Switch mode programable electronic load? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: July 24, 2015, 05:52:38 am
content: We had programmable linear power supplies and then came the programmable switching power supplies with better efficiency.Can a similar concept be applied for programable electronic load? That is, instead of disputing the consumed power on some component with a heatsink, convert it with a switch mode power supply and push the power back the the mains. This will allow smaller and quieter electronic loads with higher power capacity.Does this make any sense?

author: oldway
author_rank: Super Contributor
timestamp: July 24, 2015, 06:53:50 am
content: Yes, this concept is applicable to electronic load, but with serious limitations.I already used it to recover energy from the discharge of industrial batteries but with thyristor converters.As you need to smooth the current, you will need an electrolytic capacitor in the input circuit of such an electronic load, and this prevent to make high slew rate current variations.It can also be a challenge if you want your electronic load to work at low voltages <5V for example.

author: nctnico
author_rank: Super Contributor
timestamp: July 24, 2015, 08:49:32 am
content: Quote from: zapta on July 24, 2015, 05:52:38 amThese are commercially available. I looked at them for a battery pack cycle tester but it turned out the extra expense wasn't worth the savings in the electricity bill even though the tester runs almost 24/7.

author: tron9000
author_rank: Frequent Contributor
timestamp: July 24, 2015, 09:58:22 am
content: I think this guy does something similar:https://youtu.be/k5684mQJQRU?list=PLxaGnte1Dq0ly6tsTkCaqK3cf_viyqyEqhe's using PWM through a load changing its effective resistance.

author: Whales
author_rank: Super Contributor
timestamp: July 24, 2015, 10:18:03 am
content: This idea sounds perfectly fine. Forseeable design challenges:Input range. Unless you are using mains on the load in the first place, you will need to boost/buck and re-oscillate to match mains waveform. This will probably lead to a very limited input range, but it might be better than I assume.Bulk and cost. Up to a certain size you would probably find that traditional resistors are cheaper and better.

author: oldway
author_rank: Super Contributor
timestamp: July 24, 2015, 10:30:45 am
content: QuoteFor limited input range, as for example 12V constant current battery discharge, you can make a stepless resistive load as schematic I have already posted in another thread.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 24, 2015, 06:11:34 pm
content: In many areas you need special permit to allow feeding power back to the grid. There also need to be safty measures as not to power part of you instllation if the fuses are blow. Just for this case you may still need a heat sink as a backup anyway.As from the switching mode the load will not be as smooth as a linear one.

author: oldway
author_rank: Super Contributor
timestamp: July 25, 2015, 05:05:41 pm
content: One can also wonder what would be the interest of wanting to recover so little energy, especially if it is amateur use.To discharge a 12V car battery or to test a power supply for a half hour, it does not represent any interest.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2015, 06:09:03 pm
content: Quote from: zapta on July 24, 2015, 05:52:38 amhttps://www.eevblog.com/forum/projects/ugly-twist-to-a-programmable-load/msg666730/#msg666730

author: albert22
author_rank: Regular Contributor
timestamp: July 25, 2015, 06:45:46 pm
content: Here is a switching electronic load that uses light bulbs and pwm:http://www.edn.com/design/analog/4345737/Versatile-power-supply-load-uses-light-bulbs

author: oldway
author_rank: Super Contributor
timestamp: July 25, 2015, 08:40:07 pm
content: Quote from: albert22 on July 25, 2015, 06:45:46 pmAn important function of a dynamic load is to test power supplies with a current step to check the stability and dynamic response.A load using the switching mode principle requires a filter of the current, which makes it impossible to make a current step with a short rise time.It is therefore unusable in general.

author: TopLoser
author_rank: Unknown
timestamp: July 25, 2015, 09:22:18 pm
content: http://www.farnell.com/datasheets/1907679.pdfNot cheap though...

