Are there other types of RF loads? - Page 1
Forum section: electronics / rf-microwave

author: daisizhou
author_rank: Super Contributor
timestamp: August 30, 2024, 10:21:36 pm
content: Hi everyoneI noticed that many people test RF power using power resistors as loads to test the power supply.If you need power resistors of different resistance values, you will need a very bulky resistor box to hold them.Very bulky and difficult to transport when out and about.Are there other types of loads that could replace them?

author: wasedadoc
author_rank: Super Contributor
timestamp: August 30, 2024, 10:52:33 pm
content: RF or PSU loads? Loads for DC PSUs can use wire wound resistors. Wire wound resistors have inductance and may be quite unsuitable as RF loads.I have been known to overrun a load resistor by immersing it in a container of reasonably pure cold water.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 30, 2024, 11:11:21 pm
content: For RF you would use something like:https://btgstore.com/rfattenuators.html

author: daisizhou
author_rank: Super Contributor
timestamp: August 30, 2024, 11:22:44 pm
content: Quote from: wasedadoc on August 30, 2024, 10:52:33 pmI mean for RF power,Frequency is about 500KHZ-800KHZ，Voltage is about 8kv-20kv。Non-inductive wirewound resistor，Wirewound resistors are made using a special winding method and look very similar to ordinary resistors.

author: fourfathom
author_rank: Super Contributor
timestamp: August 30, 2024, 11:28:13 pm
content: Quote from: joeqsmith on August 30, 2024, 11:11:21 pmAnd using an open or short-circuit load on the attenuator to get loads greater and less than 50 Ohms. Just do the math. Example: a shorted 3dB attenuator gives you a 16.6 Ohm load. An open 3dB attenuator gives a 150 Ohm load. Check the attenuator power ratings.

author: CaptDon
author_rank: Super Contributor
timestamp: August 31, 2024, 12:05:26 am
content: Where on earth are you dealing with 8KV to 20KV into a dummy load and what impedance??? (20K x 20K) / 50 is a hell of a lot of power!!! What exactly are you doing? That's going to be some big resistors or a dump truck full of little ones. Your numbers seem a bit out to lunch.

author: fourfathom
author_rank: Super Contributor
timestamp: August 31, 2024, 12:22:28 am
content: Quote from: CaptDon on August 31, 2024, 12:05:26 amOh. Hey, that's only 8 Megawatts. (WTF???)

author: daisizhou
author_rank: Super Contributor
timestamp: August 31, 2024, 12:43:19 am
content: Quote from: CaptDon on August 31, 2024, 12:05:26 amI want to test the following hostshttps://www.perlong-china.com/PT300-PT300A-PT300B-H-F-Electrosurgical-Unit-pd6042484.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2024, 01:09:13 am
content: The load in that case isn't really resistive, it's a complex and variable impedance due to motion, arcing and flesh. Would a cut of pork not demonstrate it more directly?Tim

author: daisizhou
author_rank: Super Contributor
timestamp: August 31, 2024, 01:57:21 am
content: Quote from: T3sl4co1l on August 31, 2024, 01:09:13 amObjective measurement is needed, not rough measurement.I have FLUKE 8920A high-frequency voltmeter (RMS)，So I need a load

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2024, 02:16:26 am
content: bro hire a consultant for that machine, its not some hobby antenna run out of a old shedI would not even plug anything into that machine.More then likely, return to company for service. Messing with something like that is how you go to jail with just a drop of bad luck.

