How to connect a $1000 load cell to a $300 15v 1 amp power supply - Page 1
Forum section: electronics / projects

author: gumbygabe
author_rank: Newbie
timestamp: June 30, 2016, 03:07:59 am
content: :-\Hey guys,So I'm having trouble powering this load cell (http://meas-spec.com/product/t_product.aspx?id=2428) using this power supply(http://www.acopian.com/store/productdetail.aspx?q=i1866). The load cell had a 15V excitation level, which the power supply is rated to supply.My biggest problem is I don't know how to power the the power supply. I was thinking of buying a wall wart, splitting the ends and then putting the leads into their designated pins on the power supply. Would this work?L NI also don't understand what each symbol represents on the barrier strip [+V +S COM -S -V AC AC GND ].The load cell has 4 leads. 2 of which are connected to a DAQ (to measure/record data) but I don't know which pins to connect the other 2 to on the power supply. The equipment is really expensive and I don't want to risk damaging anything by putting it in the wrong place.Any advice would be appreciated and if you could recommend a source where I can teach myself that would be great.Thanks!-GG

author: tautech
author_rank: Super Contributor
timestamp: June 30, 2016, 03:37:06 am
content: Welcome to the forum.Most of what you need to know is in the datasheet linked within the page you linked:http://meas-spec.com/downloads/ELHS.pdfThere's a schematic of the part with lead designations and colour codes.Not being nasty but how did you not see this?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 30, 2016, 03:40:41 am
content: I suggest a bit of web research, just so you feel confident before you risk your hard earned dollars.In the meantime, the power supply you bought suggests that it will accept 230 VAC input. If that is your mains voltage you can connect this to the two lines marked AC. A wall wart would give some isolation, but is not really necessary. Again, some web research about safety with mains lines, etc. would give you and us more peace of mind.You would typically want to power your load cell with the +V and GND line. Those characters, not surprisingly indicate the positive output voltage of the supply and the ground. The documentation of your load cell shos which two lines are powered (+V to red, GND to black). The other two are the output, and nominally, with no force applied to the load cell, will each have half of the supply voltage on them. When force is applied, one of these voltages will go down, the other up. By a few millivolts. Since the change is so small the outputs are usually put into a differential amplifier before trying to read them with any kind of meter or A/D converter.the +S and - S lines are sense lines, used to compensate for the resistance in the power supply lines when high current is supplied. When this feature is used a second wire is run from the appropriate sense line to the point of load. The supply the regulates the voltage at the load, not at the supply output.In your application the resistive drop in your wires is not likely to matter, so you can just connect the +S line directly to the +V line and the -S line to the -V line. Connect the -S line even though you don't intend to use the negative output. It usually doesn't matter, but in some supplies leaving this open can lead to oscillations.That should cover basic hookup. Due to the small signals involved you will have to take care to avoid noise and other problems. Again, some web research will help when you run into problems.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 30, 2016, 03:48:14 am
content: I will agree with Tautech and suggest that most vendors do provide good instructions. If you go to the Acopian link you will find an instructions for use page which says what I said in a different format. If terms like sensing leads make no sense to you, a little time with google will work wonders.

author: ez24
author_rank: Super Contributor
timestamp: June 30, 2016, 05:13:54 am
content: Maybe I am missing something (probably) but seems your powering something with a $300 power supply that could be powered by a few batteries or a $20 laptop PS ??I bet if you called them, they could suggest a $25 ps. If not ask if 19v is too high (Dell laptop ps)My suggestion get some AA holders and use AA batteries 1.5v x X = 15 so 10 batteriesor some 18650 batteriesJust curious - how much and what are you weighing? How much are they, I do not see a cost.

