I am trying to run a 100W 32V Led with a DC-DC boost converter from a 6 volt battery, and I want to make it so it will shut off when the battery voltage gets low. So I am using a TIP122 transistor with the collector connected to the ground input to the Boost converter, The emitter connected to the battery ground, and the base connected to a 5.6V zener diode. The LED takes 3.3A, but even with a base current of 60mA, the transistor is only supplying 600mA.

Am I doing something wrong?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ As well as what they said - a properly rated MOSFET will do a MUCH better job for you. At say 100W/6V ~= 16A a 10 milliOhm MOSFET will drop V = IR = 16 x 0.01 = 0.16V. You cannot easily do w=this with a bipolar and CERTAINLY NOT with a darlington. And you can get 1 milliOhm <OSFETS (for more $). \$\endgroup\$
    – Russell McMahon
    Jun 25, 2015 at 2:07

2 Answers 2


The things you are overlooking and how, in a nicely ordered list are as follows:

  1. The LED takes 3.3A. 100W at 6V is 16.7A. That's assuming 100% efficiency, which is much more likely to be 80%, which would make 20.8A at 6VDC input. But even in "perfect" conditions you are trying to "melt" your transistor that has a maximum current capability of 5A.
  2. Transistors do not only have base current and amplification, but also base voltage and saturation voltage. The latter two for a darlington like the TIP122 are incredibly high. Just look at the datasheet.
  3. You are, I assume calculating the base current, because the 5.6V zener in the 1N47-- series (which is NOT the 1N4736A, that's the 6.8V one) has it's approximate voltage at 45mA, but with a reasonable margin.

To fix the situation, on 6VDC you are going to not want to use a Darlington, but another scheme, such as just a power transistor driven by another PNP transistor, so they can both saturate and actually get the saturation voltage of 0.5V or below, in stead of 2V (leaving ONLY 4V for your unit, i.e. 100W at 80% conversion over 4V gives 31.25A !!!).

Or by using a MOSFET setup.

And then make sure you properly calculate all the set-points for the transistors and zener diodes using their datasheets.

You might also want to think about some hysteresis. If you turn off the battery drain at 5.5V, for example, then the battery will lose it's drain and jump up in voltage. This can cause:

  1. Oscillations

or more likely:

  1. A transistor or MOSFET that starts hanging between on and off dissipating a BUCKET of energy to regulate the current slowly down to keep the battery voltage at the set-point.

To blatantly advertise myself, a lot of steps about transistors can be found here:


(they do ignore saturation voltages, because that makes it more difficult, but for a Darlington it's important: They will just always waste 1.5V to 3V depending on the type, no matter what you do).

And more self advertising (since it might help) an answer about adding switching hysteresis to MOSFET switches, so they turn on at one voltage and off at a voltage a little below that, to prevent stuttering:


(You can leave out the diode, that one came from the OP, but was never actually explained)

  • \$\begingroup\$ Thank you. 100W is what the LED said on the package, but I'm not sure where they're getting that from because my meter only showed 3.3A on the 6V side of the converter, which would only be 20W. \$\endgroup\$
    – Big Jake
    Jun 24, 2015 at 21:43
  • \$\begingroup\$ Could very, very probably be because the converter cannot handle taking upto 20, even 30A at the input. Or possible the battery can't handle supplying it. The LED can still easily be 30V-ish, 3.3A and rated 100W, but if your converter is a simple one it will definitely not be able to do that! \$\endgroup\$
    – Asmyldof
    Jun 24, 2015 at 22:58

The TIP122 has a Hfe (current gain) of 1000, so to get 3300mA (3.3A) out of it, will require 3.3mA into the base. The Zener diode (which is drawn backwards by the way) will "remove" 5.6v of the 6.3v supply, leaving 0.7v. Problem is, Vbe(on) in the datasheet can be as much as 2.5v...

So correct the zener orientation, and try a value more like 5v or even 4.7v.

  • 1
    \$\begingroup\$ It's worth noting that TIP122 is a darlington, so we expect the typical operating voltage on the base to be around 1.4 V, not 0.7 V like we often assume for a single transistor. (And the TIP122 datasheet has a curve showing typical Vbe around this value as well, depending on collector current) \$\endgroup\$
    – The Photon
    Jun 24, 2015 at 21:12

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