How would one make an AC Programmable load? - Page 1
Forum section: electronics / projects

author: staze
author_rank: Frequent Contributor
timestamp: September 26, 2014, 06:35:09 pm
content: All,There's all the talk of DC programmable loads... largely stemming from Dave's quick video on making one. But I've been thinking, what would it take to make an AC one?The main reason I was thinking of it was because I have a couple little enclosures with power resistors in them for dummy speakers/headphones for testing (audio) amplifiers. It would be kinda cool if you could just have one box that was a programmable load as far as AC signals were concerned. I thought MOSFETs were relatively bidirectional, but apparently not really (you can't really use them for switching AC signals without some work).Am I being blind to the obvious (probably)?

author: Jorpy
author_rank: Contributor
timestamp: September 26, 2014, 06:39:07 pm
content: I might be talking out of my ass now, but is it possible to simply have a rectifying stage before the usual circuitry?

author: staze
author_rank: Frequent Contributor
timestamp: September 26, 2014, 06:47:46 pm
content: Quote from: Jorpy on September 26, 2014, 06:39:07 pmSee, and that's what I thought... but according to someone else, it would make the load non-linear (drop across rectifier). So the impedance would vary with the input voltage...There must be some way to do it... =P

author: djacobow
author_rank: Super Contributor
timestamp: September 26, 2014, 06:53:15 pm
content: How about a transformer with a fat resistor on one side and multiple taps on the other side, with relays controlled by a uC? With a clever tap/winding arrangement of power of two ratios, you might even be able to get 8b or so of control of the impedance seen by the source.

author: penfold
author_rank: Frequent Contributor
timestamp: September 26, 2014, 07:08:05 pm
content: It depends what kinds of voltages and frequencies you're dealing with. For appreciably high voltages you could quite easily ignore a silicon diode drop or active rectifier arrangement. If you get your control right and pick the right points to take feedback from you can compensate for rectifier drops etc

author: staze
author_rank: Frequent Contributor
timestamp: September 26, 2014, 07:10:04 pm
content: Quote from: penfold on September 26, 2014, 07:08:05 pmWell, I was figuring a rather wide range, but my initial thought was as a replacement for a dummy speaker/headphone load... so we're talking a few volts... Obviously the easy solution is just a box with a rotary switch in it to select 4R, 8R, 100R, etc. But there has to be some way to do this "smart". =)

author: firewalker
author_rank: Super Contributor
timestamp: September 26, 2014, 07:17:31 pm
content: For just a simple resitive load, I guess complimentary MOSFET or BJTs could be used. Each will conduct for half the period.Alexander.

author: woodchips
author_rank: Frequent Contributor
timestamp: September 26, 2014, 08:14:17 pm
content: Have a look at the Marconi TF893A, all the work has been done. Power to 10W, impedances from low to high.Been a while since looking at one but sure it is based on a transformer.

author: staze
author_rank: Frequent Contributor
timestamp: September 26, 2014, 08:46:21 pm
content: Yup. A transformer with a crapton of taps... huh. Not exactly simple. =/

author: tom66
author_rank: Super Contributor
timestamp: September 26, 2014, 08:58:32 pm
content: You could modify a variac by putting another winding around the core, maybe? Then connect that to a resistor. A typical variac would have near zero DC impedance and response up to maybe 400Hz before inductance became a major issue.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 26, 2014, 09:00:03 pm
content: DC loads are normally constant current. That would be weird for AC. You more likely want constant resistance. To do that, make a constant current load but use an analog multiplier to multiply the set point by a scaled version of the input voltage. That makes the current proportional to voltage, aka a resistive load. A single device will be unipolar. You can use a rectifier or build two separate loads for the positive and negative half waveforms.If you only care about average power, a simple rheostat will do the trick.

author: ConKbot
author_rank: Super Contributor
timestamp: September 26, 2014, 09:41:58 pm
content: Because even specifying what the load behavior is is difficult, let alone building it. Even a simple load with a non distorted current waveform and a power factor that is anything other than 1 will have to source power during part of the AC cycle. Youre going to need a 4 quadrant power supply (source and sink positive voltages, source and sink negative voltages)A transformer hooked upto a class AB amplifier with provisions to clamp the supply rails would be a start for being able to do a programmable current waveform/power factor.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 26, 2014, 10:14:28 pm
content: A PFC front end will work nicely for higher powers and in theory can be scaled down.

