Circuit Design - Need 35mA through UV LED using a GPIO controlled switch

Question

I was trying to achieve this using a PNP transistor as a switch circuit and the voltage drop was too significant and I was only able to achieve 13mA. Circuit here

I was advised to use a FET instead. Would this be as simple as swapping the PNP for any FET? How would I design a circuit to reach 35mA through my UV LED (while being controlled by a GPIO pin)?

UV LED - 340nm: UF4VL-1H321

@SpehroPefhany I haven't selected anything besides the UV LED. My linked post above I tried the S8550 PNP Transistor but I was advised to not use a transistor and instead a FET. This question I am looking for any suggestion that will achieve the 35mA — presish, Feb 18 '20 at 20:26
That's nice. Do you care to share the UV LED part number with us? 3rd request. — Spehro Pefhany, Feb 18 '20 at 20:27
Does this answer your question? [How to calculate Resistor needed in PNP Transistor Circuit](https://electronics.stackexchange.com/questions/480118/how-to-calculate-resistor-needed-in-pnp-transistor-circuit) — , Feb 18 '20 at 20:39
@BrianDrummond That is my original question but I was advised a PNP would not achieve my results. I am looking for a circuit design where 35mA would flow through my UV LED. In that same post a FET was suggested but is that the only solution? Also would I just replace my PNP with any FET and keep the same design? — presish, Feb 18 '20 at 20:42
The accepted answer shows you how to do it with a simple NPN transistor. — , Feb 18 '20 at 20:44
@BrianDrummond I bought an NPN and it still did not work. I still only saw 13mA — presish, Feb 18 '20 at 20:47
As Brian said, it should work with the NPN, and if it didn't something else is wrong. Maybe your LED is fried. — Spehro Pefhany, Feb 18 '20 at 21:41
You need a low V(CE)sat BJT. Otherwise you'll never reach the voltage drop on that UV Led. — Ariser, Feb 19 '20 at 07:16

score 2 · Accepted Answer · answered Feb 18 '20 at 20:55

2

Okay, the 340nm version has a chance of operating properly from a 5V supply, but as there are no minimum/maximum Vf specifications it is hard to predict the forward voltage of the sample(s) you happen to have.

A MOSFET + resistor is the simplest approach. For example, an AO3400 (n-channel) or AO3401A (p-channel) will have less than 50-80m\$\Omega\$ resistance with 3.3V drive.

If you pick the AO3400, the resistor will be about 29-30 ohms. If the particular LED you have is low Vf (or damaged) the current could be more, if it is high (or damaged) it could be lower.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Try it with just the resistor first, if you can't get 35mA you certainly won't get 35mA with the transistor in there. It's possible you've damaged the LED if you can't get close to 35mA with just a 30 ohm resistor.

If you are doing a more professional design, an op-amp current sink would be preferable, but that won't help if the Vf exceeds about Vcc-150mV, assuming a 100mV resistor. Note also that if the circuit is wired incorrectly it will likely destroy the LED because it will conduct far over the rated current. The first circuit makes that much less likely.

schematic

^{simulate this circuit}

answered Feb 18 '20 at 20:55

Spehro Pefhany

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What is the benefit of the op-amp design? Also why does the the gate/base to the FET need a resistor to ground? – presish Feb 18 '20 at 21:04
1

Better control of the current since the Vf is not guaranteed to be anything in particular. The resistor to ground is in case the GPIO is set high-Z it turns off. – Spehro Pefhany Feb 18 '20 at 21:06
Thanks for the explanation. Is there specific specs I need to look for when selecting a FET that will work for my needs? Why did you suggest AO3400? – presish Feb 19 '20 at 20:00
1

It's a cheap, popular jellybean part rated for 2.5V drive, that has very low Rds(on) with 2-3V drive. – Spehro Pefhany Feb 19 '20 at 21:26
So if driven by 3.3V GPIO it will not have issues? – presish Feb 20 '20 at 20:15
1

It will have guaranteed performance. If Vgs = 3.3V it will have less than 0.052\$\Omega\$ Rds(on), probably more like 0.025\$\Omega\$ (both at 25°C). Whether that's "not issues" depends on your circuit. – Spehro Pefhany Feb 20 '20 at 20:32
What spec do I look at to determine the voltage drop? If input is 5V how do I know how much is left for my LED? – presish Feb 20 '20 at 20:41
1

0.025 \$\Omega\$ * 35mA = 875uV. – Spehro Pefhany Feb 20 '20 at 20:46
Where is 0.025 ohm from? – presish Feb 20 '20 at 21:14
1

Typical figure from Fig. 5 on the datasheet http://www.aosmd.com/pdfs/datasheet/ao3400.pdf – Spehro Pefhany Feb 20 '20 at 21:18
Is there a way to make this through hole? I would need to attach it to my circuit board. – presish Feb 24 '20 at 19:12
There is not much selection of good logic-level MOSFETs that are through-hole. You can solder an SOT-23 easily by hand. – Spehro Pefhany Feb 24 '20 at 19:13
The SOT-23 is a through hole attachment for the AO3400? Is there a link to this? – presish Feb 24 '20 at 19:17
1

SOT-23 is the surface-mount package. With a bit of practice you can solder it easily. Watch some youtube videos. – Spehro Pefhany Feb 24 '20 at 19:18

Circuit Design - Need 35mA through UV LED using a GPIO controlled switch

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