author: gumbygabe
author_rank: Newbie
timestamp: June 30, 2016, 05:46:48 am
content: I'd like to address some of the concerns and point out that I'm not completely familiar with most of the terminology in the data sheets/ instructions that came with the products. The equipment is relatively expensive and I can't afford to damage them so I want to be 100% of what I'm doing before proceeding.The load cell is measuring the pulling force on a handle during ergometric rowing, and has a capacity of 5000N (1124 lbs)@tautech: I am not comfortable yet with the data sheet and was looking for very basic help in setting this up correctly.@CatalinaWOW: I'm located in the US and our outlets are outfitted with 120VAC. Are there devices to connect my mains to the ps, or could I buy any extension cord, cut the cord, split it, and put their two lines into the pins marked AC? I'm looking into the most basic way to do this, but am having trouble finding a tutorial of some sort. Thanks by the way for the very helpful advice.@ez24: The load cell was about $1000 and the PS $300. I don't think AA batteries would be reliable for our system, not to mention that our trials for experiments could be going on for hours at a time.Thank you for the advice everyone. I'm pretty much a noob at setting up all of the electrical hardware, but have taken classes on electrical systems before. The transition is not easy, but I'm willing to learn. If anyone has specific resources you think I should check out please link them here. I'll try to update this thread as I go along with my process.-GG

author: ez24
author_rank: Super Contributor
timestamp: June 30, 2016, 06:44:30 am
content: Quote from: gumbygabe on June 30, 2016, 05:46:48 amHoly cow$1000 load cell and a $300 1 amp 15v ps. Too rich for my blood

author: Brutte
author_rank: Frequent Contributor
timestamp: June 30, 2016, 07:10:30 am
content: Indeed, 1k$ load cell.Do not experiment with that or get a 10$ version for practicing purposes.

author: ez24
author_rank: Super Contributor
timestamp: June 30, 2016, 07:14:57 am
content: Quote from: Brutte on June 30, 2016, 07:10:30 amI agree $10 plus $10 worth of batteries

author: CJay
author_rank: Super Contributor
timestamp: June 30, 2016, 07:15:42 am
content: Quote from: ez24 on June 30, 2016, 06:44:30 amWay too rich for me now as wellI have played with load cells in this price bracket before though and they worked well enough with an off the shelf £20 linear supply, a 'classic' four op-amp instrumentation amp built around a pretty much jellybean quad package (I think it was an LT part) and a fairly cheap voltage reference (I think an LM336BZ2.5But...These days, if I were looking to integrate a load cell into a test setup and had that sort of cash to spend I'd definitely be considering a digital solution, there are even load cells which are the full package and USB output, cuts out all the pitfalls of the analogue that newbies (I was at the time very new to load cells) can fall into.

author: gumbygabe
author_rank: Newbie
timestamp: June 30, 2016, 07:39:00 am
content: Before everyone else thinks I'm some sort of a baller, I should note that I did not pay for the equipment. The money was provided through grants and I'm a researcher conducting experiments. My main objective of posting here was to get redirected to some sources so if any for this application, it would be much appreciated.-GG

author: ez24
author_rank: Super Contributor
timestamp: June 30, 2016, 08:02:43 am
content: To OPI suggest you change the title of this topic to something like:"How to connect a $1000 load cell to a $300 15v 1 amp power supply"You may get more interest and therefore more answers. There is always an expert here.What I do not understand is why no wiring diagram (a real one) for the load cell. Is there a doc missing? I would call them and ask whats up. Maybe you can connect the 15v + and - to either pair of leads and read the voltages from the other pair. Thats my guess.What are you using to read this with?

