converting 1-5Vdc to 4-20mA
- Thread starter theripley
- Start date
bernie_carlton
Lifetime Supporting Member + Moderator
it only makes sense if the TOTAL resistance that the 1-5 VDC was applied across was exactly 250 ohms. If you apply 1-5 VDC across a 250 ohm resistor then you have 4-20 ma but it can't be used anywhere except a circuit which has zero resistance.
electrical_geek
Member
It is at least a theoretical way to solve the problem. If you have a 250 ohm resistor and 5 V voltagedrop over the resistor the current in the circuit will be 20 mA.
If you try to measure the current with some device you have to reduce the 250 ohms with the internal resistance of that device so the total resistance is exactly 250 ohms.
On top of that you must assure that your 5V source is capable to deliver 20 mA.
If you try to measure the current with some device you have to reduce the 250 ohms with the internal resistance of that device so the total resistance is exactly 250 ohms.
On top of that you must assure that your 5V source is capable to deliver 20 mA.
Hi electrical geek,
I have attached the technical specs of the pressure switch. It says, max.load current is 80mA so I guess it can deliver 20 mA.
What does usually integrators do when they convert 1 signal to the other, without using a signal conditioner? Signal conditioner is pretty expensive, here it cost at around Php 20,000.00
I have attached the technical specs of the pressure switch. It says, max.load current is 80mA so I guess it can deliver 20 mA.
What does usually integrators do when they convert 1 signal to the other, without using a signal conditioner? Signal conditioner is pretty expensive, here it cost at around Php 20,000.00
electrical_geek
Member
Your transducer is no problem.
The problem is more likely with your measuring device. A normal inputresistance for a current input on a PLC (or similar) is normally between 250-500 ohms. This will make your solution impossible. If you can find a current input with very low internal resistance you can use this solution. You have to keep the total resistance at exactly 250 ohms.
If I understand the specification above correct, the pressuretransmitter is capable of voltage or current output. So why don´t you use the current output directly?
The problem is more likely with your measuring device. A normal inputresistance for a current input on a PLC (or similar) is normally between 250-500 ohms. This will make your solution impossible. If you can find a current input with very low internal resistance you can use this solution. You have to keep the total resistance at exactly 250 ohms.
If I understand the specification above correct, the pressuretransmitter is capable of voltage or current output. So why don´t you use the current output directly?
I'm going to say that you cannot go 1-5 volts to 4-20 mA without a signal conditioner.
Going from 4-20 mA to 1-5 volts can be done with a resistor (because voltage inputs are usually very high impedance), but it won't work the other way--even with a low impedance analog input.
Your best bet is probably going with a 4-20 mA signal from your pressure transmitter. The cut sheet you posted indicated that the option is available.
Going from 4-20 mA to 1-5 volts can be done with a resistor (because voltage inputs are usually very high impedance), but it won't work the other way--even with a low impedance analog input.
Your best bet is probably going with a 4-20 mA signal from your pressure transmitter. The cut sheet you posted indicated that the option is available.
Tom Jenkins
Lifetime Supporting Member
As stated above, going from 4-20 mA to 1-5 VDC over a 250 Ohm resistor works fine - I've done it many times.
Don't try it the other way from 1-5 VDC to 4-20 mA. The math works out, of course, but most voltage output transmitters have a very high minimum impedence - usually in mega-Ohms. It looks like yours requires at least 1 k-Ohm. Putting the 1-5 VDC over a small resistor looks like a short circuit to the transmitter, and it will draw more current than they can put out. At the least the transmitter will drop out, at the most the smoke will escape from the transmitter.
Use a signal conditioner. AutomationDirect.com, Acromag, Action Instruments, APi, and many many others build these.
Don't try it the other way from 1-5 VDC to 4-20 mA. The math works out, of course, but most voltage output transmitters have a very high minimum impedence - usually in mega-Ohms. It looks like yours requires at least 1 k-Ohm. Putting the 1-5 VDC over a small resistor looks like a short circuit to the transmitter, and it will draw more current than they can put out. At the least the transmitter will drop out, at the most the smoke will escape from the transmitter.
Use a signal conditioner. AutomationDirect.com, Acromag, Action Instruments, APi, and many many others build these.
Last edited:
It says on the NOTE 2 that when a voltage output is selected, a simultaneous connection of switch out & current out cannot selected. I could choose current out but that would mean i would buy another pressure switch with that indication. The client who provided the switch was not taking into consideration the analog signal. So, as a technical person it would definitely be up to me to work my way.
Many might wonder why I need to convert my 1-5Vdc source to 4-20mA. My analog module Mitsubishi FX2N-2AD can only be worked with 0-10Vdc, 0-5 Vdc & 4-20mA. It just gives me headache that the switch was not current out. As all have suggested, i think it would be best to use a signal conditioner. I hope I could find an inexpensive one that is locally available. Thanks so much for your replies.
Anyway, how about converting 1-5 Vdc to 0-10Vdc or 0-5Vdc?
Anyway, how about converting 1-5 Vdc to 0-10Vdc or 0-5Vdc?
robertkjonesjr
Member
You don't need to convert 1-5VDC to use in either 0-5 or 0-10VDC. just plug it in. Do it in software - just scale it with the available math functions! You might lose a little resolution, but it's free!
bernie_carlton
Lifetime Supporting Member + Moderator
I read the specs that, if you want the switch output you give up any analog output.
I take it that setpoints can be set in the sensor to give a switched output. Would it be ok to do the level logic in the PLC and let it provide a switched condition? Or is a switched indicator right by the sensor?
Assuming the indicator is next to the sensor then you may have to either get an inexpensive 'switch only' sensor to provide the local indication or run extra wires from the PLC to a local indicator.
I take it that setpoints can be set in the sensor to give a switched output. Would it be ok to do the level logic in the PLC and let it provide a switched condition? Or is a switched indicator right by the sensor?
Assuming the indicator is next to the sensor then you may have to either get an inexpensive 'switch only' sensor to provide the local indication or run extra wires from the PLC to a local indicator.
How would I do it?(sorry if it sounded naive). The scaled value on my module is 0-4000, if I plug it to the module I will get 800-4000 & not 0-4000. The value of the switch will be compared to another signal (4-20mA) therefore comparing both minimum values (4mA & 1Vdc), it will yield 800 & 0 which are not equal values at all.
The system that I will make is an air pressure monitoring system so I have a regulator with analog in of 4-20mA. The pressure will be set on the touchpanel. The pressure switch will serve as its feedback, that is to ensure that the regulated pressure must equal to the set point. If its not equal then an alarm would output.
I hope I answer your question.
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