Electronics; Welders And Filters Flashcards
What does resistance come from on a molecular level?
Electrical energy required to free electrons from the valence shell when creating current flow.
What form of energy does resistance manifest as?
Electromagnetic radiation as heat or light
What are resistors used for, in an electronic circuit?
Control current flow
Divide voltage
What are the two groups of resistors?
Fixed
Vairable
Other names for variable resistors?
Poteniometers, trimmers, rheostats
What is the middle lead on a potentiometer called and used for?
Wiper; lead that allows variable resistance (as opposed to total resistor resistance)
What are the important characteristics to know before testing or replacing resistors?
Power (watts)
Resistance
Tolerance
Max voltage
What does the power rating of a resistor indicate?
Max power resistor can safely dissipate
What is the usual power rating of carbon resistors made of carbon compounds?
Less than 2W
How is the power rating of carbon resistors indicated?
By physical size. Smaller resistor = smaller power rating
What type of resistor has power ratings higher than 2watts?
Wire-wound resistors.
When would ‘R’ be used to indicate a decimal place for resistance value of a resistor?
For resistance values less than 1000 ohms
When would ‘K’ be used to indicate a decimal place for a resistance value of a resistor?
For resistance values between 1000 and 1,000,000 ohms
What does a tolerance rating of a resistor indicate?
Degree of accuracy the manufacturer guarantees
How many significant digits are indicated by coloured bands on a precision resistor?
3
If a resistor is connected to a DC source, what would happen to the flowing circuit properties at the instant the switch closes?
1) Voltage across resistor
2) current
3) phase relationship between voltage and current
1) Voltage across resistor raises from 0 to = source
2) current increases to a value determined by ohms law (V÷ohms)
3) current in in-phase with voltage
If a resistor is connected to an ac supply, what is the phase relationship between voltage and current?
In phase with eachother
When do resistors fail?
When their POWER ratings are exceeded
Why do we have to disconnect one side of an installed resistor to test its resistance? (For troubleshooting purposes)
To isolate the resistor’s resistance from parallel resistance in the circuit
True or false: A replacement resistor in an electronic circuit can have a greater power rating than the original.
True. But cannot be lower power rating
What is the general purpose of a capacitor in any electronic circuit?
Store and release electrical charge by way of opposing a change in voltage.
What component of a capacitor allows the two plates to be brought close together without shorting out?
Dielectric layer
What three construction characteristics affect the capacitance of a capacitor?
1) Surface area of plates
2) Distance between plates
3) Dielectric constant of dielectric layer
How would you change each construction component to increase capacitance of a capacitor?
- surface area of plates
- distance between plates
- dielectric constant
- increase surface area
- decrease distance
- increase dielectric constant
How would you connect capacitors together to decrease total capacitance?
Series.
How do you add capacitors in series?
1/C(1) + 1/C(2) + 1/C(3) = C(t)
How are capacitors generally categorized?
By the type of dielectric used in their construction.
Which type of capacitors are polarity sensitive?
Electrolytic
Tantalum
What are the most common types of dielectric materials used in capacitors?
Paper Plastic film Mica Ceramic Electrolytic Tantalum
Which type of capacitor has a capacitance range from 0.001 microfarad to 2 microfarads?
Paper capacitors
What is the voltage range of paper capacitors?
50V to 1500V
Which type of capacitor is mostly used in electronics?
Paper capacitors.
Which capacitor is widely used in electronics where rugged, reliable capacitors are required? Why?
Plastic film. Insensitive to humidity
True or false: paper is a better insulator than plastic
False
What are the capacitance and voltage rating ranges for plastic film capacitors?
5pico farads to 2 micro farads
50V to 600V
Between plastic film and paper capacitors, which would you expect to be physically smaller? Why?
Plastic film. Plastic is better insulator so dielectric can be thinner, therefore more capacity in smaller package.
Which type of capacitor is temperature stable? Why?
Mica. Because mica can withstand high temperatures.
What are typical ratings of mica capacitors?
3pico farads to 0.47 micro farads
100V to 35 000V
What other material is layered with the mica in a mica capacitor?
Metal foil
True or False: Capacity of a mica capacitor can be held to a lower tolerance than rolled capacitors
True
Which type of capacitor uses clay as the dielectric medium?
Ceramic
What are typical ratings of ceramic capacitors?
1pico farad to 1000 pico farads
50V to 2kV
Which type of capacitor offers a capacitance 10 to 20 times that of a paper capacitor of equivalent size?
Electrolytic capacitor
What is the main advantage of electrolytic capacitors?
High capacitance and small volume
True or false: polarized electrolytic capacitors can never be connected to an ac supply
FALSE. Can be connected if marked with Vac rating at 60Hz. But usually polarized capacitors not suited for ac - would explode on ac supply
What are typical ratings of tantalum capacitors?
0.004micro farads to 470micro farads
6V to 100Vdc
What do the ratings of capacitors include?
Capacitance, voltage, tolerance
What does it mean if capacitance rating on a capacitor case is given as a decimal?