author: zapta
author_rank: Super Contributor
timestamp: July 25, 2015, 10:56:54 pm
content: Quote from: TopLoser on July 25, 2015, 09:22:18 pmThey claim efficiency > 90%. Very good.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2015, 11:05:19 pm
content: Quote from: oldway on July 25, 2015, 08:40:07 pmConsider much of the supplies made are switching and even some audio amplifiers are switching as well, to be honest I am not sure what would be considered a usable short rise time.Looking at the slew rates on the BK loads, they range from 1A/us to 0.5A/us (10/90). Based on rise time, I will assume we are interested in the turn on time (10% load to 90% load).Load1 shows the home made switching load as it was during the time I made the video. I am using the Tektronix P6042 I had modified to increase the BW to look at the current. It was set to 1A. So 0.8V / 50mV = 16 16 * 1 or 16 Amps peak. 12.8A in 186uS or 69mA/us.However, the analog control signal is heavily filtered (not to mention it's based on an old Unitrode part running fairly slow). I removed the input filter and made the same measurement in load 2 which shows a rise time of 51us or 251mA/us. About half the slower BK units.But let's twist the throttle a little further on the home made switching load. Load3 is now at 1.1V peak / 50mv or 22Amps peak. So 17.6A/22uS or 800mA/uS. Consider again this is not at all based on a modern switching design.There are many reasons why I see no point in using this sort of design for a load but the rise time is not one of them.Quote from: albert22 on July 25, 2015, 06:45:46 pmThanks for the link.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2015, 11:09:21 pm
content: Quote from: TopLoser on July 25, 2015, 09:22:18 pm- Response time 10-90% load step <1.5 msVery interesting setup. I wonder how many they have sold and to who.

author: zapta
author_rank: Super Contributor
timestamp: July 26, 2015, 12:10:40 am
content: Quote from: joeqsmith on July 25, 2015, 11:09:21 pmAnd also why people are buying them, to reduce size and heat or to preserve energy.

author: oldway
author_rank: Super Contributor
timestamp: July 26, 2015, 07:03:35 am
content: Quote from: joeqsmith on July 25, 2015, 11:09:21 pmThis is useless to make dynamic tests of power supply, that's far too slow.As I said, that's the problem with this kind of technology...For example, an Hameg HMP4040 power supply has a recovery time <1 ms after a load step from 10 to 90% for return within a +/- 10 mV window.How could you mesure this if you electronic dc load has a responsive time of 1.5 ms ?Another issue to consider is the cost/benefit.This technology is expensive, is it worth the investment?Except special cases of continuous uses, such as formating of industrial battery plates, most of the time, the electronic load is used only rarely, with low power and during reduced time. (half an hour is more than sufficient to reach thermal equilibrium of a power supply).Even for testing underground trains choppers of 500 KW or more, a study had shown that it was not worth the investment and we used forced cooling resistors as load.In the winter, warm air is used to heat the factory.

author: BravoV
author_rank: Super Contributor
timestamp: July 26, 2015, 08:01:13 am
content: Quote from: oldway on July 26, 2015, 07:03:35 amAlthough I'm not an expert, have to agree with Oldway's point of view here, especially on the response time.The reason and the "main purpose" of an electronic load is to test and verify the power supply capabilities. Saving or recycling the loaded power imo is a much lower priority "feature", and it should not made the whole main task much more complicated, let alone significant cost increased in building it.Loading the PSU doesn't always mean in a continuous constant current, voltage or wattage, a dynamic loading is actually favorable as it reflects the real world condition.The load that is constantly "switching" on it's own is quite hard to tame, for example, head to this excellent project designed by one of our forum respected member ->Dynamic Electronic Load Project by Jay_Diddy_B, and look how a simple and a dirt cheap circuit capable of "torturing" at power supply at very fast period in uS (micro second), say like testing it's transient response.Again, cmiiw, not an expert here, just take it with a grain of salt.

author: nctnico
author_rank: Super Contributor
timestamp: July 26, 2015, 09:40:24 am
content: Quote from: oldway on July 26, 2015, 07:03:35 amEven with continuous use these loads are not cost effective. I have looked at these exact models for a battery pack tester in a production line.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2015, 01:39:07 pm
content: Quote from: BravoV on July 26, 2015, 08:01:13 amFor period he talks about 3ms, not micro seconds.QuoteI did not see where he defined the DSOs numbers so lets look at his SPICE. Risetime:Looking at the above, seems like 10/90 is about 2A/7us or 0.286A/us. Compare this with the BK loads at 0.5A/us for many models up to 1A/us for others and my switching load at 0.8A/us. I guess I don't understand the response time comments.Would guess most designers would use a network analyzer to verify stability. I use a screw driver but to each their own.http://www.ti.com.cn/cn/lit/an/snva364a/snva364a.pdfhttp://www.ridleyengineering.com/ridley-universal-injection-isolator.html?showall=1&limitstart=The larger loads may be for something more like burn-in. If they were running supplies non-stop, that could be a lot of heat. Need to consider the cost of removing the heat. Air conditioner, heat exchangers.... Would like to see how it is being used.

