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I have a load (from 20 LEDs with 20 resistors) that I want to switch on/off as a single block, using a GPIO pin from a micro-controller.

I have measured the current to be 145mA at 3.3V Vcc.

schematic

simulate this circuit – Schematic created using CircuitLab

I have tried switching this load using two methods:

Attempt A: N-channel mosfet

schematic

simulate this circuit

When I switch the load on with a 2N7002 N-channel mosfet, I see that only 86mA makes it through the switch.

I also tried the 2N7000 N-channel mosfet, and I see 90mA for that.

Attempt B: Darlington Array

schematic

simulate this circuit

I tried switching it through the ULN2003 darlington array, using just 1 of 7 channels, I see a measly 64mA makes it through. (Coincidentally, this part seems to be sold-out everywhere, so I cannot use it anyway.)

Am I wrong to expect more current when switched? Did I use the mosfets or darlingtons incorrectly?

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    \$\begingroup\$ I would use your 2nd schematic but not with the 2N7000. That MOSFET really needs a higher gate-source voltage than 3.3 V to fully conduct. Instead I would use an AO3400 which will have a much lower resistance when a Vgs = 3.3 V is applied. The AO3400 is an SMD component though so quite small. \$\endgroup\$
    – Bimpelrekkie
    Nov 4, 2021 at 21:33
  • \$\begingroup\$ There is also a relay you could consider -- they come latching and non-latching. No voltage drop across those so the current would be like the first case. A BJT can also be considered (not the Darlington configuration which drops a LOT of voltage.) So you should think about that, as well, before deciding. \$\endgroup\$
    – jonk
    Nov 4, 2021 at 21:39
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    \$\begingroup\$ Regarding using the Darlington transistors: these will always drop more voltage between Collector and Base (which then lowers the voltage across your LEDs + resistors and that lowers the current). You actually don't need a darlington here, your current is low enough to use a standard NPN like a 2N2222. Do add a resistor in series with the base though (without it, the base current will get out of hand), I would use 1 kOhm. \$\endgroup\$
    – Bimpelrekkie
    Nov 4, 2021 at 21:39
  • \$\begingroup\$ @Bimpelrekkie Thanks! Digikey has ao3400 in stock, although marked 'not for new designs.' Do you know if a part like that comes in array-form as well? I will be needing 5 of them on my pcb, eventually, so in a combined package would be convenient. \$\endgroup\$
    – Bram
    Nov 4, 2021 at 22:03

2 Answers 2

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Technically, you either expect devices to work better given the circumstances, or use them wrong. They are not zero ohm switches.

Case A: 2N7000 needs more than 3.3V at gate to turn on with low enough drain-source resistance to allow for more current. So too low gate voltage prevents the FET from turning fully on and there is a voltage drop over the FET, it has Vds larger than zero. This might get better by just recalculating a smaller resistor. Even at gate voltage of 4.5V, and 75mA drain current, the Vds voltage drop could be up to 0.4V.

Case B: ULN2003 channel is a Darlington transistor and these have Vce voltage drop of around 1V, so by calculating that into account you need to make the resistance smaller.

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The ULN2003A is about 30 years old. It amazes me that folks do not look up newer/equivalent parts. The best FET equivalent is the TPL7407, but this is in short supply.

However there is a FET based pin equivalent that is readily available, the ULN2003V12. The device uses an N-channel FET instead of the Darlington arrangement in the ULN2003A.

enter image description here

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There will still be an on-state V(low) limitation:

enter image description here

The ULN2003V12 is just 7 N-chan FETs, so they can be paralleled to bring the V(low) down. They also work just fine on 3.3V systems

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  • \$\begingroup\$ Thanks! Will look into those! The ULN2003V12S16-13 is one of those parts that has this weird digikey quirk: The show up in search result as "stock: 14,175 - Immediate" but when you click on it, "Stock: 0." I've asked digikey sales about it, but they give me a nonsensical answer. \$\endgroup\$
    – Bram
    Nov 5, 2021 at 19:21
  • \$\begingroup\$ Check Mouser too ...they seem to have plenty of stock. \$\endgroup\$
    – Jack Creasey
    Nov 5, 2021 at 20:08

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