W

#### William Irish

Does anyone know where I can find a 4-20ma readout display that has 1" LED, 4.5 digits, loop powered.

Thanks

J

#### Johan Bengtsson

I don't know for sure, but I seriously doubt they exist. Reason: the power needed to light a LED is not much, but you have to take it from somewhare. Taking it from the loop would require some significant voltage drop at 4mA, probably too much for anyone to bother doing one.

Of course this does depend on how bright you want the digits, and how much voltage drop you can accept.
Most standard LED:s are feed by some 10-30mA with a typical voltage drop about 1.8-2.2V.

For calculations: 10mA at 1.8V = 18mW per LED
suppose we want to be able to light about 20 LED:s
at currents near 4mA, that is 360mW, requiering the voltage drop of the loop to be at least 90V.
To that add the lost power during the conversion (to lower voltage/higher current) and the power needed for the measuring and so on.

Ok, low power LED:s exist, but even if you bring the current down to 2mA per LED you still need more than 18V of voltage drop, and that is probably too high already.

Either you have to feed it separately, or use something else than a LED, for instance a LCD.

/Johan Bengtsson

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P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
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B

#### Bruce Durdle

I don't remember seeing the original post, but MTL have some (MTL636, MTL637).

7 segment
4.5 digit
20 mm
Voltage required is 5 V at 20 mA

Their 681 series uses Liquid Crystal display and has a 1.8 v drop at 20 mA. It is classed as ":simple apparatus".

Bruce.

M

#### matt

it is not possible to power 4.5 LED digits off 4mA or less . The closest options for you would be:
1- loop powered LCD display , or
2- separate power supply LED display,
can't have them both at the same time in my experience. I'd be interested too if anyone has the loop powered LED display.

J

#### jroop

Try the Omron K3NX digital meter.
Also checkout Red Lion Controls

K

#### kvenkat1

There are even 2" high LED displays. You can see them from great distance. Check out at www.predig.com

Regards,

K. Venkatachalam

F

B

#### Bill Gausman

In an LED display, the segment currents are not additive, as they are strobed - lit one at a time by the display driver. The brightness is a
function of drive current and the on-time ratio of each strobe pulse. The average current is a function of ((on-time ratio) x (segment drive

A 3-1/2 digit loop powered LED display with large digits is in the DIGI-KEY catalog (1-800-digi-key), part number CDPM957-ND (green) or
CDPM857-ND (red). Overall size 96mm x 48mm. Maybe the manufacturer has a 4-1/2 digit in that series, as well.

Bill Gausman, SmartParts
http://www.smartparts-usa.com

J

#### Johan Bengtsson

Are those made with a LED display or with LCD?

If it is a LED display: what are the voltage required at 4mA? (probably significantly higher than 20mA, I would expect something approx 25V (5V*20mA/4mA) to get the same power for the LED:s.

Hmm, that would mean less than 5mW per LED,
interesting....

/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------

J

#### Johan Bengtsson

They are not on at the same time, that is true.

That only means you run a higher current thru the
LED for a shorter time, the amount of power required are (almost) the same. In fact it is even a little bit higher than it would have been if you actually didn't switch the current (the voltage drop are rised a little bit, not that much but anyway when the current are higher). As you say, the average current is what gives the brightness, the power needed to light two segment is twise the power needed to light one (or actually a very small amount higher when they
are strobed than when not).

That effectively means: more LED:s turned on, more
average current to the circuit driving them, unless of cource you keep the current constant and rise the input voltage instead, or if you just input enough power and burn the part you don't need in a resistor.

Regardless of how you do you have to take the power from someware, there are some ways to solve this:

1. Use an external power source (ie not take it from the loop).

2. Use very high efficient LED:s that are bright with a low average current.

3. Reduce brightness, you can always lower the average current and save power, but you loose in brightness.

4. Accept a higher voltage drop (at least at low currents).

5. Skip the LED:s and use a LCD or other method needing less power instead.

2-4 might of course be combined to get desired results.

Can we end this discussion now?

/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------

B

#### Bruce Durdle

Johan,
The MTL displays are all LCD.

Bruce.

S

#### S2

> Does anyone know where I can find a 4-20ma readout display that has 1" LED, 4.5 digits, loop powered.
>

Red-Lion Controls (www.redlion-controls.com)
What are you asking? 4-20mA current display? 4-20mA is typically a proportional feedback signal from a transmitter (sensor/electronics) 1" LED is height of 7-segment display; 4.5 digits are the number of digits displayed. LOOP powered (typ. 20mA max) is a serial (e.g. ASCII) connection to a display)....... I have NO IDEA why there are responses to this question calculating currents for LEDs. This sounds like a typical application for a serial display, OR 4-20mA proportional display!

S

#### S2

Why are you taking this to the discrete component level? This is NOT a question of ampacity; 4-20mA is a STANDARD feedback signal used for noise immunity. 20mA loop is typ. refering to MAXIMUM current in the communications loop!!!

S

#### S2

This is applications, not theory.
This is an application for a ready made display, not individual components. 4-20mA is a standard proportional I/O range; it is a current representing a value. e.g 4mA(500)=2V 2V is the actual I/O to an A/D or from a D/A converter.
20mA loop current is MAX current for a serial comm loop!

J

#### Johan Bengtsson

Well, when I read the question I understood it like this:

I have (one or more) analog signal(s) 4-20mA, going from one place to another.

I would like to conncect a display somewhere along the wire so someone can read out the signal as digits.

- well, no problem so far, it is just about measuring the current and display it as digits, lots of circuits for that exist.

Oh, by the way, I don't want to put in any extra wires for powering this device, I want the power to be taken from the 4-20mA current loop.

The problem here is that even when a LED doesn't require that much power it is probably too much for being taken from the signal wires when there only is about 4mA. This could be done, no question about that, but the voltage drop needed to get enough power out of the signal and still get enough light out of the display to be readable will be too high for most applications.

If this would be done with a LCD instead of a LED display the power requirements would be much lower and such devices do exist. Even a backlit LCD would probably be quite possible to power from the signal wire.

As I understood the original question, there was no thought of any serial protocol in the question. I am sorry if that was wrong, the original poster could perhaps answer this.

/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------

M

#### matt

Nobody was talking about discrete LED's here. It looks like I wasn't clear enough in my earlier posting. It is not possible to have a loop
powered LED display with today's technology . For those who apparently still wonder what loop powered means , it is simply a device that only requires the 4-20mA loop analog input and doesn't need a separate power supply. Another term would be passive) .

All the answers from various people who were getting down to component level were trying to explain this basic fact to the "systems" people . The reason for which the answers were all referring to the discrete LED's is because they're the main power hog in a meter display- the A/D function and microcontroller can be all powered off 100microamps or less . The LED's
can't and you either supply external power to your 4-20mA display LED's OR you select a low power technology called LCD .

The answer that suggested the Digi-Key CDPM957-ND (green) or CDPM857-ND (red) was wrong , as well as the answer complaining about the Digi-Key being just discrete LED's . They are described as 3 and 1/2 digit DIGITAL PANEL
METERs with analog 4-20mA input and LED display . They REQUIRE A SEPARATE +5VDC power supply to operate . The LCD panel meters #CDPM590-ND, CDPM190-ND and CDPM792-ND are 4-20mA analog input DIGITAL PANEL METERs which do NOT require any power supply ,being LOOP POWERED . They are however based on LCD and NOT LED .
respectfully,

Matt Tudor, MSEE
Elmar Technologies
http://personal.lig.bellsouth.net/~mariusrf