POST ISSUE 03 of 2016_10_18
Hi EF,
I have done a bit of investigation into your constant current load. Here are my conclusions and outline considerations. Let me know what you think.
(1)
APPROACH
(1.2) Suggest go for a linear approach. The PWM circuit is attractive because of its apparent simplicity and would probably be the way to go for a production model. But, by the time you sort out the switching issues, the switching circuit would not be so simple. There is also the concern about taking pulses of current from the battery and the EMC aspects. On the other hand, a linear approach has non of these problems and represents a lower risk and shorter development time, in my opinion that is.
(2)
CASE
(2.1) Case: dimensions: 540mm wide by 140mm high by 300mm deep.
(2.2) Heat sinks transistor: 16 of aluminum sheet panels, 140mm high by 140mm deep by 4mm (minimum) thick
(2.3) Heat sinks resistor: 16 of aluminum sheet panels, 140mm high by 140mm deep by 4mm (minimum) thick
(2.4) Fans: 4 of 12VV, 140mm square fans mounted vertically across the front of the case.
(2.5) The four 12V fans are connected in series to run off 48V, unless 48V versions can be procured at a reasonable cost.
(2.5) The heatsink panels mount vertically front to back on 30mm centers. The transistor heatsinks mount at the front. The resistor heatsinks mount behind the transistor heatsinks with a 20mm air gap between the transistor heat sinks and the resistor heatsinks (so that the resistor heatsinks do not heat the transistor heatsinks)
(2.8) The NMOSFETs mount on the front heatsink panels.
(2.9) The power resistors mount on the rear heatsink panels
(2.10) The rear of the case is open
(2.11) The fans suck ambient air in from the front of the case. After cooling the heatsinks, the hot air then exits from the rear of the case.
(2.12) The expelled air will be around 80 Deg C, depending on the current setting and the ambient temperature.
(3)
CIRCUIT
(3.1) 16 channels, each comprising two power resistors, one NMOSFET, and one opamp.
(3.2) The load current is set by a multi-turn potentiometer.
(3.3) The load current is accurately controlled by a conventional opamp and MOSFET negative feedback loop.
(3.4) The four fans are powered from the 48V supply line. The fan power dictates the minimum current that the load can take, but the fan power consumption could be adjustable in line with the current set. This would eliminate the error due to the fan current.
(4)
COSTS (Budgetary)
(4.1) Case and heatsinks: cost of aluminum sheet only, on the assumption that you do the metal bashing
(4.2) Fans: 4 of, 12V, 140mm square @ £8 each = £32.00 total
https://www.ebay.co.uk/itm/CORSAIR-...ffType=OrderSubTotalOffer&_trksid=p5731.m3795
(4.3) Schottky diode: 60V min, 60A min, stud or T0220 case @ £10.00 UK
(4.4) Opamp: 8 of LM358 @ £0.50 each = £4.00 total
(4.5) Resistor: 16 of 100W 4R @ £0.90 each = £14.40 total
(4.6) Resistor: 16 of 100W 8R @ £0.90 each = £14.40 total
(4.7) NMOSFET: 16 of @ £5 each = £80.00 total
(4.8) Voltage Regulator: £3.00
(4.9) Potentiometer multi-turn: £6.00
(4.10) Miscellaneous: £50.00