author: CJay
author_rank: Super Contributor
timestamp: June 30, 2016, 09:21:11 am
content: Quote from: gumbygabe on June 30, 2016, 07:39:00 amThat's fair enough, to give you much more we would need to know which DAQ system you're using.Basically though, you feed the load cell the constant voltage from the power supply on the red and black excitation wires then with no load you should have near as damnit zero output across the other two wires, if it's not you 'null' that out either in your electronics or in software.To measure the load you connect the output +/- wires to your DAQ system or a differential amplifier and then your DAQ.As you increase the load on the cell the voltage difference across the two measurement wires increases, it's proportional to the applied load and should be linear.Depending on your DAQ system you may need to amplify the cell output, relative to the input voltage, it's tiny, full scale output is only a fifth of a volt.Polarity on the load cell shouldn't matter, it's only marked so you know what polarity to expect on the output for an applied load in any particular axis, if polarity is wrong (E.G. decreasing voltage for increasing load) you can just reverse the wires on the excitation input or the signal output.

author: gumbygabe
author_rank: Newbie
timestamp: June 30, 2016, 04:01:04 pm
content: All,I've updated the title as one has suggested. Hopefully, it will catch more attention. This is the DAQ we are using (http://www.mccdaq.com/usb-data-acquisition/USB-1608G-Series.aspx).@ez24: The only doc that came with the load cell was its technical data sheet, but it doesn't have any diagrams on it. I've contacted the company but they do not provide any help with setup.@CJay: Thank you for the general overview. I'll need to look into whether or not a differential op amplifier is necessary. We have one in the lab but it's being using in another system and I don't want to disassemble their setup.I'm currently researching if a op amp will be necessary, and also how to connect the ps to the outlet. I appreciate all the feedback.-GG

author: ez24
author_rank: Super Contributor
timestamp: June 30, 2016, 06:32:22 pm
content: Edit - Deleted mistakes

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 30, 2016, 06:47:46 pm
content: Quote from: ez24 on June 30, 2016, 06:32:22 pma DAC is not a DAQ, just saying...

author: CJay
author_rank: Super Contributor
timestamp: June 30, 2016, 07:12:23 pm
content: Quote from: gumbygabe on June 30, 2016, 04:01:04 pmThe DAQ can be configured to work as a differential input with max inputs of +/- 1V, it's a 16 bit system so it *may* be OK without an amplifier but you're far better off if you can amplify the load cell signal up to somewhere near the max input level, that way you get the full range of measurement and will be able to see finer detail.I have no idea what sort of 'pull' a rower can produce so, was the load cell you've chosen, was it chosen to match the expected 'pull' of the rower?

author: ez24
author_rank: Super Contributor
timestamp: June 30, 2016, 08:33:59 pm
content: Quote from: CJay on June 30, 2016, 07:12:23 pmFrom OPQuoteI assume somewhere along the handle where the leverage force gets to 1124 lbs (whew !) Who would have guessed this.The OP's cell does not have a long plunger so I wonder how he is going to connect it.

author: CJay
author_rank: Super Contributor
timestamp: June 30, 2016, 09:04:01 pm
content: Quote from: ez24 on June 30, 2016, 08:33:59 pmI read that as the capacity of the load cell, not the applied load.

