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My homemade 1ohm +/- 0.5% Shunt 50 watt
27 May 2012 21:53
I have for a long time been missing a good Shunt that I can use with the scope and meters I have.
The Fluke 199C scope that I have can do power measurement like Watt, power factor and so on, to do this then you get the Volt on one channel and the amps via a shunt or clamp on the other channel and then it can calc the power values.
A Shunt is an very precise resistor there is connected in series with the load and then there will be an small volt different over the resistor, this then gives the amps by this formula Resistor_value*Volt_different=amps

I was having an big 1kg heat sink and was thinking that if I get some good power precisions resistors then I can build me an good Shunt there was fitting what I needed, so I was looking at EBay and found some very cool precision resistors there was 5ohm +/-0.5% 10Watt resistor and by using 5 of them in parallel so I then gets an 1ohm +/-0.5% 50Watt resistor and the best of all they was designed to be mounted on an heat sink to make better cooling.

Oversized Shunt?
yes the Shunt is very big an it will not be very often that I will do measurements of loads there can get this one to heat up, but the main reason to make it this size was because I was having the heat sink with this size and the other parts was the best I can find to fit this project and the other reason is because it is so big with the heat sink then it will be more temperature stabile and not heat up in most use and this will there for not change the resistor value and make it more precise.

Why 1ohm, what about a 0.01ohm?
I have other Shunts in the lower values but even then I have been missing a more classic 1ohm Power shunt there can give the measurement of 1 volt per 1 amp and the other reason is that some of the meters I have there you select the Shunt Volt-per-Amp setting and I was not having an shunt there always fits this, but an classic size of 1V/1A will be almost useable anywhere.

So how big loads can this shunt been used with?
By doing a simple calculation of the 50watt 1ohm resistor then it gives ca. 7.07A and at 7A the Volt drop will be 7.07V, so the Amp will be 7A max over the shunt.

This table shows how big a max load can be (Volt*7A=Watt)
Load volt
Max load watt (with PF=1)
400 V
2800 W
240 V
1680 W
110 V
770 W
48 V
336 W
24 V
168 W
18 V
126 W
15 V
105 W
12 V
84 W
8 V
56 W
5 V
35 W
3.3 V
23 W
2.5 V
17 W

Here is a photo of the finish Shunt build

The Green connector is the ground and is connected to the head sink surface, The Blue and Red connectors are just each side of the Resistor.
The wires are 1.5 mm2 and are connected to the resistors in an way that each of the 5 resistors has the exact same length of wire on each side, this is important to make sure the resistors share the load and have the same resistor value in the wires.

Here is some photos I have taken of the process of building with is showing the steps.

Drilling the holes and threading for the screws, this is for the mount of the power resistors.

Drilling the holes for the banana connectors.

Here the parts mount is done and it is ready for wiring.

Now all the wiring is done and there is made holes in the corners for the cover parts.

Here I have made some spacing part out of some 8.5mm black Lexan.

Here is the top cover there is a 3mm smoke colored Lexan.

Here is the finish version with all the parts mounted, it was very hard to get the nice edge and get the same size with a smooth surface and I think I got it very good, but I afterwards I have discovered that it donít matter so much because when this is heat up under load then the parts change size and the fit is no longer so well, but I am happy with the finish anyway and it works for my needed.

To test the performance and heat profile I have then done some testing over a long time.

First I have done an very long resistor measurement test with the Fluke 8846A 6.5 digs multimeter that I have after testing over an long time with the analyze function of the 8846A and be nulling out the test leads, I have then measured the Shunt to be 1.00449590 Ohm and that is within the 0.5% of the 1 Ohm.

To test the heat, I then put 7A load over the Shunt and then let it heat up for an long time and the Fluke 8846A is having the temperature sense on the heat sink and is showing the trend graph in the display.
The Fluke Scopemeter 199C is used to measure the Volt and Amps of the Shunt and is showing the power value of 50.1W with the 7.01A and the 7.047V there is also read by the Fluke 287 so I can see it.

If you note my HP 6632B PSU in the background is showing the volt as 7.36V and 3.501A so are that correct, this is because that PSU can only give out 5A and for this testing I need 7A, so the trick is that I have two of the HP 6632B PSUís and the other one is hidden and mounted under the table because I have limit space.
The cool thing about the PSUís is that each of them are 20V 5A but they can be connected in serial there is giving 40V 5A or they can be set in parallel to give 20V 10A, so for this test I set them in parallel and them set the current limit to 3.5A on each of them and then they share the 7A between them.

Here is a photo of the Fluke Scopemeter 199C there is measuring the 7A through the Shunt and it is showing that there is 50 Watt of power there is heating up the shunt.

Here is an graph of how hot the Shunt will get with each Amps, seems the heat sink size is fine with the 7A load as I was planning to not get higher the 80C as the limit and as the graph shows it goes to 76C for the 7A so that is good.

Here is a Fluke TI25 thermography photo of the heat at 7A and with the heat sink on top and the resistors turn down, the graph on the right shows the temperature over the green line in the image.

Same as the image above, but with the heat sink turn down and the resistors on the top.
Seems from the heat profile that the resistors are sharing the load in a nice evenly way with the temperature very close of each other.

So to test the Shunt in an real measurement I have then connected the 240V mains for the Fluke 8846A via the Shunt and powered on the Fluke 8846A, The 8846A is not set to do anything other than be powered on and then the Fluke Scopemeter is doing the measurement of the shunt and on the display you can see the AC Amp(Blue) there is following the AC Volt of the net and then it calculate the Watts(Red) there is used.

Here is an close-up photo of the Scopemeter where you can see that the Fluke 8846A meter is using 13.2 watts when it is just power on.

To get more details about the power use I have taken the other measurement of VAR, VA, PF and the Watts.
So the Fluke 8846A power measurement is 13.2 Watt, 16.8 VA, 10.2 VAR and the Power factor of 0.79 PF.

So now my new homemade shunt is done and I am happy with the outcome and think it is just what I needed.

Tooms @ 27 May 2012 21:53 | Direct link

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