27-05-2011, 10:53 PM
Just a few comments on that item - for those who may be interested.
The resistors are mounted on a large slab of finned aluminium heat-sink. On the right-hand side are two fans, whose operating voltage is anything between 6 and 30 v.d.c. Since this unit is designed to cope with d.c. and a.c. input voltages, there is a bridge rectifier to rectify the incoming a.c. to power the fans. On d.c., of course, they do nothing - but do make the input terminals non-polarised. The fact that the fans won't run at all at 5v input (or less) is not a problem, since at the minimum resistance value selectable, 0.75 ohms, at 5v, the power dissipated is in the region of 30 watts - and the since all the resistors are in parallel for this value, and each is rated at 300 watts, additional cooling is not required. At the same time, the greater the input voltage, the faster the fans spin. Of course, there is a voltage limit - the fan max. voltage is 30v.d.c.; a warning notice to this effect is clearly stated on the instructions in the lid. I did contemplate fitting a protective fuse for the fans, but my experience with these fans is that even with a fast-acting fuse, the fan burns out before the fuse blows! :@
I did slip-up at one stage.
I discovered (by consulting the RS on-line data) that the ammeter was a moving iron: therefore, O.K. for a.c. inputs. Having completed the ass'y., I blithley assumed that the voltmeter was ditto: it wasn't! 
Hence it became necessary to include another bridge rectifier to drive the voltmeter.
The fuse on the front is a 16 amps 'ordinary' type. This value is very convenient, since it 'fits in' with the ammeter scale and also the allowable wattage-dissipation figures for the highest-power/input voltage configurations. (I'll let you do the necessary Ohm's Law caculations!
)
What I now need is something similar for high-voltages: say, 50v to 500v. I need to check out a substantial assortment of mains transformers in that range. I'm currently thinking along the lines of an active dummy load. We'll see . . . .
Al.
The resistors are mounted on a large slab of finned aluminium heat-sink. On the right-hand side are two fans, whose operating voltage is anything between 6 and 30 v.d.c. Since this unit is designed to cope with d.c. and a.c. input voltages, there is a bridge rectifier to rectify the incoming a.c. to power the fans. On d.c., of course, they do nothing - but do make the input terminals non-polarised. The fact that the fans won't run at all at 5v input (or less) is not a problem, since at the minimum resistance value selectable, 0.75 ohms, at 5v, the power dissipated is in the region of 30 watts - and the since all the resistors are in parallel for this value, and each is rated at 300 watts, additional cooling is not required. At the same time, the greater the input voltage, the faster the fans spin. Of course, there is a voltage limit - the fan max. voltage is 30v.d.c.; a warning notice to this effect is clearly stated on the instructions in the lid. I did contemplate fitting a protective fuse for the fans, but my experience with these fans is that even with a fast-acting fuse, the fan burns out before the fuse blows! :@
I did slip-up at one stage.
I discovered (by consulting the RS on-line data) that the ammeter was a moving iron: therefore, O.K. for a.c. inputs. Having completed the ass'y., I blithley assumed that the voltmeter was ditto: it wasn't! Hence it became necessary to include another bridge rectifier to drive the voltmeter.The fuse on the front is a 16 amps 'ordinary' type. This value is very convenient, since it 'fits in' with the ammeter scale and also the allowable wattage-dissipation figures for the highest-power/input voltage configurations. (I'll let you do the necessary Ohm's Law caculations!
)What I now need is something similar for high-voltages: say, 50v to 500v. I need to check out a substantial assortment of mains transformers in that range. I'm currently thinking along the lines of an active dummy load. We'll see . . . .
Al.
