Schematic
Controller Installed in a Tektronix 7854
Older Tektronix 7854, 7904A, 7934 and perhaps others as well used a cooling fan based on a separate motor and controller PC board. These fan arrangements are very expensive to obtain and can be replaced with an appropriate standard computer type fan that has the DC to AC controller built into the hub of the fan.
The older fan designs used a relatively large fan blade and although the fan was noisy it was usually tolerable.
The late production units eliminated the early fan arrangement and used a standard computer type fan that has the DC to AC controller built into the hub of the fan. The fan used was a 12V DC 0.3A fan that sounds like a wind tunnel... so noisy as to be intolerable except in the noisiest locations.
This fan operates from a 15V supply in the scope and uses a 10 ohm series resistor to drop the voltage to 12V @ 0.3A.
The circuit described here is intended to temperature control this fan so that in most circumstances the fan runs at a much slower speed and is quiet. Temperature control assures that the scope is protected from overheating. Typically Tektronix specifies their scopes to operate safely up to 50 deg C or 112 deg F, and with the highest power plug-ins possible. In the vast majority of situations, the fan can be run safely at a much lower speed.
The controller used parts already available and many components can be substituted if desired. The controller is built around an LM34DZ temperature sensor that produces an output of 10 mV/deg C and an LM358 dual op amp that operates with a single positive power supply.
The first op amp is used for the temperature controller and the second op amp is used for the minimum speed setting. Diodes isolate the control signals from these two functions fed to the output power transistor.
The LM34 feeds the temperature sensor voltage [~ 0.75V at room temperature] to the non-inverting input of the first amplifier. The gain of this amp is set by the adjustable feedback resistor [20K pot, R2 + 10K resistor, R1] and the 1K, R3 resistor to ground. This GAIN setting is used to establish the fan response to increasing temperature.
R4 and R5 connected to the + 15 supply vary the BIAS level of the inverting input to set the point there the signal from the sensor fed to the first amp is large enough to drive the power transistor to increase the fan speed.
The second amp buffers the variable voltage from the series combination of R6, R7 and R8. The output of the second amp drives the power transistor through a diode and is used to set the MIN fan speed.
The output power transistor is a high current gain power transistor that is easily driven by the LM358.
The controller is set up by first raising the BIAS setting to maximum, setting the GAIN to minimum and setting the MIN speed to a suitable quiet level, as fast as possible consistent with the noise level that is tolerable... usually from 5V to 7V at the fan.
Then set the GAIN to maximum and adjust the BIAS setting to "intercept" the MIN setting [the setting when the fan speed starts to increase above the MIN setting].
If the fan speed increases too much as the scope operates and warms up, simply reduce the GAIN setting. After the scope is cool, reset the BIAS setting for the new GAIN setting.
The LM358 IC was mounted upside down on copper clad PC board; pin 4 [ground] is soldered to the board and pin 8 [V+] is held to the PC board by a .1 micro farad ceramic capacitor. The three 20K potentiometers were mounted to the board using JB Weld epoxy. The power transistor was mounted with an aluminum heat sink and a silicone thermal pad. All the wire connections and remaining components were wired between the mounted components... no other terminals were needed. The power in was accomplished with wires to a two pin header made into a plug to mate with the existing Tek two pin 15V power connector. Power out from the controller to the fan uses wires to a two pin "PC" female socket to mate with the fan connectors on the scope. Note that the 10 ohm resistor is wired to and mounted next to the fan by Tek.