Modify a temp-controlled fan circuit?

Question

This motor and its drive circuit are one unit (the circuit board is on the end of the motor opposite the impeller, in the air stream):

It seems to me that the element in the lower right corner (RT1696) is a thermistor, or such. This is what controls the motor speed?

The manual this is from describes the thermistor (RT1696) as:

RES.,THERMAL:5 K OHM,10 PERCENT

No generic pn is given. It is a little green teardrop with a red dot on top.

If I want to boost the minimum speed, or alternatively, push the entire speed range up by a few hundred rpm, do I add an adjustment pot in series with RT1696?

Or...?

Thanks,

Dave Platt · Accepted Answer

Most of those thermistors have a negative resistance coefficient of temperature. Warm them up and the resistance drops.

It looks to me as if R1695 (fixed) and R1696 (thermal variable) are setting the speed... nominal total would be 7k ohms, and it would decrease at higher temperatures.

If you were to replace R1695 with a smaller value, or perhaps a 2k pot, this would "fool" the rest of the circuit into "thinking" that the temperature is higher than it is. If this circuit is designed to fan-cool something, the fan speed would increase.

DaveC · Answer

If a 2K pot was installed and adjusted lower, this would not only set a higher minimum speed but it would add a fixed positive differential to the temperature curve of R1696?

Thanks for your assistance.

DaveC · Answer

R1695 is defined:RES.,FXD,FILM:2K OHM,1 PERCENT,0.125 WReplacing with a trimpot, use just the wiper and one leg? Or...?Thanks,

James Sweet · Answer

Depends on whether the temperature coefficient of the thermistor is positive or negative. Check it with an ohm meter and see if the resistance goes up or down as you warm it. You can then connect a variable resistor either in series or parallel to get the effect that you want. Something around 1K in series or 100K in parallel would be a good place to start. Feel free to experiment.

James Sweet · Answer

Yes, try a 5K trimpot, wiper and one leg will work, or you can connect the remaining leg directly to the wiper so that if the pot develops a dead spot, the resistance will go up to the value of the pot rather than open circuit. I would put a 1K or so resistor in series with the pot as well so that it can't be reduced down to 0.

Bob Larter · Answer

Replace R1695 with a 2K pot with 1K in series.

Eeyore · Answer

Is it an NTC or PTC resistor ? In any event a trimmer in series in place of R1695 would give you some adjustment. Say 4k7 / 5kGraham

Eeyore · Answer

It's wise to tie the wiper to the other end of the track to avoid opens.Graham

Eeyore · Answer

Alsdo a good idea.Graham

Rich Grise · Answer

^^^^^^^^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^Negative temperature coefficient of resistance. ;-)Cheers!Rich

Rich Grise · Answer

You could simply bypass the whole controller and run the fanballs-to-the-walls continuously, if it's rated for continuous duty.Have Fun!Rich

Modify a temp-controlled fan circuit?

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