author: Maxlor
author_rank: Frequent Contributor
timestamp: June 30, 2016, 09:28:34 pm
content: First, you asked what the labels on the connectors of the power supply mean:From right to left:Earth - connect your earthing wire to that. The wire should be green and connect to the earth pin of your mains cable, and if you're using a metal case for everything, also be attached to the case so that the case is grounded.N AC - mains neutral, should be connected to the gray or white wire of your mains cableL AC - mains live, should be connected to the black or red wire of your mains cable.-V - -15V output. If you have no other electronics besides what you mentioned, you can leave it unconnected.-S - sense wire for the -15V. This must be connected to -V somewhere, the easiest way is right at the power supply (it has a small metal plate for this purpose.) For more precise power regulation, run a separate wire and connect it to -V as close to your load as possible.COM - Ground, connect this to black wire of your load cell.+S - Same as -S explained above, but for +V. It's ok to just use the metal plate and run a separate wire.+V - +15V output. Connect this to the red wire of your load cell.You mentioned that you considered buying a wallwart and stripping it to power your power supply. This implies to me that you haven't realized that this power supply is supposed to be powered directly by mains. Connect it to the AC wall socket by a simple 3-wire cable (which connects to earth, N AC and L AC as outlined above.) If you don't have one, you can make do with buying an extension cord and cutting the socket end off, then stripping the insulation off the wires so you can insert them into your power supply sockets. Do think about what you're doing in this instance, as you have exposed metal contacts here with mains voltage on them. Absolutely do unplug it at the wall if you do any work at all on the system.And typically, a power supply like this would not be exposed but live inside some sort of box with the rest of your electronics, and you'd use an AC entry socket, something like this:http://www.newark.com/bulgin/bzv01-z0000-01/connector-power-entry-inlet-plug/dp/06WX9357. I do recommend you use something like this, since it has a nice switch, a fuse, and prevents you from touching any live parts.Next, the load cell. There is a diagram explaining the wire colors and functions in this datasheet here, page 3 at the bottom:http://meas-spec.com/downloads/ELHS.pdf. The load cell is a fairly simple device electrically, and you will not damage it even if you hook it up wrong, as long as you only have 15V in play. The one you picked has no overload (as in physical force, not electric power) protection though and very little margin (1.5x overrange, so 7500N is the limit over which damage may occur), so be careful in the construction of your apparatus. If you do overload it, the damage might not be immediately obvious, but it will no longer be accurate.In the load cell datasheet linked above, it also tells you what your DAQ needs to be able to handle: on page 2 there's a table that says the FSO (Full Scale Output) for your load cell is 200mV with a 15V supply. So if you apply 5000N, you get 200mV between the green and white wires., and they'll still be roughly at 7.5V above COM. It's also roughly 0.5% accurate, so if that's the accuracy you're going for, you need to be able to resolve 1mV. If your DAQs differential inputs that don't mind the 7.5V, 16bits resolution at its +-1V scale are easily enough, in fact you need just 11 bits. If you can't use the differential input, and use two single ended inputs with the +-10V range instead, you'd then need 15 bits of resolution, out of the 16 you have available. Your DAQ is pretty nice, so it should be able to just about handle that.Btw, if you have temperature changes for some reason, more than about 5°C compared to the temperature you calibrated the system at, you need to think about measuring it and compensating for it. But that can always be added later.

author: gumbygabe
author_rank: Newbie
timestamp: July 01, 2016, 07:21:44 pm
content: Hey everyone,So I was able to strip a power outlet and hooked it up the ps. Acopian was able to help out with that.@ez24 - 5000N is not the applied load as CJay pointed out.@CJay: I need to do a lot more research on what amplifier to get and more importantly why we need it in the first place. As the name suggests, I'm assuming the voltages are so tiny that the measurements we get will be too small to read.@Marlor: This is great help and practically a cheatsheet to setting my system up. I really appreciate the time you put into this. Thank you!!

author: mjkuwp
author_rank: Unknown
timestamp: July 04, 2016, 04:04:54 pm
content: Quote from: CJay on June 30, 2016, 07:12:23 pmThose are important points. my thoughts,The load range of the load cell seems quite high if you are just considering one person rowing. Unless that load cell is something really special you could have problems getting sufficient signal strength.On the other hand, the load cell is described as piezoresistive (as opposed to resistive) so this is not your typical load cell and maybe it is a bit special : )I'd guess your load cell's measurement range could be about 1/2 or 1/4 of your present range unless you are doing the whole boat?What do you want the data to go into? are you going into labview? or more simply a data file for excel? a microcontroller?

author: ez24
author_rank: Super Contributor
timestamp: July 04, 2016, 09:58:34 pm
content: Quote from: EmmanuelFaure on July 04, 2016, 09:38:40 pmMy guess - Olympics

author: engineer_in_shorts
author_rank: Regular Contributor
timestamp: July 04, 2016, 10:21:47 pm
content: Return the items and spend the money on a local guy who knows a bit about electronics, and a some computer knowledge would be helpful too.

