# Deriving power from 4-20mA current loop

Discussion in 'The Projects Forum' started by GirishC, Feb 25, 2016.

1. ### GirishC Thread Starter Active Member

Jan 23, 2009
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0
Hi,

I have designed a circuit which will be powered by 4-20mA current loop. As per my guesstimate I do not need more power at 3.3V and hence this should suffice. The circuit is based on shunt regulator TLV431B as shown below:

I have not shown a Micro connection but the Output of Opamp U3C (Pin # 7) goes to the ADC input of the Micro.

1. Based on the the datasheet the min Vref required at TLV431B is 1.234V. With this potential divider values I set that.
The output goes to 4.2V for any value of current between 4 to 20mA both inclusive. The I disconnected U2, U3 from the connection so only TLV431 is on the board but output remains unchanged.
2. The datasheet also refers to value of Vk at Ik = 10mA. With this at 4mA the output voltage is 2.5V and at 20mA it is 4.5V.
3. I replaced TLV and resistor divider by LM385-2.5 and the circuit work just fine. But then the micro needs 3.3V.

Please suggest what I could be potential problem?

Regards,

Girish

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,130
766
Are you trying to develop a 3v supply using a 4-20mA supply loop, or are you just wanting to measure the current using a micro?

3. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
Suppose the power supply to the current loop was 50VDC. Would you still get 3.3 Volts at the indicated point? I've never seen anybody put a regulator in a current loop, that's not really the purpose.

4. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,949
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U3 needs a minimum 5V supply, according to the datasheet.

5. ### GirishC Thread Starter Active Member

Jan 23, 2009
58
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@Dodgydave : Yes, I am trying to generate 3.3V and measure current as well.

Jan 23, 2009
58
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7. ### GirishC Thread Starter Active Member

Jan 23, 2009
58
0
@Papabravo : Well to answer your questions there are two thing to understand
1. The input SMJ will protect it beyond 33V
2. The source is current controlled so the voltage is function of this current.

Hence it will not hurt the shunt regulator.

@Alec_t : Input is always higher than 5V.

Interestingly I replaced TLV431 by LM431 and I get output 3.3V. But LM consumes more current than TLV according to datasheet.
It appears that TLV needs more current but datasheet fails to substantiate it and I am hence confused.

8. ### atferrari AAC Fanatic!

Jan 6, 2004
2,663
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The way I understand you, whatever you do to generate additional signals ( keep in mind you said wanting to get power from the loop) will increase the current. At that moment, whatever that loop indicates, will be not the correct value.

Are you trying to make a conversion current-voltage?

9. ### Picbuster Member

Dec 2, 2013
391
53
To complicated. one chip is doing this for you look at xtr11X series chip but I <4mA

10. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
As I understand the way it is supposed to work you can't modify the current on the loop in any way if you are supposed to sense the current in the loop. You device won't play nice with the rest of the devices.

11. ### ebeowulf17 Active Member

Aug 12, 2014
682
79
Drawing current to power something within a current loop shouldn't be a problem. The transmitter regulates current in the whole loop, so as long as additional loads don't draw enough power to prevent the transmitter from driving current high enough, there's no problem (read about loop drop in the following paper.)