Variable test load (up to 80 watts)? - Page 1
Forum section: electronics / testgear

author: Chalky
author_rank: Regular Contributor
timestamp: July 13, 2013, 12:50:10 am
content: Hi there, I'm doing a bit of testing of various lighting transformers, some of which need a minimum load to work, and some have an overload cut-out. What would be ideal is if I had a variable load thingy, let's say feeding 12V (AC or DV) in, and a big twiddly knob that let me adjust the load from nothing up to to 80W. Does such a thing exist off the shelf, or can I construct one? What's in them? Masses of high-wattage ceramic resistors? Cheers, any advice appreciated!

author: Chalky
author_rank: Regular Contributor
timestamp: July 13, 2013, 12:56:33 am
content: Is something like this the only option? Prefer something in a box with banana sockets, but can always construct that.http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-28-1225-/28-1225

author: Skimask
author_rank: Super Contributor
timestamp: July 13, 2013, 01:07:42 am
content: If you do something crazy, like search for "electronic load", you might, and I might be nuts for suggesting this, but you just might find some really good ideas and resources for the right way to procure, use, or even make a variable test load.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 13, 2013, 01:12:19 am
content: 'Rheostat' is a common name for the thingy with the big twiddly knob.Ohmite is one of the big brand names in the field. If you search ebay for 'ohmite rheostat' you'll find literally hundreds with various ohm and watt ratings. But you will probably find that the ceramic tapped resistors are a much more economical solution. I have both types and I've found that they both are useful.Ed

author: c4757p
author_rank: Super Contributor
timestamp: July 13, 2013, 01:13:59 am
content: Skimask, I think a bigass resistor is a perfectly good solution here, especially since AC was mentioned. There aren't many AC electronic loads...

author: madshaman
author_rank: Frequent Contributor
timestamp: July 13, 2013, 01:30:59 am
content: A bit off topic, but I've been thinking of making some kind of harness to turn a DC electronic load into an AC electronic load.I haven't really thought it completely through, but was thinking along the lines of a big switched-mode AC/DC converter (filtered rectifier and DC/DC converter). Then characterise the efficiency of the converter at say 50 and 60hz, then use math to back calculate the settings 'y' on the DC load given the settings 'x' in the harness.If the harness had a uC that could talk to a computer (e.g. USB or GPIB) and the load (or loads) was also connected to a computer, you could just dial in your settings to the harness and let the computer do the math (assuming you're a programmer like me, which is easier than electronics ;p)Despite how hacky this sounds, is something like this feasible? Or I am overlooking some kind of serious flaw?

author: Skimask
author_rank: Super Contributor
timestamp: July 13, 2013, 01:54:36 am
content: Quote from: c4757p on July 13, 2013, 01:13:59 amTrue, and don't get me wrong, I like a big ass resistor or any component as much as the next guy, but I've always been under the impression that an AC load is usually a light bulb, a heating element, or a regular ol' DC load with a full wave rectifier and some smoothing in front of it...

author: edavid
author_rank: Super Contributor
timestamp: July 13, 2013, 02:35:05 am
content: Clarostat 240-C power decade box is an excellent option.

author: Chalky
author_rank: Regular Contributor
timestamp: July 13, 2013, 04:09:07 am
content: Great, thanks for the replies, everyone, will check out the Clarostat, looks promising.Also, to the person who hinted I was too thick to use Google, you're not helping me, as the issue is not knowing what to search for. Had I know 'electronic load' was a meaningful term, I would have searched for that. I searched for variable load and high-power variable resistor, which got me started, so apologies again for not knowing the answer to my own question in advance. Sheesh.

author: benemorius
author_rank: Regular Contributor
timestamp: July 13, 2013, 04:35:37 am
content: Quote from: Chalky on July 13, 2013, 04:09:07 amThis may help introduce you to the subject even if it isn't exactly what you need. It's also the reason why people are likely to give you a hard time about this thread. Electronic loads have been discussed almost to death around here following the video below, but not AC loads, so you're in the clear if that's what you want this thread to be about.

