The National Conference of Volunteer Examiner Coordinators (that’s mouthful, isn’t it), aka the NCVEC, released the 2015 General Class question pool earlier this week. Since I had some spare time this morning, I decided to get started on updating my No-Nonsense General-Class License Study Guide. Because my study guide starts with section G5, that’s the section of the question pool that I started in on.
There are three parts of section G5: G5A, G5B, and G5C. G5A and G5B are exactly the same as the old question pool, but section G5C has been changed. Most notably, questions G5C17 and G5C18 were added and G5C08 was modified. These questions deal with how to convert capacitances specified in microfarads and nanofarads and picofarads.
G5C02 was changed from a relatively simple question about where to connect the source of energy to a transformer, to a more difficult question about what would happen if you connected that source of energy to the secondary of a transformer. In addition, question G5C16, which asks why the primary is often larger in diameter than the secondary was added to the question pool.
Below is the text that will be in the new version of my study guide. Critiques welcome!
Resistors, capacitors, and inductors in series and parallel; transformers
Connecting components in series and in parallel will affect their effective values. For example, if you connect resistors in series, the effective resistance is the sum of the individual resistances. A resistor in series should be added to an existing resistor in a circuit to increase circuit resistance. (G5C03)
Connecting resistors in parallel will decrease the circuit resistance. For example, the total resistance of three 100-ohm resistors in parallel is 33.3 ohms. (G5C04) 5.9 ohms is the total resistance of a 10 ohm, a 20 ohm, and a 50 ohm resistor in parallel. (G5C15) 150 ohms is the value of each resistor which, when three of them are connected in parallel, produce 50 ohms of resistance, and the same three resistors in series produce 450 ohms. (G5C05)
Inductors work the same way as resistors. An inductor in series should be added to an inductor in a circuit to increase the circuit inductance. (G5C14) The inductance of a 20 millihenry inductor in series with a 50 millihenry inductor is 70 millihenrys (G5C11), but the inductance of three 10 millihenry inductors connected in parallel is 3.3 millihenrys. (G5C10)
Capacitors, however, are quite the opposite. A capacitor in parallel should be added to a capacitor in a circuit to increase the circuit capacitance, (G5C13) while connecting capacitors in series will decrease circuit capacitance. The capacitance of a 20 microfarad capacitor in series with a 50 microfarad capacitor is 14.3 microfarads. (G5C12) The capacitance of three 100 microfarad capacitors connected in series 33.3 microfarads. (G5C09)
When working with capacitors, it’s important to take note of how the capacitance is specified. The capacitance is sometimes specified in microfarads (μF), sometimes in nanofarads (nF), and sometimes in picofarads (pF). A microfarad is 1,000 nanofarads, and a nanofarad is 1,000 picofarads. So, for example, the value in nanofarads of a 22,000 pF capacitor is 22 nF. (G5C17) The value in microfarads of a 4700 nanofarad (nF) capacitor is 4.7 μF. (G5C18)
Where it’s really important to keep this in mind is when you’re working with some capacitors whose capacitance is specified in nanofards and others who are specifed in picofarads. For example, the equivalent capacitance of two 5.0 nanofarad capacitors and one 750 picofarad capacitor connected in parallel is 10.750 nanofarads. (G5C08) 750 pF is equal to 0.75 nF, and because the capacitors are connected in parallel, the equivalent capacitance is equal to the sum of all the capacitances.
Inductors exhibit a behavior called mutual inductance. Mutual inductance occurs when a current flowing through one inductor induces a current in a nearby inductor. We use this behavior to create components called transformers.
The simplest transformer has two windings: a primary winding and a secondary winding. When an AC voltage source is connected across its primary winding, mutual inductance causes a voltage to appear across the secondary winding of a transformer. (G5C01)
The voltage across the secondary winding will be equal to the voltage across the primary times the ratio of the number of turns in the secondary to the number of turns in the primary. When the number of turns in the secondary winding is greater than the number of turns in the primary winding, the voltage across the secondary winding will be great than the voltage across the primary winding, and the transformer is called a step-up transformer.
