Wednesday, February 29, 2012

On DC Source Voltage and Current Levels and (Compliance) Limits Part 2: When levels and limits are not the same

In part 1 my colleague made a good argument for current and voltage level and limit settings actually being one and the same thing and it was really just a case of semantics whether your power supply was operating in constant voltage or in constant current mode. I disagreed and I was not ready to admit defeat on this yet. Now is my chance to explain why I believe they’re not one and the same thing.

I have been doing quite a bit of work with source measure units (SMUs) that support multi quadrant output operation. They in fact feature (constant) voltage sourcing and current sourcing modes of operation. This tailors the operation of the SMU for operating as a voltage source with a set current compliance range or conversely as a current source with a set voltage compliance range. Right at the start one difference is the set up conditions. The output voltage or current level is set to zero while the corresponding current or voltage limit is set to some value, often maximum, so that the DC source accordingly starts out in either constant voltage or constant current for normal operating conditions.

Some products feature a programmable or fixed power limits. In one product I know of, the programmable power limit acts accordingly to override and cut back the either the voltage limit when set for current sourcing, or the current limit when set for voltage sourcing. It does not do this in true real-time however. It cuts back the limit based on the level setting, as a convenient means as to help prevent the user from accidently over-powering the DUT. Alternately many auto-ranging output DC power sources exist that provide an extended range of output and voltage for a given output power capacity. They incorporate a fixed power limit to protect the power supply itself from being inadvertently overloaded, as shown in Figure 1. Usually the idea is for the user to stay below the limit, not operate in power limit. The point here on these examples is that the power parameter is an example of being a limit but not really a level.

Figure 1: Auto-ranging DC power supply power limit

More to the point is some SMUs may incorporate two limits to provide a bounded compliance range with specified positive and negative limits. Not all DUTs are passive, non-reactive devices. As one illustrative example a DUT may be the output of 2-quadrant DC voltage source which you want to force up or down, within limits, or a battery you want to charge and discharge at a fixed rate, with your test system DC source. This set up is illustrated in Figure 2.

Figure 2: Test system DC source driving the output of a DUT source

Figure 3 shows the constant voltage or voltage priority output characteristic for one particular SMU having two programmable current limits. Clearly both limits cannot also be the current level setting as you can only have one level setting. For the case of the external voltage source load line #1 (not all load lines are resistances!), when SMU voltage is less than the DUT source voltage (VEXT1 load line), the current is –ILIM. Conversely when SMU voltage is greater than the DUT source voltage (VEXT2 load line), the current is then +ILIM. In the case of the battery as a DUT this can be used to charge and discharge the battery to specified voltage levels. This desired behavior is achieved using voltage priority operation. Current priority operation would yield very different results. Understanding the nuances of voltage priority, current priority, levels, and limits is useful for getting more utility from your DC sources for more unusual and challenging power test challenges.

Figure 3: Example of a current priority output characteristic driving a DUT voltage source

In closing I’ll concur with my colleague, in many test situations using most DC sources the voltage and current levels and limits may not have a meaningful difference. However, in many more complex cases, especially when dealing with active DUTs and using more capable DC sources and SMUs, there is a clear need for voltage and current level and limit controls that are clearly differentiated and not one and the same! What do you believe?

Wednesday, February 22, 2012

On DC Source Voltage and Current Levels and (Compliance) Limits Part 1: When levels and limits are one and the same

I was having a discussion with a colleague about constant current operation versus constant voltage operation and the distinction between level settings and limit settings the other day. “The level and limit settings are really the same thing!” he claimed. I disagreed. We each then made ensuing arguments in defense of our positions.

