Tag Archives: parts

Picking The Right Output

The other day I ran across a publication from Allegro MicroSystems which was filled with an extensive list of terms and definitions for IC outputs. For instance, do you know the difference between a bipolar and a unipolar output? Well, a bipolar design allows the output to both sink and source current via controlled connections to power and ground. A unipolar output, however, can either sink or source current, but it cannot do both.

Now I don’t know about you, but I frequently confuse NPN and PNP outputs. I guess I just don’t work with these terms that often. Well, here’s one way to keep track of the difference: NPN outputs connect to the Negative supply while PNP outputs connect to the Positive supply. So just remember “N” for negative and “P” for positive! Take for example the following NPN sensor connected to a microcontroller (MCU):

NPN Sensor Wiring

The actual sensor circuitry is not shown here, only the output transistor. Now in this case, we have a unipolar output (the “Signal” line) which can only sink current to ground. In other words, this NPN sensor either connects the output to the negative supply (ground) or lets it float. In this configuration, however, the output does not float but gets pulled up to the positive supply line (V+) by a pull-up resistance. In this way, the input to the MCU is always either high (V+) or low (Gnd), but never in between (an undefined state).

For completeness, here’s one example of an actual PNP output used on a light curtain:

PNP Output ExampleThis may be a little tougher to interpret at first. However, there is one transistor directly connected between the positive supply rail (the topmost line) and the output (labeled OSSD). This means that the output may be connected directly to the positive supply as expected with a PNP configuration. This particular device also includes a pull-down resistor of 2.2kΩ. The difference here is that the pull-down may not always be connected to ground. That extra lower transistor can be disabled, allowing the output to float.

Anyway, check out that document and save it someplace – it might come in handy!

The $868.73 Op-Amp

Yes, you read that right; this here is an $868.73 operational amplifier, the Apex PA50A:

Apex PA50 Power Op-Amp

It doesn’t look like much, does it? Well at only ~1.5″ square, it’s not much larger than most power transistors. However, I can guarantee you it’s impressive. Most impressive. This is a power op-amp, which means it’s designed to deal with high currents and voltages. How high, you ask? Try 40 amps continuous (100A peak) at up to 130V. But you’re going to want a heat sink – this little guy may dissipate up to 400W internally.

JEDEC MO-127 HeatsinkNow sadly, these pictures aren’t mine, as I do not have a PA50 of my own. I also doubt I’d ever buy one (I just like to look at crazy-expensive parts on Digi-Key). However, this would be a fairly useful device. Normally op-amps are used at small-signal levels (from a few millivolts up to a few volts), and as such are only useful for amplifying sensors, performing filtering, etc. But with 40A and 130V to play with, you could drive multi-kilowatt speaker systems, go-kart motors, high-voltage transformers, etc (assuming stability with inductive loads). This baby would also make one serious output stage for a function generator, although its gain-bandwidth product is only 3Mhz. Interestingly, the datasheet lists “semiconductor testing” as the only potential application. Come on guys, I think a little more creativity is in order for such a product!

Now of course, you’re going to need one or two fairly beefy DC voltage sources to power the PA50, but I imagine they’ll cost you less than $800. And just what’s so special about this op-amp that drives up the cost so dramatically anyways? Frankly, I don’t know. Take a look at the following “equivalent schematic” provided in the PA50 datasheet:

PA50 Equivalent Schematic
Although this schematic doesn’t give values for individual components, we know that the final output MOSFETs (Q5,Q7,Q20,Q22) need to be rated for at least 100A (peak) and 130V. Well I’ve spec’d transistors like this in the past, and they’re not terribly expensive – perhaps $6-8 per device (double that if you want something really special). Even if all of the transistors in this package cost $10, that’s still just $220. Maybe there’s something costly about laying out all of this hardware in a single package? I’m certainly no expert in semiconductor design… Or perhaps the actual device is encased in 24k gold and diamond-studded? No, I guess they’re just trying to recoup development costs.

By the way, if you’re looking for high voltage instead of high current, try the Apex PA89, rated for ±600V output. It’s only $885.94 plus tax & shipping from Digi-Key. And don’t worry, I’m sure no one will question such a purchase. 🙂

When Regulators Just Won’t Cut It

Now here’s a cool part that anyone interested in electronics should know about: the V-Infinity V78xx. These small devices are DC-DC buck converters which can source up to 2A and are designed as drop-in replacements for TO-220 package linear regulators:

V-Infinity V7803-500

Why use these over standard regulators? Efficiency. A standard linear regulator reduces a given input voltage while maintaining roughly the same current from input to output. Thus, the efficiency of such a device is approximately equivalent to the output voltage divided by the input voltage. So if you wanted to regulate a 4.75V source (three AA batteries, for example) down to 3.3V, your best-case efficiency would be 3.3/4.75 = 69%. In reality this number will be slightly lower because power is consumed by the regulator’s control circuitry. Either way, this figure is pretty poor, and leads to significant power loss at high load. The following graphs compare the efficiencies and losses of a standard regulator and the V-Infinity V7803-500 (where Vin = 4.75V and Vout = 3.3V):

Efficiency Comparison (Vin = 4.75, Vout = 3.3)Power Loss Comparison (Vin = 4.75, Vout = 3.3)These results look pretty impressive to me. If we’re pulling 500mA at 3.3V, the losses in the V-Infinity converter are only 183mW – just a quarter of the 730mW lost by the regulator. What this means: no heat sink required.

So what’s the catch? Price. A single 3.3V linear regulator can be had for just one of your dollars. The V7803-500, at the time of this writing, costs $6.12. So is it worth getting? Well, that depends on your application. If we’re talking battery power, these devices may greatly extend the life of your device. If your project is running on mains power, perhaps regulators are fine, but DC-DC converters could help save the planet. The power is yours!