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2014, 10:34:20 pm
content: Quote from: staze on September 26, 2014, 06:47:46 pmThe voltage drop across the rectifier would not be a problem so much as making a constant resistance load which may be adjusted unless the AC voltages involved are low.I would use either an existing power factor correction front end like NiHaoMike suggests with a constant current load or a constant resistance active load like ejeffrey suggests.On intriguing way to make a power MOSFET operate with a constant resistance is to use a matched pair where one controls the Vgs of the other. This was commonly done with JFETs to make voltage variable resistors and I have done it myself with small signal MOSFETs in automatic gain control circuits.You might even be able to do this with back to back power MOSFETs and avoid using a rectifier.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 26, 2014, 10:47:08 pm
content: Who uses diodes, you can get ideal rectifier bridged based on mosfets and an ic.Even mosfets nowadays go above 500 Vds.

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2014, 11:08:07 pm
content: Quote from: Jeroen3 on September 26, 2014, 10:47:08 pmWhich bridge rectifier controller did you have in mind?The Linear Technology LT4320/LT4320-1 only operates up to 72 volts and does not operate below 9 volts where the forward voltage drop of the MOSFETs will still be that of the body diodes.

author: Marco
author_rank: Super Contributor
timestamp: September 26, 2014, 11:18:25 pm
content: You could use MOSFETs but you're always going to get some cross over distortion when you fully turn on the one not being used (to get rid of the body diode voltage drop).

author: richard.cs
author_rank: Super Contributor
timestamp: September 27, 2014, 08:54:43 pm
content: For low voltage loads you could use a transformer to step up to a higher voltage where you can ignore diode drops and then use mosfets inside a diode bridge. But really for audio stuff switched resistors are the way to go.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 27, 2014, 09:32:03 pm
content: For this application, dummy loads for audio amps , you only need a few specific values, so resistors are by far the easiest solution.You could use 2 back-to-back MOSFETS as an AC variable resistor, but the drive & loop control would get somewhat tricky, more so as frequency increases

author: staze
author_rank: Frequent Contributor
timestamp: September 27, 2014, 09:54:15 pm
content: Wouldn't you have to dynamically change the load for an AC signal since the resistance would change (for a mosfet) as the AC voltage varied? You'd have to limit frequency since that control loop would have some upper bounds.

author: tom66
author_rank: Super Contributor
timestamp: September 27, 2014, 10:17:40 pm
content: Quote from: 3roomlab on September 27, 2014, 09:39:57 pmEven 100kHz would be tricky.Say you need just 1 degree phase error at that frequency. Rough guess says that's 30MHz bandwidth for your control loop and to drive your MOSFET at.Ever tried to drive a ~nF of MOSFET capacitance at 30MHz? Tricky... Sure, it's not full swing from 0 to max load, but it's still a difficult task.

author: Marco
author_rank: Super Contributor
timestamp: September 27, 2014, 10:51:08 pm
content: Quote from: staze on September 27, 2014, 09:54:15 pmYou can use a variation on something like this (you also need some logic to turn the MOSFET in reverse conduction full on and take some care not to get oscillation, apparently this was not trivial for Linear with their ideal rectifier).http://3.bp.blogspot.com/-Rlfjp5Q9B0A/UJrcNdjZgNI/AAAAAAAAANo/hqhb5Eso9_8/s1600/Fig%2B5%2BCR%2BFET%2Bload%2Bckt.bmp

author: edavid
author_rank: Super Contributor
timestamp: September 27, 2014, 11:15:23 pm
content: Quote from: woodchips on September 26, 2014, 08:14:17 pmThe original US version of this is the General Radio 1840A. They are usually not too expensive.Another practical solution for audio amp testing is the Clarostat 240-C power decade box. It uses wirewound resistors, but works fine for most purposes.

