# Solutions for measuring continuity

#### Pete Repeat

Joined Oct 26, 2015
15
Good day AAC.

I work with a number of machines that use a lot of heater elements. I'm trying to reduce the time it takes to check these elements for continuity, and not by simply crawling around the machine quicker!

The way I currently do this is quite normal. I use a multi-meter and I manually check continuity on each element by disconnecting one terminal from the larger circuit. I have a lot of heater elements to check, so rather than check them all, I want something that gives me indication of faulty elements.
Can we discuss potential solutions? I will provide diagrams/pictures once the ball is rolling.

My first idea, which satisfies the project needs, would be a network of LED's that simply change state when continuity of the relative element drops out. Then I can see which elements need inspecting/changing before using the multi-meter, and this system would save me at least 1 hour per machine.
Thanks community.

#### ISB123

Joined May 21, 2014
1,236
Post the pictures.

#### cmartinez

Joined Jan 17, 2007
7,064
Posting schematics, and element specs would help too

#### Pete Repeat

Joined Oct 26, 2015
15
This is the best I can do for now just let me know if more info is needed. There are a lot more elements than shown.

I'm not sure at first guess whether fig2 should be parallel or series, but it would be between terminal and heater element.
For record, im not an experienced engineer so my knowledge of elec components and how to use them is still fairly small.
Ideally the indicator LEDs might be grouped in a control panel, something like 3m distance from elements.
This would mean a circuit for each element, so I also want to discuss how this could be done on a larger scale for say.. 400 elements. I'm on my third coffee so far!

#### Pete Repeat

Joined Oct 26, 2015
15
As for specs on the heater elements, they are 500w 230V
Other than the dimensions and length of wire (100mm) I can't really tell what other specs might be available.

#### kubeek

Joined Sep 20, 2005
5,731
It might be simpler to turn the elements on and simply check the current of each element with a clamp-on amp meter right in the distribution box. Just wrap the sense circuit around each output of the SSR and see if there is any current flowing.

#### GopherT

Joined Nov 23, 2012
8,012
Your drawing in Figure 2 will not work. There is no voltage drop across the LED. That is, the voltage of the LED's anode and cathode are the same so it will not light up.

If you add a 1 ohm resistor in line with your heater, you will have a 2.3Vac across that resistor. It will not impact heater performance since those will still have 227.7Vac across them.

Then, in parallel with the 1 ohm resistor, add (red LED and 22 ohm resistor - in series with each other). Very simple.

The LED will not light if the heater is broken because there will be no voltage drop across the one ohm resistor at that time.

Good luck

#### Pete Repeat

Joined Oct 26, 2015
15
Thanks Kubeek but for relevant UK health & safety law I'm basically prohibited from doing that. The machine needs to be isolated before I can get near the elements and their circuits, as there's a very high risk of me being cooked or crushed when it's live.
If I could have LED indicators mounted into a panel next to the operator (via long wires feeding back out from heater unit), then this would be a huge success but all ideas are welcome.

GopherT thanks, that info is great! If possible, can you help me understand more clearly by including a simple diagram of your suggestion?

#### GopherT

Joined Nov 23, 2012
8,012
@Pete Repeat

AC Line
|
|
SSR
|
|
|
|___________
| . . . . . . . . . . . . |
| . . . . . . . . . . . . |
1 ohm . . . . . . . LED
| . . . . . . . . . . . . |
| . . . . . . . . . . . . |
| . . . . . . . . . . . . 33 ohm
| . . . . . . . . . . . . |
| . . . . . . . . . . . . |
--------------------
|
|
HEATER
|
|
|
To AC neutral

Ignore the dots in the "drawing" above.

#### Pete Repeat

Joined Oct 26, 2015
15
Thanks very much, the dots help this to make perfect sense.

#### AnalogKid

Joined Aug 1, 2013
8,542
I think what gopher is suggesting is that when you are in there and have a wire already disconnected for your test, reconnect it with a 1 ohm, 10 or 20 W resistor in series and a long twisted pair wire harness going off to the LED. Simple, effective, reliable - and not isolated from the AC mains. If the remote LED and its wiring must be fully isolated, then you're looking at something like a current transformer (CT) driving the LED. The xfmr has a hole in it, so the AC mains wire is fed through it for fully-isolated, non-contact current sensing. With the right sense transformer, it could drive a back-to-back LED with no series resistor. I've used CT's but I'm not super experienced with them, so others can jump right in here, but this seems like a good starting point - passive components, no power supply, fully isolated, remote indications.

The reason for a back-to-back LED is that LEDs do not like large reverse voltages, such as what comes from an open-circuit CT. A 2-wire 2-color LED will light one color on the positive half-cycle and the other color on the negative half-cycle, but so what? All you need is a light/dark indicator, and your eye will integrate the two colors into a steady display. The output impedance of the CT will limit the current through the LED, so no series resistor needed. Maybe.
Attached is a typical panel mount part.

http://www.digikey.com/product-search/en?pv258=52&pv258=57&pv258=439&pv258=443&pv258=478&pv258=484&FV=fff40008,fff80280,2dc0728,1df00003,1df00004,1df0000d,1df0000e,1df0000f,1df00012,1df00016&k=led&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=500

A CT with wires attached so the whole project is no-solder runs around $10-$15 each. If you attach your own wires, that drops to under \$2.

http://www.digikey.com/product-search/en?FV=fff4000c,fff80124,408003c&k=transformer&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=500

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