Thread: 60 Hz hum

I once constructed a "electric heater", which was basically a large DC supply (just a full bridge and a filter cap) shorted across an array of wires. I observed something interesting and I just wanted to make sure my explanation is correct.

These were my observations at low and high power.

1. When I ran the heater a low power (with the wires not very hot) there was a distinct buzz. I assume this buzz was caused from the wires, and was due to the 60 Hz ripple that my load placed on the DC supply.

2. When I ran the heater at higher power (with red hot wires) the buzz nearly disappeared. Once again, at this power level the wires were red hot, and the heater was consuming about 1000 Watts.

I think what caused this is simply the fact that when the wires became red hot, their vibrations were dampened, because they were becoming more malleable, or moving towards the liquid state.

Also, I believe the 60 Hz hum is caused because the wires carried a magnetic field that varied in intensity. (the ripple in the DC supply) This variance of the field causes them to attract/repel eachother at a distinct frequency right? Similar to guitar strings?

You are probably right about the residual ripple being the source of the hum. Perhaps the wires expanded enough to jam together when hotter so they couldn't buzz so much?

Are you saying there was no transformer in your power supply? That being the case, what was the purpose of the rectifier - why not just short the wires together with a-c voltage? And what kept the diodes from burning out or the breaker from tripping while you were shorting 120 volts to ground?

Are you saying there was no transformer in your power supply? That being the case, what was the purpose of the rectifier - why not just short the wires together with a-c voltage? And what kept the diodes from burning out or the breaker from tripping while you were shorting 120 volts to ground?

Harold, the power supply was your typical full bridge rectifier and filter cap arrangement. I fed the full bridge with a variac to control input voltage. Of course I could have just fed the wires with AC straight out of the variac, but I just wanted to put some stuff together to learn more.

The output of this setup was just DC at whatever voltage I wanted. The length of wire that composed my heater was probably a 17 ohm load, so at around full power I was putting around 10 amps through it, and thats when the buzzing really started to go away. (like I said, at around 10 amps the wires were red hot)