Also if D1 is asked to carry significant current, it will die extremely quickly. For charging up a small capacitor it's probably fine but if for some reason it ends up supplying the full load current (say someone shorts the output) it will have a brief and explosive life, followed immediately by the lm317.
True. It has no short circuit protection. I did warn the original poster about it.
https://www.eevblog.com/forum/beginners/lm317-higher-voltage-90v-fixed-low-current/msg5341190/#msg5341190Thanks Zero999 genious! i
I will build and try it !! and comment back...
If I knew any circuit simulator i would do this test before... (wonder what would happen if):
input < 90V then ---> should be output = input
input > 100 then ---> should be output = 90V
The .asc file attached to my previous post, can be simulated in LTSpice, but there's no point because models for voltage regulators are often approximate.
Actually, the output will really be 89.2V, if the resistors are exactly 130R and 9k1, the reference is exactly 1.25V and the current flowing out of the adjust pin is exactly 50µA, but there's no point in aiming for resistor values will give exactly 90V, because the tolerance of the LM317's is too poor. In other words, 89.2V is within 1% tolerance of 90V, whilst the LM317 has a tolerance of 4%.
The LM317 has a certain drop-out voltage which is required to maintain regulation and in this case D1 bypasses it, when the input-output difference exceeds the breakdown voltage, which is 33V in this case.
Assuming the current is 250mA and room temperature and ignoring the 4% tolerance i.e. circuit will give exactly 90V out:
Input < 91.7V: the output will be about 1.7V less than the input.
Input is between 91.7V and 93V, the output will have poor regulation, but should still be close to 90V
Input between 93V & <123V: the output will be 90V and well-regulated.
Input >123V: the output = input - 33V.