author: Marco
author_rank: Super Contributor
timestamp: September 27, 2014, 11:48:59 pm
content: Quote from: 3roomlab on September 27, 2014, 10:44:34 pmIf you're going to use water you could just use a water resistor, high frequency capable, can handle lots of power ... just don't smoke near it if you have a large DC component.

author: staze
author_rank: Frequent Contributor
timestamp: September 27, 2014, 11:53:24 pm
content: Quote from: Marco on September 27, 2014, 11:48:59 pmLol!For those that don't get it, it would be liberating a fair amount of hydrogen and oxygen.Would think just some nichrome in an oil bath would be safer.

author: calzap
author_rank: Frequent Contributor
timestamp: September 28, 2014, 06:21:10 pm
content: A network of high-power BeO or AlN resistors, like those made by EMC, should work well. They are non-inductive and frequency ranges are typically DC-2 GHz. Available 2-1000 ohms, maybe more, and with power ratings up to several hundred watts. They do require heat sinking, but a shared heat sink arrangement should work because only a few would be powered at one time. Real questions are how to interconnect them (manual switches, relays, solid state, ?) and whether to use a microcontroller to achieve a specific resistance.Mike in California

author: Jeroen3
author_rank: Super Contributor
timestamp: September 28, 2014, 08:59:34 pm
content: Thyristors are often used for switching ac.You'll end up with a ladder of resistors and thyristors. Not quite the flexibility as the dc loads have.

author: johnh
author_rank: Regular Contributor
timestamp: September 28, 2014, 09:09:06 pm
content: There not programmable but its robust. Might be of overkill though.

author: staze
author_rank: Frequent Contributor
timestamp: September 28, 2014, 09:28:28 pm
content: Yeah... Was thinking load banks. We use them for commissioning data centers.Sadly, yes, overkill.One would think something like a Variac would work (I know someone mentioned it earlier), just run "in reverse". Or is that not really possible? The issue would be audio is not just a constant amplitude sine...Yeah, guessing still better to do resistors. Was thinking about repurposing my old computer water cooling gear to create a nice heat sink for a power resistor (or two).Shame there's not a good way to do this. The question was more hypothetical. :/

author: quarks
author_rank: Frequent Contributor
timestamp: September 29, 2014, 11:08:51 am
content: unfortunately I do not have a DC load.But I would just try/use a nice beefy bridge rectifier on a big heatsink (+ fan if necessary) and connect it in front of the DC load to make it a AC load.

author: tszaboo
author_rank: Super Contributor
timestamp: September 29, 2014, 11:53:46 am
content: Stepper motor + Rheostat. Nothing beats the simplicity.

