Using DC electronic load for 50Hz AC Sinewave - Page 1
Forum section: electronics / testgear

author: peter_mcc
author_rank: Contributor
timestamp: November 07, 2022, 06:39:59 am
content: I want to use an electronic load to test outputs that are 50Hz sinewave waveforms coming from a transformer. All the AC Electronic Loads I can find are really expensive.Can I run the output through a bridge rectifier and use a DC load in constand resistance mode? I realise there will be issues around the zero crossing point.I'm looking at something like the Korad KEL103/Tenma 72-13200 or Siglent SDL1020X.My only concern is how well the constant resistance mode can track the changing input voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 07, 2022, 07:22:30 am
content: Some electronic load implement the resistance mode in software. So it may be slow in adjusting the current in reponse to a changing voltage. So the load would not be very much like a constant resistance.One may be able to improve a little by using a parallel normal resistor to take over much of the current. So the electronic load with rectifier would only do part of the load.Otherwise, just a set of resistors to combine in parallel or series may well be an alternative to an electronic load to simulate resistors.

author: Fungus
author_rank: Super Contributor
timestamp: November 07, 2022, 01:26:12 pm
content: Quote from: peter_mcc on November 07, 2022, 06:39:59 amI don't see why not. It may be exactly the type of load the transformer will be connected to anyway.(unless you have a specific use case for these transformers thatisn'tthat)Quote from: peter_mcc on November 07, 2022, 06:39:59 amMost bridge rectifiers in the world have a big capacitor across the output to smooth it out.

author: alm
author_rank: Super Contributor
timestamp: November 07, 2022, 03:42:29 pm
content: How important is the shape of the current to you? Does it need to be sinusoidal and in phase with the voltage (power factor = 1)? Or does it not matter as long as the average current is correct?I don't see an issue with a full-bridge rectifier as long as the voltage is high enough that the drop in current around the +/- 1 V range doesn't matter. But I'm not convinced the load will be fast enough in constant resistance mode to track a 50 Hz signal, because many loads will implement the constant resistance control loop in software to save costs. I don't know these specific loads, and they don't seem to specify constant resistance bandwidth. I think you'll have to test it: feed a 50 Hz signal into a load through a bridge rectifier and a low value series resistor, and with a scope measure the voltage across the load and across the series resistor (careful how you connect the ground leads).If you don't care about the shape of the current signal, then a cap in parallel with the load, as Fungus, would make the job easier on the load, but would make the current draw very non-sinusoidal.

author: gf
author_rank: Super Contributor
timestamp: November 07, 2022, 05:21:08 pm
content: Are the input terminals actually isolated/floating? Or is one of them earthed?

author: peter_mcc
author_rank: Contributor
timestamp: November 07, 2022, 11:08:03 pm
content: Thanks everyone.Quote from: Kleinstein on November 07, 2022, 07:22:30 amQuote from: alm on November 07, 2022, 03:42:29 pmThat is my major concern - nobody (at least in my price bracket) seems to specify the bandwidth for "constant resistance" mode like they do for constant current/constant voltage.Quote from: alm on November 07, 2022, 03:42:29 pmI was hoping someone else had tried it! I haven't bought the DC electronic load yet and if it doesn't react fast enough it is of no real use to me.Quote from: Kleinstein on November 07, 2022, 07:22:30 amI'm trying to do production testing of a product and need 4 different currents (upper/lower bounds for on & off states). I could do it using resistors but it would be affected by mains voltage variations.I wish someone made a cheap AC electronic load - the cheapest I can find is a second hand Koritsu for about $US3k.

author: alm
author_rank: Super Contributor
timestamp: November 07, 2022, 11:18:25 pm
content: Something that could work for loads that support analog programming, or power operational amplifiers like Kepco BOP or Lambda BOSS, would be to feed them a divided (and if necessary phase-shifted) version of the voltage signal as current programming signal. In the case of a unipolar load, a rectified version of the voltage signal. I have never tried it, but it might yield better bandwidth than constant resistance mode, especially if it's CR is done in software.For a business application where time and reliability are an issue, I would not bother coming up with improvised solutions but buy a proper instrument designed to do this job.

author: peter_mcc
author_rank: Contributor
timestamp: November 08, 2022, 10:38:52 am
content: Quote from: alm on November 07, 2022, 03:42:29 pmIt needs to be reasonably close to a sine wave.Quote from: alm on November 07, 2022, 11:18:25 pmI agree in principle except that the "proper instrument" is several thousand dollars which I'd rather not spend if possible.

