LM217 voltage regulator question

[Chris Wilson]

I was looking for around 6 to 7V out from 12V in and I had a 5k ten turn pot to hand. using a cadence resistor box I found R1 at 1k gave a nice adjustment range but the spec sheets cite R1 as usually being a lot lower resistance, circa 240 Ohms. Is there anything bad in using 1k for R1 in the divider? Thanks.

[Wimberleytech]

In order for it to regulate properly, the datasheet says it needs a minimum of 10mA (worst case, with 3.5mA typical) of load current.  A resistor of 125 ohms or lower guarantees that without any additional load.  If you use 1k, then you will need to be sure that you are always sinking additional current to meet the minimum load current specification.

[magic]

My datasheet (ST) specifies 5mA worst case and that's where the 240Ω comes from.

[Wimberleytech]

My datasheet (ST) specifies 5mA worst case and that's where the 240Ω comes from.

Correct.  I was looking at a TI datasheet (actually for the 317).

[Chris Wilson]

Thanks for the very fast replies! Will an HEF4013 flip flop (which it is powering) draw that much all the time or do i need to redesign the divider? It *SEEMS* to work OK on the breadboard...

EDIT, hmm it seems the HEF4913 only draws Ua levels maybe I need to change this

IDD supply current all valid input
combinations;
IO = 0 A
5 V - 1.0 - 1.0 - 30 - 30 uA
10 V - 2.0 - 2.0 - 60 - 60 uA
15 V - 4.0 - 4.0 - 120 - 120 uA

[Wimberleytech]

Thanks for the very fast replies! Will an HEF4013 flip flop (which it is powering) draw that much all the time or do i need to redesign the divider? It *SEEMS* to work OK on the breadboard...

EDIT, hmm it seems the HEF4913 only draws Ua levels maybe I need to change this

IDD supply current all valid input
combinations;
IO = 0 A
5 V - 1.0 - 1.0 - 30 - 30 uA
10 V - 2.0 - 2.0 - 60 - 60 uA
15 V - 4.0 - 4.0 - 120 - 120 uA

Add a dummy load to make up the difference.

[magic]

Funny, mine also says 10mA for the LM317 but still recommends 240Ω without specifying it's for LM217 only. I always assumed 240Ω is enough, good to know.

[Chris Wilson]

Add a dummy load to make up the difference.

240 ohm 1/2 watt resistor OK? Although using R1 at 240 Ohms and adjusting R2 to around 900 Ohms should give circa 6V. Thank you again.

[Wimberleytech]

Add a dummy load to make up the difference.

240 ohm 1/2 watt resistor OK? Although using R1 at 240 Ohms and adjusting R2 to around 900 Ohms should give circa 6V. Thank you again.

1.25²/240  = 6.5mW  so 1/2 watt is overkill but it is fine.

[Zero999]

I was looking for around 6 to 7V out from 12V in and I had a 5k ten turn pot to hand. using a cadence resistor box I found R1 at 1k gave a nice adjustment range but the spec sheets cite R1 as usually being a lot lower resistance, circa 240 Ohms. Is there anything bad in using 1k for R1 in the divider? Thanks.

As mentioned above, the resistance needs to be lower because the LM217 has a minimum load current requirement of 5mA.

The reason for this is it's actually an op-amp and voltage reference connected to a big Darlington pair, along with some protection circuitry. The negative rail of the op-amp is also connected to the output terminal of the regulator, so in order for the op-amp to work properly, the current flowing through the output needs to be high enough. At higher currents, the op-amp still draws around 5mA, it's just the output transistor turns on to allow more current through the output terminal.