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I know this look stupid, but why high current can't substitute low voltage.

I read many articles about the difference between the two, and asked my professors, but still have some doubts and blind spots to differentiate between them.

let's say a diode, I need 0.7V to make it operate, why if I decrease the voltage below that and significantly increase the current it still doesn't operate?

I tried to visualize the current as an army (quantity) and the voltage as the power of the that army. And thought even if the power is low if I increase the number of my army I would substitute the low power.

I know this is not true, but why?

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  • \$\begingroup\$ For a diode, very little forward current will flow until you provide enough voltage to overcome the potential barrier, so attempting to increase the current in this case would be futile. You could look at a diode as being a voltage source with the voltage being the potential barrier. \$\endgroup\$ – Peter Smith Aug 11 '18 at 14:17
  • \$\begingroup\$ If you're a visual thinker, you may find the excellent videos by Eugene Khutoryansky useful to understand the basics of voltage and current; for example this one. \$\endgroup\$ – amb Aug 11 '18 at 14:18
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Seemingly your professors couldn't see or believe how big hole you had in the basics.

You cannot alter voltage and current independently. The operation of the electronic components IS how they make the current dependent on the voltage.

The operation of the component ISN't something that you can start by applying a good voltage and a good current, which you select both as you like. You apply a voltage and the component sets the current. The setting by the component happens as exprcted = the component works.

A silicon PN diode for example WORKS when it lets only a small current go when the applied voltage is much under 0,7V, say 0,2V or less

High current and low voltage is a substitute of high voltage and low current in one special case: What the power calculation equation P=U*I gives when you insert numerical values to U and I.

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Consider using the water in pluming metaphor to explain voltage and current. Using this metaphor, the voltage is the pressure in the pluming and the current is the amount of water making it through the pluming. Batteries become water pumps. Resistors become constrictions in the pipe. Diodes become check-valves which need a little pressure to open:

A diode is equivalent to a one-way check valve with a slightly leaky valve seat. As with a diode, a small pressure difference is needed before the valve opens. And like a diode, too much reverse bias can damage or destroy the valve assembly.

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The diode is a good example. The quantum physics of a silicon PN junction determine the forward voltage of the diode. You can't change that voltage by being pushy with current.

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I explained why you need 0.7 V in my answer to your question of yesterday, Why do we care about voltage and current - not power only.

"... why if I decrease the voltage below that [0.7 V] and significantly increase the current it still doesn't operate?"

You can't get significant current to flow until you exceed 0.7 V.

I tried to visualize the current as an army (quantity) and the voltage as the power of the that army.

No, the voltage would be like how many soldiers are in each line (file) pushing one behind the other. The current would be like the width of the rank (how many side by side). It's not a great analogy.

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The controls on your lab power supply are labeled as "Voltage" and "Current", and so you might think that they're setting those physical parameters directly, but they're really just limits.

The relationship between V and I is determined by the circuit, and the supply simply stops pushing when it hits one or the other limit. If you want to "push more current", then the only way to do that is to increase the voltage because that's precisely by definition how you push current.


I agree with user287001's assessment of your professors, and it's a bit unsettling to me that they would miss that. What other fundamentals (or fundamental misunderstandings) would they miss?

Or perhaps they did see and failed to address? That's also unsettling.

Or maybe they did explain it all correctly in their way of thinking and you just don't think that way. Welcome to university! A big part of it is not the subject matter itself, but learning to think differently than you're used to. Build up a library of different thought patterns and see what works.

Hint: Asperger's and Autism are often associated with technical understanding. You might want to read up on how they think and mimic that.

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why high current can't substitute low voltage.

There are situations where high current DOES substitute for low voltage, like in a transformer. But, all real-world applications have current limits, voltage limits, and thresholds. A wire that carries 15A of current is thick; hard to bend, expensive to build from copper, and requires good low-resistance terminations to connect it to anything. A wire that carries 30 kV to fire a spark plug is always fat with insulation, so that it doesn't make St. Elmo's fire (corona discharge) when active.

I have a 6V 100W light bulb: it takes about 15A to run. And, a 120V 100W light bulb takes 0.8A. They are very different designs, and extending to 0.6V or 1200V would require very different switches, wires, sockets.

let's say a diode, I need 0.7V to make it operate, why if I decrease the voltage below that and significantly increase the current it still doesn't operate?

A diode that does useful rectification of electrical power, made of silicon, has negligible power output with less bias voltage than about 0.5V. There are some RF rectifiers (point-contact diodes) that can work at much lower voltages, but burn up at high current. So, the lower-voltage/higher-current options are impractical. This is because there is a material property (the silicon bandgap) that determines the useful range for the (easy to produce) silicon rectifier.

The only way you could reduce voltage AND increase current would require a redesign of the diode itself, rather than using off-the-shelf components. Folk who DO redesign circuitry often find that their goals are inconsistent with low currents (because cosmic rays induce noise) or with low voltages (because transistor switch thresholds have as much variability as the total power voltage), or with high currents (because wiring gets too fat) or high voltages (because insulation cannot be assured). A typical CPU chip needs two or three precise voltages in order to function, and those voltages are NOT arbitrary.

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