Can someone please help me with a small circuit. Im sure it sounds simple, yet i cant exactly get my head around it.
Im required to simulate 4x 4-20mA outputs drawn from a single 5v voltage source. (the output would be manipulated with wire wound pots).
How do i isolate the current going into each terminal so that the effect of the different resistances dont affect each other?
Also, can someone please tell me the effect on a circuit with Diodes (in my case LED's) in parallel? i havent studied non linear devices in detail yet at uni, so im not quite sure how to deal with them- all i know that theres the 0.6-0.7v drop across the diode.
You need at least 5 terminals, eh? That's not much compliance for a process signal.
Visible LEDs generally have around 1.5-3.5V forward voltage drop at
20mA, depending on type type and the color.
Putting identical LEDs in parallel can work, but the current won't split exactly evenly. They are resistive enough that one can get away with it in practice, under some conditions and with some LEDs.
Its for an RTU - im just a newbie, but my manager wants to be able to simulate all 4 analogue inputs - & just looking at the unit, ive got
12v and 5v - is there a way to split and isolate the 12v voltage source or something similar so i dont have to worry about the effect of different loads? -
i dont think ill include LEDs with this unit because it would just be too much of a hassle, but thanks for the information about the LED's.
The basic 4-20 ma spec calls for a floating current source. Effective circuits needn't have unlimited compliance, but the actual compliance needs to be part of a particular spec. A reasonable spec is that the current in the loop may not vary more than V percent when the load is between a short circuit to W ohms and the negative side of the load is between +X to -Z volts relative to ground. Given reasonable numbers, a competent electronic circuit designer can come up with what you need. There are op-amp circuits that can do that if the compliance doesn't exceed their ratings.
+/-12 would make the work easier, but +12 and -5 (or vice versa) will do in certain cases. There are+% to +/-12 converters used to drive RS-232 chips, but they probably won't deliver 20 ma.
LEDs can't be run in parallel. Each needs its own ballast resistor. That's really no big deal.
Yes, it's fairly straightforward for anyone who does this kind of electronic design.
I think you're asking for four manually adjustable 4-20mA current sources with a common ground return. The compliance (maximum output voltage without losing current regulation, or to put it another way, the highest load resistance for which you can get the full 20mA out) would be in the 500-ohm range, with it getting progressively more difficult to get some part of the next 100 ohms with a 12V supply (you could always add a DC-DC converter to get a 24V supply internally if you wanted more compliance.
The output voltage will vary from 0 to close to 10V as the load resistance is changed, in order to regulate the current.
The internal circuitry is fairly straightforward, and even inexpensive parts (except for the pots, they are expensive) can yield 0.1% stability with a bit of care. I'll not get into the detailed electronic design issues here, but obviously you have to measure and regulate the currents individually on each output rather than the common (-) return.
If you want to put an LED in series with each output, it will work fine, however you will lose a bit of compliance. A blue or white LED might have more than 3V drop.
From a straight "I" standpoint the supply needs AT LEAST 20 mA X 4 = 80 ma, so a supply that will do 5V ( or 12 ) AND 80 ma is the ABSOLUTE MIN. With standard sizes something like 100 - 250 ma TOTAL LOAD rated supply as to allow for future add on channels. If your supply is heavy enough "I" you could use the 12V to feed a separate 5V regulator IC + filter caps for each channel to provide POS side channel supply voltage isolation, like an LM7905.
From a straight "I" standpoint the supply needs AT LEAST 20 mA X 4 = 80 ma, so a supply that will do 5V ( or 12 ) AND 80 ma is the ABSOLUTE MIN. With standard sizes something like 100 - 250 ma TOTAL LOAD rated supply as to allow for future add on channels. If your supply is heavy enough "I" you could use the 12V to feed a separate 5V regulator IC + filter caps for each channel to provide POS side channel supply voltage isolation, like an LM7905.
Taking that a step further, to be able to support a 100-ohm burden with one side grounded, only 2 volts is needed, plus whatever drop the current source (likely a voltage-to-current converter) uses up. For 500 ohms, the requirement is 10 volts. Wire drop and however much the load floats above ground has to be added to that.
It's quite possible to build the circuit using a single supply. If there are no monolithic voltage-to-current converters intended for the application, the design time will be less with a bipolar supply that allows the use of traditional op-amp voltage-to-current converters. For a one-off, the dual supply will be cheaper than anyone's time.
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