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I need to make a test PCB for an LED drive circuit that can drive a variety of different single LEDs. The forward voltage (Vf) of the LED could be anywhere from ~2.2V up to ~6.5V. I'd like the test PCB to be able to simulate the range of possible Vf.

Rather than using switches (likely FETs) and putting a variety of LEDs on the test PCB, it would be great to find something (a device, a circuit) that is a "variable Vf diode" or "voltage controlled diode." Does such a thing exist?

My searching and (limited) EE knowledge so far suggests that an PN junction (a diode) gives a constant Vf, that adding another junction (BJT transistor) allows for current but not Vf control, and that adding a 4th junction (thyristors, SCRs, triacs, ...) doesn't help either. A FET can be thought of as a voltage controlled resistor and a Varicap is a voltage controlled capacitor, both of which are close but not what I need.

For what little I know about device construction, it seems like there should be a device that uses a voltage or current to bias up a PN junction, changing the width of the depletion region and therefore changing the bias required (Vf) to "punch through" with carriers into conduction. Then again, this sounds a lot like a BJT, which I don't think will work.

I guess putting a heater under a diode could give some Vf control, but that seems like it would have poor control (and be inefficient and sort of silly).

It feels like I'm missing something obvious here. Any thoughts on devices or circuits that can function as a voltage-controlled, forward-voltage diode?

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  • \$\begingroup\$ That sounds like a simple VC-CC lab variable power supply which can either limit in voltage or current. \$\endgroup\$ – Damien Jan 3 at 10:56
  • \$\begingroup\$ There are single chip constant current drivers for LEDs. What current do you want? \$\endgroup\$ – JimmyB Jan 4 at 0:28
  • \$\begingroup\$ The current can range from 300mA to 1.2A. I've got a few different circuits that all work pretty well, but I want to do more testing (and later quality control) to see all of the details. \$\endgroup\$ – Casey Jan 4 at 20:37
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schematic

simulate this circuit – Schematic created using CircuitLab

Something like this?

It's not a single device, but it is simple(ish).

Heatsink Q1 for the LED current dissipation.

Choose R1/R2 to give you a suitable ratio between the 'diode' voltage and your control voltage, the values shown give you 3:1.

D1 is to prevent reverse breakdown of Q1 BE junction, should you use a high voltage supply to the op-amp.

Use a darlington for Q1 if you want to sink a lot of current. Even with a darlington and D1, you ought to be able to get down to 2.2v if your opamp output goes down to near -ve rail.

Put a small resistor in series with the load if you want to increase the impedance to mimic a real LED load, this sink will be very low impedance.

Bonus component. Add R3 to alow you to monitor the current with a voltmeter. Obviously it doesn't measure the current through the other resistors (they could be increased somewhat), or in the base of the transistor (1-2% in a power PNP, <0.1% in a power darlington), but it measures most of it with an accuracy comparable to your DMM and its own tolerance. If you do increase R1 and R2 above 100k or so, put a few 10s of pFs across R1, to ensure stability.

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  • \$\begingroup\$ The information on the design and purpose of each component (such as D1, added cap across R1, Darlington, ...) is very helpful. \$\endgroup\$ – Casey Jan 4 at 21:04

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