Why did Voyager have to shunt unused electrical power and radiate as heat?

Question

Reading about Voyager in The oldest computer (not) on Earth I saw the image below, where is shown a science instrument calibration panel and shunt radiator.

According to the November 1980 NASA News Release Voyager Backgrounder:

A .36-square-m (4-square-ft.) shunt radiator/science calibration target faces outward from the propulsion module truss adapter toward the scan platform. The dual-purpose structure is a flat sandwich of two aluminum radiating surfaces lining a honeycomb core. Through power collectors and emitter resistors between the plates, any amount of the electrical power from the radioisotope thermoelectric generators can be radiated to space as heat. The outer surface also serves as a photometric calibration target for the remote-sensing science instruments on the scan platform.

Why would it be necessary to build a system to radiate unused electrical power as heat? According to this table each of Voyager's three MHW-RTGs started with 2400 W of thermal power producing only 160W of electrical power. I assume that using or shunting the 160W of electrical power results in 160W less radiated power, but why was providing a maximum 7% drop in radiated power worth adding the weight and complexity of this electrical and radiation system?

Answer 1

It's not about reducing radiated power. The shunt is used to keep the bus voltage constant when the power draw changes.

A similar system was used on Cassini:

Other portions of the power system include the power control boards, with 192 solid-state power switches (SSPSs), a radiator to “shunt” excess, unused power as heat to space, and a shunt regulator assembly, which (1) tells the radiator how much power to shunt and (2) maintains the spacecraft bus voltage at 30 volts.

It's done this way because a shunt is simple: it's just a switch that diverts power to the shunt resistor. The alternative is the use of an electronic system that regulates bus voltage. In this alternative, all input power is routed through electronic components, and you get a higher component count an some losses.

Direct Energy Transfer (DET) systems dissipates unneeded power – Typically use shunt resistors to maintain bus voltage at a predetermined level

Peak Power Trackers (PPT) extract the exact power required from the solar array – Uses DC to DC converter in series with the array – Requires 4 - 7% of the solar array power to operate