High-power, low-voltage dummy load.

[Skywave]

Now and again, the need arises to test low-voltage power sources - predominately PSUs and transformers. My usual approach is simply to measure the voltage and current when various load resistors are connected. Of course, this requires an assortment of power resistors - and sometimes the power rating required of these can be quite considerable. Normally, for me, this is not really a problem, but the need to keep wiring up & then re-wiring these resistors can get rather tiresome.
For quite some time now, I have owned four 3-ohm 300-watt high-power resistors of the well-known aluminium-clad variety: 'Arcol' manufacture. Often, these have been used for the above need. However, during a recent tidy-up, I came upon a steel box, some meters and a few other bits & pieces - and another idea was born. :idea: Wire them together, via switches, to make a semi-variable dummy load.

And the associated pictures give the end result.

Al. / May 27, 2011 //

Update: Following this Forum's earlier re-structure, pictures now reinstated:

[attachment=3492][attachment=3493][attachment=3494][attachment=3495][attachment=3496][attachment=3497]


Al. / Dec. 6, 2011 //

[AlanBeckett]

That looks splendid Col.
Alan

[Skywave]

Hi Alan,
You've done it again! Its Al.!
Never mind - thanks for the accolade.

Al.

[AlanBeckett]

I know, I know.
It's late, and I've got some variety packs of Wychwood beer.
I'll bring you one by way of compensation
Actually, it's a peculiarity of this Forum that I could go back and change it.
Alan

[Skywave]

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.

[Skywave]

I know, I know.
It's late, and I've got some variety packs of Wychwood beer.
I'll bring you one by way of compensation
Actually, it's a peculiarity of this Forum that I could go back and change it.
Alan

Would that change be the name . . . or the beer?
Mine's 'Old Empire' . . . which, regrettably, I'm currently out of stock of. :@

Al.

[AlanBeckett]

Al,
It's all marston's these days, and as my mate Spock works at the Brewery I've laid in stocks against just such an eventuality
Alan

[AlanBeckett]

Al,
There's only me and you on line so I think it must be time to go to bed
Alan

[Yorkie]

Al,
There's only me and you on line so I think it must be time to go to bed
Alan

Apart from which, at that time of night, should you not already be tucked up in bed, and supping Ovaltine - not Wychwood!?

Nice job Alan - as you say, by using the resistors in series or parallel, you can get a nice combination for various power levels. I've sometimes use car headlamp bulbs strung together, but it's all a bit makeshift. And having the meters in circuit too is better than spragging leads from a multimeter across or in line with the equipment being tested.

Another nice example of bringing together items from the 'spares box' gathered together in the expectation that one day perhaps, they may come in useful for some as yet not thought of piece of gear. In this instance, that day arrived!

Enjoy your home-brewing.

David

[Skywave]

Thank you David.

Yes, I am a bit of a magpie, but OTOH I have been 'doing' electronics in one form or another for over 50 years now (employment & hobby), so one tends to accumulate stuff. And since I was brought up in a non-too-wealthy family in the 'make-do-and-mend' culture of the 1950s probably explains enough.
Incidentally, the case came from a small local firm who supplied domestic burglar alarm systems; I used to do the occasional repair job for them. When they folded a few years back, I bought a lot of their then redundant stock at a very good price; the box in this project was part of that purchase. That bit is believable - but this bit isn't: the meters came out of a skip! I can't remember where the resistors nor where the fans or heatsink came from; they've all been here a very long time.

Finally, I've added the cct. as per below.

Al.