Power Supply thoughts needed

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jonjonr6
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Power Supply thoughts needed

Post by jonjonr6 »

I have a couple new ideas about power my PC via 12v DC. The key to remember is that the PC needs to support sleep mode, and still need sto draw under about .1A.<p>First thought, I have an ATX power supply that has a very simple circuit design. I could remove the rectifier, bypassing the 120v AC input, and supply +60v and -80v DC from DC-DC converter or transformer(s). <p>Second thought, the PC uses only the +5v VSB when is sleep mode. I could use a 7805 to regulate a constant lead of the battery to keep it powered. I could then use a standard ATX power supply and a DC-AC converter (which I already have) to power it. A timer relay could interupt the +12v to the DC-AC so it is dead after the computer goes to sleep.<p>Any ideas or opinions? I am still working on determining exactly how much power the +5VSB draws for my system. I do not know how much power the modified ATX power supply would need.<p>Thanks,
Jon
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Joseph
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Re: Power Supply thoughts needed

Post by Joseph »

With yor first idea, one thing to be sure to check first is how the AC power input to your ATX is handled. Many computer SMPSs have a 120/240 switch. If yours has such a setting option, then the DC supply to the power MOSFETS will be 320 volts. If you try to operate it on +60, -80 volts, those MOSFETS will need to work with twice the current the design calls for and therefore, may have to dissipate 4 times the heat due to their channel resistance.<p>Your second way seems to make more sense. I am far from expert on the internal workings of computers though (or external either :eek: ). Take care.<p>[ June 18, 2003: Message edited by: Joseph Meisenhelder ]</p>
jonjonr6
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Re: Power Supply thoughts needed

Post by jonjonr6 »

Thanks Joseph,<p>Actually, I measured the voltage at the rectifier, which is where I got the +60/-80 ratings. It's a small/cheap power supply, but it works fine to power my AMD K6-2 500 processor and Socket 7 system board. A laptop hard drive will be used. No floppy, CD, or additional cards will be used.
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Joseph
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Re: Power Supply thoughts needed

Post by Joseph »

Ok, hmm...I was thinking the ATX power supply was using a MOSFET totem pole to drive the power transformer. It may more likely use another way like flyback.<p>I tend to like the possibility of running the ATX power supply off of the inverter. You may possibly be able to increase the efficiency of the inverter by passing its output though a semi-resonant circuit like a capacitor feeding into an isolation torroidal transformer. The capacitor would have to be something like a high voltage polypropylene motor starting type. These are just some further thoughts.
jonjonr6
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Re: Power Supply thoughts needed

Post by jonjonr6 »

I've had a chance to further test my system board. Some of the power outputs to the board do not turn off during suspend to RAM, so my idea of keeping the +5VSB live and turning off everything else during suspend mode will not work. Also, this board is too old to support hibernate. So, I'm back on my very first idea of building a simple power supply. I'm not really any sort of engineer, so I'm sure my design (when done) will be crude, but I think it won't be too hard to use 78xx and 79xx regulators and some power transistors to provide the power. I have a 1/4F and a 1F capacitor at my diposal to help smooth the power system of the car.
I'm not certain how to get negative DC voltages from a single battery though.
A crude power supply is fine, if I blow up the board, no big deal.
Thanks,
Jon
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Joseph
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Re: Power Supply thoughts needed

Post by Joseph »

The easiest way to get negative voltages is by means of a charge pump. You charge a capacitor, then pull its positive terminal low, and lastly capture the negative pulse from the negative terminal with a diode. Then you can run that voltage after filtering through negative regulators.<p>Did you know that mp3 players supposedly exist with an internal hard drive which can hold over 1000 songs? I saw one on a computer show on PBS.
jonjonr6
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Re: Power Supply thoughts needed

Post by jonjonr6 »

Thanks,
I'll do some searching for a schematic of a charge pump circuit.
I actually have a hard drive based portable MP3 player. I even have it wired into my car stereo. However, it has it's drawbacks, it's not exactly easy to use, ocassionally pops and clicks, the batteries are limited, and on. Simce I have some extra computer gear, I can use some of it to build a better player. I eventually hope to have touchscreens in my car to run navigation and video.
There are also car MP3 players with laptop harddrives inside an indash player. The nicest one is more like a CD changer, with a small box under the seat with a removable laptop hard drive. It comes with a USB cradle to connect to your computer. However, these units are very expensive for what you get. You can buy a brand new 2.4GHz computer for the same price.
Thanks again,
Jon
jonjonr6
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Re: Power Supply thoughts needed

Post by jonjonr6 »

I have a diagram to use a MAX1044 charge pump to take +9v and increase it to +33v.
http://www.geofex.com/circuits/+9_to_33.htm<p>This looks like it might be useful. However, the MAX1044 will only take 10.5v DC in, and I don't know how high you can increase the voltage considering the loss across the diodes. Would it be possible to use a "super charge pump" that will take +12v in and get it up to +60v out?<p>Thanks.
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Joseph
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Re: Power Supply thoughts needed

