dimmer switches use current

[jetlag]

I hope Ive have been telling customers correctly when they call for me to install dimmer switches to save on electrical bill. I advise them that dimmers dont save anything . If my electrical theory serves me right a dimmer is nothing more than a variable resister in series with the light load. The dimmer causes a voltage drop across it the same as a light bulb and gets hot the same way. We used to make home made testers by wiring 2 light bulb sockets in series each bulb recieved 60 volts on a 120 circuit and would burn dim yet the total watts was the same as a single bulb . If the bulbs burned normal the circuit was 240 v . I still use those testers sometime because they put a load on the circuit and wont read phantom voltage like a tester. The point Im making is the dimmer is the same as a bulb tester it uses its share of the voltage drop and the rest goes to the light circuit.

 

[Umlaut]

Old dimmers (and some that you can still probably find today) are rheostats; they're just variable resistors which do dissipate heat. And you're right; approximately as much energy goes into heat as would have been saved in dimming the lights.

Modern dimmers us a TRIAC and a small microprocessor. The TRIAC is a semiconductor switch that can handle alternating current, and pretty decent current ranges. The dimmer works by modulating the electricity given to the lamp over time. A lamp that turns itself off and on so it's on for a millisecond and off for a millisecond uses about 50% of the energy as a lamp that's on all the time.

 

[jetlag]

thanks Umlaut

thanks Umlaut

Old dimmers (and some that you can still probably find today) are rheostats; they're just variable resistors which do dissipate heat. And you're right; approximately as much energy goes into heat as would have been saved in dimming the lights.

Modern dimmers us a TRIAC and a small microprocessor. The TRIAC is a semiconductor switch that can handle alternating current, and pretty decent current ranges. The dimmer works by modulating the electricity given to the lamp over time. A lamp that turns itself off and on so it's on for a millisecond and off for a millisecond uses about 50% of the energy as a lamp that's on all the time.

I wont be convinced untill I connect a watt meter to a lamp and see for myself. I think the cutting on and off will cause the voltage to splike in between cycles and the flickering although it is to fast to see is wasting current but maybe not as much as the rheostats. If what you say is true the dimmer should not get hot. All the ones I know of at the supply house get hot and have heat sinks and instructions on how many can go in a gang box.. Are the new ones you speak of that way ?

 

[paul]

Tell them they'll save more money by installing timers.

 

[electricmanscott]

I hope Ive have been telling customers correctly when they call for me to install dimmer switches to save on electrical bill. I advise them that dimmers dont save anything .

First, why try to talk them out of spending money, that's crazy talk.

Second, you have been misinforming your customers.

Read all about it....

 

[curt swartz]

Jet, imagine controlling 8-75 watt recessed lights with a 600 watt dimmer. If you dim the lights so they are only operating at 25% of full output do you really think the dimmer will be dissipating 450 watts of heat? You would have some pretty bad burns if you came in contact with the dimmer.

 

[MikeyinLA]

Yup, dimmers save $$ both in energy and lamps (your lamps will last longer when dimmed)

Hook up a dimmer to a work light at home and clamp on your amp meter to see for yourself.

 

[Buck Parrish]

Jet, imagine controlling 8-75 watt recessed lights with a 600 watt dimmer. If you dim the lights so they are only operating at 25% of full output do you really think the dimmer will be dissipating 450 watts of heat? You would have some pretty bad burns if you came in contact with the dimmer.

I get a lot of complaints about dimmers being to hot. The heat from the dimmer switch worries the customers. They think it is going to burn up.

 

[jetlag]

Jet, imagine controlling 8-75 watt recessed lights with a 600 watt dimmer. If you dim the lights so they are only operating at 25% of full output do you really think the dimmer will be dissipating 450 watts of heat? You would have some pretty bad burns if you came in contact with the dimmer.

I think you are correct but if are are dimming 600 watt they will get very hot , pull the cover off and check. I just hate to see power wasted that way in the form of heat in the outlet box. that has to take a toll on the insulation of the conductors in the outlet and also derate them.

 

[macmikeman]

I think you are correct but if are are dimming 600 watt they will get very hot , pull the cover off and check. I just hate to see power wasted that way in the form of heat in the outlet box. that has to take a toll on the insulation of the conductors in the outlet and also derate them.

Transistors, Triacs, and SCR's all build up heat when performing the operations they were designed for. The board manufactures often require the use of heat sinks for full wave rectifiers for just such reasons. That heat is not the type produced by the inductance of the old style rheostats, which employed resistance to lower the voltage. There are some losses due to the circuit that come out as heat however, but not as much as what the natural process of the electronic switch produces.