author: macboy
author_rank: Super Contributor
timestamp: September 29, 2014, 03:16:20 pm
content: Quote from: NANDBlog on September 29, 2014, 11:53:46 amLet's think about programmable loads for a second.ForDC, you generally would want your load to operate in one of three modes:constant resistance(current proportional to source voltage, load power is then proportional to V2),constant current(load power is then proportional to source V). andconstant power(current proportional to 1/V).Since it is fundamentally a DC load and "DC" is contant voltage by definition, the load typically operates in all modes as a constant current dump, with the current set-point programmed according to the mode as above, and only needing to be adjusted occasionally (whatever than means, maybe between few times per second, or more, or much much more, depending on your requirements). Usually, an analog control loop takes care of controlling the current to match the set point current. For a DC source with AC (ripple or noise) superimposed, the above modes still make sense, as long as the DC>>AC.For trueAC, the only sensible mode of operation isconstant resistance. The resistance (not the current) is the 'programmed' parameter. (Think about what would happen near/at zero-crossing with a constant current... the load becomes a short-circuit and still fails to deliver the required current, not what you intend I think). Of course, an AC load could do a modified "constant RMS power" mode where the RMS value (as opposed to the instantaneous value) of the power would be constant, with the AC load adjusting its constant resistance parameter to meet the requested power. Or you could do "constant RMS current" again by calculating the required resistance for the present input voltage, and applying that resistance. But any AC load still fundamentally a constant resistance load.So, given that, an AC load is going to be very different from a DC load. Certainly, it is not a DC load with a rectifier on the input, whether a diode bridge or an ideal rectifier network.Obviously the easiest AC load is a resistor, or a variable resistor. A bank of power resistors with a creative array of (mechanical) relays to switch them into various configurations could do the job, but the adjustment will be rather granular. If you want fine adjustment, you need more complexity.I would imagine that using a power amplifier output at the 'ground' side of the resistor network (instead of direct connection to ground) would provide you the ability to create an finely adjustable resistance by adjusting the output level of that amplifier. It would provide a sort of back-emf for the resistor that would reduce the current through it, effectively reducing the resistance of the AC load. A power amplifier is a four-quadrant device, capable of sinking or sourcing current at any output voltage, unlike the simple single FET/BJT of a DC load, which is a one-quadrant device (it can only sink current with positive input voltages). A power op-amp-like audio amp chip like a LM3886, capable of a minimum 7A continuous output (within its SOA) could do this duty. The power amp would only be called upon to provide a few volts of adjustment to fine-tune the resistor bank. They could be paralleled for more current when necessary.

author: zapta
author_rank: Super Contributor
timestamp: September 29, 2014, 03:32:56 pm
content: Quote from: macboy on September 29, 2014, 03:16:20 pmFirst, it may also be useful to have constant impedance, not just resistance, in case you want to simulate non resistive loads. Second, theoretically it's possible to have also a constant (RMS) current and constant (RMS) voltage. Something along these lines:Let's say your source is 60 hz sinusoidal mains. You digitally lock phase on it and now at any given time you can compute the expected current for that phase and adjust your variable load (e.g. MOSFETS) accordingly. You end up with sinusoidal current of the target RMS regardless of input (RMS) voltage.

author: mikerj
author_rank: Super Contributor
timestamp: September 29, 2014, 03:42:28 pm
content: This might be a possible application for a magnetic amplifier.

author: macboy
author_rank: Super Contributor
timestamp: September 29, 2014, 04:19:42 pm
content: Quote from: zapta on September 29, 2014, 03:32:56 pmGood point.QuoteI did mention that alreadyBut you must understand, that the load will be programmed to operate at a constant resistance(/impedance) according to RMS input voltage and desired RMS operating current (or power for that matter). Yes, the 'constant' resistance will need to be varied over time, as the RMS input voltage varies over time. This is fundamentally different to how a DC load typically operates.

author: David Hess
author_rank: Super Contributor
timestamp: September 29, 2014, 04:50:15 pm
content: Quote from: macboy on September 29, 2014, 03:16:20 pmThis is a common configuration when it is desirable to terminate a transmission line into a lower value resistance so the voltage compliance of the termination can be raised without raising the supply voltage. For instance on the driving side, a source terminated driver normally requires a gain of 2 and twice the supply voltage. By reducing the resistance and adding feedback, the output impedance of the driver is artificially raised to make up for the lower resistance of the source termination and the necessary gain and supply voltage are reduced.

author: zapta
author_rank: Super Contributor
timestamp: September 29, 2014, 04:56:43 pm
content: Quote from: macboy on September 29, 2014, 04:19:42 pmSorry about that ;-)Quote from: macboy on September 29, 2014, 04:19:42 pmWhat I describe earlier allows the momentary current to be readjusted immediately, no need an over time RMS measurement, assuming you know the shape of the signal (e.g. sinusoidal) and you are phase locked to it.In a sense it is a generalization of what DC loads do, they assume some signal shape (flat), derive the expected momentary current (always the same) and adjust the actual current.

