mike48
30th November 2014, 02:44 PM
Resistive Load for PSUs and Batteries
I needed a resistive load to load up a 12 volt system to chase voltage drops in cables, connectors, earth runs, battery capacity testing, PSU loadup, and for other uses.
I always wanted to make one of these.
The requirement was to have current draw of 2.5/5.0/10.0/20.0 amps at 12 volts, and to be cheap and robust.
It should be able to draw up to 10 amps all day, and 20.0 amps for about an hour, before heat becomes a problem.
I used Whites Products 1.6mm 304 stainless steel "Garden Tie Wire" as the resistive element, available ex Bunnings for $13 for 15 metres. It is quite resistive, able to handle high currents, and relatively corrosion free.
This wire tested (over 5 metres) at approx 0.445 ohm per metre, and the measured diameter was 1.57mm.
I used a scrap piece of 8mm fibre cement paneling, 400 x 150 mm as the base, (free from Bunnings as it was a packing piece for FC sheet), and found some scrap aluminium angle as strengthening pieces.
I painted the FC base with Cabots water based clear PU paint.
I wound three windings of 19 and 9 and 9 turns, after doing the mathematics for the electrical length around the FC base, to give 2.4 and 1.2 and 1.2 ohms.
For 12 volts, all turns in series draws 2.5 amps, 19 turns draws 5 amps, 9 turns draws 10 amps, and the two 9 turn windings in parallel draw 20 amps.
These windings didn’t need to be exact for me, but turned out quite accurate and within 5%.
Caution on the wire resistance, as the product wire material will vary from time to time, and yours might be slightly different. Measure your wire resistance before making up a unit.
You could use steel tie wire, but you will need much longer lengths.
My wire was only very, very slightly magnetic, even with a strong rare earth magnet (less magnetic than normal 304), so I don’t know what exact type of SS it is, but it doesn’t matter for this application.
After all, it is “garden variety”!!!
My device is prototype no 1.
Not really a thing of beauty, but works well and is sturdy.
If I make another one, I would make the FC base notches at ¼ inch spacing instead of 5mm, as in mine.
If you add a 12 V muffin fan, you could probably run the load at 20 amps all day.
cheerio, mike
332858332857
I needed a resistive load to load up a 12 volt system to chase voltage drops in cables, connectors, earth runs, battery capacity testing, PSU loadup, and for other uses.
I always wanted to make one of these.
The requirement was to have current draw of 2.5/5.0/10.0/20.0 amps at 12 volts, and to be cheap and robust.
It should be able to draw up to 10 amps all day, and 20.0 amps for about an hour, before heat becomes a problem.
I used Whites Products 1.6mm 304 stainless steel "Garden Tie Wire" as the resistive element, available ex Bunnings for $13 for 15 metres. It is quite resistive, able to handle high currents, and relatively corrosion free.
This wire tested (over 5 metres) at approx 0.445 ohm per metre, and the measured diameter was 1.57mm.
I used a scrap piece of 8mm fibre cement paneling, 400 x 150 mm as the base, (free from Bunnings as it was a packing piece for FC sheet), and found some scrap aluminium angle as strengthening pieces.
I painted the FC base with Cabots water based clear PU paint.
I wound three windings of 19 and 9 and 9 turns, after doing the mathematics for the electrical length around the FC base, to give 2.4 and 1.2 and 1.2 ohms.
For 12 volts, all turns in series draws 2.5 amps, 19 turns draws 5 amps, 9 turns draws 10 amps, and the two 9 turn windings in parallel draw 20 amps.
These windings didn’t need to be exact for me, but turned out quite accurate and within 5%.
Caution on the wire resistance, as the product wire material will vary from time to time, and yours might be slightly different. Measure your wire resistance before making up a unit.
You could use steel tie wire, but you will need much longer lengths.
My wire was only very, very slightly magnetic, even with a strong rare earth magnet (less magnetic than normal 304), so I don’t know what exact type of SS it is, but it doesn’t matter for this application.
After all, it is “garden variety”!!!
My device is prototype no 1.
Not really a thing of beauty, but works well and is sturdy.
If I make another one, I would make the FC base notches at ¼ inch spacing instead of 5mm, as in mine.
If you add a 12 V muffin fan, you could probably run the load at 20 amps all day.
cheerio, mike
332858332857