Silicon Chip – May 2019

siliconchip.com.au Australia’s electronics magazine May 2019 97

I built a 12V 10A power supply
for general purpose use, but most of
the time it is used for float charging
a 12V lead-acid battery which runs
LED lighting. The supply needs a cool-
ing fan when under heavy load but I
didn’t like the fan running continu-
ously when it’s only putting out a few
hundred milliamps to keep the battery
charged during the day.

I decided to add circuitry to switch

the fan on when the supply is deliver-

ing more current but rather than use a

shunt to monitor the load, I came up

with a simpler solution.

This circuit monitors the ripple volt-

age on the capacitor bank that’s sup-

plied by the main rectifier and which

feeds the regulator portion of the sup-

ply. The higher the load, the greater

Switching cooling fan based on power supply load

the ripple, so this is a good way to

control the fan.

I’m using an LM386 amplifier (IC1)

to amplify the ripple. VR1 provides a

means to adjust the amount of amplifi-

cation and therefore the load threshold

at which the fan switches on.

The output of the LM386 is fed to a

diode charge pump based on D1 & D2

which results in a DC voltage propor-

tional to the AC ripple voltage.

This is applied to the coil of RLY1,

so once the output of the charge pump

exceeds the relay’s switching thresh-

old (around 3V for the 5V relay), it

latches on, lighting LED1 and power-

ing the fan.

A thermal switch on the heatsink

is used as a failsafe, to power the fan

if the heatsink temperature gets too

high, eg, if the load is somehow draw-

ing very high current pulses which are

too brief to trigger the ripple-monitor-

ing circuitry but still high enough to

cause considerable heating.

If the thermal switch closes, the fan

runs at full speed; the rest of the time,

it’s either off, or running at a slightly

reduced speed due to the 82W series

resistor. This reduces noise while still

providing adequate cooling in most

usage cases.

To set up the circuit, draw a constant

load from the supply at the level you

want the fan to switch on (I chose 3A),

then rotate VR1 full anti-clockwise

and then slowly clockwise until the

fan switches on, as indicated by LED1.

Assuming your supply has an adjust-

able current limit, you can do this by

shorting the outputs and then adjust-

ing the current to the desired level.

Trevor Vieritz,

Burpengary, Qld ($65).