 

[wwhitney]

A rheostat would still save power: P = (V * V) / R. So when R increases, then P decreases.

Cheers, Wayne

 

[kwired]

I hope Ive have been telling customers correctly when they call for me to install dimmer switches to save on electrical bill. I advise them that dimmers dont save anything . If my electrical theory serves me right a dimmer is nothing more than a variable resister in series with the light load. The dimmer causes a voltage drop across it the same as a light bulb and gets hot the same way. We used to make home made testers by wiring 2 light bulb sockets in series each bulb recieved 60 volts on a 120 circuit and would burn dim yet the total watts was the same as a single bulb . If the bulbs burned normal the circuit was 240 v . I still use those testers sometime because they put a load on the circuit and wont read phantom voltage like a tester. The point Im making is the dimmer is the same as a bulb tester it uses its share of the voltage drop and the rest goes to the light circuit.

Jet, imagine controlling 8-75 watt recessed lights with a 600 watt dimmer. If you dim the lights so they are only operating at 25% of full output do you really think the dimmer will be dissipating 450 watts of heat? You would have some pretty bad burns if you came in contact with the dimmer.

I think you are correct but if are are dimming 600 watt they will get very hot , pull the cover off and check. I just hate to see power wasted that way in the form of heat in the outlet box. that has to take a toll on the insulation of the conductors in the outlet and also derate them.

The semiconductor dimmers do lose some heat but in the example that curt used they would not produce 450 watts of heat, that would be very hot compared to what the dimmer switch acutually gives up as heat.

Putting two loads in series does not result in same amount of total power in the circuit either. In the OP if the two test bulbs were both identical 100 watt bulbs and were in series across a 120 volt load they each have 60 volts across them but the total power used would not be 100 watts it would be 50 watts (this is assuming resistance is constant at all temperatures which it is not in an incandescant lamp).

In curts example if you were to use a true resistor for dimming 600 watts of lamps to 25% your results would be like this:

600 watts @ 120 volts = 5 amps

600 watts / 5 amps = 24 ohms of resistance on the lamps (we again will assume that resistance will be constant at all temperatures to simplify the problem)

25% of 5 amps = 1.25 amps - this will be our target amperage for 25% of the 600 watt amperage.

To achieve 1.25 amps of current through a 24 ohm resistance with 120 volts of input you must put 72 ohms of resistance in series with it.

30 volts will drop across the 24 ohm resistance, and 90 volts will drop across the 72 ohm resistance.

The power consumed by the lamps will be 37.5 watts and the power consumed by the resistor will be 112.5 watts. The dimming resistance will consume more power than the lamps being dimmed but the total power consumed by the 120 volt input will only be 150 watts so there is still a reduction in energy used.

 

[jetlag]

The semiconductor dimmers do lose some heat but in the example that curt used they would not produce 450 watts of heat, that would be very hot compared to what the dimmer switch acutually gives up as heat.

Putting two loads in series does not result in same amount of total power in the circuit either. In the OP if the two test bulbs were both identical 100 watt bulbs and were in series across a 120 volt load they each have 60 volts across them but the total power used would not be 100 watts it would be 50 watts (this is assuming resistance is constant at all temperatures which it is not in an incandescant lamp).

In curts example if you were to use a true resistor for dimming 600 watts of lamps to 25% your results would be like this:

600 watts @ 120 volts = 5 amps

600 watts / 5 amps = 24 ohms of resistance on the lamps (we again will assume that resistance will be constant at all temperatures to simplify the problem)

25% of 5 amps = 1.25 amps - this will be our target amperage for 25% of the 600 watt amperage.

To achieve 1.25 amps of current through a 24 ohm resistance with 120 volts of input you must put 72 ohms of resistance in series with it.

30 volts will drop across the 24 ohm resistance, and 90 volts will drop across the 72 ohm resistance.

The power consumed by the lamps will be 37.5 watts and the power consumed by the resistor will be 112.5 watts. The dimming resistance will consume more power than the lamps being dimmed but the total power consumed by the 120 volt input will only be 150 watts so there is still a reduction in energy used.

Well i didnt word that well two equal resisters in series will reduce the total watts to 50% each resister contributing 25 % . But if one is a resister and the other a bulb of egual resistance, the resister uses the same amount of power as the bulb , thats what I was trying to get across. If you started with a 60 watt bulb ,it will be reduced to 15 watt but the resister will also use 15 watts which is wasted. But its true dimmers are not made as straight resisters any more , but they still waste a lot of power, why do you think they come with a heat sink ? I have had them to melt insulation in the outlet box

 

[jetlag]

A rheostat would still save power: P = (V * V) / R. So when R increases, then P decreases.

