinking a resistor could do it if they had one that could cope with the heat of chewing up 50% of the power.


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## Radman (Oct 4, 2005)

Probably just a typo, but you have your switch abbreviations switched up. SPDT would be 3 poles, DPDT 6, SPST 2, DPST 4. (Double/Single)Pole(Double/Single)Throw.

Oh and that pun was totally unintended.


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## Mayhem (Oct 5, 2005)

Yes - a typo on my part - thanks for pointing that out and I will edit the original post to correct it.


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