Can I put DC electronic loads in series and/or parallel to increase power dissipation over a single load?

Frequently Asked Questions (FAQs)SummaryOften, your load requirements will exceed the capability of a single N330xA load module. This article will explain how you may do this, and operations to avoid.QuestionCan I put DC electronic loads in series and/or parallel to increase power dissipation over a single load?AnswerSpecifically, a DC electronic load is a two-terminal electrical instrument that draws power from a DC source. Loads are used to test DC sources. Any device that has a source of DC output power, such as a DC power supply, a DC-to-DC converter, a battery, a fuel cell, or a solar panel, can have power drawn from it with an electronic load as shown here: Keysight's current family of stand-alone electronic loads is our modular N3300A family. Please see the link below in attachments for more information on these products: As an example, to test a fixed-output DC power supply that is rated for 20 V, 5 A, 100 W, you would connect the power supply output to an electronic load with ratings that are equal to or greater than the power supply ratings and that can draw a constant current from the power supply. Since the power supply is regulating the voltage (20 V), the load must regulate the current it draws from the power supply (up to 5 A). If your DC power supply is a constant current source, the load must be capable of drawing power while regulating voltage. You can set most electronic loads to draw power by regulating either constant voltage (CV) or constant current (CC). You can also set many electronic loads to regulate constant resistance (CR) across their input terminals. If you need more power dissipated than one load module can do by itself, can you "stack" them? In parallel, certainly. Multiple loads work well in parallel assuming you are using CC or CR modes. However, the third mode of load operation CV (constant voltage) does not allow parallel operation for the same reason you shouldn't put batteries or Zener diodes in direct parallel. One battery or Zener diode will try and "hog" all of the current, unbalancing and overloading that component. Same with electronic loads. However, in CC or CR mode, paralleling is easy. You connect them as in the figure below, and if you are in CC mode, for example, you control the loads separately for whatever current you'll need from each one. The model N3306A load module can handle up to 120 A and up to 60 V (600 W max). Let's say you have two of these loads connected to a 10 V source and you need to draw a total of 100 A (1000 W) from the source. Since each load has a max power limit of 600 W, at 10 V, each load could draw 60 A. To get to 100 A, you could program each one for 50 A, or even 60/40 if you like. However, if the power supply to be tested has a higher output voltage than a single electronic load can handle, you may be tempted to put multiple load inputs in series to accommodate the higher voltage. After all, you can do this with power supply outputs to get higher voltage….why not with loads? Putting electronic loads in series can cause one of the load inputs to be exposed to a voltage beyond its capabilities that could result in damage to the load. You are putting loads in series because a single load does not have a high enough voltage rating to handle the voltage of your DC power source. But since one of the load inputs could become a low impedance (nearly a short circuit) during test, all of the voltage from your DC source could appear across the other load input in series. There are several scenarios that can result in this destructive situation. To understand these scenarios, you first have to understand how an electronic load works. Loads work by controlling the conduction of FETs across their input terminals. The control is realized by using a feedback loop to adjust a measured level (such as the input current) so that it equals a reference level (such as the set current). When you put multiple electronic loads in series to accommodate higher voltage, one problem scenario occurs when you set both loads to operate in CC mode. You set the same current on both loads. The exact same current flows through both loads (see figure below), but due to small errors in the accuracy of the settings, the real set values will never be exactly equal. Therefore, one of the loads will be trying to draw a higher current (Load 2 in the figure) than the other load (Load 1 in the figure). Since Load 1 will limit the current at the lower value (9.99 A in this example), Load 2 can never attain its real set point (10.01 A in this example). So its internal feedback loop continues to tell the FETs to conduct more and more current until the FETs are fully on looking nearly like a short circuit. This results in nearly all of the power supply voltage appearing across the Load 1 input which can damage it. See what happens in the following figure: If you operate one load input in CC and one in CV, at first this looks like it will result in a stable operating point. However you have to think about how you get to that stable operating point. If you set the loads first before you connect the voltage, before the voltage is applied, the CC load is not satisfied (no current is flowing) so it goes to a short and the CV load is also not satisfied (no voltage is present) so it goes to an open. When the test voltage is applied, all of the voltage initially appears across the open CV load and can damage it. There are other procedures to follow that could temporarily result in a stable operating point (such as slowly increasing the test voltage if you have that ability), but if any fault condition occurs in any of the loads, they try to protect themselves by either turning the FETs on hard (a short) or opening the FETs. In either case, the large destructive voltage will appear across one of the loads in the series connection resulting in damage. The electronic loads are very good about protecting themselves during some overloads. Things like overcurrent, overpower, overtemperature, and reverse voltage are easily handled. The overvoltage protection circuit is set at a predetermined voltage level, which cannot be changed. If the overvoltage circuit has tripped, the module will attempt to limit the voltage level by drawing current from the dc source. The module limits the value of current drawn such that the resulting power is within the power rating. However, as noted above, the usual scenario results in module damage. Please don't attempt to operate the load modules in series!AttachmentsClick here for a copy of the N3300A data sheet as referenced above.