1 second UPS

"stefanv" schreef in bericht news:huydnbb8YeRLAAreRVn snipped-for-privacy@giganews.com...

Stefan,

This is exactly why I advised to refresh your basics. Maybe the confusion comes from the word "power". AFAIK it's used meaning both "force" and "energy" which are two different things. If you have 110V on your outlet and a fuse of lets say 16A then you have 110*16=1760W available. But that's not what you pay for. (Unless you have a minimum contribution.) You have to pay for the part of that 1760W * the time you use it. That's what you find on your bill.

So if you need 250W for half a second, you need 125Ws. (What's that in kWh? :) That's not much energy compared to the amount on your bill. Even a small battery can produce it - but not within half a second. To store it in a capacitor on the other hand requires quite a huge capacitor. Calculate for your own, using the CV/2 rule.

To summerize the lecture :)

Energy = Force * Time -> [J] = [W] * [s] (or [Ws] but not [W/s] !)

petrus bitbyter

No, the energy is 0.5 times the capacitance times the voltage squared. or C*V*V/2

It is practical to power a computer for a second from a large capacitor, but that capacitor would be perhaps somewhat larger than a normal PC power supply.

Chris

I think if you open up the PC power supply and find the terminals of the main input capacitor, these terminals could be brought to a shrouded (finger proof) insulated connector on the back of the power supply. You could then build a grounded box containing a bank of capacitors which would be connected essentially in parallel with the internal capacitor of the PC power supply. The larger capacitor could keep the supply running for a second or more. In order to avoid very large surge currents when the PC is first turned on, a high-power series resistor should be inserted so that the external capacitor charges gently. A diode across the charging resistor would allow the fairly large current to flow out of the capacitor when powering the PC.

Hence: a) my warning not to build it until and only when the dangers are fully appreciated since it is a potentially dangerous circuit, and

b) the series resistor which prevents any significant surge current above what the power supply would normally produce.

Chris

would

is

this,

The 'main input capacitor' will be at several hundred volts (depending on OP's location). This is therefore a very BAD idea, given the OP has little idea of what's going on in these supplies. This is without even considering how the input side of the power supply is going to react to the addition of several extra amp's of surge current.

Ken

...

like

For the amount of money it would cost to get a qualified tech to do a job like this safely (i.e., hack a Chinese PC power supply), you'd be better off to just get a good UPS.

Good Luck! Rich

Aw, need some refresh too. Forgot to square the voltage. Nevertheless the

petrus bitbyter

"petrus bitbyter" schreef in bericht news:4399ae34$0$24454$ snipped-for-privacy@dreader24.news.xs4all.nl...

Error: CV/2 should be CV^2/2

petrus bitbyter

A dangerous suggestion, and useless against multiple short outages. The power distribution system here attempts to reroute itself when there is a failure, but sometimes it can't. in the process the power will go off then back on every couple seconds till the controller decides that there is an overload, or shorted line.

Wiring inside switched-mode power supplies is certainly dangerous when worked on by an incompetent, but the capacitors are not significantly more dangerous than any other power wiring connected to the AC mains, except that one must remember to wait before opening the cover. Either source of power can be lethal and has about the same voltage. Rather than discouraging someone from taking up an interest in electronics I would encourage them to learn why something is dangerous so that they can become able to work safely with that technology. If that doesn't suit you, then pay someone else to do it all for you and sell it to you in a nice box.

If you read the OP's post, it sounds like the system he proposes is not inherently safer than what I suggested, and I believe that my suggestion is less complicated. If you believe that less high voltage wiring is safer than more high voltage wiring then it is possible that my suggestion is safer than the OP's suggestion. Either suggestion could be made as safe as the original PC power supply by putting it in a grounded metal box, and following the relevant standards for the construction of electrical equipment. If there are particular things that you think need to be pointed out from those standards, then I think that would be one constructive way of helping to ensure that any equipment constructed as a result of this thread would be safe.

On the other point, it sounds like the OP has an inverter that could power his PC during a long train of repeated power outages, but that the delay to switch over during the first outage is too long for the PC's power supply. I think that provided the inverter starts up on the first outage and does not switch back until the normal power has been restored and stable for a certain period of time, the PC power supply may indeed benefit from a larger capacitor.

Chris

doing stuff on output side of PSU voltages !!! ================================ mine idea: !

to computer-> 20pin conn. | | 2x 10A shottky_diode, few Ohm/few_watt Res. |-------I>]-------------\\/\\/\\/\\---| | |

->from--|--------[

Other drawbacks:

2): With my proposal, the regulating action of the switched mode power supply means that the output voltages will stay in spec whilst a significant fraction of the energy in the input capacitor of the PSU is used up because the duty cycle of the PWM chip adjusts to compensate for the decaying input voltage as the input capacitor is discharging, so adding capacitance on the input side is fairly effective. On the other hand if you add capacitance on the output side of the power supply, the output voltage starts to drop as soon as the power cut occurs, there is no regulating action during the power cut. This means that you have to add capacitors with a much higher energy storage capacity if you add them on the output side of the power supply. A smaller energy storage capacity would be needed if adding capacitors on the input side.

Still if you have an aversion to voltages above 100 Volts, and if you can afford some of those nice Epcos hundred Farad capacitors, then your scheme could work (minus the diodes of course).

Chris

I tried it & I must admitt, that is to toss it away ... Well, it (kind a) works (helps a bit), but caps should be much much greater! (much more than 10x bigger for a second!).

Hello The point is that you dont want a *qualified tech* ,you'd want to do it yourself.