# TLV431 problem

Discussion in 'General Electronics Chat' started by Dritech, Jun 12, 2014.

1. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi,

I am using the TLV431 shunt regulator. With the output I connected a potentiometer with a resistance of 500 to 400 ohms. When using the Excel calculator provided by Texas Instruments, I came with the following resistor values:

R1 = 8.7 ohms
R2 = 35700 ohms
R3 = 12400 ohms

(Note: Input voltage = 4.9V to 5.1V , output voltage = 4.8V)

http://www.ti.com/tool/tl431calc

When connecting the circuit to a power supply, the SC indication LED was turning on. I then increased the value of R1 to approx 1kohms (If I remember correctly). This time there were no short circuits.

I then did the following calculations to determine the value of R1:

Vout = 4.8V therefore 4.8V / 400ohms = 12mA

R1 = (Minimum input voltage - output voltage ) / (output current + 1mA for TLV431 saturation)

R1 = (4.9V - 4.8V) / (12mA + 1mA)

R1 = 8 ohms

8ohms is very close to the one provided by TI calculator, so what is causing this short circuit please?

Any help would be highly appreciated.

2. ### crutschow Expert

Mar 14, 2008
13,052
3,244
What short circuit?

3. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
With 8.7 ohms as R1, the benchtop power supply was indicating a short circuit (SC led). The current consumed was also maximum.

Mar 14, 2008
13,052
3,244

5. ### ronv AAC Fanatic!

Nov 12, 2008
3,292
1,255
Your divider for the reference pin is wrong. You want that point to be 2.5 volts at your desired output voltage.

6. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi,

Thanks for the reply. That shunt regulator is the TLV431 not the TL431.

7. ### pilko Senior Member

Dec 8, 2008
213
20
"Note: The 35Kohms resistor is formed from a 22Kohms fixed resistor and a 20K trimmer in parallel"
Your parallel combo is 10.5K max.

8. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,809
1,105
They should be in series, not parallel.