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 Post subject: Making low value resistors
PostPosted: Feb Fri 26, 2021 11:49 pm 
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Location: Yukon, OK USA
I needed a couple of 10 ohm resistors that could handle 10 watts and had none. However, there is some nichrome wire in my collection. You can't solder it but mechanical connections work. I measured it until I hit 10 ohms. That was about 8 inches. I coiled it around an allen wrench and put the ends under some solder tabs mounted on a piece of MDF. This allows you to make up an odd value resistor that can handle some current. I use nichrome to make ignitors and smoke generators for magic props. This was needed to measure current of a tube in the cathode line by measuring voltage drop. I will replace them with actual resistors in the final build but they let me get the test and adjustment done quickly.


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 Post subject: Re: Making low value resistors
PostPosted: Feb Sat 27, 2021 2:55 am 
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Back when we could "shop" at land disposal sites I used to collect the old spring like stove elements and use them like this.

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 Post subject: Re: Making low value resistors
PostPosted: Feb Sat 27, 2021 3:37 am 
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lwhite46 wrote:
Back when we could "shop" at land disposal sites I used to collect the old spring like stove elements and use them like this.


Hair blow driers can be found for $1 at thrift shops. That might work too.

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 Post subject: Re: Making low value resistors
PostPosted: Feb Sat 27, 2021 8:07 pm 
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Nichrome is nice for load resistors, but poor for current measurement "shunts." Too much resistance change with temperature.

Rich


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 Post subject: Re: Making low value resistors
PostPosted: Feb Sat 27, 2021 11:15 pm 
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Rich, W3HWJ wrote:
Nichrome is nice for load resistors, but poor for current measurement "shunts." Too much resistance change with temperature. Rich


Thanks Rich. In this case it is temporary until my order of resistors gets here. Good fact to remember.

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 Post subject: Re: Making low value resistors
PostPosted: Feb Sun 28, 2021 2:27 am 
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Rich, W3HWJ wrote:
Nichrome is nice for load resistors, but poor for current measurement "shunts." Too much resistance change with temperature.

Rich

From what I have read that isn't true. Nichrome has a much lower temperature coefficient than any common metal. There are special alloys that are better, of course. But for general work, Nichrome is fine. It was one of the first low coefficient materials used to make wire-wound precision resistors. If you check the name plate of a heating appliance and calculate the resistance when the wire is red hot and then measure the cold resistance, there isn't much difference.

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 Post subject: Re: Making low value resistors
PostPosted: Feb Sun 28, 2021 7:44 pm 
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Nichrome can make a good resistor if you run it at very low current so it doesn't heat. It's chief virtue is that it has a much higher resistance than copper or brass.

High current shunts are usually some kind of copper alloy. Five Amp shunt:

https://www.rammeter.com/ram-meter-inc- ... rent-shunt

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They are massive because the goal is to minimize heating for the sake of accuracy.

Rich


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 Post subject: Re: Making low value resistors
PostPosted: Mar Mon 01, 2021 2:50 am 
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Location: Oswego, NY, USA
The copper alloy leaf(s) which is soldered into these DCA shunts end brass blocks is usually referred to as a "manganin". If one is sending through one of these shunts anywhere near their capacity, then it's a good idea to take the ready sooner than later, to avoid possible heating effects reading errors. Many lower Amps reading DCA meters already have installed inside their cases the shunt; but larger Amps meters (usually above around 25A) normally will require connecting to them a shunt, like the one pictured above, because the larger Amps shunts are too large and heavy to be installed inside the meter's case. On both Simpson and Triplett analog VOM's, their 10 or 12 DC Amps range is supplied by a makeshift (mechanically attached at the factory any-which-way that works) DCA shunt which is simply a fairly long (up to 10 inches long in some models) thick-diameter solid wire made of manganin; one can easily spot it inside the VOM case; nothing else looks anything like it. If anyone buys a higher Amps meter online; it's good to know that the needed shunt may or may not be installed inside the meter's case; & many online sellers simply don't know that the shunt might be missing, because they don't understand DC Ammeters or haven't bench tested the unit.


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 Post subject: Re: Making low value resistors
PostPosted: Mar Tue 02, 2021 2:05 am 
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Manganin is another low temperature coefficient alloy that, like Nichrome, was used in early precision resistors. They are probably still in use.

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 Post subject: Re: Making low value resistors
PostPosted: Mar Wed 03, 2021 8:38 pm 
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Would this wire work to rebuild ballasts?


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 Post subject: Re: Making low value resistors
PostPosted: Mar Thu 04, 2021 1:15 am 
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If the ballast were just a resistor, you could rebuild it with commercially made resistors more easily. If it is a true ballast that actually regulates the current, then you could use resistors if the input voltage were constant, otherwise you need a real ballast.

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 Post subject: Re: Making low value resistors
PostPosted: Jun Sat 05, 2021 4:24 pm 
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Manganin works great because it's stable and you can solder it, but it can be hard to buy or expensive. It is not a good material for high temperatures. Something else that works well is Constantan (aka thermocouple) wire. It has a high voltage coefficient against copper but that's not usually a problem because you'll have copper leads on both sides. With a decent flux you can solder it, though not as well as manganin. To get a precise value easily, measure out a long piece. say one to ten feet and measure that. From that you can get ohms per inch and if you measure short length, it should be near dead on.


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