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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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Hi Everyone, nice to see one of the US guys visiting our humble abode..
Voltage doesnt bother the IBC at all, it will happily run 5v motors or 35v motors. its *current* that gets things warm.
The Fets have an internal resistance (a very small one - .005ohms). if you suck lots of current through that resistance, a small voltage drop develops across it and the power that the heatsink needs to dissapate is that voltage drop multiplied by the current. so effectively Power dissipation goes up as the square of the current, or P =Isquared *R.
If you are using RC car type motors, they are designed to run at low-voltages (7.2-9.6v) but at very high currents, which will make the IBC work much harder than getting an equivalent amount of power out of a higher voltage (12 or 24v) system.
To get a 200watt drive from a 24v drill motor, you only need to feed it 8.3 amps, which means the IBC only has to dissipate .345 of a watt per Fet (2 per motor).
To get the same power (200 watts) from a 7.2v (less than 1/3 of 24 volts) motor you need to feed it 27.7 amps (3.3 times as much current), and as the power dissipation goes up as the *square* of the current, the IBC now needs to radiate 3.85 watts per Fet or more than *11* times as much heat for the exactly the same output power. No wonder it gets hot..
What all this means is trying to get the same power out of high current low voltage motors stresses the controller a hell of a lot more than getting the same power out of a higher voltage lower current motor does. If you are running 4 of these low voltage motors, you are working it *hard*.
We have 4WD 4 x Drill Motor powered 12Kg (twice your weight) robots running on 24v with the IBC in a sealed, unventilated box without a problem. They get warm, but they handle it ok, provided they dont have to maintain a stall.. so higher voltage and lower current draw motors is definitely the better direction to go.
Failing that, give it a Fan. The IBC's heatsink is designed to absorb surges of power with high thermal mass, but doesnt have much surafce area to radiate sustained power input away into space (we didnt want to have big finned heatsinks hanging off it), so if you are constantly drawing high currents through it, some cool airflow across it will really help)
edit: corrected some stuff ups in my calculations, squared, not doubled
Last edited by Spockie-Tech on Thu Jul 15, 2004 2:32 pm; edited 2 times in total
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Thu Jul 15, 2004 11:52 am |
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