Re: the best power regulator
 

but for Iout wont it only be equal to the Iin of what ever if hooked to the output of the 7805, so the Iin would be equal to the Iin of the 7805 + the Iin of what ever is on the other side?

Re: the best power regulator
 

Manoel is correct about Iin and Iout being equal. You have a good point that they aren't exactly equal however. Iin of the 7805 is usually small compared to the load. I've seen people use the factor of Iin=1.01Iout (although that was for more efficient linear regulators then a 7805). I'd use something higher for safety, but it doesn't matter if your Iout is low.

Re: the best power regulator
 

The data sheet you gave claimed the output capacitor wasn't necessary for stability, only transients... So now I'm a bit confused... Perhaps you were refering to the input capacitors? But even then the data sheet says they are only necessary "far from the power supply". :confused:

NOTE: I'm not a EE, I'm not trying to be smart, I just want to be educated if I'm wrong :).

Re: the best power regulator
 

the regulator works by acting like a variable resistor. Depending on how much current your load is taking, it changes its resistance to keep the voltage steady.

So depending on how fast it can respond to changes in the load, it might become unstable unless you put the caps on both sides.

If the input is located within inches of a battery or a filtered output from a transformer and rectifier, then you might not need the input cap

but we are talking about what? 5 cents? why not put the caps on both sides, build the thing once and you are done with it.

Re: the best power regulator
 

Well the issue was more of: I don't understand why you would need seperate (parallel) capacitors for filtering transients and stability... Especially when the data sheet claims you don't need them for stability. I'm just a little confused :confused:

Re: the best power regulator
 

The default or standard caps for a 7805 are a 10uf aluminum electrolytic cap on both the in and out. Could you get by with out them? Depends what the regulator is driving. A pure resistive load like a light bulb wouldn't need them. Any digital electronics need a well filtered power supply. I always use at least a 100 uf on the output.

Re: the best power regulator
 

I know that... I said they were needed for transients (generated by digital logic and that). But what I don't get is having seperate capacitors for stability.

Re: the best power regulator
 

Guys,
I have to step in here and add a few things. As to the 78XX series of regulators, they are very sophisticated devices and include temperature compensation, voltage feedback and over current protection circuits. In power supply design using this series, keep leads on everything short so that series resistance and inductance don't cause variations in the output. The negative side of all capacitors, the regulator and the power common (the negative side of the bridge rectifier or transformer center tap if part of an AC supply) should all be tied together at one point. Since the 78XX series are active devices they can oscillate if not handled properly. Although your AP physics has taught you that capacitors in parallel add together, this is one of the cases where that is not exactly true. Different capacitor types act differently at high frequency than they do at low frequency. As such, use the cap type specified in the data sheet. It is not uncommon to find a large electrolytic in parallel with a small ceramic cap. The electrolytic supplies current to heavy loads and acts as a low impedance source at low frequencies while the small ceramic cap prevents high frequency oscillations and prevents phase shifts from existing from input to output. As a power device, a 7805 can become a very effective transmitter producing a 5 volt square wave of RF energy up into the 100kHz-1 MHz range that will drive you digital circuitry nuts. A big problem for designers using a 7805 from a 12 volts source is getting rid of heat. In the previous examples, drawing 350 ma, the device is being asked to dissipate 2.45 watts minimum. Without a heatsink, the little package gets very hot and the protection circuits cut in to protect the device. When breadboarding your design be careful around the tab of the case, it can get hot enough to burn the tender parts of your hand. Many designers will use the case of their project or the chassis as a heatsink.

Re: the best power regulator
 

Al speaks of power supply filtering black magic :^)

the reason why real caps 'dont exactly add in parallel' is because every cap has some series resistance and series inductance.

At low frequencies these second order parameters have no effect. But at higher frequences they become significant.

This is why a small value cap performs better at high frequencies, because it is physically smaller than, lets say, a big old 1000µF 16V electrolytic can.

A small cap will have much lower series resistance and inductance.

Hope this makes more sense now.

Re: the best power regulator
 

Ah ok, that confirms what I read yesterday when I decided to just go find out. Thanks :).

EDIT: And sorry for kind of derailing the thread :o.

Re: the best power regulator
 

The cap issue is of minor concern, the real problem is the heat that running on a 12 volt supply will generate. What is the regulator driving. How many ma? Is this for a first robot project or something else? There are many advantages to using a separate 7.2 or 8.4 battery pack for the electronics and let the 12 volts drive the motors. For a First project, the PWM ports can supply 7.2 volts to an addition electronics board. This comes from the nicad battery that is plugged into the robot controller. The cmu cam was supplied this way. This power has to be shared with all devices plugged into the PWM outputs. The details are in the robot manual.

Re: the best power regulator
 

but how ma can the pwms supply?

Re: the best power regulator
 

Ops, I thought I saw this in the robot manual. The Analog power pins can provide a total of 1 amp 5 volts regulated. It is fused. First recommends a limit of 50 ma per pin. This is in the first 2005 robot manual. The only place I found a reference to the power available to the PWM pins is in the IFI question and answer for the RC. A total of 4 amps at 7.2 volts is available on the PWM pins which is supplied by the backup battery. There is no mention if it is fused. The victors do not draw from the pwm pins. I don't know about spikes. I believe I read that the radio modem is powered by the backup battery. The CMU camera uses this power but I didn't find a total amp draw for the camera and boards. I also couldn't find an amp dram at 7.2 volts for the hitec servos. Still there would seem to be plenty of power at 7.2 volts available for most additional electronics. A 5 volt regulator would be allot happier if supplied by the PWM pins instead of the 12 volt main feed. One caution- there should be only 1 ground path. On the CMU the ground is threw the rs232 port.

Re: the best power regulator
 

who sed i am using it with the robot controller?

i want to use them for my custom robot i have built

Re: the best power regulator
 

Before things get too far here, a 78XX series regulator needs at least 2 volts across the regulator to operate as a stable voltage source. (this varies as a function of load current) Using a 7.2 volt battery as the input will not give you the regulation you might need for your custom circuit. When the input drops below the 2 volt differential, the output goes down as well. (usually in bad and unpredictable ways.) Although the backup battery on the RC does supply power to the servo outputs, it is also supplying a variety of other things like the main processor, radio modem and other keep alive circuitry ( and this year the CMU and all of it's related servoes, about 250 ma max.) so I don't suggest using that power on an FRC robot using an RC. In addition, the robot electrical manual requires that custom circuits be fed through their own circuit breaker off the 12 volt buss.

Since this application does not use the RC all of that is mute. The cap issue is not a minor concern when using digital custom circuits. The capacitor recomendations are part of the output impedance set point for the regulator. As the frequency increase so does the output impedance. The caps stabilize the output impedance and prevent the regulator from being influenced by rapidly changing loads. Depending on the type of digital circuits you are using (TTL, CMOS) they have different current usage parameters and anything that affects the current provided affects the switching points and the outputs. As Ken said above, we are talking some black magic kind of electronics at this point, follow the data sheets.

Finally, both the Victors and the Spikes draw primary operating power from the power input pins, but the control signals (PWM and relay control) are supplied by the 5 volt regulator on board the RC.