Cheers, Wayne

The total watts is reduced but the rheostat still uses its share , when the rheostat is set at same resistance as the bulb they both use equal power and the rheostat power is wasted

 

[Umlaut]

I wont be convinced untill I connect a watt meter to a lamp and see for myself. I think the cutting on and off will cause the voltage to splike in between cycles and the flickering although it is to fast to see is wasting current but maybe not as much as the rheostats. If what you say is true the dimmer should not get hot. All the ones I know of at the supply house get hot and have heat sinks and instructions on how many can go in a gang box.. Are the new ones you speak of that way ?

The voltage doesn't spike. I'm not sure why you assume it would. You can use an oscilloscope to verify this for yourself.

Solid state dimmers get warm because the semiconductor is switching. At the instant it turns on, it's a pretty high resistance, then quickly (in a matter of microseconds) comes down. It's never quite zero, though -- still a few milliohms. Since that happens a few dozen times a second, the TRIAC has a little bit of heat to dissipate.

If you're concerned about heat being wasted energy, I hope you're using solid-state lighting. Even without a dimmer, incandescent bulbs convert only 20 to 30% of their input energy into light. The rest is wasted as heat by the bulb. If you use CFL or LED, then you're in much better shape, but still not quite 50% efficient.

 

[kwired]

All conductors also have resistance and give up heat as a result, that is why we must derate them when installing multiple conductors in a raceway.

With the dimmers you still have ineffeciencies in the lamps and circuit. If you want to reduce energy given up as heat by not using a dimmer remember that you are giving up heat at the lamp (and probably more heat) if you are not giving it up at the dimmer.

This to a certain extent is also really only a problem during cooling seasons, during heating seasons it reduces load on the heating system.

If you have electric resistance heating system wasted heat from other electric equipment is saved heat from the heating system at the same cost per watt hour of heat.

 

[winnie]

IMHO selling dimmers for 'energy saving' is almost as much a scam as selling power factor correction to residential users.

While it is quite true that using a dimmer will reduce the energy consumed by an incandescent lamp, you will also greatly reduce the efficiency of that lamp. A 100W lamp 'dimmed down' until it uses 50W of energy will produce far less light than a 50W lamp at full power.

If your goal is decorative, or 'mood lighting', or control for other reasons, than dimmers are great. But if you want to save energy, then IMHO simply turning off lights is a better approach.

-Jon

 

[M. D.]

I wired a pool on a piece of property that has no other structure Just the pool and the service pole.. There is a 500 watt quartz light on the pole connected through a dimmer I was surprised to see how much the meter slowed when the dimmer was employed..
My brother who maintains this pool decided not to use the timer this year,.. He's been running the pump on low 24\7 the electrical bill dropped by a considerable amount from this year to last .. I'm not sure why perhaps the lack of starting inrush?
Also I used a timer for Christmas lights on year ,. my bill increased. Why you ask ? I figure it was because the timer never forgot to turn them on

 

[jetlag]

thanks winnie

thanks winnie

IMHO selling dimmers for 'energy saving' is almost as much a scam as selling power factor correction to residential users.

While it is quite true that using a dimmer will reduce the energy consumed by an incandescent lamp, you will also greatly reduce the efficiency of that lamp. A 100W lamp 'dimmed down' until it uses 50W of energy will produce far less light than a 50W lamp at full power.

If your goal is decorative, or 'mood lighting', or control for other reasons, than dimmers are great. But if you want to save energy, then IMHO simply turning off lights is a better approach.

-Jon

Thank you winnie , every body got on my case . good point, if you are happy with 1/4 the light for 1/2 the power then I guess that could be twisted to say you are saving power.

 

[kwired]

I wired a pool on a piece of property that has no other structure Just the pool and the service pole.. There is a 500 watt quartz light on the pole connected through a dimmer I was surprised to see how much the meter slowed when the dimmer was employed..
My brother who maintains this pool decided not to use the timer this year,.. He's been running the pump on low 24\7 the electrical bill dropped by a considerable amount from this year to last .. I'm not sure why perhaps the lack of starting inrush?
Also I used a timer for Christmas lights on year ,. my bill increased. Why you ask ? I figure it was because the timer never forgot to turn them on

When was the dimmer installed? Add to that, a pump on low speed likely draws much less power than on high speed. Loading of a centrifigal pump is not linear with the speed. It could easily 25% of the energy at a low speed than it uses at a high speed.