Building an AHU simulator - Need to simulate digital and analog inputs

Discussion in 'Analog & Mixed-Signal Design' started by rfthe2nd, Jun 21, 2016.

1. rfthe2nd Thread Starter New Member

Jun 21, 2016
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Hey ya'll,

I'm building a panel to test the programming of the ddc controls for a fairly complex air handling unit,
I'm just posting to get some guidance on how to simulate the inputs using potentiometers, switches, and pushbuttons..

I have 27 digital inputs to simulate and 18 analog inputs.. i want to use the pots for the analog inputs and I can configure the universal input on the controller for 0-10v.. I guess my main concern would be how to source power for all of these inputs? I have a 24vdc power supply for the controller that I am using. I can probably get a 24vac one for the inputs..I cant upload pictures or schematics for this project.

any ideas?

2. Alec_t AAC Fanatic!

Sep 17, 2013
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Welcome to AAC!
It's a bit tricky to advise when you haven't told us what inputs (voltage, current, frequency, pulse width, .....) the controller is expecting, what the input impedances of the controller's inputs are, signal limits and tolerances, etc .
A clear, easily-readable schematic would be more useful than a picture.

3. rfthe2nd Thread Starter New Member

Jun 21, 2016
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Well they are all universal inputs.. I think i'm gong to set them all to accept a 0-10v signal, just for the simulation. There are a bunch of 4-20ma signals, and 2-10v signals for the real application though.. I cant provide a schematic sadly, because this is for work and I'm in the earlier stages of designing it anyway..

4. Papabravo Expert

Feb 24, 2006
11,069
2,152
Most industrial I/O is 24VDC. You can supply +24V and GND to the simulator and use comparators with open collector outputs to simulate inputs. For outputs you just need a high side or low side switch (BJT) as appropriate.

5. rfthe2nd Thread Starter New Member

Jun 21, 2016
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Hmm, well can't I just supply 24vdc to the potentiometers and adjust each to vary the output between 0-10v? I think i'll need a limiting resistor to get it down to 10v right?

I'm trying to figure out if I'll need a separate 24vdc power supply for each pot though?

6. crutschow Expert

Mar 14, 2008
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One power supply can power everything. Why would you think you need more than one?

To limit the pot output, just put a resistor in series from the power supply to the top leg of the pot.
For example if you want a maximum of 10V from a 24Vdc supply with a 1k ohm pot, then you would put a 1.4kΩ resistor between the +24V and one pot leg with the other (CCW) leg grounded.
The wiper output will then go between 0V and 10V.

rfthe2nd likes this.
7. rfthe2nd Thread Starter New Member

Jun 21, 2016
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Like this right? Could I do something similar and produce a 4-20mA signal or how about a 2-10v?

Last edited: Jun 21, 2016
8. rfthe2nd Thread Starter New Member

Jun 21, 2016
12
0
To clarify:
I know I will have to have 24vdc power supplied and thanks to @crutschow I've figured out how to produce a variable 0-10 v signal.. now i need to figure out how to produce a variable 4-20 mA signal from the 24vdc being supplied using a potentiometer to adjust the signal..

Secondly: Can I just put the pots in parallel off of a single 24vdc power supply or will I need multiple power supplies? I'm pretty sure I can put the six 0-10vdc sinnals in parallel but I think I will need separate power supplies for the 4-20mA signals..right?

9. Papabravo Expert

Feb 24, 2006
11,069
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You only need the one +24V supply and you can do all of that.

10. rfthe2nd Thread Starter New Member

Jun 21, 2016
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Alright, well I guess what I'm wondering is how do I make a 4-20mA potentiometer... whats the simplest way to take the 24vdc power supplied and produce 12 parallel outputs that each vary between 4 and 20 mA when I adjust the pots..