author: daisizhou
author_rank: Super Contributor
timestamp: August 31, 2024, 02:26:08 am
content: ;DDon't worry, this is a pet hospital device.It has only served animals and never worked on humans.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: August 31, 2024, 05:09:34 am
content: Quote from: CaptDon on August 31, 2024, 12:05:26 amLooking at his frequency range its no different to AM broadcast and those giant wire wound resistors is exactly what they use fr dummy loads. Banks of them in a cabinet with cooling with spares to match. He needs 500ohms that can deal with1kw a few big wire wounds would do that easy.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: August 31, 2024, 05:19:01 am
content: Quote from: daisizhou on August 31, 2024, 02:26:08 amThose big wire wound resistors will work for what you are testing, the machine puts out 300 watts into 500 ohms, build a 500ohm dummyload that can sink 1000w and you will be golden, as long as you are aware of the inherent danger of those power levels and related voltages because high power RF hurts LOL ask me how I know.If you cannot source them in China, you might be able to find high wattage flange mount RF resistors in the 200 to 400W range and mount them up on a suitable heatsink in series and parallel configuration to get close to the resistance you need with excess power rating. I built my dummyload using 4 200W 200 ohm flange mount resistors off ali express.https://www.aliexpress.com/item/1005006901914907.html?spm=a2g0o.productlist.main.5.6d186y806y801D&algo_pvid=256c4d1f-f4a6-4365-a5b7-da281e7bafb1&algo_exp_id=256c4d1f-f4a6-4365-a5b7-da281e7bafb1-2&pdp_npi=4%40dis%21AUD%2119.29%2119.29%21%21%2112.91%2112.91%21%402103080717250819045168173e8d87%2112000038662949312%21sea%21AU%210%21ABX&curPageLogUid=biTKTYIj90Oo&utparam-url=scene%3Asearch%7Cquery_from%3A5 of these in series mounted on a giant heat sink would work fine.

author: daisizhou
author_rank: Super Contributor
timestamp: August 31, 2024, 05:31:29 am
content: Quote from: vk4ffab on August 31, 2024, 05:19:01 amThanksI can purchase that type of resistor and get it.I hope to find a better alternative item,Because those resistors are too large and inconvenient to carry around,Is there a better solution?

author: CaptDon
author_rank: Super Contributor
timestamp: August 31, 2024, 01:33:58 pm
content: Here is some advice from a BioMed tech of a 900 bed hospital. An ESU generates specific waveforms based on task. Notice the machine can do several types of electro-surgical procedures. That machine looks like an exact knock-off of stuff I tested in 2005-2010 in the states. We had a specific testing device which probably cost as much as the ESU itself. We tested each function at probably 5 different power levels and the calibration tester gave a direct reading of power in watts. Notice also most of the specs are at 500 ohms but there is also a function to be tested at 100 ohms. A 225 watt (one of the standard sizes of ceramic resistors) 500 ohm resistor with the N.I. specification (non-inductive) could work for you since these units were meant for short bursts of use. But, to be on the safe side get a pair of 225W 1000 ohm N.I. resistors and parallel them for 500 ohms. A single 225 watt 100 ohm should be fine for the other mode. The waveform can be very strange and I am not sure how you expect to get a meaningful 'RMS power output' measurement unless the meter you use to measure the signal can do some amazing math functions.

author: LM21
author_rank: Regular Contributor
timestamp: August 31, 2024, 01:55:14 pm
content: There is a special oil to cool high power loads. Transformer oil or something similar. But those are not hobby projectsRadio stations have them

author: CaptDon
author_rank: Super Contributor
timestamp: August 31, 2024, 06:37:26 pm
content: Don't get too crazy with exotic dummy loads. As I stated, the ESU's are used intermittantly in surgery and you really only need to activate it long enough to get a meaningful power output indication. You're not cutting a cow in half or seaming two halves back together. Your kit needs only exactly what I stated, a pair of 1000 ohm resistors in parallel, 225 watts each, standard size AND a single 225 watt 100 ohm resistor. Again, I am not sure your true RMS meter will give a valid result but may be close enough for power verification within about 10% which should be fine. No need to overthink the dummy load. You really should think about purchasing a 'real' ESU tester / calibrator. If you do proceed with the RMS voltmeter you should do a data entry spread sheet or a chart that shows RMS power vs. voltage at 500 ohms and at 100 ohms.

author: daisizhou
author_rank: Super Contributor
timestamp: August 31, 2024, 10:42:21 pm
content: Quote from: CaptDon on August 31, 2024, 06:37:26 pmThankshttp://www.metek.com.tr/main/web/urunresim/ESUComparisonChart.pdfI was lucky enough to find the link above.As far as I know, the latest model is Fluke QA-ES III。But it looks no different than QA-ES II,In terms of function, a frequency display has been added, that's all.In my opinion,The above wire-wound resistors are used as loads in 454a, FLUKE QA-ESII, and QA-ESIII hosts.This makes all the hosts too bulky and inconvenient to carry.I wonder if there is a better loadout to replace them completely

author: abeyer
author_rank: Frequent Contributor
timestamp: September 01, 2024, 01:36:28 am
content: Quote from: CaptDon on August 31, 2024, 06:37:26 pmThat sounds like a challenge

author: boB
author_rank: Frequent Contributor
timestamp: September 01, 2024, 02:19:25 am
content: Fans cool down power resistors pretty well.

