I'm using a power resistor in series with a string of 4 pcs. 1N5380B zener diodes as a low budget voltage limiter (in front of a MOSFET regulator). The input voltage is 600 VDC, the voltage across the zener string is 480 VDC, rising to 540 VDC when the zeners get warm (hot).
The current in the zener string is approx. 30 mA. This would mean 0,9 Watt per zener. Their power rating is 5 Watt up to 70°C, 3 Watt @120°C and 1 Watt @ 175°C. However I think they become way too warm and will not last a long time. They cannot be touched (power down!)
I don't think the 1N5380B is the best choice for this purpose, in spite of it's power rating.
Any suggestions what alternative zener to use for this? I seem to need more power (and less voltage drift).
Regards, Gerrit
The current in the zener string is approx. 30 mA. This would mean 0,9 Watt per zener. Their power rating is 5 Watt up to 70°C, 3 Watt @120°C and 1 Watt @ 175°C. However I think they become way too warm and will not last a long time. They cannot be touched (power down!)
I don't think the 1N5380B is the best choice for this purpose, in spite of it's power rating.
Any suggestions what alternative zener to use for this? I seem to need more power (and less voltage drift).
Regards, Gerrit
Though it may be rated for a dissipation of 5 watts, the 1N5380 is a rather tiny device. Try to cut the leads to their absolute minimum, as some significant heat is conducted to the next sink via them, and/or clamp cooling fins around them.
Best regards!
Best regards!
120V x 30mA = 3,6W no wonder they get hot.
You may dissipate 3W if you keep the lead under 75°C (junction to lead 25°/W), that means cooling !
The datasheet uses 10mA to define the 120V for 1,2W.With junction to ambiant (without extra cooling) 85°/W gives junction temp (85 x 1,2)+25=127° is below the 150° max.
That is 12k 3W series resistor from 600V.
Mona
You may dissipate 3W if you keep the lead under 75°C (junction to lead 25°/W), that means cooling !
The datasheet uses 10mA to define the 120V for 1,2W.With junction to ambiant (without extra cooling) 85°/W gives junction temp (85 x 1,2)+25=127° is below the 150° max.
That is 12k 3W series resistor from 600V.
Mona
Kay is right practical experience shows they heat up,check out-
1N3008B
1N3008B - SOLID STATE - Zener Single Diode, 120 V, 10 W
1N3008B
1N3008B - SOLID STATE - Zener Single Diode, 120 V, 10 W
Use an active circuit with a mosfet or bipolar transistor. You could use Zeners at minimum possible current (about 1 mA), so they wan't became too hot, and so be much stable.
Look here, for example: Transistor-Zener Diode Regulator Circuits
Transistor would need some heatsink of cause.
Last edited:
Yes, I will add a power MOSFET, rated for 800 VDC and max. 190 Watt dissipation. That should work!
Regards, Gerrit
Regards, Gerrit
Don't forget to add protective gate-source 15-18 V Zener to Mosfer (it there isn't in-built one).
Yes, I know. I have several MOSFET power supplies in use and allways use a zener for protection.
Regards, Gerrit
Regards, Gerrit
The datasheet I found says 1.47W for a standard FR4 pcb and reasonable traces, 85C/W
5W is for infinite heatsink on short leads, where a thermal resistance of 25C/W is quoted.
In practice this means 0.75W is a sensible dissipation limit to go for (60C rise), anything higher use a package with heatsink tab or a stud-mounted.
5W is for infinite heatsink on short leads, where a thermal resistance of 25C/W is quoted.
In practice this means 0.75W is a sensible dissipation limit to go for (60C rise), anything higher use a package with heatsink tab or a stud-mounted.
