voltage divider with zener?

Thread Starter

joewales44

Joined Oct 8, 2017
150
i need to produce about .6volt from a 12 to 24volt power source to control adjust pin on driver. i get what i want with a 20k and 1k when voltage is 12volts. i want to use a zener to reduce 24volts down to 12. if a zener will work how do i connect, between the voltage divider resistors and ground or does it have to be in front of both voltage divider resistors? if it must go in front of both, when i add another resistor to limit current to zener, will current be too low through divider?
 

dl324

Joined Mar 30, 2015
8,952
How much current will you draw from the 0.6V? How close to 0.6V does it need to be?

Your shift and return keys don't seem to be working...
 

AnalogKid

Joined Aug 1, 2013
8,108
Yes, the zener needs to be in front of both.
Yes, you need to add a current limit resistor for the zener.

To calculate the added resistor, start with the zener current. The datasheet for the part will have a typical or test current value. To reduce heating, you can safely use a design value of 1/2 the test current. So if the test current is 10 mA, then the design current is 5 mA. Your divider network takes about 0.6 mA, so we'll increase the zener current 1 mA to a value of 6 mA cover that. Using Ohm's Law:

R = E / I
R = (24-12) / 0.006
R = 12 / 0.006
R = 2000 ohms

That's it. For this example, the zener will have 6 mA of static current through it. Its load cannot draw more than 1 mA so the load won't starve out the zener.

ak
 

dendad

Joined Feb 20, 2016
3,023
If it is "about 0.6V", why not try a rectifier diode like 1N4007 forward biased with a series resistor from the 24V?
No other Zener required. It all depends on how much the "about" is and the current you need to draw.
 

Thread Starter

joewales44

Joined Oct 8, 2017
150
anywhere from .5 to .7 volts is ok. very low current. just enough to control adjust pin on pam2861. i'm not sure how to use the 4007 idea. would it work for 12 volt supply also? voltage can be from 11 to 26 volts from 1 input connection.
thanks
 

dendad

Joined Feb 20, 2016
3,023
anywhere from .5 to .7 volts is ok. very low current. just enough to control adjust pin on pam2861. i'm not sure how to use the 4007 idea. would it work for 12 volt supply also? voltage can be from 11 to 26 volts from 1 input connection.
thanks
Just put a 2K2 resistor from the supply to the IN4007 (or just about any other diode) anode, and the diode cathode to 0V.
Adding a 100nF cap across the diode is always a good idea.
Then you get the 0.7V across the diode. Easy :)
I just had a look at the pam2861 data sheet. Are you only looking to set the LEDs at a low brightness? 0.7V will give you a few % of the max brightness.
I'll check back in the morning. It is 1:30AM here and I should be asleep!
 
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dendad

Joined Feb 20, 2016
3,023
If you want full dimming range, 0.7V will not be enough.
It needs to go from 0 (or 0.3) to 2.4V.
Here are some ideas...
pam2862_VSET.jpg
An LED can be used in place of the 2.4V zener, just select the one to get less than 2.5V drop across it.
The bonus is you get a power on indicator :)
 
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Thread Starter

joewales44

Joined Oct 8, 2017
150
i like to report results after receiving help.
got the board built with 2k resistor in front of zener as suggested by the Kid.
i used a 20k and 1k voltage divider after the zener. it works great.
i also laid out the board for a 5k pot also. it works perfect too from full brightness to off. it only goes up to 2.3 volts but i like a safeguard any way. 2.5 was max.
thanks everyone.
 
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