DC Current indicator using LEDs

[MWTinOK]

Hi folks,

I'm trying to build a current indicator to test various input devices. The voltage will range from 12 to 24V DC. I need to be able to indicate 0-20 mA. I can do voltage switching for 12V or 24V or build a separate circuit for different voltages. The thing I need is to be able to use a dot or bar display to indicate the mA currents.

Thanks in advance.

 

[kripacharya]

you can maybe use LM3914/ 3915/ 3916. the data sheet has many examples. you can check national and texas instruments

 

[MWTinOK]

you can maybe use LM3914/ 3915/ 3916. the data sheet has many examples. you can check national and texas instruments

I've looked at the datasheets and examples from those, but they all show indicators from variance in voltage. I need the indicators to change from variance in the mA supplied to the circuit.

 

[kripacharya]

I've looked at the datasheets and examples from those, but they all show indicators from variance in voltage. I need the indicators to change from variance in the mA supplied to the circuit.

there is a simple component which converts current to voltage. it is called resistor. you can combine this with opamp too if you need to change polarity of the voltage.
A 250 ohm resistor will convert 0-20mA to 0-5V

 

[crutschow]

there is a simple component which converts current to voltage. it is called resistor. you can combine this with opamp too if you need to change polarity of the voltage.
A 250 ohm resistor will convert 0-20mA to 0-5V

That's OK if you don't mind a 5V drop in the power to the circuit whose current your are measuring.

In not, than more typically a shunt resistor value is used that gives 50mV to 100mV at full scale current which is then amplified to the desired voltage. A 5Ω resistor will give 100mV at 20mA. There are also dedicated ICs that are designed to convert a high-side current into a ground based voltage.

 

[kripacharya]

That's OK if you don't mind a 5V drop in the power to the circuit whose current your are measuring.

In not, than more typically a shunt resistor value is used that gives 50mV to 100mV at full scale current which is then amplified to the desired voltage. A 5Ω resistor will give 100mV at 20mA. There are also dedicated ICs that are designed to convert a high-side current into a ground based voltage.

i think this is a current loop circuit from some sort of transducer, and not a power supply circuit. Of course we can only guess on this from information given and our own knowledge, because no circuit outline has been posted. making a power supply for just 0-20ma would be very trivial.. is it not ?

 

[tpetar]

Consider using OpAmp between input and LM3914.

 

[kripacharya]

Consider using OpAmp between input and LM3914.

this is good suggestion, but what is the parameters we have to consider to make choice ?

 

[tpetar]

**broken link removed**

 

[MWTinOK]

i think this is a current loop circuit from some sort of transducer, and not a power supply circuit. Of course we can only guess on this from information given and our own knowledge, because no circuit outline has been posted. making a power supply for just 0-20ma would be very trivial.. is it not ?

Correct, I am measuring a transducer output. We have a device that we connect our equipment to that can be a 0-10 mA output or a 4-20 mA output. Many times the polarity is also reversed, so I will have a two-color LED prior to the measurement circuit to indicate reversed polarity. Yes, I understand all this can be done with a multi-meter, but I'm trying to make a convenient tool for those in our company who don't have a lot of practice with a multi-meter to diagnose this circuit setup quickly.

 

[kripacharya]

**broken link removed**

? this is too complicated circuit which i cannot understand in the context of this thread. why are you measuring millivolts ?? the requirement is to measure current in the mA range. please explain what this circuit does, and how the opamp is required for current measurement and interface to the lm3914.

the input current on pin5 is ~5nA, so it will not load the measurement anyway. why we need opamp at all ?

 

[tpetar]

? this is too complicated circuit which i cannot understand in the context of this thread. why are you measuring millivolts ?? the requirement is to measure current in the mA range. please explain what this circuit does, and how the opamp is required for current measurement and interface to the lm3914.

the input current on pin5 is ~5nA, so it will not load the measurement anyway. why we need opamp at all ?

Please dont make mess in threads, you dont helping thread poster to make solution. I think you should try to help others with some suggestions not to be critical on every post of other users. Lets put thread owner to say opinion.

I just give one example of opamp with LM3914. User could adjust to his needs or to get idea for his problem.

 

[kripacharya]

I'm trying to make a convenient tool for those in our company who don't have a lot of practice with a multi-meter to diagnose this circuit setup quickly.

sounds like an excellent concept.

the polarity reversal can cause some trouble. good idea with the coloured leds, but only problem is that with the lower side of current, the leds will not light up properly. i assume you plan to connect them in series to the current loop ? typical leds need 10mA - 20mA for decent brightness

right now i cannot think of a simple effective way to solve this in simple ways, but i have 1 - 2 ideas

- - - Updated - - -

Please dont make mess in threads, you dont helping thread poster to make solution. I think you should try to help others with some suggestions not to be critical on every post of other users. Lets put thread owner to say opinion.

I just give one example of opamp with LM3914. User could adjust to his needs or to get idea for his problem.

i only asked for explanation of your circuit, and how it measures current, and why opamp is needed. all this is only to help poster, since you did not give any details anywhere.
my apologies if asking a question on this forum is a bad thing to do.

sorry

 

[tpetar]

i only asked for explanation of your circuit, and how it measures current, and why opamp is needed. all this is only to help poster, since you did not give any details anywhere.
my apologies if asking a question on this forum is a bad thing to do.

sorry

LM3914 cant measure small values direct, according to datasheet. With opamp he can adjust offset of measurement. Circuit is not mine, I give original link, description you can see on that page.

my apologies if asking a question on this forum is a bad thing to do.

Asking question is not problems, of course this is forum, but I have impression that you jumping through forum and siking my posts just to give some reply on it. I dont understand that. I think that is better to give that energy into making solution for thread problem.

 

[kripacharya]

but I have impression that you jumping through forum and siking my posts just to give some reply on it. I dont understand that. I think that is better to give that energy into making solution for thread problem.

i try my best to give meaningful suggestion in many many posts on many many topics. i do not remember if you are there everytime also, i think not.
why you say such things ? i always try give best idea i can have, with all details and diagrams and calculations always.