author: oldway
author_rank: Super Contributor
timestamp: July 26, 2015, 04:30:53 pm
content: QuoteThis is a very specialized application only for switch mode power supplies factories.Not even 1% of the electronic load market.QuoteIn winter and in cold weather, the hot air is used to help plant heating .When the heating is not useful, we reject the hot air outside.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2015, 05:28:40 pm
content: Quote from: oldway on July 26, 2015, 04:30:53 pmI don't know what the market breakdown is for electronic loads. Other applications may be for testing generators or part of a load for engine testing. For home use the only need I have ever had for one was to show an example of a switching design.Quote from: oldway on July 26, 2015, 04:30:53 pmRight, reject the hot air outside causing global warming.I can see where there would be cases that recovering this energy would make sense. I would like to see some of these installations.They have a little video that gives a little background on them.http://www.elektroautomatik.de/en/aboutus.html

author: C
author_rank: Super Contributor
timestamp: July 26, 2015, 06:07:01 pm
content: Might want to Google "jim williams load" and read the followingyou getLinear's Application Note 104Load Transient Response Testing for Voltage Regulatorshttp://cds.linear.com/docs/en/application-note/an104f.pdfDesign a 100A Active Load to Test Power Supplieshttp://cds.linear.com/docs/en/article/WMSstory_From%20EDN%2009-22%20NoAds.pdfLinear's Application Note 133A Closed-Loop, Wideband, 100A Active Loadhttp://cds.linear.com/docs/en/application-note/an133f.pdf

author: oldway
author_rank: Super Contributor
timestamp: July 26, 2015, 06:25:15 pm
content: QuoteWe are speaking about electronic loads for DC, not of grid inverters.DC generators and motors (not alternators + rectifier) are tested by the energy recovery method using 2 identical generators mechanicaly and electricaly coupled so one works as a motor, the other works as a generator.By control of field of one of the 2 generators, you can control the circulating current.No need of a DC load.The only wasted energy are the electrical and mechanical losses.QuoteProblem of global warming is so big that power losses of a 2 or 4KW electronic load used once a week to test a 500W power supply you just repaired does not matter at all.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2015, 06:54:28 pm
content: Quote from: oldway on July 26, 2015, 06:25:15 pmI was thinking an engine (gasoline/diesel) being attached to a dyno and using the load to dump the energy from the absorber. Basically what you describe but I have been at locations where they dumped the energy from the absorbers into large resistive banks. The last place I worked at we just absorbed the heat in the windings then removed it with water.QuoteNow that I think about it.... I have actually needed to build a variable electric load once. In the following video, I am taking the voltage off a 3 phase generator, rectifying it, then feed that into my DC linear load. This is being used to test the torque/power output of a small brush electric motor.I posted about this toy dyno here if anyone is interested.http://www.msuk-forum.co.uk/topic/81512-self-built-rc-dyno/page-11

author: zapta
author_rank: Super Contributor
timestamp: July 26, 2015, 07:20:18 pm
content: Quote from: oldway on July 26, 2015, 07:03:35 amCan't you absorb the fast load increase into a capacitor? This is what switch mode supplies do in the other direction.Switch mode power supply and load seems to me to be dual problems with similar challenges but in the opposite directions, so a solution in one can hint for a solution in the other.

author: oldway
author_rank: Super Contributor
timestamp: July 26, 2015, 07:56:58 pm
content: No, the challenges are not similar...To explain this, let us give an example: you can't measure a 10 ns rise time with a 20Mhz oscilloscope...electronic load must be 10x or more faster than the power supply you want to test. (dynamic test)

author: electr_peter
author_rank: Unknown
timestamp: July 26, 2015, 09:37:32 pm
content: Besides very complicated design, it is simply not cost effective to recover energy to mains from electronic DC load. The only real advantage is reduced need for cooling (some of electronic loads with energy recovery linked above have ~10 kW rating, so heat is significant issue).Similar problem occurs during testing of jet engines (or gas generators) of various kinds. When put into test cell, jet engine converts massive quantities of fuel to massive quantity of noise, heat and gas flow - wasted energy, that is. To recover this energy huge investment in infrastructure is needed which is completely uneconomical and very complicated. Also, remember that testing phase consumes many orders less fuel than engine in operation. So nothing to save here.

author: oldway
author_rank: Super Contributor
timestamp: July 27, 2015, 06:02:22 am
content: I continue with the idea that the principle of dissipating all the power in semiconductor is not the best.I choose rather the principle of variable rheostat combining power resistors with semiconductors.Much of the power should be dissipated in power resistors.The radiators are then reduced and problems of semi-cnducteurs operating in linear region with high dissipation would be reduced.