It is understood to be in micro farads.
.22 = .22 micro farads
What does each digit represent in a three-digit coding system?
1 & 2 = significant digits
3 = multiplier
In pico farads
How many farads would a capacitor have if it had “184” on the case?
0.18 micro farads
What are the letters and associated tolerance ratings for capacitors?
J = 5% K = 10% M= 20%
What is “WVDC” indicate on a capacitor data sheet?
Working volts DC; maximum (peak) safe voltage without danger of breaking down dielectric
What does higher voltage rating of a capacitor mean for physical characteristics?
Larger size. (Requires thicker dielectric)
How does voltage and current change over time in a capacitive DC circuit?
Voltage starting near-zero reaches a Max of 99% of the applied voltage in 5 time constants
Current starting at a max (capacitor acts as a short circuit at moment of connection) decreases to near 0
In a capacitive dc circuit, current flows until ______________.
Capacitor is charged
Explain a capacitor in “steady state” and its practical application.
Fully charged and blocking DC current. Can be used as a COUPLING DEVICE to block DC and pass AC.
How much does a capacitor charge (%-wise) each time constant?
66.6% of the remaining capacity (therefore never reaches true max)
What is the rate at which a capacitor charges and discharges based on?
Capacitance
Resistance of current path (independent of applied voltage)
Describe the transient state of a capacitor
Increasing voltage and decreasing current during charging.
How is a capacitor discharged?
When a load is connected in parallel with the capacitor.
How long can a capacitor supply voltage and current to a load?
Depends on circuit time constant. 5 time constants to discharge
On discharge, what does the “R” in the RC time constant represent?
The load
If a load has a low resistance it will draw (high/low) current and discharge the capacitor (faster/slower)
High current
Faster discharge
What causes capacitive reactance in an ac circuit?
The constant store and release of charge by the capacitor
What two circuit characteristics affect the capacitive reactance of a capacitor?
Frequency of the supply
Capacitance of the capacitor
How is capacitive reactance related to frequency and capacitance?
Inversely related.
Capacitive reactance (increases/decreases) when added in series.
Increases. X(c1) + X(c2) + X(c3)
Capacitive reactance (increases/decreases) when added in parallel
Decreases 1/X(c1) + 1/X(c2) + 1/X(c3)
Capacitance (increases/decreases) when added in series?
Decreases. 1/C(1) + 1/C(2) + 1/C(3)
Capacitance (increases/decreases) when added in parallel?
Increases C(1) + C(2) + C(3)
Describe the phase relationship between voltage across a capacitor and current through a capacitor when an ac source is applied
Current leads voltage.
How do you connect a capacitor in order to block DC to a load?
Connect capacitor in series with the load
How do you drain a capacitor in a controlled manner?
Place a discharge resistor in parallel with the capacitor before working on the circuit
What circuit property does a capacitor oppose?
Change in voltage
What circuit property does an inductor oppose?
Change in current
What does Lenz’s Law state?
The direction of the induced emf is such that any current resulting from it develops a flux that opposes any change in the original flux.
How does an inductor affect peak values when current increases in a circuit?
Inductor opposes the increase and lowers peak values
What do you call devices that have the property of inductance?
Inductors, coils, or chokes
How is an inductor formed?
When a length of wire is wound into a coil
What is the inductance of a coil affected by?
Number of turns (N; direct)
Permiability of the core (u; direct)
Cross sectional area of the core (A; direct)
Length if core (l; inverse)
L = (N²uA)/l
What are the two most important ratings of an inductor?
Inductance and current ratings
Constant store and release of charge causes ______________.
Constant store and release of magnetic field causes _____________.
Capacitive reactance.
Inductive reactance.
Inductance (increases/decreases) when inductors are connected in series?
Increases
Inductance (increases/decreases) when inductors are connected in parallel?
Decreases
Inductive reactance (increases/decreases) when inductors are connected in series?
Increases. (Basically increasing the length of conductor)
Inductive reactance (increases/decreases) when inductors are connected in parallel?
Decreases (multiple paths for current to flow)
Describe the phase relationship between voltage across an inductor and current through an inductor
Current lags voltage
Which quadrant would the supply voltage phasor land in a resistive-capacitive circuit?
4th
Which quadrant would the supply voltage phasor land in a resistive-inductive circuit?
1
Why are inductors called choke coils?
They suppress a sudden rise and decrease in current
How would an inductor be connected with respect to a load, in order to regulate current?
In series
Describe the effect a choke coil has on the peak voltage values in a circuit.
A choke coil reduces the peak voltage values by opposing current with a cemf. Opposing both a rise in current on the increase of a pulse, and the drop in current when the pulse attempts to drop to 0.
Where does the “peak” of the inductive circuit waveform line up when superimposed on a regular pulsating DC circuit?
Inductive peak lags regular pulsating DC peak. Peaks on decrease of current.
When does the greatest change in current occur? How does a choke coil react?
When circuit is opened.
1) Inductor attempts to oppose this change with cemf spike that may be greater than the source voltage (depending on the inductance of the coil)
2) Inductor seeks to maintain current flow by racing across an open switch.