Post by Joseph »

For increasing voltage more than a couple of times, a converter using an inductor is simpler. But then, it is often more practical to use a transformer, which is two or more inductors magnetically coupled together.<p>If you just want to roughly multiply voltage, the easiest way I know is to use a half-bridge circuit like the one I uploaded to the location below. http://f1.grp.yahoofs.com/v1/0Jz5Pr5Gn2 ... ooster.GIF<p>[ June 25, 2003: Message edited by: Joseph Meisenhelder ]</p>
jonjonr6
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Re: Power Supply thoughts needed

Post by jonjonr6 »

Thanks for the design. However, someone said (in another topic) that transformers do not work with DC.
Are there transformers available that will take my 12v DC in, and give me a 1:10 boost? Are they available center-tapped to I can get a negative voltage too?
Thanks
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Joseph
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Re: Power Supply thoughts needed

Post by Joseph »

Transformers are a major device in most DC to DC converters. Those circuits basically chop the DC voltage, pass it through a capacitor to convert it to AC, feed it into a transformer to change the voltage, then pass it through one or more diodes to convert it back to DC. The voltage output will approximately be multiplied by the ratio between number of output turns on the transformer divided by the number in the input. So, if the input side, the primary, has 5 turns, the output side, the secondary, needs 50 turns to get 10 times more voltage out.<p>You can get the output to be the negative of the input by turning the rectifier around and charging the negative terminal of a capacitor.
jonjonr6
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Re: Power Supply thoughts needed

Post by jonjonr6 »

So what you're saying is, that to increase DC voltage, I'd need to use some sort of switching device to turn the DC into pulses, then pass that through a capacitor, then to a transformer (where the voltage is actually increased), then rectify it back into DC. Hence, I'm back to using my DC/AC invertor and a standard ATX P/S, which is what you recommended in the beginning, I think.
Well, I guess I'll have to play with some ideas. I still want to use standby mode, but I think it will be tricky to work that out.
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Joseph
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Re: Power Supply thoughts needed

Post by Joseph »

Yep, that is what it boils down to. What Dean may attest to is that there is not enough teaching in the US on basic circuit theory, but too much emphasis on using or maybe assembling computers.<p>Much of the lack of knowledge of circuit theory is traceable to the advent of surface mount components in packages with many tiny leads. The concept is ridiculous. If manufacturers are going to make such small components, they should assemble them into usable circuits with interfaces to the outside world that are proportionate to human physiology. Otherwise they should simply construct the entire product in one package with the few input and output leads needed for use.<p>Long live through-hole sized components. They still can be surfaced mounted if desired.
rshayes
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Re: Power Supply thoughts needed

Post by rshayes »

Since you are using an automobile electrical system as a power source, you will want to isolate the output power from the automative electrical system. This is best done by transformer coupling. Linear regulators and charge pumps working off of the 12 volt supply will not provide isolation.<p>You may need two separate power converters, due to the standby mode. A converter capable of several hundred watts output will probably require several watts or more to power its control and drive circuits. These circuits require about the same amount of power even if the output load is very low. Ten watts drawn from the AC line is negligable, but that same ten watts will discharge a car battery in a couple of days.<p>The power supply that you have can probably be used with a DC input. The input circuit is probably a bridge rectifier or voltage doubler with large filter capacitors. The advantage to this is that you don't have to design a multiple output power supply. That's already been done. The DC to DC converter may not even have to be regulated, since the power supply is already tolerant to the normal AC line voltage variations. This avoids feedback loops and stability problems in the DC to DC converter. Heathkit made severals models of DC to AC converter for operating equipment from battery supplies. These were push-pull designs with square wave outputs. Some of these might be available on eBay. In the 1960's, "QST" and other ham radio magazines had several articles describing how to build these supplies. Sometimes public libraries subscribe to these magazines, and either keep the back issues or have them on microfilm.<p>The standby supply will need to deliver 5 volts at low power with good efficiency. Small DC to DC converters are available, try the DigiKey and Mouser catalogs. They might also be available from surplus sources, such as All Electronics and similar companies.<p>The main supply will have to be switched on and off. Probably a relay is the simplest method. A relay meant for controlling headlights might be suitable if the current is not excessive. A drive circuit to operate this relay from the motherboard control signal will be necessary. An optical isolator is probably a good idea for this signal.<p>Modifying the original computer supply for 12 volt input might be possible, but it would require rewinding the transformer and a fair amount of circuit design.
bruinbear714
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Re: Power Supply thoughts needed

Post by bruinbear714 »

If you want to build your own, the simplest (and not so simple if you don't have a strong electronics background) method would be to get a PWM controller chip like the TL494, or SG3525 and a gate drive chip such as an IR2110 to drive your switching fets. Pull an old transformer off a big car amp, wind your own windings, and add supply caps. Walla. Cheap power supply.<p>The pwm controller chips even have a shutdown pin to which you can tie a remote line.
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