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sinara-hw / Thermostat

2-channel temperature controller
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D12 Zenner diode draws too much current #92

Closed maciejprzybysz closed 3 years ago

maciejprzybysz commented 3 years ago

During thermal tests of the Thermostat, we noticed that the 1SMA5914BT3G Zenner diode (D12) gets warm. The photo from the thermal imaging camera also shows that significant amount of power is emitted on the R35 resistor. FLIR2399

Measurement with a multimeter shows that, depending on the device (several pieces checked), the R35 resistor drops voltage to ~ 3.15-3.30V. The same voltage was measured at diode D12. 2021-02-02_16h14_48

After desoldering the diode and examining it, it turned out that at 3.15V the diode current is over 50mA. In the datasheet you can find this specification: 2021-02-02_16h17_08

This Zenner should be changed to a more suitable one. The absolute maximum rating on the AD7172 inputs is 3.3V+0.3V so Vz(max)<3.6V and current at 3V3 should be much less than 50mA. I checked distributors stock and didn't find such diode in SMA case (same as 1SMA5914BT3G), so we ordered this one for testing: PLZ3V3B-G3-H

It should be solderable on existing boards, but changing footprint in the next review should be considered.

gkasprow commented 3 years ago

This issue was fixed already

maciejprzybysz commented 3 years ago

This issue was fixed already

Ok, I had to miss this issue. This change is not pushed on latest commit nor is it listed on v2.2 changes issue, so I thought there was still a problem.

gkasprow commented 3 years ago

This is still a work in progress

gkasprow commented 3 years ago

I pushed all the recent changes

gkasprow commented 3 years ago

fixed