Where are inductors used?
In circuits where sudden rise or fall of current is undesirable
A circuit where an inductor is used must be able to withstand ________ that the inductor introduces into the circuit.
Spikes (noise or dirty power)
What are the three steps to test for an open circuit on an inductor?
1) turn off circuit
2) remove at least one lead to remove coil from cct
3) use ohmmeter to test resistance.
What reading would you get on an ohmmeter if a coil was short circuited?
Infinity
Where are filters used with regards to a circuit’s rectifier?
On the output of a rectifier
What is another word for the “ac component of a voltage”?
Ripple
What does a %ripple value indicate? How do you calculate it?
Indicates how “clean” the dc voltage is.
Compare rms value of ripple with DC avg.
%ripple = (V(rms) of ripple/ V(dc avg.)) × 100
We generally prefer %ripple to be a (high/low) number?
Low. (Smoother dc voltage)
What is the simplest filter circuit?
A capacitor connected in PARALLEL with the output terminals of a rectifier and in parallel with the load.
When does a capacitor filter charge?
When recified voltage rises toward maximum
When does a capacitor stop charging?
At Vpeak
When does a capacitor discharge its energy into the circuit (load)?
As the recifier voltage begins to decrease.
What does the amount of ripple at the load depend on?
Capacitance of capacitor
Resistance or impedance of the load
True or false: the conversion factors for calculating average voltage of recifiers can still be used once the circuit is filtered.
Hell no
A capacitor discharges faster with a load of (high/low) resistance.
Low
There is (more/less) ripple when a capacitor filter is connected to a greater load (ie less resistance)
More.
A capacitors filtering effectiveness (decreases/increases) as load increases
Decreases.
How is a chole filter connected with regards to a recifier and load?
In series with the output of a recifier and in series with the load.
Which types of cores can a choke filter have?
Air or iron
True or false: there is no phase shift between output voltage of the rectifier and the load voltage due to the inductor in the circuit.
False.
What two factors determine how well the choke filters the output?
Inductance of the choke
Amount of current
The filtering effect of a choke (increases/decreases) as the load increases. Why?
Increases. Choke reacts more to a larger load because a larger load tries to create a larger variation in current.
Compare the filtering effects of a choke and a capacitor at no load.
At no load a choke has virtually no filtering effect.
At no load a capacitor is a perfect filter (maintains voltage at peak value)
How can the noise from a choke coil be filtered out?
+ Capacitor in parallel with the load
What does an L-section filter consist of?
Capacitor and choke
What advantages does an L-section filter offer?
Capacitor good for filtering small loads
Choke good for filtering large loads
Capacitor suppresses spike produced by choke
Where is a capacitor connected within an l section filter?
Across the output AFTER the choke
Another name for an L section filter
Choke input filter
What is the basic process of resistance welding
Use high current and pass it through a resistance
What are three examples of resistance welding machines?
Seam welding machines
Spot welding machines
Box welding machines
What are MIG and GMAW?
How does this type of welding work?
Metal inert gas
Gas metal arc welding
They are arc welding processes. Use high heat of an electric arc to melt metals and add extra metal to a joint.
Compare the specs of small and large welding machines
Small: 120V ac supply, 100A dc welding. 20% duty cycle
Large: 240V single phase or higher with 3 phase
What are the main components of GMAW welders?
Step-down transformer (120 or 240 to 18 or 25)
Rectifier (convert ac to dc)
Filter (requires steady dc)
How does the transformer of a welder set the current levels?
Using voltage taps on either the primary or secondary windings
What device is used to protect diodes in a welder from high voltage spikes?
A varistor, or surge suppressor.
What type of filter is used in the welder shown in figure 46?
Choke. (Not capacitor)
A high heat setting on a welder would (increase/decrease) the number of turns on the primary?
Decrease
Using the welder in figure 46, describe how you would attain the following heat settings:
1) high
2) high medium
3) low medium
4) low
1) both switches up
2) s2 up, s3 down (to connect additional s3 winding)
3) s2 down (to connect full s2 winding) and s3 up (to disconnect additional s3 winding)
4) both switches down (max amount of windings connected)
How do diode current ratings and reverse voltage ratings of a welder with a bridge rectifier compare to a welder with a centre-tap recifier?
Diode current ratings are the same
Working reverse voltage rating has to be doubled for centre-tap because secondary voltage is doubled.
What type of welder has four taps on the primary to supply four secondary heat windings?
120V input centre-tap recifier welding machine
What size capacitor is used in a 120V input centre-tap rectifier welding machine?
53000 microfarad
Which types of welders use both capacitors and inductors for filtering purposes? Which do NOT use both?
120V centre-tap rectifier welding machine
240V input welding machine
NOT
120V single phase bridge rectifier welding machine
Why is the capacitor in a 120V center tap welding machine connected to the rectifier with large conductors?
The current into the capacitor is high and requires low resistance connections.
What current rating is associated with a 240V welding machine?
185Adc
