Sorry, that should be power resistors.

May I suggest you speak with Component Shop ( 01248 719353 ) . They produce 2 high power voltage reducers ( 10.5v/40v or 16/40v input) each of which will easily give you the 3 volts and will handle up to 7 amps (8amps with addition ventilation ) and they only cost £8.95 each . They may even be able to supply higher capacity units to order . Hope this helps . Trevor

Get a high power 12V motor control module.

Cheap as chips, feed it from the PSU and set it to give you 3V output.

Trevor and Neil's suggestions are worth a try because they're cheap, but otherwise not ideal.

• Neil's Motor control units are pulse width modulated. The problem in this application is they don't reduce output voltage. Instead they switch on and off rapidly and the energy delivered to the motor is varied by changing the ratio between on and off times. This system works well on motors and filament lamps, but I think it's dodgy on a low impedance load like a resistance soldering unit especially if the electrode is moved. Say the electrode is 1ohm and you want 30W of heat: that's 5.5A at 5.5V. If instead you apply 10V across 1ohm, the power supply will deliver 10A or 100W continuous. To reduce that to 30W of heat, the PWM duty cycle would be 30%. So far so good but the energy is delivered in the form of 10V/10A pulses. I think this is undesirable because the extra volts are more likely to spark and the extra current more likely to arc. (Do you know what the resistance of your electrode is?)
• Trevor's suggestion is a buck unit. The trouble with these devices is they prefer to work into a steady load, which an RSU isn't. My feeling is that one would stall and/or burn out if used for any length of time on intermittent load. I've no idea what happens if you pair them up for extra current.

But hey, it's not going to break the bank if one or both ideas don't work out. Neither idea is unsafe. Be really good to know what happens in practice. Please give one or both ideas a go and report back.

If you can find or make one, the least troublesome way to do resistance soldering is with a suitable transformer. Shame that's getting hard &/or costly. There's a real need for a cheap alternative, so good luck.

Dave

Edited By SillyOldDuffer on 22/06/2018 18:08:32

I think that using AC would give a simpler solution. obtain a toroidal transformer that is not filled with resin in the centre. The secondary voltage is not that important as you will wind a few turns of thick wire for the 3 volt secondary. From the figures you quote a 30 VA rated transfprmer should be enough but I suggest using at least a 60 VA one to give some power in reserve and also make it easier to wind the extra secondary. You can put a light dimmer in series with the primary so you can adjust the secondary voltage. You could use an old microwave oven transformer instead of a toroidal one but it will involve more work removing the high voltage secondary.

Les.

Let's do some simple arithmetic.

Original requirement was to provide RSU with about 3 volts, it would draw approx. 10 amps. So absorbed power is 30 watts. Possibly a bit light but it feels in the right ball park. Think of a 30 watt soldering iron – would this cut the mustard? Note the phrase " of the order of 10 amps" – properly this only says the current is somewhere between 1 amp and 100 amps! Need to nail this down a bit to do some meaningful calculations, so take the nominal values for the time being.

Equivalent resistance of the RSU is then 0.3 ohms (ish).

So we want to supply the RSU from a nominal 12 volt source. PSU needs to see 1.2 ohms total load for the load to draw 10 amps.

RSU accounts for about 0.3 ohms, so we need an additional 0.9 ohms in series with the RSU to end up with a circuit drawing 10 amps when supplied with 12 volts.

0.9 ohms isn't a preferred (standard, purchasable) value, so stick with 1.0 ohms as series ballast resistor for the time being. Moderates the load current to 9.2 amps.

Power dissipated by 1.0 ohm ballast resistor is 85 watts (I squared R), but only for the duration of the current flow. Suggest that the current flow will be less than 50% of the time, so average power dissipation is say 40 to about 50 watts. That said, the ballast resistor needs to have sufficient thermal mass that it doesn't reach its melt-down temperature during the time the current is flowing.

Suggest the ballast resistor needs to be one of those aluminium bodied plate mounted things, if you are going to buy one then stick with 100 watts dissipation, mount it on a moderate sized heat sink, try it and see. Remember that by adding a second resistor in parallel you reduce the overall value of the resistance, (product over sum rule for parallel resistances) and adding a resistance of 10 ohms in parallel will give you an effective resistance of (near enough) 0.9 ohms. The parallel trimmer resistance dissipates much less power – approx. 10 watts – so this "trimmer" resistor can be of a smaller power dissipation.

Putting two 1.0 ohm resistances in parallel halves the effective resistance i.e. 0.5 ohms, plus 0.3 ohms inside the RSU makes the load 0.8 ohms, supplied by 12 volts draws 15 amps, heading towards the max output of your fancy PSU. Dissipated power in EACH resistor is now 7.5 (volts) squared over 1.0 (ohms) equals 56 watts, but RSU terminal voltage rises to 4.5 volts. RSU absorbed power now 4.5 (volts) times 15 (amps) equals 68 watts, i.e more than doubled.

I've used the nominal 12 volts value of the input voltage so you can use the voltage adjustment of your "decent bench power supply" to fine tune the heat input.

Lots more arithmetic possible if we know the terminal characteristics of the RSU more accurately.

Hope this helps Simon

Why use DC in the first place? What you need is a chunky transformer with a mains primary and a few turns of thick secondary to generate a few volts but plenty of amps; then feed the primary from a variac to adjust the current. If you want to measure the actual current, get a current transformer, cheap enough from RS. Don't start messing with series resistors to control the current, they just get hot.

I was going to suggest that with most larger transformers there is usually enough room between the outer winding and the laminations to thread an extra secondary to give you the few volts that you need, but after reading your last post I won't. I did that with the large auto-transformer that supplies the 18V for my workshop power supply.

Ian S C

Regarding AC vs DC … It's interesting to note that American Beauty uses only AC

**LINK**

http://www.international-tooling.co.uk/Pages/American_Beauty_Pages/ATI_Power_Units_All.html

Whether for cost & convenience, or for technical superiority, I know not.

MichaelG.

Here's something for you to contemplate, Dave:

Decades ago, I was given an 'XP Welder' <phew, glad I didn't pay for it>

This contraption, which was also marketed as the 'Rawlplug Welder's Mate' , allowed one to arc weld [using flux-coated rods] using car batteries … The handle included a buzzer solenoid which kept the arc going, and avoided sticking.

The interesting bit is that it also featured the facility to mechanically lock-out the buzzer … a carbon rod then replaced the welding rod, to allow resistance brazing.

MichaelG.