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I'm working on RD194 revH board to build a dimmable led driver with 27,2V and 0,6A output. According with DER278 to improve efficency I racalculated the transformer with a RM10 core.

From the original BOM of the RD194 I changrd the following components:

- transformer RM10 with primary splitted

- LNK407EG

- output diode BYV32E-200

- R6 feedback resistor 172KOhm


If I leave the original RM8 transformer, all is OK, also the output current is OK (630mA). When I replace the original RM8 transformer with the new one (RM10) the device goes in autorestart mode (the led string "flashes" about once per second).

I wound a new RM8 transformer (with the same pinout of RM10 and primary splitted), but the problem persist, while If I replace the original transformer all is OK.

So the problem is the construction of the transformer or the pinout.


The no-load condition with the new transformers seems to be identical to the old transformer (the board goes in OVP)


Please can you help me to fix this problem?


Thank you


Ghiotto Roberto

첨부 파일Size
TANDEM_RM8B_00.pdf48.35 KB
TANDEM_RM10_00.pdf48.14 KB
TANDEM_RM10_00_0.pdf48.14 KB

Please check your winding orientation. You may have terminated the windings incorrectly otherwise the unit will work with a new RM8 transformer. I don't know exactly what to change because the attached trf spec is different from the original RD-194 (referring to secondary windings), and there is no schematic. Try interchanging the secondary winding termination, or please send the schematic for further checking.


thank you for suggestion, I've solved the problem: the device entered in OVP,
because the bias winding was slightly higher than the other transformer.

I'll rewind the transformer with a lower bias voltage or use a zener with higher clamping voltage.


Thank you Sir

I'm now testing the driver with a triac dimmer. There's a lot of low-frequency noise (I think 100Hz) coming from the driver (with the original RM8 transformer the dimming was "silent"). Have you any suggestion about this?

Thank you!

Try to varnish the other transformer if the original RM8 trf you have has varnish. Other components especially the EMI filters may contribute to audible noise as well when you dim with leading-edge triac dimmers due to large current spike once the triac fires.

The original transformer isn't varnished, while the new RM10 transformer is varnished, so it's quite strange this behaviour!. Further, the bulk capacitors C2-C11 are metal film type (Panasonic ECQ series).

I think could be useful to increase the value of R13 or C6-R12: what do you think?


Have you other suggestions?


Thank you

Please check that the new transformer does not saturate. This could explain why it is even noisier despite having varnish.

Or perhaps the core materials are different. Try better core materials, because if the peak currents are similar, then it should have the same noise level. 


If it's not related to the transformer, you can try to reduce the value of the passive bleeder C11 or C1. You can increase R13 value but check thermals and efficiency. Also verify dimming performance if you change these components.