author: ez24
author_rank: Super Contributor
timestamp: July 04, 2016, 10:28:39 pm
content: Quote from: engineer_in_shorts on July 04, 2016, 10:21:47 pmI think research money is non refundable by design.

author: CJay
author_rank: Super Contributor
timestamp: July 05, 2016, 11:15:07 am
content: Quote from: gumbygabe on July 01, 2016, 07:21:44 pmYou *might* not need the amplifier, depends how much load you expect to see and it may also be possible to increase the output of the load cell with careful mechanical design of your rig, repositioning it closer to the fulcrum for instance, to give a higher output that 'fills' more of the DAQ range, of course you'd need to add a calibration factor if you want absolute instead of relative measurements.The voltages are small but they're well within the measurement range of the DAQ as long as you take a little care with wiring to ensure electrical noise isn't interfering with your signals (you can check this by watching the DAQ output when there's no load applied).Amplifying the signal as close to the load cell may be advantageous if you experience noise problems.

author: gumbygabe
author_rank: Newbie
timestamp: July 05, 2016, 04:34:17 pm
content: @mjkuwp: The data will be exported as a .csv- I'll be inputting it into another program along with some other data@EmmanuelFaure: I can't say much more due to an NDA. Sorry.@engineer_in_shorts: ez24 is correct.@CJay: Would you anticipate more noise at lower or higher loads? You mentioned that a higher output will "fill" the DAQ better. Does a higher output mean the same as a higher load? We're expecting loads to be on the lower end of the load cell's capacity

author: alsetalokin4017
author_rank: Super Contributor
timestamp: July 05, 2016, 06:36:05 pm
content: Thank you Simon, good call.

author: ez24
author_rank: Super Contributor
timestamp: July 05, 2016, 06:55:39 pm
content: Quote from: gumbygabe on July 05, 2016, 04:34:17 pmOP I believe you have not said which cell you have - is it ELHS-T4 @ 2,000 lb? I wonder if you are using it in tension.I believe the specs state 200mv full scale. @ 2,000 lb ? Seems a good multimeter could read accurately these readings. So for 1.000 lb it would be 100mv, 500 lb - 50mv, 100lb 10mvI assume the readings are linear because I do not see otherwise. I also assume a low level at which the meter would not work ( 1 lb ? )I think someone here could answer the lowest reasonable voltage a meter can read reliably(good topic to start)In your budget a good multimeter that records CSV seems reasonable.Take a look herehttps://www.eevblog.com/forum/testgear/multimeter-spreadsheet/If a multimeter is possible, you could start a new topic with the specs for the multimeter and see what you come up with. If so ask about connections to reduce stray voltages.Here is a meter that can connect and record to a PChttps://www.amazon.com/Greenlee-DM-830A-Digital-Multimeter-1000/dp/B003TO5YUU/ref=pd_sim_sbs_469_3?ie=UTF8&dpID=41jZ7xbP5wL&dpSrc=sims&preST=_AC_UL320_SR174%2C320_&psc=1&refRID=09A5YJT7CAYXTRW1N3XYI do not know the format (I cannot download the manual ?) but I guess it can get to CSVYou could place the meter close to the sensor and the PC can be far away because it is connected via RS-232 (brain dead - is Rs-232 - 4-20 ma ? My first sign of the Big A)Greenlee is made by Bryman, which is a respected brand on this forumGlad you are hanging in there - a lot of newbies get turned off by the response

author: CJay
author_rank: Super Contributor
timestamp: July 05, 2016, 07:51:02 pm
content: Quote from: gumbygabe on July 05, 2016, 04:34:17 pmNoise: Neither, I would expect noise to be the same at both levels, the smaller the signal (thus the smaller the load aplied) the more of a problem noise will be.Load cells can be pretty good with respect to noise but you need to ensure they're rated accordingly and that's not just maximum load.It's not impossible to use a cell rated for 5000N to measure a peak load of 100N for instance but it's an awful lot more tricky to separate the signal from the noise than using one that's rated for 100N to start with because a 100N cell will give full output (200mV), a 5000N cell will output a fiftieth of full output (4mV)

