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Sense resistor for fan current

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iXod:
I'm using a Microchip TC649B:

http://ww1.microchip.com/downloads/en/DeviceDoc/21755C.pdf

to PWM a 48vDC, 1.6A BLDC 2-wire fan.

The examples in the datasheet use a 12v fan but since the supply for the fan and IC can be separate I'm using a 48v fan for better CFM (and I have it in my junque box).

The sense input to the IC calls for a resistor in series with the fan to give a pulse sense signal in the range 50mV-90mV.

I'm having a bit of a difficult time finding a resistor or shunt. 50mV would require 0.031 \$\Omega\$ ; 90mV would require 0.056 \$\Omega\$ . This is a through-hole board, but a prototype, so even SMD I could adapt.

I thought about using whatever resistor/shunt I can scare up and add a dropping resistor between the sense resistor/shunt and the sense input pin of the IC.

Any reason this dropping resistor configuration wouldn't work?

Thanks.

Renate:
Are you building a leaf blower?
You've powered this thing up already and determined that the noise level is acceptable?
Just asking.

Yeah, with 48V a bit of voltage drop won't kill you.
OTOH, 1 ohm is 2.5 W
0.1 ohm will get you down to 1/4 W

iXod:

--- Quote ---You've powered this thing up already and determined that the noise level is acceptable?

--- End quote ---
Hence PWM...


--- Quote ---Yeah, with 48V a bit of voltage drop won't kill you.
OTOH, 1 ohm is 2.5 W
0.1 ohm will get you down to 1/4 W

--- End quote ---
I don’t see an answer to my question there.

Will there be any issue placing a dropping resistor between the sense resistor and the IC sense input? See datasheet, p.26, for example schematic. There’s no input impedance listed.

Renate:

--- Quote from: iXod on August 26, 2020, 07:05:38 pm ---Hence PWM...
--- End quote ---
Some fans don't take too kindly to being operated under 20% power.


--- Quote from: iXod on August 26, 2020, 07:05:38 pm ---I don’t see an answer to my question there.
--- End quote ---
Well, your source impedance is 1 ohm or less.
We don't know what the input impedance is.
In worst case you will only need a series resistor instead of a divider.

ledtester:
See page 19 of the datasheet for information on selecting Rsense and Csense.

Update: If I read Renate's remarks correctly, just try Rsense of 1 ohm with a 3 W power rating, or a 0.1 ohm resistor with a 1/4W power rating. The 90mV figure is a threshold -- i.e. a minimum -- it's okay if the voltage across Rsense is greater than that.

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