For sensing the DC bus voltage of SMPS, MELF resistors are usually chosen over typical chip resistors (as in this question).
Why is that?
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\$\begingroup\$ I don't think that MELF parts are "usually" used. The ones in the linked question are used for the power rating. I think you need to find better examples. \$\endgroup\$– JRECommented Nov 25, 2021 at 13:26
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\$\begingroup\$ I have seen MELF resistors in many power supplies. I've also seen that 1206 high voltage SMD resistors are widely used but I'd like to know what are the advantages of MELFs, being more expensive. \$\endgroup\$– Ultra67Commented Nov 25, 2021 at 13:35
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1\$\begingroup\$ Metal film resistors were used for accurate and stable measurement. MELF is metal film resistor with cut-out leads. Wire-wound and metal-foil resistors provides similar properties, or better. But, the cost of production is much higher than metal-film. Nowadays, other type of packaging options are getting popularity, along with the technical advances. \$\endgroup\$– jayCommented Nov 25, 2021 at 14:12
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2 Answers
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Directly from:
https://www.electronics-notes.com/articles/electronic_components/resistors/melf-smd-resistor.php
MELF resistors are used because of their long term stability, moisture resistance, reliability, and the resistance to the effects of temperature cycling experienced in the soldering processes used today.
MELF (Metal Electrode Leadless Face resistors) are used where higher reliability and performance than that provided by normal SMT resistors are needed.
The MELF SMD resistors are used for a number of reasons:
MELF resistors provide a high level of reliability.
A MELF resistor has a more predictable pulse handling capacity than other SMD resistors.
MELF resistors can be manufactured with tolerances as tight as 0.1%.
They can be manufactured with very low levels of temperature coefficient, sometimes as low as 5 ppm/°C.
Overall the cylindrical construction of MELF resistors provides an optimal power rating and pulse load capability related to the mounting space.
Reflow oven's RoHS processes work at high temperature in the range 240°C to 260°C.
For sensing the DC bus voltage of SMPS, MELF resistors are usually chosen
I've seen many Switch Mode Power Supplies with bleeding resistors, between LINE and NEUTRAL, in 1206 pacakges. They had 2 resistors in series.
MMA 0204 HV and MMB 0207 HV Vishay's MELF resistors, for example, have a high operating voltage of 1000 V.
That's why MELF resistors provide a high level of reliability.
answered Nov 25, 2021 at 13:33
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\$\begingroup\$ Right. Those are generic reasons to use MELF resistors. What specifically would make a MELF better for the example in the question? \$\endgroup\$– JRECommented Nov 25, 2021 at 13:57
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MELF resistors can get pretty big in physical size compared to most other surface mount packages so that means wide terminal spacing so they tend to be able to handle more voltage without arcing. Like a leadless version of a leaded resistor which is also physically large. That's all.
They also handle heat due to pulses or surges well due to large etched surface area.
