I have a 45Volt DC power supply but i need to drop this to about 40 Volt in order to power a LED driver. The maximum current i assume that will be drawn is somewhere around 100mA. My PCB is 12mm wide and 100mm long. MAximum component hight is 10mm.
The series zenner should work fine. Thanks. Somehow it slipped my mind, i was thinking of parallel zenner only.
Does the LM317 work at these voltages ?
Does the LM317 work at these voltages ?
LM317 input to output voltage difference allowed is 35 volts maximum. If there is input bypass, which means if there r any capacitors across the supply of the driver(which is quite possible), they will short the output for a very short time. This condition will drop more than 35 volts across the 317 and will become RIP.
On a second thought, why not use a resistor.
Gajanan Phadte
On a second thought, why not use a resistor.
Gajanan Phadte
Last edited:
Thank you all for your suggestions.
I think i will try astankov's circuit first. Thanks a lot.
As for the input capacitor bypass, it is obvious a small cap is needed.
I think i will try astankov's circuit first. Thanks a lot.
As for the input capacitor bypass, it is obvious a small cap is needed.
Ok go ahead but just to remember you dont need the 390R resistor - it is used to simulate 100 mA load current.
Be careful with MOST regulators. It is very easy to exceed Vin-Vout without thinking about it.
If you assume that the load will have a capacitor across the supply - then Vout at Switch-On will invariably be 0V all-be-it for only a short period of time.
If Vin exceeds 40V with Vout = 0V then the 317 regulator will DIE.
Most of simplistic solutions will be destroyed if there is a bypass cap: when you connect the supply, the cap is not charged, and zener/pseudo-zener/regulator will have to sustain the full charging voltage and current, for some microseconds or milliseconds.
This is normally sufficient to destroy them, if not the first time then after a reasonable number of attempts.
If the current is realitivily constant at 100mA, why not just use a simple 50 Ohm 1W resistor? If there is no need for specific voltage regulation, then KISS.😉
If you really do want to use a regulator then a simple RC Zener arrangement is what you need.
A simple diode chain or a resistor will do the job though.
A simple diode chain or a resistor will do the job though.
5v 100ma? 10mm 1w amber+white+R, which is 2v+3v and a tiny current drop for protection. That seems a bit more elegant than lighting up black diodes. 🙂
-or-
5v zener + R parallel to another 5v zener + R parallel to another 5v zener + R, but unfortunately, diode current handling doesn't double in parallel, it only increase 1.5x.
-or-
Or a series string of 1n5407, since their surge capacity can fairly easily handle charge up of the bypass caps.
-or-
5v zener + R parallel to another 5v zener + R parallel to another 5v zener + R, but unfortunately, diode current handling doesn't double in parallel, it only increase 1.5x.
-or-
Or a series string of 1n5407, since their surge capacity can fairly easily handle charge up of the bypass caps.
- Status
- Not open for further replies.