SparkFun Forums 

Where electronics enthusiasts find answers.

General project discussion / help
Did you make a robotic coffee pot which implements HTCPCP and decafs unauthorized users? Show it off here!
By stone
I want to use this circuit with 12vdc instead of 24.

So, I want my output(right side) to be 12vdc with the
variable milliamps, based on the microcontroller input.(left side)

Can anyone explain which resistors may need to be changed
to make this happen? (Or any other changes)

This is way past blinking the led and I’m a bit lost here.

I want to at least have some idea before I start to
take measurements and make things hot and smokey.

(I think it will work just the way it is, but I'm not sure)

You do not have the required permissions to view the files attached to this post.
By lyndon
Try it and see. My gut tells me it'll probably work (at 15V, I'd be more certain), since it's only 20mA max, so the transistors don't need to be saturated. It's probably designed for 24V supply because that's a common supply voltage for industrial equipment.

That said, I'm sure you could find a simpler 4-20mA converter online. Perhaps I should ask the obvious question: what are you trying to control?
By stone
lyndon wrote: Mon Jun 04, 2018 10:10 am That said, I'm sure you could find a simpler 4-20mA converter online.
First, thank you so much for being un-greedy with your knowledge, generous.

Second, it's taken me about 10 years to find this thing.
Where might I look for a simpler 4-20ma converter online?
I might as well switch before I get started,
although I have already ordered some parts.
(Using 1-5v from a pic or arduino)

If this circuit does not work correctly. Does this mean I would
get my 4-20ma, but then have too much voltage reduction below 12vdc?

If this is correct, what would I need to change, the transistors or the resistors?

If I understand the circuit, at least partially, the 10K resistor network
is just used because it's a big resistance and because it matches the other 10K resistor.

I'm a little foggy on transistors. Is saturation, the point where the transistor
switches 'ON'? If so, I don't know how to select a more useful or correct transistor.

I'm in trouble when I try to venture beyond blinking the LED.
And all I remember is P=IE and E=AR. So when it comes to figuring out how
to select a transistor, it may as well be quantum mechanics.

Thanks for your generosity.
By stone
jremington wrote: Mon Jun 04, 2018 11:50 am 4-20 mA interfaces are industry standard since about 1950, and many companies sell them. There are also interface ICs that are dedicated to the task, for example from Analog Devices: ... 20-ma.html

See this article: ... -interface
Oh man! These Analog Devices have 'Lectrolytes!

Wow, where am I,
and where are my pants?

I thought I was out of the forest when I discovered the 4-20mA Loop,
but this is a full beam out.

You know I'm thankful.

fwiw, I made my mistake by trying to learn in a general way
about controlling valves, in my case. When I picked out the
valve I wanted to use, it described the 4-20mA control clearly
in the data sheet. And now I'm here. I'll get off your shoulders now.
By lyndon
There are more modern ways of control. You can probably find a valve to suit your needs that's digitally controlled and avoid the whole 4-20mA debacle since you're starting with an Arduino.
By stone
Thanks Lyndon
I'll check into direct digital valves.
I'm still going to try to build this circuit.
I'd like to see how well it works. I spent about $40
for the parts. The precision resistor array was $19.

But thanks to good suggestions here, I found a 4-20mA
board for $2 on ebay. So when I go to actually control
my test valve, I'm going to go from arduino to DAC to 4-20 board,
and see how that works out.

You have all been very helpful, thanks.
long long title how many chars? lets see 123 ok more? yes 60

We have created lots of YouTube videos just so you can achieve [...]

Another post test yes yes yes or no, maybe ni? :-/

The best flat phpBB theme around. Period. Fine craftmanship and [...]

Do you need a super MOD? Well here it is. chew on this

All you need is right here. Content tag, SEO, listing, Pizza and spaghetti [...]

Lasagna on me this time ok? I got plenty of cash

this should be fantastic. but what about links,images, bbcodes etc etc? [...]