# 4mA - 20mA Current Limiting Circuit in ratio with 5 to 36 volts DC.

Discussion in 'Homework Help' started by rmrjr22, Aug 25, 2015.

1. ### rmrjr22 Thread Starter New Member

Apr 10, 2011
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Objective is to design a current limiting circuit with a variation input voltage between 5 volts - 36 volts. The output current must not exceed 21 mA at 36 volts. Using a PZTA56 PNP transistor, LM234 Current Limiter IC, 1.2 Zener Diode. Cant seem to simulate it or replicate the outcome on a breadboard. Any help is appreciated!

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2. ### #12 Expert

Nov 30, 2010
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Could you indicate which is the 5V to 36V location and where is the output?
(Using MSPaint.)

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3. ### rmrjr22 Thread Starter New Member

Apr 10, 2011
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Circled in Red will be a change in DC voltage ranging from 5 volts to 36 volts max. It is expected that when voltage is at the minimum 5 Volts, 4 mA will be the output. When max 36 volts is reached, 20 mA will be reached.

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4. ### #12 Expert

Nov 30, 2010
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Where is the output?

5. ### rmrjr22 Thread Starter New Member

Apr 10, 2011
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Out of the LM234 Adjustment Node. "R"

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6. ### #12 Expert

Nov 30, 2010
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You marked the output as a voltage. I thought it was supposed to be a current.
Are you saying that a 4ma to 20 ma output across a 68 ohm resistor is where the result is not what you desire?
4 ma x 68R = 0.272 volts
20 ma x 68R = 1.36 volts

7. ### rmrjr22 Thread Starter New Member

Apr 10, 2011
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My mistake, the "Vout" is meant to be Iout. The zener diode limits the Vout voltage to about 1.3 Volts. The output current when the voltage is adjusted from 5 volts to 36 volts input, should be 4mA to 20mA proportionally. The desire is controlled current output. When R3 is 66.5 Ohms, Iout = 20mA. When R3 is 325 Ohms, Iout= 4mA. How do I make the current regulator act as a current regulator without having to change R3 value.

8. ### atferrari AAC Fanatic!

Jan 6, 2004
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Where the circuit comes from? Is it your creation?

Any datasheet used as reference?

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
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I can tell it's not from the datasheet.

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Apr 10, 2011
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11. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Welcome to AAC!
Why did you opt to use an LM234 (which is rated as programmable up to 10mA only)? Is Q1 intended to provide most of the 20mA current?

12. ### rmrjr22 Thread Starter New Member

Apr 10, 2011
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That is correct Alec

13. ### rmrjr22 Thread Starter New Member

Apr 10, 2011
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The LM234 has a higher temperature operating range.
I would like for it to work in somewhat extreme compound temperatures. (-10 C to 70 C)
The LM234 is flexible in this area and has the capability to handle the voltage range.
If there is another component better suited for the task, any help would be appreciated.
Just want to make sure I am not missing anything in getting full functionality out of the LM234.

14. ### Alec_t AAC Fanatic!

Sep 17, 2013
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I'm unclear as to whether you're trying to make a voltage-controlled 4-20mA current-loop circuit, constant-current circuit, or current-limiting circuit (i.e. any current up to a controllable maximum) as the thread title says? Can you confirm?
I'd be looking at using an op-amp rather than an LM234.

15. ### rmrjr22 Thread Starter New Member

Apr 10, 2011
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Current Limiting circuit up to about 21 mA. Proportional to the rise in voltage. For example, at 5 volts input, the current will be measured at about 4mA, then at 36 volts maximum, the current will reach 20 mA. So as the input voltage increases, as does current. But only within those ranges. I have a hundred LM234's lying around, but not many PZTA56 PNP transistors. The PNP's purpose is to make up the additional 10mA the current adjuster can not. Leveling off at 20 mA when voltage reaches 36 volts.