convert 240 to 120 volts

[Leo]

I have 120 Volts feeding the thermostat and another 120 volts feeding the heater. I turn the heater on just a little, and it kicks out a lot of heat and when the bedroom heater is on with it the two poll 20 trips. How do I convert the 240 volts to 120v

[jimbrown]

Well in the USA you don't covert 240 to 120 as you do in other countries using transformers. In the USA 240 is simply two 120 lines tied together.
It sounds to me like you got two electric heaters on one circuit breaker and those two heaters could be pulling 2500 watts each for 5000 watts which is overloading your 20 amp circuit.

If the heaters are 240 heaters and you lower the voltage to 120 they will pull twice the amperage and your situation will be worse.

[Pat]

Sorry, Jim but you are slightly kinda totally wrong around the edges in this instance. [img]/forums/images/icons/wink.gif[/img] [img]/forums/images/icons/wink.gif[/img]

Use the formulas I=E/R and P=IE where:

I = Amps (of current)
E = volts (of electromotive force)
R = Ohms (of resistance)
P = Watts (of power)

Example is for illustration and not intended to be actual values of a specific appliance.

If you have a heater that pulls 12 amps at 240 volts then that is 2880 Watts. The running, i.e. hot resistance of the heater in this example would therefore be 20 Ohms.
If you supply 120 volts to the heater instead of 240 then it will produce about 720 Watts and pull about 6 Amps. The use of "ABOUT" in this example is because the heating elements change resistance with their temperature and running them on a reduced voltage changes their resistance and therefor the Watts of heat they produce and the amps they pull but not enough to make a big fuss.

Your erroneous statement:

"If the heaters are 240 heaters and you lower the voltage to 120 they will pull twice the amperage and your situation will be worse."

Appears to assume that the power (watts) would somehow be constant and the current would have to double to meet that requirement. This is NOT the case.

If the OP cares to be more forthcoming with information as to his installation we could probably be more helpful.

Pat

[jimbrown]

Well Pat I guess that on purely resistive loads that is true. And I did make the assumption the wattage would be the same and of course that is not true. Not sure about AC currents on motors and other appliances. I ain't studied that stuff for a long time. I have a 1000 watt radio transmitter next to my desk and I assure you that if I switch it from 240 to 120 it positively pulls twice the current to make the 1000 watts of output.

[Pat]

Jim, You are not telling the whole story and will mislead the uninitiated. When you "SWITCH" your gallon to 120 vac vice 240 vac you reconfigure the xformer so that its output remains unchanged. Same output at 1/2 the volts input will of course require twice the amps. This is definitely not the same as the heaters case I discussed.

Since we did not discuss changing the frequency of the supplied power in the heaters case then whether or not it is PURE resistance or not is not an issue. There will be some distributed capacitance but negligible for practical reasons at 60Hz. Depending on the physical layout of the heater element (spiral wrapped wire or what) there may be significantly more X sub L than X sub C but still mostly negligible.

At a fixed frequency the reactance of a circuit (heater or whatever) is not subject to change with supplied voltage so is still the simple case. Yes, motors get a little more tricky. First you have to understand the characteristics of the particular type of motor. Is it series brush type, parallel with brushes, induction, brushless, or what. I bought some really cheap used 240 volt muffin fans and put them in pairs wired in series to 120 volts and they barely self started and ran really slowly and quietly just as I needed.

Even a series string of tungsten filament light bulbs exhibit non-linear resistance depending on the temp of the filaments. While two similar bulbs in series will divide the power equally the total power in the ckt will not quite be off a factor of two from the power of one bulb. If you put enough bulbs in series such that the filament just barely glows red the results will be way off from the linear model due to the change in resistance of the filaments with temperature. I have seen circuits with a small light bulb in the cathode path of a radio tube as a feedback mechanism which relied on the non-linear resistance that varied with temperature.

OK, drop the other shoe, what is the radio gear? My computer room/ham shack is wired for 50 amps 240 VAC in case I got a linear amp but the circuit is UNUSED unfortunately.

Pat

[jimbrown]

Nothing like hijacking a thread but don't think anyone cares.
Pat all my radio gear is old and I seldom even turn it on any more I find the Internet and these type of forums far more relaxing. Radio is and always has been sort of hostile at least to me. I got a shed full of stuff that I never even unpacked when I moved into this house.
I do have in my office for lack of a better description an old Icom 730 that I won at a ham fest (my $4 radio) hooked to a Heathkit SB 1000 (single 500Z) that goes out to a Hygain vertical up on a 20 foot mast with tuned radials for guys. I have another Icom , a yaesu FT 101, Denton GL 1000 amp , VHF stuff, teletype machines and oscilloscope out in the shed, plus a whole bunch of antennas my wife says go up after she dies. I hereby leave all threads about electricity and circuits to you. Well maybe.

[Pat]

I'm not real active either. I have a Drake x-cvr (7 line) in a box unopened for 8 yrs. I still have VHF and UHF in my truck and some HT's. Been thinking of putting up a simplex repeater.

The internet helps dilute my radio fever (for HF) and there isn't much VHF and UHF activity here, at least compared to out west in general and SOCAL in particular with all those wonderful repeater clubs and mountain top sites.

Recently got my interest stirred up a bit. A local club wants me as a guest lecturer on a number of topics.

Please, don't expect me to handle all the traffic, you keep on chiming in!

Pat

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] Er.... ah, Pat; do those club guys KNOW that if they ask you the time, you will reply with detailed instructions on how to build an atomic clock? [img]/forums/images/icons/smile.gif[/img]

[Pat]

Dave, That plus a brief review of the history of horology with highlights of the competition and prize for a usable ships chronometer.

Pat