Concept Questions: Current and Resistance?
A fuse is a device designed to break a circuit, usually by melting when the current exceeds a certain value. Fuses are widely used in electronic equipment, but have been replaced by circuit breakers in household wiring. In the "old days" people would sometimes replace a blown fuse with a penny, which happened to be the same size as a fuse. Was this a safe practice? Why
High-voltage power supplies are sometimes designed to have a rather large internal resistance as a safety precaution. Why is such a power supply with a large internal resistance safer than one with the same voltage, but lower internal resistance?
Can all combinations of resistors be reduced to series and parallel combinations?
Old-time Christmas tree lights had the property that, when one bulb burned out, all the lights went out. How are these lights connected, in series or in parallel? How could you rewire them to prevent all the lights from going out when one of them burned out?
Tagged with: blown fuse • christmas • christmas tree • circuit breakers • electronic equipment • fuses • high voltage power • high voltage power supplies • household wiring • internal resistance • old time christmas • parallel combinations • power supply • resistors • safety precaution
Filed under: Parallel Electronic Circuits
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Hi Alekz,
A penny can carry a great deal more current than a fuse, so the wiring in the home would not be protected from melting and causing a fire. The idea of the fuse is to be the weakest link in a safe, protected environment (Fuse box). Therefore, replacing a fuse with a penny is NOT a safe practice.
In the case of a short-circuit, if the power supply has a high internal impedence (resistance), the circuit voltage can drop across the internal resistance, thus reducing the current and reducing the opportunity for circuit damage. If the power supply has a low internal impedence, the short circuit current is not limited and can cause severe damage to human life.
The third part of your question is ambiguous. Do you mean can all values of resistance be created by using standard values in series and/or in parallel. If this is what you mean, then yes, they can.
The Christmas lights were connected in series. Rewire them in parallel, but be careful since in series, each light can only tolerate a voltage equal to a small portion of the total voltage (for 10 lights, each one operates at 1/10 of the mains voltage). If you simply rewire them all in parallel, they will all burn out since instead of 11/22 volts across each, there will be 110/220 volts…kapop!!
A fuse is used because it conducts up to a certain voltage and then breaks down, becoming non-conductive. A penny conducts until…well, it just conducts. Not a good practice.
V=IR. Same voltage. Lower resistance means higher current – P=I^2*R, so power increases rapidly with increasing current. Higher power = more heat dissipated = less safe.
If you have enough time on your hands, perhaps.
If a light is a resistor consider what happens if it stops passing current. In parallel the current has other branches to travel to complete the circuit. In series a single open circuit kills the whole thing.