That 2.0 Amp is not the maximum allowable current; it is the Zener test current-- i.e., the current at which the Zener voltage is tested and specified.been looking for some high power zener diodes and found the BZY91C13
which is 100 watt but the data sheet says 2amp ?
Can anyone explain that please
thanksThat 2.0 Amp is not the maximum allowable current; it is the Zener test current-- i.e., the current at which the Zener voltage is tested and specified.
The old British motorcycles were regulated by a dirty great Zener, which I think was 100W.been looking for some high power zener diodes and found the BZY91C13
which is 100 watt but the data sheet says 2amp ?
Can anyone explain that please
thanks for looking
You need enough voltage headroom to accommodate VGSthr - the TL431 can carry 100mA as long as you don't exceed dissipation, so there's no particular advantage in using a MOSFET over using a BJT.Another option to emulate an accurate, high-power zener is to use an inexpensive TL431 programmable shunt regulator with a power P-MOSFET current booster as shown in the LTspice simulation below.
The voltage divider R1 and R2 can be adjusted to give any "zener" voltage from 2.5V to 36V (show set for 13V in the simulation with a simulated current of 0-10A).
Their value is selected to give 2.5V at the TL431 control input for the desired clamp voltage.
A pot can also be used to adjust the voltage.
This voltage is more stable and accurate than a typical zener, with a much lower dynamic impedance and temperature coefficient.
Of course, the transistor must be mounted on a heatsink to handle its dissipation, which equals the zener voltage times the current.
View attachment 152464
If you want the BJT to carry more than a couple amps, it would need to be either a Darlington stage or a MOSFET.the TL431 can carry 100mA as long as you don't exceed dissipation, so there's no particular advantage in using a MOSFET over using a BJT.
Everythings a load of compromises - a Darlington can need as much voltage as a LL MOSFET. For 100w equivalency, a power MOSFET isn't particularly outstanding.If you want the BJT to carry more than a couple amps, it would need to be either a Darlington stage or a MOSFET.
For low voltage equivalent zener voltage (6V or so) you would need a logic-level MOSFET with a Vth(max) of ≤2V.
What is?For 100w equivalency, a power MOSFET isn't particularly outstanding.
You can make a Sziklai hybrid with the TL431 - but probably not for that much current. I use it in the shunt regulators for my DIY E-cig chargers.For the lowest equivalent Zener voltage setting, you could use a Sziklai (complementary) Darlington Pair in place of a MOSFET, which only requires about 0.7V of headroom added to the ≈2V minimum required by the TL431.
The 2N3055 has a 10A maximum rating.
View attachment 152491
Your figures did not post.I found this circuit on the web
Background,
in the late 1960/70s British bikes had a permanent magnet alternator...
... so I would like to make the simplest circuit possible to emulate the original
alternator excess output with engine running and lights on is about 20 watts (I always ride with lights on)
without lights its nearer 60+ watts
I found this circuit on the web, is it viable if so what value and spec of components would I need
thanks for your help
You can probably use the 2N3055 (or the TIP3055) for the NPN in your circuit. Either one will have to be on a large heat sink. I assume that you already have a heat sink that the power Zener was mounted on. You might be able to make it work for the '3055.D1 zener voltage would be about 0.7V less than the voltage you want to regulate (probably about 14V for a 12V battery, so a zener voltage of 13.3V.
You would use a standard value of 13V±5%.
The value of R1 is not critical, so 10kΩ should work fine.
For a more accurate voltage setting and regulation, you could use one of the TL431 circuits I posted above.