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Author Topic: How does power supply regulate amperage Mike Croaro Master Film Handler Posts: 394 From: Millbrae, CA Registered: Apr 2005 posted 12-25-2009 10:46 PM                     Hi Folks: How exactly does the output knob on a power supply regulate the amperage of a lamp? Does is regulate the aperage directly, or does it regulat the voltage to the lamp which then determines the amperage? Just curious, Mike  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 12-26-2009 12:45 AM                     Even though you got two different types of rectifiers, both work in the same basis: On diode rectifiers, you're changing the secondary voltage output on the three transformers before that current is going into the diode bank which converts the AC current to DC. Switchers rectifiers (which is as the same as a power supply on a computer) convert the incoming AC to a pulsatng DC then fed into the switching circuit, which converts the pulsating DC to a square wave at 60hz. Then this current is fed into transformers to be converted again to a smooth DC which the DC output is controlled by a 'volume control' located on the switching circuit. Both types of rectifiers are designed to handle certain min and max output ratings in which the bulb rated usage should be between the minimun and maximum ratings of the rectifier. Thus, you're actually controlling voltage, but it's the amperage that is the most common readable gauge to use .. and this goes back to the carbon arc days. Why amperage readings, is that we want to see the "pressure" of the current going thru the bulb, not the voltage. -Monte  |  IP: Logged Michael Voiland Expert Film Handler Posts: 219 From: Naperville, IL US Registered: Aug 2009 posted 12-26-2009 01:21 AM                     quote: Monte L Fullmer Why amperage readings, is that we want to see the "pressure" of the current going thru the bulb, not the voltage. Amperage is the amount of electrons flowing by a curtain point so wouldn't that be the quantity. As voltage is the potential difference or the push / pressure of a circuit.  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 12-26-2009 01:39 AM                     Guess, we can use the analogy of a garden hose to this topic then: the inside diameter being the voltage and the flow of water out of the hose being the current. -Monte  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 12-26-2009 09:25 AM                     Err...voltage and current are inexorably linked. You can't really change one without the other changing. When we say "Xenon lamps are current-controlled" what we really mean is that there is a very small voltage variation for a rather large current variation. Change the voltage by a little, and the current can change a lot. Change the current by a little, and the voltage change is almost imperceptible. So really it comes down to how you think about it. You can think about the rectifier changing its output voltage to change the lamp's applied current. This tends to make sense to me, but it's not the only legitimate interpretation. It is true that you can't really think about the current without knowing the characteristics of the load (the lamp), whereas its easy to think about the voltage without the load. But of course, the load characteristics of the lamp are radically different before and after striking... Monte, switching power supplies do not switch at 60 Hz. They switch a thousand times faster (in the kHz), and then that switched voltage is filtered to look like smooth DC, but there is always some ripple (just as there is with traditional high-reactance power supplies). --jhawk  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 12-26-2009 10:08 AM                     Generally speaking... If you are talking about electricity going through a transformer, changing the ratio of primary windings to secondary windings (AKA: "Taps") will vary the voltage coming out of the secondary, assuming the voltage going into the primary is constant. Current generally obeys Ohm's Law. Now, if you have a regulated power supply there are many kinds of components which can be used to ensure the output stays within a certain range, it is possible to regulate current. Some of those power supplies can be very complicated. When a power supply is built, regardless of the type, it will engineered from the ground up to supply voltage and current within certain ranges. Therefore, in operation, resetting the taps will vary the voltage and the current according to the laws of physics/electronics but, if the device is operating the way it is supposed to, you should not see current or voltage go outside the limits that the xenon lamp can handle.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 12-26-2009 10:10 AM                        For years I have hoped (given that we measure both voltage and current on the meter board) that we could get a direct reading in WATTS. Should be simple with the addition of only one chip. We already have the digital display and the volt/amp switch. Further, a limit could be preset internally to turn off the power if current exceeded a certain level, depending on the bulb and power supply. (For example 2200 watts on a 2000 watt bulb.) Louis  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 12-26-2009 10:19 AM                     Irem does that on their "EX" series switch mode supplies.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 12-26-2009 10:29 AM                     Louis, why is a display of watts useful? A reasonably wide display of voltage is helpful in diagnosing striking problems (is the no-load voltage sufficient to strike?) and once you've struck, as long as the voltage is near the nominal operating point (e.g. 28V for a 2k), it's not very interesting. Current is what the bulb is actually rated in, and is what matters for ohmic heating of components (I^2*R) and I think is also what the luminous flux is proportional to. It's what the bulb datasheet rates the bulb in terms of. As the bulb ages, the cathode tip erodes (from striking), and that causes the operating voltage of the lamp to rise. If you maintained constant-current as this happened, then the power draw (watts) of the bulb would grow without the brightness growing. (Of course, that's not what actually happens. Since the rectifier probably tends to maintain constant-wattage output, meaning as the bulb's required voltage increases its current draw changes and it's necessary to adjust the rectifier's current control to get the same current through the bulb...; and of course there's also the fact that the same current will produce lower brightness from a bulb as it ages...blackening? not sure what the mechanisms there are...) So why do you want watts? Sure, it's the product of volts*amps, but if, e.g, your voltage is at 28.0V and you are running at 80A, that's 2240W, but still perfectly OK on a 2k bulb. And if your voltage rises to 30.0V and you're still at 80A, that's 2400W, but it's still within the current-control range of the bulb. On the other hand, if your current rose to 85A (the maximum by the datasheet) and your voltage was still at 28V, then your power would be 2380W, but you would *not* be ok (redlining the bulb). So 2400W could reflect a reasonable current with a high-but-acceptable voltage, which is fine. But it could also by a normal voltage with a too-high current, which is not fine. I guess cooling capacity of the lamphouse is measured in watts, but one would think that a reasonably-designed lamphouse would deal adequately with the full range of a 2k lamp (famous last words...) --jhawk  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 12-26-2009 12:39 PM                     Actually the latest version of the Strong switcher is based on a 60khz oscillator. Although I repair a gob of these I've never bothered to look at the exact switching frequency with a scope... Mark  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 12-26-2009 02:10 PM                     60 kHz = 60 Hz * 1000 not exactly the same...  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 12-26-2009 05:08 PM                     Monte, I just have to clarify your hose analogy for the sake of future readers of this thread. In Ohm's Law, Current equals Voltage divided by Resistance. (I=E/R) To use the garden hose analogy, The pressure of the water is the Voltage, The cross section area of the hose is the Resistance and the water flow is the Current. This is an easy analogy to use if the components are properly identified. I hope this is helpful. KEN  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 12-26-2009 05:39 PM                     I often use the "cars traveling down a freeway" analogy where the number of cars passing a given point every second equals "current", the average speed of the cars passing that point equals "potential" and the number of lanes on the freeway is resistance.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 12-26-2009 06:53 PM                     The problem with the hose analogy is that the bigger the hose the LESS resistance to flow so it has an inverse relationship and that is where the analogy breaks down. KEN  |  IP: Logged Monte L Fullmer Film God Posts: 8367 From: Nampa, Idaho, USA Registered: Nov 2004 posted 12-26-2009 07:01 PM                     Whoops, I forgot to put the "k" in the "60" kilohertz thingey ..(my bad on that one) - why John had to make the correction. Thx Ken, I knew that I was missing something with that analogy. Even though I knew of the "PIE" formula to get the wattage. -Monte  |  IP: Logged

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