author: Robert763
author_rank: Super Contributor
timestamp: October 01, 2024, 11:46:49 am
content: I'd Use two 1K Arcol FPA350 resistors on a small fan cooled heatsink. Connect them in parallel for 500 ohms.If you need to check at 100 ohms add a extra 100 ohm one.https://www.farnell.com/datasheets/2572861.pdf

author: daisizhou
author_rank: Super Contributor
timestamp: October 01, 2024, 12:47:10 pm
content: Quote from: Robert763 on October 01, 2024, 11:46:49 amThis thick film resistor looks good.But I am not sure whether it can be insulated for a long time under the 3kv withstand voltage.Also, whether it can maintain stability during long-term operation and whether the consistency is the same.Can the resistance value accuracy be controlled to 1%-2%?

author: Randy222
author_rank: Frequent Contributor
timestamp: October 01, 2024, 02:40:51 pm
content: Where does kV come from? That unit puts out around 400vac at max output power. (400v 500ohm 0.78amp 300watt)"500 ohms" in the specs for "load" is deceiving. And with the spec sheet, the unit is only around 37% efficient between input and output.Is that resistive load (Rdc) or is that a Z spec? Take for example the large wirewound item for load, When the machine is running 330kHz you could have mostly inductance to present the machine a "500ohm" load as Z, but I bet ya the machine goes poof with full inductive load.My guess is, there's little inductance or capacitance in what the tool end touches, the load is likely mostly Rdc. I would stay away from wirewound.An alternative is a bucket of cold salt water, with electrode spacing just so to show 500 ohms. I am not 100% what salt water looks like in terms of the components of Z at 330kHz 430kHz and 1MHz. Perhaps using a VNA or the like to test the LCR at those frequencies.

author: daisizhou
author_rank: Super Contributor
timestamp: October 02, 2024, 12:15:20 am
content: NHL-175-07E is a special wirewound resistor with almost zero inductance at high frequencies.Input signal:Frequency 400KHZ-850KHZ, Voltage:800kv-2500KV，Current: 0.05ma-0.5A Common resistance range 150 ohms-300 ohms

author: Randy222
author_rank: Frequent Contributor
timestamp: October 02, 2024, 02:32:32 pm
content: Quote from: daisizhou on October 02, 2024, 12:15:20 amOk, but the machine specs say 300w into 500 ohms load.I listed the math and specs in my last post. The load needs to handle 300w and about 1amp to survive any power testing you do. If vswr is horrible then some of the power does not turn into heat in the load.The AP winding, as stated in the Wiki you attached, says that winding has low inductance, not no inductance. What is "low"? many big watt wirewounds in that ohm range can be in mH area, so is low 0.1 to 1mH? That's plenty of inductance for 1MHz. The Vishay spec sheet does say it's uH but varies with ohms and wattage.I still would be more worried about attaching an inductive load to that machine, power coming back to the machine port could cause the machine to burn up.In the Vishay MRA series PDF, it says:QuoteAP winding is simple, two coils, each wound in opposite magnetic direction, the mag fields cancel each other out, but note in the attached PDF, parasitic inductance and capacitance is not eliminated, it just says greatly reduced. I would want to LCR the NHL before attaching to the machine. I attached the model of all resistors, so again, I would like to see a NHL on a good LCR meter.These are all questions to investigate, but perhaps the AP wound loads may work for you.

