Voltage drop question

JeffKiper · Nov 28, 2011

OK I am stuck on a job with (1) 4~20 mA transmitter and (6) devices on the loop, 48 VDC power supply.
OK dumb math
6 devices 4 mA =6VDC drop on the loop.
6 devices 20 mA=30VDC drop on the loop.

My transmitter is only rated for a 40 VDC max input. If my loop ever gets to 4 mA I will have an over-voltage condition.

So I need to a 6 Volt drop to ensure I don't have an over-voltage condition if they take a few devices out of the loop. How do I get there.

If I do Ohm's law R=V/I
6 V/ 0.004 A = 1500 Ohm's
6V / 0.02A = 300 ohm's

I probably didn't explain this well I am tired

iant · Nov 28, 2011

do it the other way -

 V -----[ RES ]------[4 ~ 20mA Loop]------ V

Your resistance needs to see an 8 volt drop to maintain Maximun 40 Vdc. (around 400 Ohm)
I recomend you calculate it for a lesser voltage (32 or 24)

no create 6 Parallel circuits - the same

JeffKiper · Nov 28, 2011

Ian The transmitter has a minimum voltage of 10VDC so If I get a 20mA signal I will drop 30 VDC. See the fine line I am trying to balance?

iant · Nov 28, 2011

By using a resistance is series concentrate on the Value of R
the Supply Voltage will be 48 V
Voltage Drop on R is Max Current x R = 0.004 x 2000 = 8V Drop (40 Vs max)
or 0.02 x 2000 = 40 Volt Drop (10 Vs Min)

No issue with minimum voltage.
I would recomend you consider a Zener Diode to Prevent an Over voltage Spike

Mickey · Nov 28, 2011

Consider these or something like them.

4-20 mA Current Loop Splitter / Retransmitter

http://www.laurels.com/tls.htm

http://www.laurels.com/tls.htm

iant · Nov 28, 2011

Hi Mickey - they don't appear to be rated for 48 Vdc (30 Vdc Max)

Mickey · Nov 28, 2011

iant said:
Hi Mickey - they don't appear to be rated for 48 Vdc (30 Vdc Max)

He would not need 48VDC if he used one/couple of those. I think the only reason for the increased voltage was to support the increased load.

JeffKiper · Nov 28, 2011

Mickey is correct the original guys used a 48VDC supply because the had so many devices in series that the caused a significant voltage drop on the loop. I got the job of a just replacing the existing trip alarms with new ones. We all know how that goes the system was installed in 1999 so everyone of the alarms are obsolete and we had to get new devices.
I dropped the ball and didn't do a loop calculation because hey we are just replacing the alarms.
you all know what these jobs are like.

iant · Nov 28, 2011

Your probably right - I was remembering that some servo systems use 48V and their power supplies don't have a lower voltage available

iant · Nov 28, 2011

too bl--dy right Jeff - then you cop the blame - B-gger

JeffKiper · Nov 28, 2011

Yes last minute stuff on startup. I get screwed

iant · Nov 28, 2011

the resistor Calc's I have given will work Fine -
If Possible make 6 seperate loops and use 6 seperate Series CCts

JeffKiper · Nov 28, 2011

I only have 1 transmitter so I can't split the loops up.

iant · Nov 28, 2011

Ahh - Me Not Understand - Me Stupid -

It may still work Out - I'll get back to you shortly

JeffKiper · Nov 28, 2011

Not stupid at all

Voltage drop question

JeffKiper

Lifetime Supporting Member + Moderator

iant

Lifetime Supporting Member + Moderator

JeffKiper

Lifetime Supporting Member + Moderator

iant

Lifetime Supporting Member + Moderator

Mickey

Lifetime Supporting Member

iant

Lifetime Supporting Member + Moderator

Mickey

Lifetime Supporting Member

JeffKiper

Lifetime Supporting Member + Moderator

iant

Lifetime Supporting Member + Moderator

iant

Lifetime Supporting Member + Moderator

JeffKiper

Lifetime Supporting Member + Moderator

iant

Lifetime Supporting Member + Moderator

JeffKiper

Lifetime Supporting Member + Moderator

iant

Lifetime Supporting Member + Moderator

JeffKiper

Lifetime Supporting Member + Moderator

Similar Topics