He based his argument on the case of a DC power supply that has both constant voltage and constant current operation. I’ll agree that is a reasonable starting point. As a side note there is a general consensus here that if it isn’t a true, well regulated constant voltage or constant current, whether settable or fixed, then it is simply a limit, not a level setting, end of story. He continued “if the load on the power supply is such that it is operating in constant voltage, then the voltage setting is the level setting and the current setting is the limit setting. If the load increases such that the power supply changes over from constant voltage operation into constant current operation then the voltage setting is becomes the limit setting and the current setting becomes the level setting!” (See figure 1.) He certainly has a good point! For your more basic DC power supply that only operates in quadrant 1 capable of sourcing power only, the current and voltage settings usually interchangeably serve as both the level and compliance limit setting, depending on whether the DC power supply is operating in constant voltage or constant current. The level and compliance limit regulating circuits are one and the same. Likewise with the programming, there are only commands to set the voltage and current levels. There are not separate commands for the limits. I might be starting to lose grounds on this discussion!
Figure 1: Unipolar single quadrant DC source operation

However, all is not lost yet. The DC power supply world is often more complicated than just this unipolar single quadrant operation just presented. Watch for my second part on when the levels and limits are not necessarily one and the same.

Friday, February 17, 2012

The economics of recharging your toy helicopter

While on a business trip visiting customers in Taiwan back in December, I got a toy helicopter as a thank-you gift from one of my coworkers (thanks, Sharon!). This toy helicopter is fun to fly and is surprisingly stable in the air.

Flight time is about 7 minutes, and the battery recharge time is about 40 minutes. It can be recharged from a powered USB port by using a wire that came with the toy that has a USB connector on one end and the helicopter charging connector on the other end. Or, it can be recharged from the six AA alkaline batteries inside the handheld controller via a wire that exits from the controller. Thinking I did not want to prematurely drain the controller batteries, I typically used the USB charging method by using my iPad’s 10 W USB power adapter plugged into a wall outlet. So I got to thinking about which charging method was more economical: charging from a wall outlet or from the batteries. Luckily, I have test equipment at my disposal that can help me answer that question!

Recharging using AC power via USB power adapter
Using one of our Agilent 6812B AC sources, I captured the AC power used during a recharge cycle. I used the AC source GUI to take readings of power every second for the charge period and plotted it in a spreadsheet (graph shown below). I found that the power consumed started at about 2.2 W and ended at about 1.2 W roughly 41 minutes later. The energy used during this time was 1.1 W-hours. Where I live in New Jersey, the utility company charges about 15 cents per kilowatt-hour, so 1.1 W-hours of energy used to charge the helicopter costs fractions of a penny (0.0165 cents = US$ 0.000165). This is basically nothing!

Recharging using controller battery power
To analyze the current drawn from the controller batteries, I used one of our Agilent N6705B DC power analyzers with an N6781A SMU module installed. I ran the battery current path through the SMU set for Current Measure mode and used our 14585A Control and Analysis software. I captured the current drawn from the six AA batteries in the controller during the helicopter recharge cycle. These batteries are in series, so the same current flows through each of the six batteries and also through the SMU for my test.

For the recharge period (about 43 minutes using this method), the software shows the batteries provided 173 mA-hours of charge to the helicopter. A typical AA alkaline battery is rated for 2500 mA-hours, so that means I would get about 14 (= 2500/173) charge cycles from these six batteries. If you shop around for high-quality AA batteries, you might find them for as low as 25 cents per battery. Since the controller takes six of these, the battery cost for the controller is $1.50. If I can recharge the helicopter 14 times with $1.50 worth of batteries, each recharge cycle costs about 10.7 cents (= US$ 0.107). This is 650 times more expensive than using the AC power method, so I will continue using the wall outlet to recharge my toy helicopter! How about you?
Note that with the AC power recharge method, you pay for the kilowatt-hours you consume from your utility company. With the controller battery power method, you pay for the mA-hours you consume from your batteries. Consider this: if you choose the AC power method, you will save US$ 0.106835 per recharge cycle. That means after just 2.81 million recharge cycles, you will have saved enough money to buy yourself a real helicopter worth US$ 300,000, so you better get started now!