author: Chalky
author_rank: Regular Contributor
timestamp: July 13, 2013, 05:11:24 am
content: Quote from: edpalmer42 on July 13, 2013, 01:12:19 amWonderful, a big Ohmite variable looks great, cheers for that! Will start with one of those and see how I get on.http://www.ebay.com/itm/OHMITE-RHEOSTAT-4-5K-OHMS-150-WATTS-/321162984105

author: SeanB
author_rank: Super Contributor
timestamp: July 13, 2013, 06:09:57 am
content: As you are doing lights simply take a set of GU5.6 holders and mount them to a piece of aluminium and have a set of 20W lamps to hand, and use a switch to turn them on as needed. Then you can go 20, 40, 60, 80W as needed and have it compact ( and toasty and bright) as well as having a realistic load profile on turn on.

author: megajocke
author_rank: Regular Contributor
timestamp: July 13, 2013, 08:56:46 am
content: Quote from: Chalky on July 13, 2013, 05:11:24 amA 4500 ohm 150 W rheostat can't take more than 0.18 A so doesn't seem very suitable for your application.

author: Chalky
author_rank: Regular Contributor
timestamp: July 13, 2013, 09:35:30 am
content: Thanks SeanB, yeah, I initially thought a single socket and some various wattage bulbs that I could swap, before deciding that variable would be good, as I good see at which point these transformers cut out. The row of a few 12V bulbs is good, I reckon that's the fall-back option if I can't get something else going. Let's face it though, nothing else is going to look as good!Thanks for pointing that out, Megajocke, I guess those are designed for much higher voltages. Shame, will keep hunting for something that can take up to 7 amps or so at 12 V.Cheers!

author: Spikee
author_rank: Frequent Contributor
timestamp: July 13, 2013, 11:16:07 pm
content: A constant current load that can do 80+ watts is not that hard.You need:10 turn pot + shunt resistor + opamp + mosfetI probably had a schematic for you of my constant current load that can do ~120 W using one mosfet. But has been tested using multiple mosfets up to 63A and 200+ Watt .I'll search for the schematic now ...Edit:I found the schematic , this is for ~400W dissipation and up to 60A or something (need higher power shunt).https://dl.dropboxusercontent.com/u/100819959/constant%20current%20load/Constant%20current%20EEVblog_Spikee.pdfYou can just remove op-amp B,C,D and Mosfet B,C,D for your application.More information can be found here:https://www.eevblog.com/forum/projects/did-i-forget-anything-constant-current-load/And on my YouTube channel (signature) you can see some video's of it.b.t.w. i should really finish that design an put it on a pcb and add digital monitoring and control + use precision op-amps ...

author: Skimask
author_rank: Super Contributor
timestamp: July 14, 2013, 01:35:45 am
content: Quote from: Chalky on July 13, 2013, 09:35:30 amSomething anything in an automobile?Maybe a...headlight...

author: Chalky
author_rank: Regular Contributor
timestamp: July 14, 2013, 03:51:10 am
content: Quote from: Skimask on July 14, 2013, 01:35:45 amYeah, I reckon that might work, will check out the wattage, main issue is that's fixed not variable, but if I can get a few test loads of differing wattages then that should suffice.

author: Skimask
author_rank: Super Contributor
timestamp: July 14, 2013, 07:30:38 am
content: Quote from: Chalky on July 14, 2013, 03:51:10 amMix it up...Average headlight is probably worth 5 amps. Taillight/markers, a couple amps. Dome light, maybe 1/2 amp.

author: Chalky
author_rank: Regular Contributor
timestamp: July 14, 2013, 07:38:40 am
content: Am also wondering why halogens need such high frequency when using AC, and whether that high frequency will mean certain types of loads will be unsuitable.

author: SeanB
author_rank: Super Contributor
timestamp: July 14, 2013, 09:40:33 am
content: They don't need high frequency AC, it is just that using a high frequency makes the magnetic parts a lot smaller, lighter and lower cost. Energy transfer per cycle is fixed, but with higher frequency you transfer more units per second, thus per second the transfer units can be smaller and thus the core area smaller, the copper turns less and the cost ( most important to the manufacturer) lower. That you are making a very noisy broadband RF emitter is immaterial to him, it is up to the end user to get bitten by this.