What size resistors do I use for 110 lighting?

Question

I am making a large VU meter ran off of a high voltage color organ and in order to make the display move I need to put resistors between the lights, what size risistors are recomended?

Answer 1

We need to know more about how high a voltage, what volt and power rating for the lamps, and why resistors are needed to make the "display move." With those answers and some arithmetic you question is easy to answer.

Answer 2

As defined, a VU meter has an internal resistance of 600 ohms and a full scale deflection of plus 3 at 2 milliwatts ac, and an intermediate deflection of zero at 1 milliwatts ac and a 0.3 second TC reponse time. It is a kind of absorption milliwatt meter, and not very practical. The device that is normally used bridges (has high internal resistance) a line which has 600 ohm termination and measures the effective ac voltage. If the line is not 600 ohm terminated, then the indicated decibels are not an accurate measure of milliwatts.

What i suspect you have is not a VU meter at all, but a meter movement with a VU scale printed on it. That being the case, it is missing a whole lot of necessary parts. Most likely it is not ac, but rather a dc movement. It will need an instrument style bridge rectifier, an isolating capacitor to block dc from the rectifier from seeing the external circuit, a capacitor across the meter movement to establish the proper time constant, a voltage dropping resistor to keep most of the outside voltage outside.

so much to figure out, so little data to work with. my guess is the meter movement will have full scale at 1 ma dc current. If we are talking 120 volt bulbs in the color organ, then let that be the zero point on the dial, or 0.7 ma. To get 0.7 ma draw from 120 volts, the resistance needs to be about 180 kohms. however if the meter is more sensitive, it could have full deflection at only 50 mircoamps, that case would call for a resistor size of 3300 k ohms, quite a difference. The power is inconsequential, but a half watt resistor is physically large enough to give good spacing between the leads at this voltage. The blocking capacitor has to be rated for 120 VAC cross line application and sufficiently large to not impede current at 50/60 hz, or about 0.5 uf. this is normally a plastic type of the kind used in telephone circuits (ringer is 90 volts ac). it could be sized at 0.1 uf if the movement is a more sensitive type. Since it is not possible to know how the grounding is applied to the light bulbs, it would be decidedly safer to put a blocker on both leads to the meter. The time constant capacitor is sized according to the internal meter movement resistance for 0.3 second. If the movement is 5000 ohms, then that would be 60 uf. this could be a dc type electrolytic with a 10 volt rating, polarize meter and cap according to the dc from the rectifier.

If you got all of this understood, then carefully circuit test. These voltages are lethal, so be prepared for anything and put your hands in nothing when connected to power.

Answer 3

Since you are messing about with 'high voltage' I would assume you are competent enough to deal with electrical installations.

However, if you are unable to determine suitable resistor values for your project I have my doubts.

Legislation was recently passed relating to electrical installations and only 'Competent Electricians' are allowed to do such work..

If you are conversant with Domestic Installations Part P of the current Building Regulations then by all means go ahead. Otherwise seek professional help with your project.

Answer 4

r=v over i


v=volts
I =Amps