author: Fungus
author_rank: Super Contributor
timestamp: November 08, 2022, 12:17:18 pm
content: Quote from: peter_mcc on November 08, 2022, 10:38:52 amIn that case there's probably no shortcut.I guess you could buy a selection of big power resistors, put them in parallel in different combinations, and ... heat them up.It's not as easy as dialing a number on a screen though.Quote from: alm on November 07, 2022, 11:18:25 pmMaybe there's a reason for that.

author: nctnico
author_rank: Super Contributor
timestamp: November 08, 2022, 01:58:43 pm
content: To the OP:What kind of range & adjustment accuracy do you need? If making a DIY solution is an option, then you could consider building an AC load using resistors and opto-relays (or conventional relays). By putting more or less resistors in parallel, you can create different resistance values.For one of my customers I have designed & build a bunch of high power DC loads that need to run 24/7. These are about 2* 1.2kW in a 3U 19" case. I have used resistors bonded onto a metal base (WDBR series from TE):These can run hotter compared to resistors (smaller fan cooled heatsink) and are way more reliable as well.In my case I have used MOSFETs but for AC purposes the easiest way is to go for (opto) relays based on MOSFET or Triac. Be sure to add some TVS diodes as well; at high currents power resistors have enough inductance to create a nasty spike at turn off.

author: alm
author_rank: Super Contributor
timestamp: November 08, 2022, 03:00:51 pm
content: If your power requirements are modest (dozens to maybe 200W), there are power resistance decades, like the Clarostat 240C and those made by Time Electronics, that are a more convenient to use than a bunch of power resistors and clip leads. I believe there are also versions with relays that can be automated. You do need to pay attention to how much power they can handle depending. Usually each decade is rated for a certain current, so make sure you stay within that. For example for the Clarostat the 1 Ohm decade can handle 5 A, so at 1 Ohm it can handle at most 5 A^2 * 1 Ohm = 25W, while with the first decade set to 9, it could handle 5 A^2 * 9 Ohm = 225W.I did a quick test with a Kikusui PLZ-300W load which implements constant resistance using an analog feedback loop (see manual with specs and theory of operationshere) set to 180 Ohm, a standard bridge rectifier and a HP 33120A function generator set to 50 Hz sine 20Vp-p (with 50 Ohm output impedance). I wouldn't want to use it for AC testing. Yellow is the AC voltage going into the bridge rectifier, blue is the current going into the bridge rectifier measured with a current probe (Tek P6302 + AM503B, vertical scale on scope is correct), gray is the voltage from the bridge rectifier going into the load (same scale as yellow), and white is the current from the bridge rectifier flowing into the load (same scale as blue). So clearly the load is quite slow in responding to the abrupt voltage changes even at low current levels, like the overshoot after the zero crossing:The bridge rectifier itself is not a problem as long as your voltage is high enough, and could be improved somewhat if you'd use Schottky diodes instead of standard rectifier diodes. Here is the same test showing the AC voltage and current when replacing the load by a 150 Ohm resistor:

author: unknownparticle
author_rank: Frequent Contributor
timestamp: November 08, 2022, 03:15:01 pm
content: Very coincidentally I was going to try this as a load for testing audio amps, think I'm going to test it out on a relatively low powered single board power amp first to see what happensI have an old school DC electronic load by ACDC Electronics, a real beast from the '70's that can handle upto 750 watts, and also a Kikusui 300 watt unit which is much more recent. Should be interesting.

author: alm
author_rank: Super Contributor
timestamp: November 08, 2022, 03:26:47 pm
content: I'm a little bit surprised it's performing so badly, because the PLZ-300W manual (page 16, fig 2-3-4) shows a flat impedance up to 1 kHz.

author: unknownparticle
author_rank: Frequent Contributor
timestamp: November 08, 2022, 03:42:42 pm
content: Interesting. I have the PLZ-303W, I'll check the specs on that.

author: peter_mcc
author_rank: Contributor
timestamp: November 10, 2022, 10:52:00 am
content: Quote from: alm on November 08, 2022, 03:00:51 pmThanks for doing that - unfortunately it is as I expected and didn't workI've come up with another solution using a fixed resistor bank and a programmable AC power supply which I have.Thanks everyone for your input!

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: November 10, 2022, 11:45:43 am
content: Good that you found a solution.I was noodling around about a programmable constant resistance load for another project and I think I've come up with something worth trying. I need to build it and see if it actually works. Might help you next time