When the number of turns in the secondary winding is less than the number of turns in the primary winding, the voltage across the secondary winding will be less than the voltage across the primary winding, and the transformer is called a step-down transformer. For example, the voltage across a 500-turn secondary winding of a transformer is 26.7 volts if the 2250-turn primary is connected to 120 VAC. (G5C06)
By reversing a transformer’s windings, that is connecting an input voltage to a transformer’s secondary winding and connecting the primary windings to the output circuit, you make a step-up transformer act like a step-down transformer and vice versa. For example, if you reverse the primary and secondary windings of a 4:1 voltage step down transformer, the secondary voltage becomes 4 times the primary voltage. (G5C02) In effect, it becomes a step-up transformer.
Doing this is not necessarily a good idea, however. Current in the primary winding of a step-up transformer is higher than the current in the secondary, and to accommodate the higher current of the primary, the conductor of the primary winding of many voltage step up transformers is larger in diameter than the conductor of the secondary winding. (G5C16) If you use a step-down transformer as a step-up transformer by connecting the input voltage to the secondary winding, the wire in the winding may not be able to handle the higher current and burn out.
Transformers are also used to transform impedances. The impedance ratio is also related to the turns ratio, but the transformation is equal to the square of the turns ratio. The turns ratio of a transformer used to match an audio amplifier having a 600-ohm output impedance to a speaker having a 4-ohm impedance is 12.2 to 1. (G5C07)
David Ryeburn, VE7EZM and AF7BZ says
“The voltage across the secondary winding will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary.” is incorrect (sentence wasn’t completed correctly). Two ways to fix it are “The voltage across the secondary winding will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary, multiplied by the voltage across the primary winding” or “The ratio of the voltage across the secondary winding to the voltage across the primary winding will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary.”
“For example, if you reverse the primary and secondary windings of a 4:1 voltage step down transformer, the secondary voltage becomes 4 times the primary voltage” is confusing because what you mean when you say “secondary” is “new secondary” which is the same as “old primary”. Try this instead: “For example, if you take what was intended to be a 4:1 voltage step-down transformer and connect the input voltage source to what was supposed to be the secondary and take the output from what was supposed to be the primary, the output voltage will now be 4 times the imput voltage.”
The sentence “Current in the primary winding is often quite high” is, without qualification, not correct. If a transformer is intended to be a step-up transformer then the current in what was intended to be its primary will be larger than the current in the other winding, so the (intended) primary will be wound with larger diameter wire. But you’re talking before this about what is intended to be a step-down transformer that you worried about connecting backwards. Whether your worries are justified depends upon the circumstances.
Some numbers will make this more clear. Suppose we have a well-engineered step-down transformer, with winding A intended to be connected to a 120 V source and winding B intended to deliver 30 V to the load. Winding B is made with wire size adequate to allow 400 mA of current to flow through winding B into the load without overheating. Then 100 mA will flow though winding A, and it too is sized so as not to overheat when this happens. (I am assuming 100% efficiency here; in a real transformer the product of current and voltage for the primary would be a bit larger than that for the secondary.) The insulation on winding A is adequate for 120 V and the insulation on winding B is adequate for 30 V. Also the core of the transformer is of adequate size not to go into saturation under these conditions. But now suppose we turn the transformer around so as to use it as a step-up transformer. Whether that will be a bad idea depends upon how we do it.
If we connect winding B to a 30 V source and connect a load which wants to see 120 V across winding A, and if that load then draws 100 mA or less, all will be well. But if the load draws more than 100 mA all may not be well.
Worse yet, if we connect a 120 V source across winding B with the goal of making 480 V available across winding A, several bad things may happen. The wire on winding A still is rated to carry 100 mA safely; if the load then connected across winding A is of lower resistance than 4800 ohms, when 480 volts is applied to it, it will draw more than 100 mA and the current through winding B will exceed 400 mA. One or both of these windings may not have been wound with large enough wire to cope with this situation. Also the insulation on winding A, adequate for 120 V, may be inadequate for 480 V, and similarly the insulation on winding B, adequate for 30 V, may be inadeuqate for 120 V. Finally with the transformer operating at a higher power level than intended, the core may saturate.
I haven’t seen the actual exam question whose answer is supposed to be supported by your explanation, so I don’t see exactly how to modify things; how the question was phrased and what the incorrect and correct answer choices are will determine that.
Dan KB6NU says
Great feedback. Thanks!