BENCH POWER SUPPLY - Help!

[Andy A.]

According to IFI's published data, here (.pdf), the typical draw of the RC is between .75 and 1.5 amps, likely very dependent on the radio's draw. So far as I know servos are still powered by the back up battery. Maximum voltage is 15, and minimum is listed as 7, although I've noticed that radio performance degrades quickly as voltage drops below 12. If you operate with a tether then the OI will also be powered by the RC.

The power supply you've got should be capable of running the RC easily. The RC will draw exactly the same current for a given voltage regardless of the supply. However, as others have noted, switching power supplies are not always the cleanest sources, and the high current rating of this supply suggests that it's not intended to power sensitive micro electronics like that in the RC. If possible it would be nice if you could use a 'scope to look at the output and ensure that it is 'clean' and the supply is calibrated. Failing that a multimeter will at least ensure that the voltage it creates is in fact what it says it should be.

Most teams power the RC using one of the supplied batteries while testing. I would recommend doing the same, and using a fast acting fuse or breaker with a 3 to 5 amp rating, regardless of the supply, to safe guard both the RC and the supply. A charged battery should be capable of running the RC for several hours.

-Andy A.

[Al Skierkiewicz]

The Radio Shack power supply is just fine for most of your experimenting. Please keep in mind that this supply may have a problem starting some motors. You might be able to power the RC and the compressor and you might be able to run the Globe motor and possibly the Fisher Price but trying to turn the Chalupa (CIM) motors might prove to be too much current. Remember that a non moving motor is in stall and that means 133 amps for the small and 96 amps for the large Chalupa motors. If you try, the power supply will shut down on high current and the reset may require a power off to restart.

As always, check the polarity of your wiring, twice, before applying power. Many of the electronic devices cannot stand a reverse voltage.

[EricVanWyk]

DO NOT POWER ANY BIG MOTORS OFF OF THIS SUPPLY.

The key point about your bench top supply is that it is a switching regulator. This means that its output looks like a diode followed by a capacitor. Therefore, it will only supply power, it will not sink power.

This is all well and good for powering your control system, as it (should) never attempt to push power back into the supply. However, remember that a DC motor is also a generator. If you were to run a heavy motor full forward and then stop, the energy that is held in the shaft will get pushed back through the victors to your supply. The capacitor will attempt to soak that current, but its voltage will shoot up. The diode will then be reverse biased, effectively eliminating your supply from the circuit.

The end result is that your 12V rail just jumped to some unknown value. If this new voltage is high enough, badness 10000 will occur.

I haven't managed to do this with the KOP, but I've done it before quite painfully on another project. Typically, this will occur while you are presenting.

If you have them laying around, a couple FHCs (Freaking Huge Caps) can help mitigate this, as well as handle the kick over problem.

[Al Skierkiewicz]

Erik,
You are right that switching supplies are a little more unforgiving of motor loads, but the output circuit is essentially the same as an unregulated linear supply. i.e. AC transformer winding feeds a diode rectifier which in turn feeds a low pass (capacitor) filter to smooth the DC voltage. The only difference is that the high frequency of the switching supply does not require the large (high value) capacitors needed to form a 60 Hz low pass filter since their operating frequency is much higher. Unfortuantely, feeding FHC is like starting a motor, the inrush current to charge the caps may trip the supply.

[mneary]

Why not use a battery? You can use the bench supply to keep it charged.