11. Papabravo Expert

Feb 24, 2006
11,069
2,152
You need several pieces of information to do it the simple way. First piece is you need to know the resistance used by the 4-20 mA sensor. This is commonly ~250 Ω (249 Ω being the closest 1% value.). 2nd we need to pick a value for the pot. It should be a linear taper rather than an audio taper. Wire the wiper of the pot to one end of the pot. Third we need to pick a fixed resistor. Using +24 Volts, a fixed resistor, the full value of the pot, and 249 ohms, come up with a solution that yields 4 mA. The solution set is not unique. With the pot at the other end of its range, it is effectively removed from the circuit, so the +24Volt supply and the fixed resistor and the 249Ω resistor will allow a current of 20 mA.

Example:
$\frac {24}{0.020} =1200 \;\Omega$
$1200 \;\Omega\;-\;249\;\Omega=951\;\Omega$
$\frac{24}{0.004}=6000 \;\Omega$
$6000\;\Omega\;-\;951\;\Omega\;-\;249\;\Omega=4800\;\Omega$

This suggests that a 5K linear taper pot should do the trick.

Last edited: Jun 21, 2016
12. crutschow Expert

Mar 14, 2008
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4,324
If you want a 4-20 mA current that is relatively unchanged by the load resistance then you could use a transistor in a constant current configuration controlled by a pot.
Is it a requirement to have 4-20mA into different load impedances?

13. KeepItSimpleStupid Well-Known Member

Mar 4, 2014
2,012
350

These, http://www.ebay.com/itm/like/400531403623?lpid=82&chn=ps&ul_noapp=true or similar may come into play. The dial helps you to be able to set a value.

A 10 V high power reference is what I'm thinking of depending on the answer to Q.

Based on the input Z, you can select the pot value and the current requirements. 18 5 K's in parallel. You might be able to getaway with 100K or 10K.

Q2: what are the inputs protected to? 24 VDC?

Last edited: Jun 21, 2016
14. Alec_t AAC Fanatic!

Sep 17, 2013
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1,451
How accurate does the 4-20mA have to be? The transistor method suggested by Crutschow (post #12) will be fine providing you don't need high accuracy, but will drift a few % with temperature change. Greater accuracy would involve more complex circuits.

15. rfthe2nd Thread Starter New Member

Jun 21, 2016
12
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Here is all the info on the input for my controller... I just need a simple circuit to produce a variable 4-20 mA output using a potentiometer that simulates the signal from a sensor.. like an air flow sensor for example.. I need to be able to simulate about 12-20 different inputs to the controller at the same time and I want to be able to use only one or two 24vdc power supplies at the most... if it helps, I'm using this power supply: http://www.mouser.com/ds/2/260/SDR-240-SPEC-806261.pdf

16. Alec_t AAC Fanatic!

Sep 17, 2013
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How many is 'a bunch'?
Can you use a single 4-20mA loop current for all the 40-20mA inputs, or do you want individually controlled loop currents for each input?
Any idea what "Galvanically isolated sensors resp. switches must be connected" means in that attached info?

Last edited: Jun 22, 2016
17. crutschow Expert

Mar 14, 2008
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Can you find out what the 4-20mA loop impedances are?

18. rfthe2nd Thread Starter New Member

Jun 21, 2016
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@Alec_t I definitely need individual loop currents for each of the inputs, and I'm not sure what that means either..

19. KeepItSimpleStupid Well-Known Member

Mar 4, 2014
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With a 20K input Z, every voltage input is going to need an OP-amp buffer. We have to work on a selection. Without any info on input protection, i'd have to suggest a lower supply voltage for the buffers.

Current loop can be funny to deal with because of the positioning of the power supplies Can you post a diagram for that?

If you can configure the inputs to 0-20 mA that may make it easier to deal with and as long as you don't have to go to exactly zero and you can make one side common. it's then easy to inject a 0-5V or 0-10v source across a fixed resistor.

It's harder, but not impossible to come up with a 1-5V source (4-20 mA) or a 2 to 10 V source.

Note that 249 is 5/249 or 20 mA. Funny how that works.

20. hp1729 Well-Known Member

Nov 23, 2015
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What kind of digital inputs would you like to simulate? TTL? CMOS? Open Collector? Tri-state? Switches alone are a crude simulation.