4-20mA potentiometer - Page 1
Forum section: electronics / projects

author: gaminn
author_rank: Frequent Contributor
timestamp: October 26, 2022, 07:44:32 pm
content: Hi,I'm looking for 4-20mA "potentiometer". I found only this device:https://www.tme.eu/details/cm22-4-20ma/panel-mount-accessories/cobi-electronic/Is it the only 4-20mA potentiometer in the world or am I unable to find them? Thanks for any hint.

author: jkostb
author_rank: Regular Contributor
timestamp: October 26, 2022, 08:05:41 pm
content: You can build yourself very easily a 4-20mA by use of a LM317 and a potentiometer. Is also much cheaper.

author: gaminn
author_rank: Frequent Contributor
timestamp: October 26, 2022, 08:33:34 pm
content: Thanks, but I would like to buy integrated industrial device.

author: pqass
author_rank: Super Contributor
timestamp: October 26, 2022, 08:33:48 pm
content: See:http://www.reuk.co.uk/wordpress/electric-circuit/lm317-current-calculator/Enter the target output current for 4mA and 20mA to see what resistances you'll need.Spoiler: Replace fixed resistor in the link above with the following 500R pot+resistors (varies from 65R to 320R).Also, add a diode before the LM317 for protection. Easy peasy lemon squeezy.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 26, 2022, 10:35:37 pm
content: Quote from: gaminn on October 26, 2022, 07:44:32 pmYou may google "4-20ma current loop transmitter", "4-20ma current source" or "4-20ma process meter".

author: SmallCog
author_rank: Frequent Contributor
timestamp: October 27, 2022, 02:42:14 am
content: What are you trying to accomplish?If you're looking to test an input then there are tools as mentioned above.I also have a whole bunch of these in my drawer for simulating processes, they're cheap and I don't trust them to be super accurate but handy for testing functionality:I reckon I paid about $10 each for them at the time.If you're looking to have a control input for an operator then perhaps something like this?https://oceancontrols.com.au/CMC-060.html

author: gaminn
author_rank: Frequent Contributor
timestamp: October 27, 2022, 10:43:47 am
content: It will be used for variable frequency drive control. I would like to replace "voltage" potentiometer with "current" potentiometer. One to one replacement. There is not much space inside control panel do embed any voltage to current transmitter box.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 27, 2022, 03:51:50 pm
content: DIY transmitter from LT app note. Easy

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 27, 2022, 10:41:26 pm
content: Quote from: gaminn on October 27, 2022, 10:43:47 amThere is no practical advantage to a current based device over voltage unless you need to route hundreds of feet of cable in high noise environments where you’re trying to solve an existing problem. If you need the resolution of a ten turn pot, the non-linearities of a current source may well trash your linearity budget.Precision 2 to 5K wire wound pots on a +10 volt reference provided by the drive has been the standard of both DC and later AC variable speed drives for fifty years or more for one single reason: it works. You need a pretty good reason to do what you’re suggesting as it has some significant down sides. Cost, complexity and reliability are the first three things that come to mind.

author: gaminn
author_rank: Frequent Contributor
timestamp: October 29, 2022, 08:06:25 pm
content: Thanks for your explanation. I just automatically assumed, that 4-20 mA current loop would be less susceptible to noise as I need to route 3 metres of vfd to potentiometer cable. Linearity is not important. All cables are of course shielded and motor and control cables are routed at least 20 cm away from each other.

author: RogerThat
author_rank: Regular Contributor
timestamp: October 29, 2022, 09:41:25 pm
content: With 4-20mA you can easily detect cable breaks and errors...not to mention the signal you send will always arrive exactly the same, with voltage you may have drops.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 31, 2022, 07:23:38 pm
content: It it depends on what the noise type and level is. Either way current or voltage signalling need proper rf bypass and surge protection.4-20mA is common in power plants and industry where the control room is far away from the hot action.The current loop lets you power the remote end without extra psus or cabling. Thats why 0-20mA is not used much now there are low power sensor signal conditioners.4mA at up to 24V is very handy.

author: mushroom
author_rank: Regular Contributor
timestamp: November 01, 2022, 02:00:23 am
content: There's no need for 4-20mA or 0-20mA control. I am using VFDs in my workshop (total = 4), with long cables for commands and power, with no shielding, and with absolutely no problems. On the picture you can see two cabinets. The top one powers 3 machines that can be selected, in a star topology, the longest cables being 7 meters. The bottom one is for a milling machine (2 VFDs), longest cables are about 3 meters. The last one (not on the picture, with a chinese crap) is for a lathe and also has 7+ meters cables. Zero problems. Keep in mind the analog inputs are filtered and the logic ones are filtered + debounced ! I only use the 10V source of each VFD, wire it to the machine command buttons and pot, and send back the pot voltage to the VFDs (meaning 2 x 7 meters). Again no shielding, no problems.There's tons of urban legends about VFD input sensivity to EMF. Just test : you will make things simpler and cheaper. Simpler means more reliable. I even routed the unshielded cables side by side, power and control (they are just just separated, because routing low voltage and high voltage in the same rails is bad pratice (safety, different isolation classes).I've been using VFDs this way for 15+ years with zero problems. Not even one.Oh, forgot to say : no problems with WiFi and Zigbee in the house.