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multiplying two analog signals

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iggy

New Member
Hello!

I've build a regulated, adjustable power supply. Unfortunately, there isn't much space left for the heat sink - it's too small. However i can stil mount a fan to help cooling. I got the fan, a small but very efficient and unfortunately very noisy. I decided to make a circuit to control the fan. The basic idea is to keep the fan off when forced cooling isn't neccessary, and to regulate the fan's rpms (thus the cooling) regarding to the power dissipating in the supply. This can be achieved with two circuits. The first one is a voltage controled pwm - i've done it, works fine. The second circuit is a multipyer. That one should multiply two (analog) voltages. The 1st (X) is the voltage drop on the regulator and the 2nd (Y) is (also a voltage) proportional to the current flowing trough the regulator. The second circuit would then give an output proportional to the dissipating power (P=X*Y). Any ideas? Is it possible to multiply two analogue signals and have an analogue output to drive the fan??

Thank you!

iggy
 

Phasor

Member
It's perfectly possible, but probably overkill. Why not just mount a thermistor on the heatsink (in series with a fixed resistor), and use the voltage between them to control your PWM?
 

kinjalgp

Active Member
Yes it is possible to multiply two analog signals using operational amplifiers.

The circuit includes

I/P-1----> Log Amplifier 1-----> | Adder |-----> Antilog Amp.-----> O/P
I/p-2----->Log Amplifier 2-----> |______|

The output is multiplication of two analog I/P's.

You can find log amp., adder, antilog amp. in any op-amp text book.

But doing all this stuff just for cooling is useless. Its better to change the position og reglator IC and mount it on appropriate heatsink.
 

iggy

New Member
kinjalgp said:
Yes it is possible to multiply two analog signals using operational amplifiers.

The circuit includes

I/P-1----> Log Amplifier 1-----> | Adder |-----> Antilog Amp.-----> O/P
I/p-2----->Log Amplifier 2-----> |______|

The output is multiplication of two analog I/P's.

You can find log amp., adder, antilog amp. in any op-amp text book.

But doing all this stuff just for cooling is useless. Its better to change the position og reglator IC and mount it on appropriate heatsink.
I'm beating my head against the wall right now. I'm a student since i was born - almost 20years. I've had passed about 15 exams in mathematics and i didn't came across such a simple idea. I feel terrible. Ashamed, really. Thank you.

I know it would be much better to have a suitable heatsink, but unfortunately this is not an option and neither is moving the regulator - it can't be done. The power supply is enclosed in a 100*160*40 <mm> case togather with a 50W toroid + various servo circuits. It's getting crowded in there. The regulator itself has a current + power + current protection, an aditional thermistor is not needed. I also plan to make the "power meter" output aviable for external measurement, since i can't include anything else on the front panel than (max 5) 1mm diodes.
And a nother thing... i want to hear (the fan) when the supply is heavily loaded. Instantly. Since a thermistor + heatsink + regulator need time to heat or cool down, it would not be the way i wanted it.... I will stick to the plan.

Thank you for your help!

iggy
 

mechie

New Member
iggy said:
Since a thermistor + heatsink + regulator need time to heat or cool down,
This sounds super for an increasing load - the fan will effectively anticipate a temperature rise ...
But if a large load is suddenly disconnected ... can you cope with the thermal lag still dumping heat from the regulator ?
Maybe a thermistor should still be used as an additional input (fan runs to highest demand, multiplier or thermistor) to ensure you catch the tail end of this heat lag.
 

iggy

New Member
mechie said:
But if a large load is suddenly disconnected ... can you cope with the thermal lag still dumping heat from the regulator ?
Maybe a thermistor should still be used as an additional input (fan runs to highest demand, multiplier or thermistor) to ensure you catch the tail end of this heat lag.
Yes, I thought about that. I concluded this wasn't so critical for my everyday use. However you are right, I should include a nother thermistor to control the fan in such an event. It's not much work anyway.

Tnx to all of you!

iggy
 

Roff

Well-Known Member
You can make a low frequency analog multiplier with a pulse width modulator (the sawtooth and comparator), an analog switch, and a lowpass filter. Here's a block diagram:
 

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