# Help required

Discussion in 'The Projects Forum' started by justinesmithies, Aug 17, 2013.

1. ### justinesmithies Thread Starter New Member

Aug 4, 2013
5
0
I am using an LM334z run from a 12vdc supply to fix the output current to a max of 2Ma . What value pot would i need to put in series with negative side to be able to vary the current from the regulated max of 2Ma to 0Ma ??

Any help would be brilliant.

Justine

2. ### odinhg Active Member

Jul 22, 2009
65
15
You will need to read the datasheet here:

At page 6, it explains to you the relationship between $I_{SET}$ and $R_{SET}$.

We get the following equation from the datasheet:
$I_{SET}=\left(\frac{V_{R}}{R_{SET}}\right) * \left(\frac{n}{n-1}\right)$

$V_{R}$ and the ratio $n$ between $I_{SET}$ and $I_{BIAS}$ can be found in Figure 9 and Figure 12 respectively.

Your $I_{SET}$ is 2mA. So for 25°C and 2mA we get:

$V_{R} = 63mV$
$n = 16$

$I_{SET}=\left(\frac{0.063mV}{R_{SET}}\right) * \left(\frac{16}{16-1}\right)$

$2mA=\left(\frac{0.063mV}{R_{SET}}\right) * \left(1.067\right)$

Now you just have to calculate your unknown value of $R_{SET}$