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This topic comprises 3 pages: 1 2 3
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Topic: How does power supply regulate amperage
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Bill Yacey
Film Handler
Posts: 23
From: St. Albert, Alberta, Canada
Registered: Feb 2011
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posted 03-03-2011 11:13 AM
I realize this is an old post, but I thought I might have something to add here. In a simple electric circuit with a fixed resistance (load) value, current will increase or decrease in a linear manner as the voltage goes up or down. The resistance is the constant.
Arc lamps which are a variety of lamp that operates by ionizing the gas between the electrodes. Neon and other gaseous filled lamps operate on the same basic premise of ionization. One of the important characteristics of this type of lamp is that it tends to self regulate the voltage applied across the electrodes once the arc is struck. In trying to increase the voltage across the lamp, the voltage will not increase in a linear manner, in fact it remains for all intents and purposes a constant.
Most of the deviation from this constant is due to electrode erosion which changes the gap length as the bulb ages during use, and also by contamination withing the bulb envelope changing the characteristics of the arc conduction.
However, upon trying to increase the voltage, the current will readily increase, utilizing more power and generating more light output yet the arc will still clamp the voltage and prevents it from rising.
Early voltage regulator circuits actually used neon or other ionized gas bulbs for a constant voltage reference prior to the introduction of zener diodes in electronics.
As an example, if the voltage across the bulb is 24VDC after the arc is struck and stabilized, and the current measured is 68.5 Amperes, using ohms law we come up with a resistance value of the arc at .35 ohms. 24Vdc divided by x resistance in Ohms equals the current in Amperes.
In the real world according to Ohms Law, something has to change before the current can change if the voltage is constant. This only leaves the resistance value. By increasing the current, this causes the ionized gas resistance to lower, or conversely to go up in resistance as the current is decreased.
I hope this helps to explain the current vs voltage theory regarding lamp supplies.
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Robert Minichino
Master Film Handler
Posts: 350
From: Haskell, NJ, USA
Registered: Dec 2005
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posted 03-03-2011 12:42 PM
Any power supply is built specifically to attempt to hold its output to a particular range of voltage, current, and/or power.
Most familiar power supplies are approximations of voltage sources, which mean they try to hold their voltage constant as the load changes by varying current. Arc lighting power supplies (and welders) are approximations of current sources, which mean they try to keep their output current constant as the load varies by varying their voltage. A constant-power supply is somewhat exotic, but attempts to supply a constant power into the load by varying voltage and current together.
For the simplest cases, Ohm's law says that the voltage drop across a load is proportional to the current through it, with the scale factor being a fixed property of the load called resistance. Ohm's law doesn't apply in every case, but it is useful even where it doesn't strictly apply. We can use Ohm's law in a case where the resistance isn't constant by specifying the resistance at a given current.
The resistance of most real devices changes, usually with the current passing through the device. A wire, a simple resistor, and an incandescent light bulb will all have a higher resistance as they gets hotter, and they get hotter with more power dissipation, which generally means more current. This results in somewhat non-linear behavior, but it's self-limiting, since a higher resistance with a common voltage source means that less current flows (back to Ohm's law here).
As Bill explained above, Xenon bulbs, and other arcs, aren't ohmic devices, and their resistance actually decreases with increased current. This means that as you pass more current through the arc it becomes a better conductor. If you hook an arc up to a voltage source, it will draw more and more current until the voltage source is maxed out. This seems to be an application crying out for a current source, particularly since lamp brightness is related to current, and we want to keep the brightness constant.
A voltage source "likes" an open circuit (infinite resistance), because it doesn't need to supply any current to get the desired voltage across its terminals, and it "hates" a short circuit (zero resistance), because it would need to supply infinite current to get the desired voltage across its terminals. Non-ideal voltage sources (i.e. real ones) vary from this behavior because they 1) can't supply infinite current; and 2) let the voltage vary a little as the load varies. We generally represent the first shortcoming as a specification sticker slapped on the supply (5V @ 3A), and the second we represent as an ideal resistor in series with an ideal voltage source.
A current source "likes" a short circuit because it doesn't need to produce any voltage difference between its terminals to get a given current to flow, and it "hates" an open circuit because it needs to supply infinite voltage to get a current to flow. Here, current sources fall from the ideal by 1) not being able to supply infinite voltage, and 2) allowing the current to vary a little as the load changes. We represent the first shortcoming as the current source's open-circuit voltage, and the second as a resistor in parallel with an ideal current source.
In an arc power supply, the open circuit voltage is the striking voltage, and the voltage drops as the arc starts, because the load resistance drops with the higher current.
A really neat trick is that a non-ideal voltage source can be implemented as a non-ideal current source, and vice versa. A 10A current source with a 50V open circuit voltage can be represented as a 50V voltage source in series with a 5 ohm resistor. When this current source is shorted, you get 10A. Of course, if the resistance is higher, the current is actually lower, so this is not an ideal current source, but it can act like a current source for a given range of loads.
This is exactly how the current was regulated in the old motor-generator days with carbon arcs. Large rheostats were placed in series with a DC generator (which is a very good approximation of a voltage source), and the resistance was varied until the arc current was approximately correct. The high-reactance supplies were way better, because instead of wasting a lot of power in the rheostat, they used a specially-wound transformer that had a lot of "virtual" resistance that didn't result in actual power dissipation. Even better are the newer switching power supplies that use transistors and feedback to monitor the output current and vary the output voltage based on the current set point. These use transistors that are either on or off, so very little power is dissipated in them (if they're on, there's little voltage across them, so P=0*I = 0, ideally, and if they're off, no current flows P=V*0 = 0), and an inductor smooths the pulses into a nice constant flow. Since the transistors switch at a very high frequency, the size of the magnetics are much smaller than the 50 or 60 Hz transformers.
But all the power supplies work on the same principle: they try to keep some aspect of their output constant as the load varies.
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