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Our lab has an old New Brunswick Model G25 incubator shaker. Its temperature control function is broken lately. We would like to replace the two copper capillary thermostats with modern digital temperature controllers. Such as this one on Amazon.

I found its manual. On page 32, it has the circuit diagram. I found an RCAS bridging the "control" thermostat 1TAS-2 (pic 1). In the manual of another similar model, I learned this is a resister capacitor in series circuit (1mF+100Ω, pic 2).

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My questions are:

  1. What is the purpose of this bridging RCAS design? To provide a low constitutive current to the heater circuits? To prevent spark or surge from the thermostat?

  2. Will it affect my attempt to replace the capillary thermostats with digital controllers?

Thank you very much. I have rudimentary electrical skills. Any suggestions on this project will be appreciated.

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  • \$\begingroup\$ Did you ever end up completing this project? I've just been told about a G25 with thermostat problems and was thinking the thermostat was at fault and needed replacement. \$\endgroup\$ – Dustin Wheeler Oct 8 '19 at 20:45
  • \$\begingroup\$ @DustinWheeler yes, the project was completed soon after. It works perfectly fine. Very reliable and precise temperature control. I put the sensor on the chamber wall between blower and wind outlet(?), sticking out into the air. Make sure you buy the right controller. We bought the Fahrenheit-only controller while we need 37Celcius. \$\endgroup\$ – laviex Oct 9 '19 at 6:02
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RCAS is intended to extend the life of the contacts by absorbing arc energy during shut off to the motor. It must be used on the new relay. ( plastic cap and power resistor , same values )

Although the thermostat has a 10A relay and the G25 is rated at 10A , 10A relays are only rated for heater loads and not inductive motor loads which generate a hotter arc when the motor acts as a generator shutting down.

So if you can replace it with a 25A relay , somehow, or get a thermostat to drive a 25A relay, or use the 10A relay to drive a 25A DC relay using the internal supply voltage then it will last longer. A reverse diode is needed across the DC relay coil to suppress an impulse voltage during shutoff from inductance.

Remote location of the thermostat may pose noise and attachment problems but needs to be thermally attached like the copper tubes perhaps with thin epoxy.

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  • \$\begingroup\$ Thank you for fast and thorough answer. The thermostats only control the 400W heater; shaker motor is constantly running with a button switch. So I assume we will be OK with 10A? Thanks for the thermo sensor attachment suggestion, we will try to attach it reliably and close to sample in the chamber. \$\endgroup\$ – laviex Mar 4 '18 at 6:48

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