author: daisizhou
author_rank: Super Contributor
timestamp: October 02, 2024, 11:42:46 pm
content: Thanks.Choose this type of resistor. I disassembled the inside of the FLUKE 454AeBay auction: #https://www.ebay.com/itm/156438371870?mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338722076&customid=&toolid=10050or QA-ES IIeBay auction: #https://www.ebay.com/itm/276667385654?mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338722076&customid=&toolid=10050&amdata=enc%3AAQAJAAAAoHoV3kP08IDx%2BKZ9MfhVJKkPqUEIcBjkpTgfbm4bNOB6c2e23MTxPFgPVgyPD44fX9w5%2BiDqYScPlcPFxc90uGosqAQDKvv6xeYSvd6fFxQHiy2gwnyLra0FKm03Fv%2FpXR%2F9rUvohW0S43Vh5BYinCjN68OA%2B3mrU%2BA1BW8Uw3nTtC1Q4Q9TuX90LdfgDhUh1TBbMkf8jXHDiYl3dr6%2BzZI%3Dhost, and they all chose this type of power resistor.I found these consoles to be very large and inconvenient to carry.Among them, power resistors occupy the main part.So I tried to find out if there is any alternative to them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2024, 01:43:24 am
content: AP construction turns a single helix (or if you like, two parallel helices interleaved, but essentially the same thing for EM purposes), into a crossed pair, meaning the wires overlap every half-turn. If you imagine the overlaps welded together, the resulting shape is like a single-cell chicken-wire fence, wrapped around, and scrunched down (along the axis) until the wires are nearly parallel.Still another way to put it, to reinforce the idea of the crossings: suppose we wound both strands simultaneously, with counter-rotating heads, but every half-turn the heads are swapped, putting a twist in the pair, and the the wire that descended into that crossing, rises away from it on the same side. Then repeat at the top, and so on until the resistor is complete. Each individual strand makes a cylindrical zig-zag the whole way along, never making turns all the way around.We can further express this geometry as a parallel-wire transmission line, where the wires from one crossing, go down opposite sides and then back up again (as the next turn), making a shorted stub; the inductance of this is half the inductance of a single stub, because the two act in parallel; and a little bit less further because the turns are closely spaced to each other.The inductance of this sort of accordion structure, more or less comes to ballpark mu_0 * turns * circumference / 4, and turns = length / pitch. So, not quite proportional to length, it depends on wire size (and thus pitch) too.Very broadly speaking, things have inductance proportional to length, you can work with that---you just need to know what the proportionality constant is, which depends on geometry of the thing.Put in entirely different terms, it's two solenoid windings nested; if they were parallel [same winding direction], the geometry would be a wound-up parallel-wire TL shorted at one end, so the inductance is half the wire length; instead they're counter-wound, which makes a loop between turns so there's less cancellation, but if the pitch is small, it's not much difference.Roughly speaking, expect 1/10th the inductance or less of an inductive type. Good to a few MHz, rather than a few 100 kHz.Tim

author: Randy222
author_rank: Frequent Contributor
timestamp: October 03, 2024, 02:28:08 pm
content: Quote from: T3sl4co1l on October 03, 2024, 01:43:24 amIf you took 20ft of thin insulated #22-24 wire, folded it in half (10ft) twisted it "tight" like "twisted pair", and then single wrapped that around an air core, you end up with a coil where the bare ends are on same side of the core, current flows into one wire with mag field right, gets to end of the coil and returns back on "other" wire with mag field left.Those closely "coupled" local mag fields should cancel each other?But then I ask, what about flat coils where every 360deg the direction folds back around to go the other way? What about basic air core where every turn folds back 180deg so the odd # coil mag field is opposite from even # coil?And for this thread, why a wirewound at all, what about resistance wire like electric stovetop element? You can get those cherry red without issue.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2024, 07:14:57 pm
content: Exactly. Field superposition never ceases to apply, so we can view it as two large fields canceled out. We can equally well arrive at the result by considering the field within the TL, which drops off quickly with distance, and so the placement of the TL itself (whether straight-line or wrapped up like a coil) doesn't matter much.And indeed, a zigzag path acts like a stack of TL stubs, so has less inductance than if that same length were looped around. Whether laid flat, or coaxial arcs on a flat plane, or straight lines on a cylinder.Tim

author: boB
author_rank: Frequent Contributor
timestamp: October 04, 2024, 01:45:25 am
content: When I was a novice ham in the mid 1960s, we used incandescent light bulbs to check and tune our 75 watt transmitters and there was never a problem.Incandescent light bulbs are not constant impedance over their hot-cold range so I'm not sure why they were recommended to be used this way. But they worked great !I wonder if a large I L Bulb would work for this 500W source ? You can get them to 1000 W and above still I believe from industrial sources.We use up to 1500W but mainly 500W bulbs in our inverter testing.boB

author: Randy222
author_rank: Frequent Contributor
timestamp: October 04, 2024, 04:10:40 pm
content: Quote from: CaptDon on August 31, 2024, 12:05:26 amI think that was the rated of the NHL wirewound.The equip spec sheet indicates 400vac into 500ohm, max 1MHz. I assume it's rms ac.