author: Marco
author_rank: Super Contributor
timestamp: September 29, 2014, 06:14:56 pm
content: Quote from: zapta on September 29, 2014, 03:32:56 pmYou will have to adjust that MOSFET really hard to get it to the negative resistance necessary for the part of the cycle where the current is flowing against the voltage ...You need a powered amplifier to emulate non resistive loads.

author: robrenz
author_rank: Super Contributor
timestamp: September 29, 2014, 06:32:55 pm
content: A bipolar operational power supply like a Kepko BOP or the Lambda BOSS(thread here)could possibly be a good start on a AC load that could be set up to follow and load an AC input signal.

author: eneuro
author_rank: Super Contributor
timestamp: September 29, 2014, 07:56:30 pm
content: Quote from: mikeselectricstuff on September 27, 2014, 09:32:03 pmDo you mean AC mosfets switch not fully open?This is low power version of such switch, so only 3 AC switches in pararell- 6 mosfets needed for this prototype 1inch2 PCBs eachIt is up to designer to choose right amount of pararell mosfets and its RDSONs to be able avoid power loses in mosfets body diodes-keep them not conducting even when switch is fully open at desired maximum power

author: zapta
author_rank: Super Contributor
timestamp: September 29, 2014, 08:26:04 pm
content: Quote from: Marco on September 29, 2014, 06:14:56 pmYes, that's right. My description above was for resistive constant current or constant voltage AC loads. If you want non resistive load with arbitrary phase, you may also need to source current to achieve the momentary target current or voltage.

author: zapta
author_rank: Super Contributor
timestamp: September 29, 2014, 08:28:11 pm
content: Quote from: robrenz on September 29, 2014, 06:32:55 pmDave mentioned once a 'four quadrant power supply'. Is it a similar concept?

author: tszaboo
author_rank: Super Contributor
timestamp: September 29, 2014, 08:36:29 pm
content: Quote from: macboy on September 29, 2014, 03:16:20 pmExcellent case for my one line! Anything which rectifies the AC will not be a good load, as diodes conduct or they dont. You cannot pull current from the rectified AC when your bulky capacitor's voltage is above the mains voltage. It needs to be a resistive load, or an inductive load, analog, something which works down to the diode drop level and below.Now there are some really nice rheostats out there, that is why I was mentioning them. And it is not that hard to make a mechanical system from off the shelf parts, shafts, timing belts (hope that is the right term). And it will work nicely, the resolution can be even higher than a normal PWM or a 12 bit DAC would make. A a 200 step/rev stepper is alone 7+ bit, even that the rheostat could make is a bit non linear.I mean sure probably there is some twisted PWM circuit with active, separately powered PFC and huge inductors and transformers... Or you can just put some 60W incandescent light bulbs on relays and make a stepper controlled rheostat for the resolution. One is a weekend project, in the other getting rid of the stability problems could take forever.Quote from: zapta on September 29, 2014, 08:28:11 pmFour quadrant is something which can source or sink current with positive or negative voltage

author: robrenz
author_rank: Super Contributor
timestamp: September 29, 2014, 08:51:09 pm
content: Quote from: zapta on September 29, 2014, 08:28:11 pmYes, but these are more like a mega op-amp that can follow an bipolar input up to about 20kHz depending on the V and A rating. Check the link out.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: September 29, 2014, 09:31:17 pm
content: I use a 12A variac followed by a space heater. Works well enough for my purposes. There is a small amount of leakage inductance from the variac, of course, but this is usually small enough to be negligible at high currents.

author: ivaylo
author_rank: Frequent Contributor
timestamp: September 30, 2014, 06:36:50 am
content: Quote from: robrenz on September 29, 2014, 08:51:09 pmThe Kepco will actually do this out of the box. The current modulating input is already inverting, you just feed it the same AC source you want to load (so positive voltage generates negative current and vice versa, i.e. load), scaled to the 10V input the Kepco takes. Probably through a transformer for galvanic isolation and all. Good idea!