Thread: Electronics ModIO question

Hi Tim,

If you want to reduce the scale of the input, you need to set up a voltage divider.
Voltage divider - Wikipedia, the free encyclopedia

The easiest way is to use 2 resistors of the same value, say 4.7K ohm. The result is that 1/2 the input voltage will be presented to the ModIO. So a 7V input will be scaled to 3.5V.

You can change the 2 resistor inputs to get 5V for a 7V input. There are online calculators that you can use. But scaling it by 1/2 is usually OK and easy.
Voltage Divider Calculator

BTW, what is the input thatis producing the 7V?

Cheers,

Peter.

The values shown should be suitable, but may need to be proportionally scaled depending on the input source impedance. The diode is optional, but will provide some peace of mind in case the input rises above 7V.

Thanks Guys,

Peter the input producing the 7V is the internal voltage divider in my plasma cutter.
Steve I will use the complete schematic since I already have all the parts.
Cheers,

Tim.

Hi Tim

you may find that a standard 10% tolerance 5.1 volt zener will conduct "early" at say 4.7 volts and clamp off the high end of your 0-5 volt range.
That is, for 0-5 volts input, your ModIO "sees only 0-4.7 volts( worst case on the low side).

If that happens in your situation, and you really need that high end linearity arpound 5 volts, consider using a 6.8 volt zener (type 1N4736 or equivalent), as the zener is only being used for ModIO input protection. This should clamp in a range of 6.1 to 7.5 volts and not interfere with the voltage divider action of the two resistors.

cheerio, mike

Thanks Mike,

I will bear that in mind. As soon as I get the cable sorted for my plasma I will try to measure the output from the built in voltage divider over a range of cutting conditions to better determine the operating range and circuit requirements.

Cheers

Hi Tim

there are some other more obscure implications of using this circuit, which in part, is why the others giving advice on this post have asked for details of the "source".

Also, there is a remote possibility of external and spurious influences affecting your circuit eg things called hum loops and RFI.

If you have any unexplained results when using the circuit, you may have to add a few components.

Also, from the ModIO User Manual, it is uncertain how the ModIO Analog Input encoding reacts to a voltage just higher than 5 volts as the ModIO internal reference voltage is 5 volts, and the encoded data may revert to an "undetermined" state, with unstable results.
If that happens, there may be value in adding a variable resistor to the circuit to set the maximum voltage (ie 7.0 volts in gives 5.0 volts out, when all connected up), or deliberately choosing a zener which clamps at say 4.9 volts to limit the encoding.

It is amazing how just three components can cause such a huge amount of design implications, but it all has to be viewed as a "system" of 1 external device, 2 voltage level translator, 3 ModIo.

Hope that this doesn't complicate things for you.

I don't have a ModIO; I am just reading the User Manual and drawing some conclusions.

Please let us know how you get on.

cheerio, mike

Thanks again Mike.
As usual I have more questions. Today I made the cable and measured the voltage over a range of fairly extreme conditions.
I found that the voltage runs between approx 2.5 - 4.0v under all the conditions of the experiment but on arc initiation, and arc break there appears to be a spike that I was unable to measure. This shouldn't affect the application or the brain, but I am concerned about the possibility of damaging the ModIO.
I am happy now that the signal will remain in the operating range without adding resistors.
Would the 5.1v zenner diode alone be adequate protection?
Or should I look at optoisolation?

Cheers,

Tim.

Hi Tim

hmmm...
If your source ouputs a changing DC voltage proportional to say the normal speed of a CNC head traversing on a work table (as distinct from very fast eg "speeding bullet" fast), then I suggest that you use Steve's circuit, minus the 10K resistor, and changing the series 3K9 to 1K0 ohms.
Retain the 5.1 volt zener, and make sure the polarity is correct, ie the cathode "band" painted on the diode end is the "bar" end shown in the circuit
Steve has it correct.
Incorrect polarity wont harm anything, but it will not work correctly, as it will hold the ModIo input down at approx 0.5 volts.

A 1 watt zener (eg 1N4733 5.1V) is a bit more electrically rugged than smaller 400 milliwatt types. This type number is normally written on the diode.
The 1 watt zener will safely take care of any spikes up to approx 900 volts, but you cannot allow any intermittent contact type connections here, or large spikes may destroy your ModIO input (at least you have three!).

