If nobody has anything, I could try writing up a decent text description for wearcomp.org, not that I'm the worlds' best tech writer or good at making things clearer or anything <G> Quick outline: Think of Amps (flow rate) as how fast the water's flowing out of the spigot (power source). The voltage is the pressure of the water source; The resistance is how hard whatever you attach to the spigot, resists water flow (So if the water just splashes onto the ground, all water's "wasted" and you have a short-circuit condition; If you have a sprinkler on the end of a hose, that's "some" resistance; If you put a sprayer head on the end of a hose & it's totally closed, that's "open circuit" or infinite resistance. (Sorta strange, we normally think in terms of "conductance" or how easy it is for water to flow!) We understand current fairly well though, from watching hoses & creeks & so on in real life. Power used in a component is defined as the voltage across that component, times the current through that component (that's an instantaneous power, or power rate, like gallons per hour.) Total power capacity would be that times hours it can source that much power flow, like gallons. Voltage is measured in Volts. Resistance is measured in Ohms. Current is measured in Amps. Power (Flow Rate) is measured in Watts, calculated 1 Watt = 1 Volt * 1 Amp or 10 Volts * 0.1 Amp or whatever. Power (Total Capacity) is measured in Watt Hours (1 Watt * 1 Hour). You'll see Amp Hours etc. on batteries, that number, multiplied by the nominal voltage of the battery, gives you a pretty good idea of the total power available in that battery. One common mistake is to confuse Power (Instantaneous Flow Rate) with Power (Total Capacity) - don't do it <G> Gallons per minute are not gallons available <G> Capacitors are voltage storage devices, sort of like a dam or "air hammer" tube, they tend to average out voltage. Some capacitors are "Polarized" (You want to make sure the voltage you apply to them matches their polarity, or they tend to do nasty things like blowing up or oozing their contents into your circuitry.) Inductors are current smoothing devices (sort of like a flywheel), they tend to smooth out currents. DC-DC Converters use inductors to modify one voltage into another through a variety of sneaky means; The important things are to know the peak current draw of your circuit, to have that much current (at least!) available in a DC-DC Converter, and to have smooth enough voltage output from the DC-DC Converter for your use. Efficiency of a DC-DC converter tells you how much power is lost in this conversion (Don't expect 100% efficiency, but more efficient IS better, that total Power Capacity of your battery is being wasted as heat in the converter, constantly, if you have a low efficiency converter.) Expect to see capacitors in that DC-DC converter to supply enough power to the DC-DC converter to run it, and to smooth out the resulting voltage(s). Diodes are one other common component, I'll introduce them because they're mentioned in the power bridge. There are several types; All have a "Cathode" and an "Anode", usually the Cathode is marked with a bar or stripe (For historical reasons, Cathodes are ofen marked K on circuit diagrams, Anodes as A.) Regular Diodes only let current go through them one way, electrons flow from Cathode to Anode. Not the other way at all, unless you over-power the diode with way too much voltage, then "bad things happen". Germanium Diodes are pretty obsolete (Used mainly in VU meters); They only take about 0.3-0.4 volts to "prop the valve open", but suffer from the problem that they tend to melt and fail at far cooler temperatures than Silicon diodes do. So let's avoid those. Silicon Diodes are what you usually see in diodes, or full wave bridges; these have a 0.7 volt forward drop (it takes .7 volts to "prop them open" so to speak.) Schottky Diodes are a slightly different construction style of diode, these have the advantage that they only have about 0.45 volts forward voltage drop (Remember that in bridging your batteries so you can run several in parallel, that .25 volts worth is taken "off the top", before any DC-DC conversion, etc., so though Schottky Diodes are usually slightly more expensive, I really like them for that job.) Another kind of Diode, Zener Diodes, act as a regular diode, plus, when you run them backwards, they'll let current flow after a certain "breakdown" voltage is reached. These are used to keep electronic parts safe from electrostatic discharge, to regulate voltages (somewhat inefficiently, but they do work), and so on. You want to make sure you don't exceed the power rating of Zeners, particularly (a 500mW 12 Volt Zener is only capable of a half-watt energy dissipation - that's 41.66mA - before it self-destructs, often as not shorting itself.) With all diodes, you do dissipate power in them, across that voltage drop, depending on how high the current through the diode is (For a big stereo power supply that pulls 15 Amps through a diode, you may well see the diode heat-sinked, to pull the power away from the diode & keep it from melting.) Common Equations: Ohm's Law (E=I * R) defines that E (voltage) = Current (I) times Resistance (R), the Volt, Amp, and Ohm are defined together here, and work pretty well together <G> If you know you have 12 volts across a component and are drawing 1/2 amp through it, you have a 6 ohm resistance, this way. (It gets more complex with inductances and capacitances, but I can only do so much in a few k of text! <G>) P = I * E defines that Power = Current times Amps. Enough for today, we have a lightning storm passing through & it's time to shut down the LAN for a while, so I don't lose any computers! Mark,Newhall wrote: > > OK, time for a Stupid Question (TM). > > I'm very interested in building a wearable computer, but my hardware > konwledge isn't quite up to speed; I've traditionally been more of a > software guy. I can swap out CD-ROM drives on my tower and so forth > without a problem, but amps/DC-to-DC/etc. are things I know of only as > hazy outlines, so to speak. > > Do you all know of any sites I can go to that cover this sort of thing, so > I can bone up on my knowledge? I'd (obviously) prefer something with a > computer focus rather than a generic electrical engineering bent, but > whatever you can think of would be greatly appreciated. > > I understand that I can't read a few webpages about watts and converters > and magically become a computer hardware expert, but I need to start > somewhere. :-) I'm currently building a keyglove > (www.wearables.ml.org/keyglove.html), but that's at the very limit of my > ability. > > Thanks, > > Gurney > > -- > Subcription/unsubscription/info requests: send e-mail with subject of > "subscribe", "unsubscribe", or "info" to
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