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I'm making an isolated communication/power module PCB, and I came across a problem that the B0505 isolated DC-DC converter has poor output regulation (+15% to -7.5% under load), and my HCS-LSP hall effect current sensor has requirement of 5V +/- 5% with 10mA rated power draw.

I don't think that the power draw from the DC-DC converter will be so high that it will dip below 5V, as it's only 3v3 MCU reading the hall effect sensor and a voltage divider.

I've read about zener diode voltage regulators somewhere, not sure if that would be the best choice, I've also have some TL431s.

I am aware that the schematic could be prettier. The double resistors/diodes/capacitors are only for SMD/through hole components alternatives.

schematic

What is the best way to power the sensor I have with the DC-DC converter I am using? How do I handle the fact that the maximum output voltage of my converter is higher than maximum input voltage of my sensor?

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    \$\begingroup\$ So you've made a bunch of statements, is there a question to go along with them? \$\endgroup\$
    – GodJihyo
    Commented Feb 13, 2022 at 3:30
  • \$\begingroup\$ get a better dc/dc converter. The one you specified can be got with a better regulated output from various vendors. \$\endgroup\$
    – Kartman
    Commented Feb 13, 2022 at 9:50

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Using zeners is a bad idea in this application -- zeners are not very precise -- a random "5V" zener I found on google ("BZT52-5V1") was actually specced to "4.8 - 5.4V @ 5mA". Also, zeners need some minimum current to operate, which is very hard in your case - in the worst case, you will have almost no voltage drop on zener.

If you are doing a lot of those, my advice would be to either get a DC/DC converter with better regulator; or get a model with 9V or 12V output and use linear regulator to regulate it down to 5V. The linear regulator approach is even shown as "Recommended circuit" in the B0505's datasheet!

If you really want to use the parts you have, you can use TL431 regulator in the "shunt mode" to limit the voltage to a bit below 5V (see section 10.2.2, page 20 of TL431 datasheet). But I would not use this in production design -- if using the cheap converters, the actual output voltage may end up worse than datasheet says, so you don't want on published margins being exactly correct.

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  • \$\begingroup\$ this is a one-off pcb, I've made the optocoupler part on perdboard but it looks hideous and doesn't work. I will order some B0509. When I was ordering the 0505 I forgot that the hall sensor needs 5v, hence the lack of 5v rail. I would just want a way to develop further in the meantime, so I'm looking for some kind of easy bodge. \$\endgroup\$
    – Oxmaster
    Commented Feb 15, 2022 at 14:24
  • \$\begingroup\$ For the short-term development you can just measure the output voltage, and if it is in the right range, use as-is without extra regulation. And if it is too high, add more load resistance until it becomes in range. This is not a good idea when assembling multiple boards, but is OK for one-off prototype. \$\endgroup\$
    – theamk
    Commented Feb 16, 2022 at 16:39

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