• I have 18W, 36 LED lamp fixture which I cannot change.
  • Originally it has 2x parallel 18off 5730 LEDs and 300mA driver. It works for some time, but diodes starts burning. I thought that the problem is 300mA when one of the LED strings gets faulty - remaining parallel string cannot handle 300mA and it ends up with both branches destroyed. Below table describes 5730 LED which I bought to replace burnt diodes: 5730 ratings

Solution which turns into problem:

  • I manage to re-wire all 36 LEDs in series (=3.3Vx36=118,8V, 120mA) & renew faulty diodes.
  • I bought driver based on BP9833A and assembly everything altogether. Please see below picture of my BP9833A driver:LED driver BP9833A
  • After re-installation of fixture two LEDs get burnt during power-on. I repeat previous steps and got another few LEDs burnt (not only those which I renew). Finally I install 3x in series 390 varistor parallel to the LEDs. It allows me to start-up the driver without harm to the diodes. With UT203 multimeter I have: 115-120mA, 117Vdc. Driver idle voltage is ca. 180Vdc. When loaded it is switching off around 120Vdc. Nearly perfect except this idle voltage which I suppose should be not the case as I have the load connected. BP9833A application
  • ROVP has 10kOhm, resistor on pin CS seems to be fine as driver stabilises around 120mA.
  • I thought my bad experience is a flaw of BP9833A. I bought driver based on MT7854BD with lower maximum voltage. Please see below picture: LED driver MT7854BD
  • I reduce string to 24 LEDs (=3.3Vx24=79,2V, 120mA) as it cannot handle more than that - it simply stays in safe mode. Current stays around 120mA, loaded voltage 75Vdc. Surprisingly same fault happens without varistors. Few LEDs are damaged again during power-up phase. It seems to be worse when single pole wall switch is in use. They survive few starts when switched by Schuko type plug with round pins on my bench.


  1. Why do I burn 150mA diodes with 120mA driver during power-up phase?
  2. What can I do to limit excessive initial voltage/current in both BUCK converter designs during start?
  • \$\begingroup\$ What peak current have you measured though the LEDs during start-up, hot and cold? \$\endgroup\$
    – winny
    May 24, 2020 at 21:06
  • \$\begingroup\$ That is a little problem as I have only rather simple UT203 multimeter. I measure current with clamp - it seems way to slow to show startup peak. Will try to arrange oscilloscope. \$\endgroup\$
    – gustawo2
    May 24, 2020 at 21:09
  • \$\begingroup\$ You need an oscilloscope. \$\endgroup\$
    – winny
    May 24, 2020 at 21:10

1 Answer 1


I think it's because you attempt to use these constant power (18W) power supplies at or near their maximum voltage. It should be used closer to the minimum voltage. And/or because you use constant power and not constant current power supply. When you try at the upper voltage limits, the driver doesn't see 18W even thought it outputs 120 mA and rises voltage much higher in order to reach 18W.

You need a constant current power supply for exactly 120 mA (Not 100~240 mA or the likes) with a power range between 15W to 30W. Then theorically it should outputs a voltage within the LED limit. The goal should be to keep 120mA, not 18W or x volts.

Besides, I advise you to divide the series and use two or even three series in parallel, each with their own suppy. It's an extra cost but it's much safer against electro shocks. At 120 VDC it's already quite dangerous.

  • \$\begingroup\$ Both sources were ordered for exactly 120mA - in data-sheet you will find equations to calculate resistors around chip to obtain constant current ie for BP9833A it will be a resistor on CS pin. In fact both really stabilise around 120mA (measured with clamp UT203). Will try the idea to utilise them closer to lower ratings. \$\endgroup\$
    – gustawo2
    May 24, 2020 at 21:19
  • 2
    \$\begingroup\$ Maybe it doesn't react optimally when you use than at the upper limit. Another suggestion: Add a large capacitor on the DC side. Something like 100µF or 470µF. \$\endgroup\$
    – Fredled
    May 24, 2020 at 21:28

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