Adjustable resistive divider resolution improvement - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 17, 2015, 01:17:35 pm
content: I have breadboarded a very simple circuit to create an adjustable voltage limiter using an TL431 adjustable Zener. For the divider resistors I have been using a single multi turn 5K pot, slider to the centre. This was instead of a fixed and an adjustable resistor, R1 and R2, as in the shunt regulator schematic from the device's info pages, attached. Even with a 10 turn pot adjustment is quite coarse. I want to input 1 to 5V and limit output to a maximum of 4.27V. How can I arrange a finer resolution of adjustment please? Thanks.

author: mikerj
author_rank: Super Contributor
timestamp: February 17, 2015, 01:43:12 pm
content: I presume you mean LM431 (more commonly TL431)?You can shift the operational range of the pot simply by adding a fixed resistor in series with one or both sides of the pot.What is the output voltage range you wish to achieve, you mentioned 4.27v max, but what minimum value? Obviously it can't be lower than the 2.5v reference

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 17, 2015, 01:43:38 pm
content: Quote1 to 5v input??QuoteMake the value of R2 higher, and put a resistor in series with the pometer, one side to the plus.

author: nadona
author_rank: Regular Contributor
timestamp: February 17, 2015, 10:53:01 pm
content: I would put the 5K pot parallel to a fixed 500ohm R1 and make R2 as 500ohm. It will have about 10 times finer resolution than using the pot as R1.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 18, 2015, 11:59:30 am
content: Quote from: JohnnyBerg on February 17, 2015, 01:43:38 pmThe OP is talking about a limiter... 1-5V input, 1-4.27V output.i.e. input < 4.27V goes straight through, input > 4.27V is limited to 4.27VBasically using the TL431 as an adjustable precision zener.Quote from: nadona on February 17, 2015, 10:53:01 pmThis gives you a very non-linear solution, doing huge jumps at the end of the scale.As previously suggested by mikerj and JohnnyBerg, you want this, instead:The basic range can be set with R1 and R2, and the adjustable can trim the range.Vmin= 2.5*(1+R1/(R2+Radj))Vmax= 2.5*(1+(R1+Radj)/R2)So if Radj= 5k, and you wanted adjustable range of Vmin=4V and Vmax= 4.5V then4 = 2.5*(1+R1/(R2+5000))4.5 = 2.5*(1+(R1+5000)/R2)solve that. Here's a screen capture using Mathomatic to solve it.You can see from the solution below, R1 = 27000, R2 = 40000, Radj= 5000, and you get an adjustment range of 4 to 4.5 Volts output. If it's a 10 turn pot, you get 0.5 V/10 turns = 0.050 V/turn or 50mV per turn of the pot.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 18, 2015, 12:10:34 pm
content: Codeboy: yes, that's what I am doing. The current limiting resistor in the + rail I am using is 68 Ohms, and it keeps the max current on the device well below the limits for the 431 if someone shorts the output. I will test with your values, many thanks for working that out and showing how the results were achieved!A

author: codeboy2k
author_rank: Super Contributor
timestamp: February 18, 2015, 12:25:15 pm
content: You're welcome. Those numbers are just one solution, that works with your 5k trimmer.You also need to check that the resistances chosen can also supply enough current to the Ref input. The datasheet says the Ref input consumes max 4 uA. So 4V/(R1+Radj+R2) = 56 uA ... it's enough current available for the reference.If the solution resistances did not supply enough current to the Ref input you would have to scale the resistances.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 18, 2015, 12:46:45 pm
content: Yes, I was thinking of that last night, so if the divider resistors were of too high a value, their *division ratio* may be good, but their *current load* might be inadequate for proper regulation?I see circa 1mA draw when resting, with just a digi multimeter on the output. I guess that would have been OK had I left the values as I had them?May I put another question to you please? This is post is a spur from another at :https://www.eevblog.com/forum/beginners/could-someone-please-expalin-how-this-quad-op-amp-is-being-utilised/new/?topicseen#newA poster there suggested the three terminal Zener as a simpler way of achieving what i was looking for. I now see an Australian commercial device for this purpose that not only voltage limits a 1 to 5V input at a user selectable level, but if the input goes *OVER* a user selectable level it then "lets through" the input voltage to the output, triggering a fail safe circuit in the control ecu it outputs to. I have never seen one of these more sophisticated devices, so have no idea what circuitry is used within, but I was told that the limiting is a three terminal Zener, like the circuit we are talking of now. How might they have simply achieved the over voltage "let through" ability?The device's advertising blurb is attached, thanks again! Wish I had listened more in my maths classes, as you get old it's hard to learn new skills...

author: bugs
author_rank: Contributor
timestamp: February 19, 2015, 05:57:53 pm
content: q1 is always on :-)

author: codeboy2k
author_rank: Super Contributor
timestamp: February 19, 2015, 06:18:30 pm
content: Quote from: bugs on February 19, 2015, 05:57:53 pmShit yah.That's what I get for whipping up something fast like that in a response. It was never a good solution anyways.Removed.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 19, 2015, 06:25:14 pm
content: I built it on breadboard this morning, before work, and spent the day trying to work out why, even with a decade resistance box in place of the 5.6K or whatever it was, failed to get any sense from it. Now I know whyNo hard feelings, and sincere thanks for the help. It actually made me think more about what was happening than if it had actually worked out of the box. No worries!Any other ideas? Cheers.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 19, 2015, 06:32:26 pm
content: Quote from: Chris Wilson on February 19, 2015, 06:25:14 pmIf you've built that old circuit, then flip the zener diode and the resistor top to bottom, make the resistor 1.2k and make the zener a lower voltage if you have one, like 4.2V. Then the zener drops 4.2V, the resistor drops about 0.65V, the whole thing drops 4.8-5V and starts conduction at about 0.5mA or so. Again need to trial and error with the resistor, and it's still not a good solution.Something like this:You might need to decrease R3 (slightly) to get enough current to allow the zener to conduct at it's zener voltage, and then you might need R5 to limit the current to the transistor, so that it doesn't turn on too early. You need R3 to drop about .6 to .8 volts to turn on the transistor. So that means, with 1 mA current, R3 = 0.6/0.001 = 600 ohms to 0.8/0.001 = 800 ohms. The 1.2K value I chose only allows about 0.6 mA to flow, but this might be enough to turn on the zener.That's another off the cuff fix to the "always on" problem. But I'm not breadboarding it nor simulating it.