The 1k0 resistor limits the zener current, protects the sensor from accidental feedback from the ModIO, and contributes to filtering of unwanted signals.
It is safe to use Steve's 3K9 resistor, only the slightly higher value may affect ModIO response to fast change.
You may have to add 1M0 polyester capacitor across the zener later, to add more filtering if required, but this value is a compromise with the ModIO response to fast changes.

The ModIO handbook on page 57 shows that there is an internal 1K5 resistor, implying that the Analogue Input impedance is 15K ohms or higher, and this (together with the internal 470p capacitor), provides some filtering of your analog signal.

Good luck

cheerio, mike

Sorry but again with the questions,
If I have the 400mw zeners can I use 2 or 3 in parallel to offer the same protection as the 1w?
Also, I'd say the spikes I saw are more of the speeding bullet type and i'm not sure if the multimeter couldn't change ranges quick enough or if the reading was out of it's range when it showed 'OL'

Appreciate your patience,

Tim.

Hi Tim

Yes, you can safely use two matched voltage 400 mw zeners in parallel, and gain an increase in power (almost twice), but lose a little of the electrical "ruggedness" of the larger single 1 watt type.

If you are using a digital voltmeter (DVM) to "see" the spikes, then electrically they are "slow".

Most meters have a "range hold" function which may help, but normally, a digital meter's response is slow in itself.

A DVM will not "see" a fast spike, and you have to use another type of equipment to monitor those.
A zener diode will protect against slow and fast spikes that you will experience in your system.

cheerio, mike

Today I finally got time to build the input circuit and hook everything up for some testing. After a short time I could see in my brain there was no input signal coming from the plasma. I started at the beginning and tested each part of the circuit and finally disconnected it completely and measured the voltage output from the plasma, a fairly steady 2.5 volts.
As soon as I connect the negative side of the signal to the ModIO ground the 2.5v disapears.
Why does this happen and how does one overcome this?

Thanks,

Tim.

Hi Tim,

Sounds like a power supply ground referencing problem, or you circuit is pulling the 2.5V signal down.

Can you post the complete circuit of your setup?

Cheers,

Peter.

Hi Tim

as Phomann says, it could be a reference problem where the earth or negative from one part of the circuit doesnt "suit or match" the earth or negative from the ModIO.

Check that the output voltage from the plasma sensor, when disconnected from all external circuits is positive 2.5 volts to ground.
You have to actually measure it, as you may have a plasma wiring reversal with say a red wire incorrectly connected to negative and the black to positive.
A DVM meter with red prod to plasma voltage output and black prod to plasma earth or negative should read say "2.500" and not "-2.500" (that - or minus sign means reverse polarity, which means you may have a polarity problem).
This assumes that your DVM meter has red prod lead plugged into the DVM meter "+" socket, and the black prod lead plugged into the "-" DVM socket.

Check also that you have the zener diode in circuit with the "unbanded" end of the component connected to negative or earth, otherwise it will pull the plasma voltage down to approx 0.5 volts or so.

We are all assuming that your plasma output voltage uses a negative earth system to match the ModIO negative earth input sense.

I have to go away for some time now.
If you are still having problems, you are then at the stage of posting more info about the plasma voltage circuit, and or the whole circuit of plasma and ModIO.
Post it all; dont assume anything.

Sorry if I am using too low a level of tech talk.
Pity you arent in Brisbane,

cheerio, mike

Thanks for the words of wisdom. When I discovered I had no signal after measuring the voltage output from the plasma it was opposite polarity to what I thought it should be, so I reversed the connection but still had no signal. In the end I disconnected the circuit with the zeners and measured the voltage straight from the plasma at around 2.5v while cutting until I jumped the - side to the ModIO - and with the + only connected to the multimeter the voltage dissapeared.
Hopefully you can see from my sketch that the arc transfer signal is running through the ModIO and this is working fine.
One thing I didn't try is disconnecting the Pot. Could this be the problem having it grounded to the same place as the plasma signal?
During testing I used a second identical Pot instead of the plasma on the An In, but I am now using a different one to those.
I am also attaching the pages describing the voltage divider from the plasma manual.

Cheers,

Tim.