Why aren't electronic loads designed this way? - Page 1
Forum section: electronics / projects

author: Corporate666
author_rank: Unknown
timestamp: March 14, 2013, 06:46:59 pm
content: I was driving to work today thinking about a design I'm working on for a boost converter, and thinking of how the inductor works to regulate the current, etc.Then I was thinking about my 300-watt electronic load that has a giant heatsink for power dissipation.There are lots of constant current PWM controller/regulators out there... couldn't such a regulator just have the output shorted to ground, then let the regulator figure out the duty cycle. There would be very little heat generated in such a setup. I haven't taken apart my e-load to see exactly how they are creating the load, but I've seen small examples of such regulators which provide 25W+ dummy loads... so it seems applicable to commercial electronic loads.How come this isn't done?

author: AndyC_772
author_rank: Super Contributor
timestamp: March 14, 2013, 07:02:40 pm
content: In your boost converter, the inductor stores energy which is drawn from a power source, and transfers it into the load. Although ideally it doesn't get hot itself, the load does.The issue is simply that you can't cheat the laws of physics. If you want a device that can draw 300W from a power supply, then that 300W has to go somewhere - and all energy eventually ends up as heat.What you could do, perhaps, would be to have the dissipative element as a fixed resistor - say, 10 Ohms - and then use a switched mode converter to adjust the voltage across that resistor so it ends up drawing the correct amount of power depending on the input voltage and how the load has been programmed. The switched mode converter itself might not get hot, but the resistor still would.If you simply shorted the output of the switched mode supply, the energy would simply get turned into heat in the ESR of the inductor and other components.

author: mzzj
author_rank: Super Contributor
timestamp: March 14, 2013, 07:13:29 pm
content: Quote from: AndyC_772 on March 14, 2013, 07:02:40 pmYou can "cheat" without breaking the laws of physics by feeding the power back to the power supply input.This way its actually possible to test something like 20 kW switch-mode power supply at full load using just 2kW from the AC-outlet.

author: Corporate666
author_rank: Unknown
timestamp: March 14, 2013, 08:00:20 pm
content: Thanks Andy.I had a look at my notes from last night (I was reading something about e-loads on switching supplies in Elektor) and I was incorrect that the output was shorted to ground - rather, as the post above this states, in the example I saw, the output was fed back into the input. I'm still trying to wrap my mind around this one... as you said, there is no free lunch, and if the output is fed back into the input, it would reduce the load from mains, but from the article, it states 300W can be consumed without being dissipated. Other than the magnetic field of the inductor and normal losses... how?

author: Rerouter
author_rank: Super Contributor
timestamp: March 14, 2013, 08:05:24 pm
content: mzzj that is a very clever idea, using a step up switcher to counteract the supply your testing, though that would mainly only be feasible cheaply for dc, you only loose the losses of the inductor and switch + the efficiency of the device, allowing you to recoup a lot of the power,

author: AndyC_772
author_rank: Super Contributor
timestamp: March 14, 2013, 08:07:55 pm
content: That's a really good idea, hadn't thought of that!I was thinking of a dummy load in terms of a (logically) simple device which can be connected across the output terminals of a power supply, and which will just simulate the behaviour of whatever device the supply is designed to power.Powering the EUT from its own output is a great idea, but it means the "dummy load" must be a larger, more complicated system which is more akin to the type of inverter you might use to run mains appliances from a car battery - but with the ability to draw some additional 'top-up' power from the mains to make up for the losses in the various power supplies.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 14, 2013, 08:20:11 pm
content: You can use a grid tie inverter to regenerate the power back into the source, but making it work with a wide range of input voltages is difficult and it's only cost effective if it's going to be used for hours a day.That said, there are ways to dissipate the heat without large heatsinks. One of the most obvious is to use incandescent light bulbs or heating elements to radiate most of the heat. (Most of the energy is radiated heat, with a small amount of light and some convected heat.)For limited duty cycles, a phase change material like wax can allow the use of a smaller heatsink.

author: penfold
author_rank: Frequent Contributor
timestamp: March 14, 2013, 09:24:40 pm
content: In my experience electronic loads are normally built around linear devices to reduce any ripple and additional noise added to the system under test.It is also possible to attain improved load bandwidths and slew rates using linear devices without much cleverness as opposed to having to use high bandwidth high power switchers.I general scheme I have seen used is when the system just needs to dissipate (as in soak testing etc) a lot of tungsten bulbs get used. When the load needs to be changed often to verify regulation - electronic load. To measure ripple, nothing beats a resistor, they just do their job, and rather well most of the time.

author: c4757p
author_rank: Super Contributor
timestamp: March 14, 2013, 10:13:06 pm
content: Quote from: penfold on March 14, 2013, 09:24:40 pmYes, thank you! I wouldn't want to take actual measurements off a system under load by some sort of inverter. I'm not just trying to stress-test it, I could dump the power into a lamp if that were the case. Lord knows we've all got enough miscellaneous light bulbs and broken toasters lying around to source some beefy resistances that don't mind getting hot as hell. I want to measure and quantify its performance under load.