4 - 20 mA power supply.

Hi everybody,
I am currently involved in a project that includes several pt100 that I intend to connect to an arduino UNO by using a current loops. I guess that the power supply (24 V) I need has to have some specific characteristics.
Can I use a switching one?. Do I need an "old fashioned" linear one?. Would one of those cheap ones used for LED strips do?.
Thanks in advance for your advice.
Regards

Vicente ..

Can I use a switching one?

Yes.

Would one of those cheap ones used for LED strips do?.

What specific one? Some of these are constant current and that is not what you want.

Grumpy_Mike:
What specific one? Some of these are constant current and that is not what you want.

Thanks Mike,
I didn't know they -some of them- were constant current; in fact I mentioned them as an example of "cheap" ones ...
As power requirements are low (6 x 20 mA; 24 V), and -I know- linear ones may be, up to a certain point, "improved" by adding capacitors, my understanding is that they -linear- will not present noise problems ... I am not sure if switching ones will have noise problems.
Thanks again.
Regards
Vicente ..

How are you planning on converting the pt100 "signal" to a 4-20mA current loop?

adwsystems:
How are you planning on converting the pt100 "signal" to a 4-20mA current loop?

Hi,
Something like this:
https://www.ebay.es/itm/SENSOR-EXTERIOR-AKTIV-PT100-TRANSMISOR-DE-TEMPERATURA-0-10V-Voltaje-universal/291799526540?hash=item43f09b6c8c:g:~5AAAOSw3ydVixDe
(If you look at the description it says that is configurable "0-10V" - "4-20 mA". By the way, I wonder if this "german made" transmitter is industrial grade).
Thaks.
Regards.
Vicente ..

You'll also need some precision 250ohm resistors to convert the 4-20mA into a 1-5v input to your UNO and you'll also need to think about how you connect these resistors to the common ground point since you'll be sharing current paths between numerous devices.

In the industrial world a separate power supply is generally used for each transmitter to minimise problems.

The transmitter is unlikely to be 'industrial grade' at the price they are selling for.

jackrae:
You'll also need some precision 250ohm resistors to convert the 4-20mA into a 1-5v input to your UNO and you'll also need to think about how you connect these resistors to the common ground point since you'll be sharing current paths between numerous devices.

In the industrial world a separate power supply is generally used for each transmitter to minimise problems.

The transmitter is unlikely to be 'industrial grade' at the price they are selling for.

Hi,
Many thanks for your advice.
Have you at hand any reference for a "good" transmitter?.
Thanks again.
Regards
Vicente..

What makes you think it's not a "good" transmitter.
They offer a 2 year guarantee and a host of ancillary support items so obviously not a piece of far eastern cheap junk.
"Industrial grade" means different things to different people. Having spent many years in heavy industry my idea of "industrial grade" generally means metal housings with expensive components and loads of certification documents, but to others it may mean something used in anything other than a domestic environment.
The vendor's website offers a dialogue link so why not ask them directly about the quality of their product.
As an aside, is your proposed UNO of 'industrial grade'

my understanding is that they -linear- will not present noise problems …

No all components generate noise. It is just that a linear regulator generates less noise than a switching one.

Capacitors on a linear regulator are not optional they must be used otherwise they will oscillate.

jackrae:
What makes you think it's not a "good" transmitter.
They offer a 2 year guarantee and a host of ancillary support items so obviously not a piece of far eastern cheap junk.
"Industrial grade" means different things to different people. Having spent many years in heavy industry my idea of "industrial grade" generally means metal housings with expensive components and loads of certification documents, but to others it may mean something used in anything other than a domestic environment.
The vendor's website offers a dialogue link so why not ask them directly about the quality of their product.
As an aside, is your proposed UNO of 'industrial grade'

Jack,
Thanks for your post.
You're right: 'industrial grade' is not an accurate specification.
In fact the arduino I am going to use is a "pro mini" (sparkfun; has the same uController than the UNO). I power it through a Mean Well P.S. and behaves quite well over long periods of time in, say, lab conditions. By this time I have some units working with analog voltage sensors (LM35), and results are quite satisfactory. Of course I have added some electronics to filter the signals and protect the chips.
The choice for current loop is to connect sensors over a longer distance (20 m).
Thanks again.
Regards.
Vicente ..

Grumpy_Mike:
No all components generate noise. It is just that a linear regulator generates less noise than a switching one.

Capacitors on a linear regulator are not optional they must be used otherwise they will oscillate.

Mike,

Thanks for your advice.
In fact I was wondering if it is worth (just thinking on) deeming a linear one as an alternative …I’ve never worked with loop current transmitters, so I don’t know how sensitive they are to noise.
Perhaps the best idea is to buy a “serious” transmitter and check how it behaves with a switching PS. Let’s see …
Regards.
Vicente …

If you want a simple rugged pre-made ready-to-go RTD to 4-20mA converter, then I suggest the Burns Engineer TL21. Been around a lllooonnnggg time...for a reason. Simple robust. Works with pt100 RTDs.

Burns TL21 spec sheet.

I found a couple on ebay.com for 30-40USD.

adwsystems:
If you want a simple rugged pre-made ready-to-go RTD to 4-20mA converter, then I suggest the Burns Engineer TL21. Been around a lllooonnnggg time...for a reason. Simple robust. Works with pt100 RTDs.

Burns TL21 spec sheet.

I found a couple on ebay.com for 30-40USD.

¡Many thanks!

(Observe the opening "¡": to give more emphasis)

Regards.
Vicente ...

On long current loops you should use twisted pair wires so that any noise that is introduced is common to both legs. You need to identify if the module you intend to use is a 2-wire (combined power and current signal) or a 4-wire (separate power and isolated current signal) device. Although the latter is more complex, it is easier to use, especially if you are using a single power supply to power multiple modules.

Hi everybody,
Finally the project is developed, built and working.
I’ll try immediately to upload the sketch and comment it …
(I’ll try to embed the picture in the post following the -as usual- excellent instructions from Robin. Let’s see …)

Bingo!! (Thanks again Robin).

The logger has been working for two weeks up to now. Conditions are not too tough, but it is an industrial environment: system is monitoring and logging the temperature -six channels- data for a thermal treatment (60 ºC). As you can see a Mean-Well power supply has been installed for the current sensors (24 V); it is rated 625 mA and works fine.

As this is the opening question of the thread I deem it is "SOLVED". Needless to say, I'll answer any question concerning the issue that anybody may ask.

Thanks everybody.