LM317 Higher Voltage 90v fixed low current

[ejeffrey]

The problem is that your resistor values are off.  The voltage divider ratio is 81:1.  With the 1.25 reference voltage that sets the regulator for 101.25 V.  With your input only at 100V the regulator can't do that so you get Vin-2V.

You need to hit a voltage divider of 72:1. 

[marce002]

hks ejeffrey  will try with 130ohm to achieve your suggested ratio and report back, thks

[Zero999]

The load is not capacitive it is just a kind of signal to controller (ebike), to fix 90v in the red thin "power switch bat sense line" to tell de controller that the battery remains at 90 vol when 100V are present, and not lower power to motor. if the signal is less than 90V then no problem (means I need to recharge)

What do you think is inside it?

There are bound to be some decoupling capacitors.

My issue here is what i have said before, the output is almost identical to the input less 2 vol , for eg if input is 100v, the output is 98,3V (instead close to 90v),  it is like it follows the input -2v , not fixed at 90v at all.

so i thought was related to the lm317 or missing zener, now i do not know what can be wrong, sorry to bother.

Test it at a lower voltage. Connect it to a low voltage PSU, say 5V to 24V. Set it to 1.25V out by connecting the output to the adjust pin, via your 130R resistor and connect the adjust pin to 0V. The output should read close to 1.25V, give or take 50mV.

That will confirm the LM317 isn't fried.

The others who've commented about the output voltage being set to high are also right.

R1 is four 470 resistors in parallel

470/4 = 117.5 Ohms

R2 is two 4k7 resistors in series
4700 + 4700 = 9400 Ohms

V_OUT = (1+R2/R1)*1.25 = (1+9400/117.5)*1.25 = (1+80)*1.25 = 81*1.25 = 101.25V

Actually, it'll be a bit higher, since I didn't take into account I_ADG which is 50µA though R2. V = IR = =0.00005*9400 = 0.47V, so 101.25 + 0.47 = 101.72V, call it 102V.

To summarise, the resistor values you've chosen will give 102V, so the LM317's voltage will not be regulated, until the input exceeds 105V or so and then it'll be 102V, rather than the desired 90V.

[marce002]

thanks a lot, worked perfect. Thanks zero999 and ejeffrey and all, with a value around 125ohm i have perfect constant 89.9V when desired!!! thanks, and no more post.