I am an AP Physics student in high school, and I am in a group that is doing a project that will involve a bank of capacitors taken out of used disposable cameras hooked up in parallel. We need to charge the capacitors. My friend had an idea of charging them with a power supply taken out of a computer. I am wondering: is this safe and will it work? The power supply is rated at 115 and 230 volts; the capacitors are averaging 100 micro farads and 330-440 volts.
Electronic – Charging capacitors with computer power supply
capacitorpower supply
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stevenh already said it pretty good, but most likely they are decoupling capacitors.
Decoupling capacitors, or bypass capacitors, are capacitors meant to smooth power flow into specific parts of the circuit or into specific ICs. Changing power demands will create a "sag" on the power supply as it changes to meet output current demands. This pulls down the voltage. These capacitors will act as "local storage" to the load during a transient event that effectively masks the sag on the power supply to the load being bypassed/decoupled.
In a very dumb downed way, think of it like a pipe. One end is your power supply, and the other end is your load. The power supply adjusts itself to supply what the load is demanding. If the load changes, it might temporarily take enough water (power) out of the pipe to the point where the pipe isn't entirely full. The pipe not being full is the equivalent of your voltage sagging. This is what happens all the time on a power supply... load changes, and the voltage sags slightly as the power supply tries to supply enough current to meet demands... then eventually the voltage comes back up once the power supply has changed its output current to meet demands.
Now, a decoupling capacitor is like adding a big tank on top of the pipe. When the pipe is full... the capacitor can't empty any of its water out. However, when the load gets big enough and the power supply can't supply it quick enough... the tank lets some of its water out to keep the pipe full until the power supply can supply the given current.
As far as why they are different values, different parts of the circuit will require different amounts of power. Usually you'll see big caps (in the tens of microfarads, in this case, those big 100uF ones) near the power supply output itself... I usually see this referred to as "bulk"... this is for really big transients that pull a lot of power. Smaller values are for things with smaller current draws.
There's also some math behind the capacitance, I believe, in regards to how fast the capacitor can give up its energy for a transient event. Smaller capacitance being better for high-frequency transients, etc.
Battery with nominal voltage 12V usually is charged with a little higher voltage. If this is lead-acid battery - that voltage should be 13.8 - 14.4V.
Computer power supply is definetly not designed to be connected to battery. Partially discharged 12V battery can have voltage higher than 12V. If you connect it to computer power supply - you may feed power supply with energy. Power supply will "see" too high voltage on its output and will try to lower it to 12V. Depending on design of that power supply - bad things can happen.
You would need some circuit to limit current or some battery charging circuit anyway. Depending on battery type and size - you will probably damage battery or power supply. Battery can even explode, so never try charging when you can't measure current.
Many (if not all) computer power supply units can't work without some minimum load. You shall not power on computer power supply without load.
If you have no multimeter and no experience with electronics at all - I would recommend you to buy battery charger for that specyfic battery type (chemistry) and size.
Best Answer
A computer power supply is really not suitable for charging photo flash capacitors to near working voltage. It may be safe, if you use a series resistor to limit the current and take care as to the polarity(!), but it won't charge them to anywhere near full voltage, and since energy is proportional to voltage squared, the energy storage will be very low.
Charging photo flash capacitors to near working voltage, particularly in parallel or series groups, is not a suitable project safety-wise for high school students, particularly unsupervised, no matter what some of us may or may not have survived in our youth.