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I'm trying to make a fairly simple circuit, which powers a few LEDs in series using a single BuckPuck 3021 (https://www.ledsupply.com/content/pdf/led-driver-luxdrive-buckpuck_documentation.pdf)

The BuckPuck provides a 5V 20mA 'reference' pin, which is intended to be used to power control circuitry. The BuckPuck output is controlled by an input pin, which is active low (0-5V.) The control input pin has quite a low input impedance (1.5Kohm,) and as I was intending to drive this from a NI-DAQ's analog output I decided to put in a unity gain buffer opamp to compensate for this.

I made the attached circuit, and have had a few problems that I just can't figure out. The main one is that the buffer opamp keeps blowing up, after which its output becomes ~0V and the LED turns on constantly.

  • With no control input connected, the opamp's input is pulled up to 5V. Due to the 1.5K input impedance of the BuckPuck, the opamp is still unable to output 5V and so its output ends up being about 4.6V, which is still enough for the LED output to turn off. Could this difference between the inverting and non-inverting pins of the opamp (4.6V/5V) be damaging the opamp?
  • I am powering the circuit with a Vcc of 24VDC, and connecting the LED output to 8 LEDs in series - LZ1-00G102. These have a forward voltage of 2.7V, for a total series forward voltage of 21.6V.

I had intended to connect a bunch of these control circuits in parallel, with them all sharing the Vcc, GND and CTRL_IN (opamp input) pins. The circuit is blowing up opamps even when connected without additional circuits, so it seems like this isn't the problem at this stage.

Circuit Schematic

Does anyone know why my opamps are blowing up? The LEDs seem to be totally fine, as does the BuckPuck. I can't figure out what's causing this.

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  • \$\begingroup\$ Did you measure the circuit voltages? What does it read at the op-amp supply? \$\endgroup\$
    – Justme
    Jul 5, 2022 at 6:06
  • \$\begingroup\$ The op-amp supply (the REF pin of the BuckPuck) is pretty stable at 5V, even with the op-amp fried. When the op-amp dies, its output ends up constant at about ~20mV, resulting in the LED being constantly on. One thing that I found a bit confusing is that with the LEDs on, the voltage between LED+ and LED- is about 24V. this is higher than the expected 21.6V combined forward LED voltage, and I've checked this at the LEDs themselves to ensure it's not cable losses causing the extra drop. \$\endgroup\$
    – Llewellyn Sims Johns
    Jul 6, 2022 at 3:38

1 Answer 1

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The datasheet says in figure 11, 14 and 16 that an external voltage provided for the control pin must be relative to LED-, not relative to VIN-. This is ugly here because your circuit GND cannot be VIN-.

The OpAmp will not help here. I recommend to use an optocoupler to avoid all sort of trouble related to this supply problem.

It is not clear what happens if you use more than one of these modules in parallel. VIN+ and VIN- will be fed from a common supply and you probably can't connect all LED- to one logic GND.

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  • \$\begingroup\$ Ah, good catch, thanks Jens. That is very annoying, it hadn't even occurred to me that the CTRL might be referenced to the LED- pin, isn't that highly unusual? I briefly tried connecting LED- to GND on a breadboard and the circuit seemed to work without immediately blowing up. I can see, however, that even if that works then it wouldn't be compatible with setting them up in parallel as it would mean connecting all of the LED- channels together. If I want to do that then I might need an opto-isolator or to move to a different LED driver altogether, which I am strongly considering \$\endgroup\$
    – Llewellyn Sims Johns
    Jul 6, 2022 at 3:13
  • \$\begingroup\$ @LlewellynSimsJohns You are by far not the first, who went into this trap. It is an absurd, nearly unusable remote control interface. A PWM driven optocoupler with a big capacitor at the control pin may work, but full light at startup is not nice either. \$\endgroup\$
    – Jens
    Jul 6, 2022 at 3:30
  • \$\begingroup\$ @LlewellynSimsJohns -- If this answer meets your needs and/or solves your problems and/or answers your question, please mark it with a check-mark to make it the answer that you have accepted. This gives "points" to the one who worked hard for you, and is a way of thanking them and cluing other in on a useful answer. \$\endgroup\$
    – MicroservicesOnDDD
    Jul 11, 2022 at 23:58
  • \$\begingroup\$ @Jens, if I could trouble you slightly further - Part of the reason I am using the Buckpuck for this project is its advertised ability to pulse the LED quickly and at variable brightness by applying a very short pulse of between 0-5V at the CTL pin. I am hoping to be able to turn the LED on for ~1 millisecond, and then off again. I haven't used optocouplers before, but would it be viable to use one for this purpose? I would still need to use a buffer opamp as well as a new 5V regulator as my control source is only able to supply 1mA. \$\endgroup\$
    – Llewellyn Sims Johns
    Jul 12, 2022 at 2:46
  • \$\begingroup\$ @LlewellynSimsJohns If you only need pulsed light a digital coupler can be used. The Buckpuck ca provide the supply for the coupler output side. If you need both, fast response and linear control, the design becomes interesting. You can create a 100 kHz PWM signal forwarded and isolated by a coupler like TLP2710 and followed by a low pass RC filter of e.g. 3kHz to form a proper and fast linear control voltage. \$\endgroup\$
    – Jens
    Jul 12, 2022 at 16:59

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