author: Back2Volts
author_rank: Unknown
timestamp: July 06, 2016, 04:21:22 am
content: This statement in the datasheet called my attention:Care should be exercised to isolate your ELHS fromoff-axis loads.I read it as meaning that the mechanical design may require some attention to ensure accurate transmission of force and prevent cell damage by the athlete.

author: CJay
author_rank: Super Contributor
timestamp: July 06, 2016, 08:40:23 am
content: Quote from: Back2Volts on July 06, 2016, 04:21:22 amSame here, the load, tension or compression, needs to be perpendicular, twisting or bending the cell in other axis will damage it.

author: fubar.gr
author_rank: Unknown
timestamp: July 06, 2016, 10:33:48 am
content: What's so special about that power supply and it costs $300? It doesn't even have overvoltage protection as a standard, it comes as a $30 option?

author: dannyf
author_rank: Super Contributor
timestamp: July 06, 2016, 10:47:58 am
content: Obviously, you have to dollar match the load cell and the power supply, since the dollar figures play so prominently in the title.So cut the load cell in 1/3 and then you are alright.

author: CJay
author_rank: Super Contributor
timestamp: July 06, 2016, 11:04:51 am
content: Quote from: fubar.gr on July 06, 2016, 10:33:48 amIt's a pretty good spec linear PSU, built so it will meet it's design specs easily and continue to meet them for a *long* time.It's not an unusual price for that sort of item, they're designed to be built into industrial equipment where reliability is paramount so the price reflects that.No, the OP didn't need that sort of spec but they've got it and unfortunately I get the feeling that they've been dropped in the deep end a bit with this project and are trying to make the best of the situation.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 08, 2016, 10:20:55 pm
content: Quote from: CJay on July 06, 2016, 08:40:23 amI have been away for a while. To the OP. You appear to have your basic questions answered, but if you have not done so already, you need to spend some time clearly understanding your problem. You need to know what peak loads you are expecting, and the smallest loads you care about. Then you will know how much signal you will be dealing with. Will you see the 200 mV full scale value? Do you care about fractions of a pound difference in force? Answers to these questions will inform what measurement system you need to apply to your load cell (and also tell you if the stability of your power supply is sufficient to support your needs. If you are interested in fractions of a pound you will need very good stability to separate gain variation from actual load changes.) Your need to be coy about details of your measurement make it difficult to give specific advice, but the start to any measurement systems design is to clearly define the questions you are asking.

author: ez24
author_rank: Super Contributor
timestamp: July 09, 2016, 01:05:24 am
content: Quote from: CatalinaWOW on July 08, 2016, 10:20:55 pmGood advice

author: N2IXK
author_rank: Frequent Contributor
timestamp: July 09, 2016, 01:49:52 am
content: How about taking some of your grant money, and hiring an electronics tech who actually knows something about instrumentation?A load cell requires a very stable excitation voltage (usually 5 or 10 VDC), and a high gain amplifier on the output to boost the millivolt level signal to something that will make maximum use of the full resolution of your DAQ system. Typically 0-5V or 0-10VDC.Typically, a laboratory test application would use a ready-made strain gage signal conditioner module. It will supply the excitation and amplify the load cell output signal, as well as allow for offset and drift compensation. A basic industrial unit is something like this:http://www.omega.com/pptst/DMD4059.htmland it only costs $75 more than the absolute overkill power supply you needlessly bought.A more polished, laboratory grade unit is available here:http://www.ectron.com/prods/428.phpAnd a dirt cheap hobbyist grade device here:https://www.sparkfun.com/products/13230which might be all you need if you can deal with a digital output to a microcontroller.