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| Thread ID: 97972 | 2009-03-06 03:04:00 | Calling electronics experts | pcuser42 (130) | PC World Chat |
| Post ID | Timestamp | Content | User | ||
| 753873 | 2009-03-06 03:04:00 | I'm starting to ease myself into building simple circits, and my first project is a USB battery (a USB port hooked up to some batteries :D). I've seen the many 9v batteries and 7805 voltage regulator ones, however Dad suggested I use AA batteries instead, as they should last longer. I'm using rechargables for this, as the original idea was that I wouldn't need regulators, as 1.2v*4=4.8, just enough for USB (5v with a 5% tolerance). However, my NiMH batteries give out not 1.2v, but 1.35v! :annoyed: This comes out to 5.4v, which is too high. So how would I drop the voltage? I know I could use a resister, but the voltage would drop as the batteries drain, and a 7805 requires at least ~7v to operate. :( |
pcuser42 (130) | ||
| 753874 | 2009-03-06 03:32:00 | You sure that 1.35v isn't just a surface charge? | ubergeek85 (131) | ||
| 753875 | 2009-03-06 03:34:00 | I also measured the voltage of the USB port itself, which did come out to 5.4v (I actually measured that first). | pcuser42 (130) | ||
| 753876 | 2009-03-06 04:05:00 | Will you be using an old computer's USB ports? The official USB spec (if i remember correctly) says that by default it provides 100mA. If the device requires more, it must properly ask the OS for more current (up to 500mA i think). I dont think devices are supposed to just grab 500mA. I'm not sure you'd have to check. It may also be unrelated to what you are building... Its why some USB devices (cup warmers, fans etc) which do not have any controllers in them can ruin usb headers or the motherboard itself... |
utopian201 (6245) | ||
| 753877 | 2009-03-06 04:20:00 | I don't think 5.4 volts will be any problem at all so there is no real need to drop that tiny over-voltage. Modern electronics is very tolerant of minor supply variations so don't try to over-complicate your initial project. A 9 volt battery and 7805 is overkill, or what we in the trade might call "creeping elegance". Cheers Billy 8-{) 44 years in electronics and still going strong! |
Billy T (70) | ||
| 753878 | 2009-03-06 04:27:00 | Will you be using an old computer's USB ports? The official USB spec (if i remember correctly) says that by default it provides 100mA. If the device requires more, it must properly ask the OS for more current (up to 500mA i think). I dont think devices are supposed to just grab 500mA. I'm not sure you'd have to check. It may also be unrelated to what you are building... Its why some USB devices (cup warmers, fans etc) which do not have any controllers in them can ruin usb headers or the motherboard itself... I'm using a backplane, simply because it can be screwed onto something. :p I don't think 5.4 volts will be any problem at all so there is no real need to drop that tiny over-voltage. Modern electronics is very tolerant of minor supply variations so don't try to over-complicate your initial project. A 9 volt battery and 7805 is overkill, or what we in the trade might call "creeping elegance". Cheers Billy 8-{) 44 years in electronics and still going strong! I don't want to risk my MP3 player. :p |
pcuser42 (130) | ||
| 753879 | 2009-03-06 04:44:00 | I don't want to risk my MP3 player. :p Measure a few plug-packs and you'll soon get over that worry. Your MP3 player won't die on 5.4 volts! My MP3 player gets 5.4 volts from the 5 volt mains plugpack that came with it, and 6.4 volts into the 5 volt external power socket from the four AA alkaline portable external battery supply that also came with it, until the cream goes off the batteries. That is a 28% over-voltage, no problem. You are talking about 8%, if that. Get the picture? Electronics is more robust and voltage-tolerant than you think, it is spikes that kill, not minor over-voltage excursions. Cheers Billy 8-{) |
Billy T (70) | ||
| 753880 | 2009-03-06 10:22:00 | not all nihms are that high are they?....... or doesnt the 1800 2400 stuff affect the voltage | ZapperBoy10647 (11988) | ||
| 753881 | 2009-03-07 00:21:00 | I'm starting to ease myself into building simple circits, and my first project is a USB battery (a USB port hooked up to some batteries :D). I've seen the many 9v batteries and 7805 voltage regulator ones, however Dad suggested I use AA batteries instead, as they should last longer. I'm using rechargables for this, as the original idea was that I wouldn't need regulators, as 1.2v*4=4.8, just enough for USB (5v with a 5% tolerance). However, my NiMH batteries give out not 1.2v, but 1.35v! :annoyed: This comes out to 5.4v, which is too high. So how would I drop the voltage? I know I could use a resister, but the voltage would drop as the batteries drain, and a 7805 requires at least ~7v to operate. :( check out the DIY show over at systm. they did a good show about making USB chargers. revision3.com it will depending on what your charging as to if you will have damage from 5.5V but in most cases you will be OK. if you are charging IPOD stuff you may need to setup a few "trick" resistors onto the data lines, the IPOD need a small voltage on the data lines before it will charge. the best design will be a DC-DC up converter, this is a lot more work to build, but the results are very good. 3V in and 5V out, even works at 2.7V in from rechargables. |
robsonde (120) | ||
| 753882 | 2009-03-08 08:43:00 | If you're still keen to drop the voltage, place a silicon diode in series (in the forward bias direction) eg 1N4001. It has a forward voltage drop of 0.6 volts. Diodes are a great device for dropping voltages. |
porkster (6331) | ||
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