Readings not the same as the theory

Thread Starter

Jeanguypataterubberboot

Joined Aug 29, 2016
10
I have this setup on the bench and I just wanted to compare computed readings from calculated ones and I am confused. First of all I am not a professional, just a guy trying to learn electronics. I have a 40w heater in series with a fan. The total current draw of both is 164mA. I measured the resistance of the heater to be 320 ohms and the fan to be 185 ohms. Mains voltage is 117v. Here is where I get confused:

Calculated heater voltage: 164mA x 320 ohms = 52V
Calculated fan voltage: 164mA x185 ohms = 30V so why do I get a measured reading of 65V at the fan when the reading of 52V at the heater seems to workout?

The only thing I can think of is that my reading for resistance is wrong. Does the resistance of an electric fan motor change when it runs?

Thanks for your responses.

PV
 

Tonyr1084

Joined Sep 24, 2015
3,680
Good explanations about the motor. The heater - on the other hand - is a resistive element. Assuming you don't have a fan to blow warm air. The resistance of the heater will change as it gets hot. The resistance will go up when hot.

You're trying to compare a resistive load to a reactive load. Personally, I don't think I could calculate it - IF it can be done. I would assume that the 40 watt heater runs on 120 VAC, meaning (40 ÷ 120 = ) 0.333 amps. Therefore when the heater is in operation the resistance of the heating element should be (120 ÷ 0.333 = ) 360Ω.

If your heater has a fan, and that's what you're trying to read - the heater may be using the heating elements to act as a resistance for the fan, thus, the fan may operate on a lower voltage (actually lower current). My wife's hair dryer runs basically a 12 volt motor on 120 VAC. BUT! the way the heater is wired up it may be using the heating elements as either a current limiter OR as a voltage divider. Just how yours is wired - we can't know without knowing more about the heater or seeing a wiring diagram. At least I can't.
 

BR-549

Joined Sep 22, 2013
4,938
"I have a 40w heater in series with a fan." How much current can you put thru that fan? How much current do you need for the 40W heater?

In series the current is the same.

The heater and fan will have a voltage rating. They need that voltage....not half of it.
 

Ylli

Joined Nov 13, 2015
799
The fan becomes a fairly complex load. Not only is is resistive, it is also reactive (inductive). In addition, an electric motor will generate a back EMF (voltage) when running - that will make it appear to have a higher resistance.
 

Sensacell

Joined Jun 19, 2012
2,456
Ohms law can get you into trouble with strange loads.

Take a mains voltage incandescent light bulb, measure it's resistance with an ohm meter.

Now work out the current and wattage from the reading, using ohms law.
The answer you get will be wildly off, with some lamps, the value might be 10 X what's printed on the lamp.

The filament is non-linear load, the resistance increases dramatically when it gets hot.
 

Thread Starter

Jeanguypataterubberboot

Joined Aug 29, 2016
10
Good explanations about the motor. The heater - on the other hand - is a resistive element. Assuming you don't have a fan to blow warm air. The resistance of the heater will change as it gets hot. The resistance will go up when hot.

You're trying to compare a resistive load to a reactive load. Personally, I don't think I could calculate it - IF it can be done. I would assume that the 40 watt heater runs on 120 VAC, meaning (40 ÷ 120 = ) 0.333 amps. Therefore when the heater is in operation the resistance of the heating element should be (120 ÷ 0.333 = ) 360Ω.

If your heater has a fan, and that's what you're trying to read - the heater may be using the heating elements to act as a resistance for the fan, thus, the fan may operate on a lower voltage (actually lower current). My wife's hair dryer runs basically a 12 volt motor on 120 VAC. BUT! the way the heater is wired up it may be using the heating elements as either a current limiter OR as a voltage divider. Just how yours is wired - we can't know without knowing more about the heater or seeing a wiring diagram. At least I can't.


Thanks for the reply. I switched the fan with a 40w soldering iron and everything works out now. I've started reading about reactance and I've got a long way to go. Pretty fascinating.
I appreciate you guys in this forum helping me out.

PV
 

Thread Starter

Jeanguypataterubberboot

Joined Aug 29, 2016
10
Ohms law can get you into trouble with strange loads.

Take a mains voltage incandescent light bulb, measure it's resistance with an ohm meter.

Now work out the current and wattage from the reading, using ohms law.
The answer you get will be wildly off, with some lamps, the value might be 10 X what's printed on the lamp.

The filament is non-linear load, the resistance increases dramatically when it gets hot.

I learned that today. Thanks.

Nothing like practical experiments. :)
 

MisterBill2

Joined Jan 23, 2018
4,068
The resistance of the heaters will change with temperature, and just like was explained, the DC resistance of the motor is not the same as it's impedance to AC power. And if you want more confusion, measure the resistance of a light bulb, and then the current it draws when it is lighted. Temperature plays a large part in the resistance of many things. also impedance, which includes resistance
 
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