n00b power supply questions! - Page 1
Forum section: electronics / beginners

author: djcrunkmix
author_rank: Contributor
timestamp: March 25, 2011, 06:32:05 pm
content: Hi guys,First post. They say there aren't any stupid questions but I think these are pretty stupid. I am really new to electronics and I have been looking for a decent lab power supply to start building circuits.1) Is the current "drawn" or "pushed"? Lets say I connect a circuit with a total resistance of 10 ohms between +12V and GND. So that means, according to the Ohms law, the circuit will "draw" I = V/R = 12/10 = 1.2A of current. So when I design the circuits, do I need to make sure the path of lease resistance is at least 10 ohms if the power supply can only provide 1.2A max? I understand constant voltage power supplies where the voltage remains the same, but the current "drawn" depends on the circuit connected to the power supply. But, what about constant current power supply? What if I want to make sure that the voltage is +12V in addition to throttling the current to 500 mA (for example)?2) Resistors are rated as 1/4 watt, 1/2 watt, etc. So, if I have a power supply capable of supplying 10A and connected to a 12V circuit with a total resistance of 10 Ohms (which yields a total current draw of 1.2A); what rating resistors should I use in the circuit? I know that power is V*I and that should equate to 12V * 1.2A = 14.4 W. Do I need to use 14.4 watt resistors or is it the voltage across individual resistors that is used to determine the wattage rating in an arbitrary circuit?3) I know ATX power supplies can supply insane amounts of current. What safety precautions should I take? What if I accidentally short out the +12V and GND lines on my circuit board? Is there a way to install a big ass variable power resistor to throttle the max current supplied by the ATX PSU?4) I am incredibly confused with electronics safety...I have taken college courses on electronics as a mechanical engineering student, but they NEVER taught us about personal safety. I know that a few milliamperes of current can kill a human being. I guess the resistance of the skin/body is enough to handle low voltage devices (like 12V) so that the resulting current isn't huge. But, at what voltage should I be very careful? (24? 30? 48? 120 V?). When working with ATX power supplies, can you die if you touch +12V and GND lines with dry hands? What about a car battery? So this ties back to the question of whether the current is "pushed" or "drawn"? Or may be I am an idiot5) I have a Fluke 87 multimeter with 10A fuse. So, if I connect it in series between the +12V and GND lines from a ATX power supply that supplies 30A, will it burn out the fuse?

author: Neilm
author_rank: Super Contributor
timestamp: March 25, 2011, 07:22:31 pm
content: Well, we were all new once.1) Current is drawn from a supply. Assuming that you have bought a bench PSU rather than tried to design one, the PSU will output a voltage and the circuit will draw the current. If the current the circuit draws is too much for the supply, the supply will reduce the voltage until the current draw is the maximum the supply can produce. Therefore you don't have to design the circuit to limit the current draw.2) Resistors have two ratings - both power and voltage.2a) Power. The power rating limits the current that can flow through a resistor. Therefore, you should specify the power of the resistor from the current expected and the resistor value. After all, you may be wanting to measure the current flow, so you would trade the resistance against the power dissipated by the resistor to allow you to get the best voltage across the resistor to measure.Figuring out the power from current and resistance can be done with a quick bit of algebra. As you pointed out, P=V*I. As V=I*R, replacing the V in the first equation gives you P= (I*R)*I which simplifies to P=R*I^2. (Power = resistance multiplied by the current squared.2b) Voltage. Each resistor has a maximum voltage it can handle before it breaks. Generally, this is in the order of 200V for through hole components so will probably be irrelevant for a n00b.3) See point 1 – the PSU should be designed to survive a short circuit on the output.4) The value that authorities regard as dangerous voltages depends on the conditions, if you are stood in a wet field it is very low (16V springs to mind but that may be for animals in that field). It is also dependent on how much power is available at that voltage. If you are hit by an audible static spark the chances are that was more than 8kV – however there was very little energy there to cause problems. In dry conditions, I would say that anything over 50V presents hazards. A car battery represents a virtually unlimited current supply so this is also a potentially dangerous situation - it is possible to melt a spanner if it shorts out the terminals.5) If you have set it to measure current then yes you will blow the fuse – a fuse is designed to break when too much current flows through it.YoursNeil