Hi everyone, a question from Australia.

A couple of years ago at work, I rescued three very nice PSUs ordered incorrectly and destined by the powers that be to be thrown out.

They are Schroff Maxpower 124 units and I wish to use one of them to construct a polystyrene hot wire cutter. The data sheet for these is found here:
http://docs-europe.electrocomponents.com/webdocs/0ef8/0900766b80ef89cb.pdf

To work correctly, a cutter requires the ability for it's PSU current to be regulated so as to set the correct heat of the wire. I have had a bit of a look around for a circuit to achieve this however could not find one specifically for 24V.

It appears to my untrained eye that the current is set to 4.2A, but then it makes mention that over 50degC the output current should be reduced. Does anyone have some advice on a simple external current regulator circuit for this unit, or whether it can be regulated using it's native connections?

Also I do not need three of these units, so if anyone is interested in one or two of them, make me a reasonable offer (they are unused and still in their original factory plastic wrap). If someone wants all three, perhaps I will just use the cash to buy a decent variable current PSU and save the mucking about.

Any comments would be much appreciated.

Cheers!

 

What you can do is to construct a simple On/Off switching circuit for the heating elements, i.e. you are changing the duty cycle of the current and thus changing the average power delivered to the heating element.

The output of your existing power supply is 24V at around 4A, this output can then be controlled by a MOSFET in series with the heating element, with the MOSFET driven by a 555 timer powers via a 7812 voltage regulator. The variable resistor varies the average power to the heating element between 5% and 95%, suitable for any kind of heating element.

The component are easily obtainable and the circuit is not difficult to build. Let us know if you want to go down this path.

 

It appears to my untrained eye that the current is set to 4.2A, but then it makes mention that over 50degC the output current should be reduced.

You need to nail this down a bit more, so that you know exactly the current range you'll be operating in. The strategy above is an excellent one, but as offered it will control from 0% to 100% of the power available from the PSU. I believe you'll want to ensure that no dial setting will ruin your cutter. You may also want much tighter control, for instance from 3A to 4A "full scale". You can use the same basic approach, but with some adjustments to suit your application.

 

Hey thanks for this suggestion, it makes a lot of sense rather than 'dumping' excess wattage.

The element is just a wire, so unless the current is too high (and it melts) a fairly course adjustment will be fine.

Thanks again, and if anyone is after two power supplies, I'm happy to give them away for a pretty cheap price. Make me an offer.

Cheers,

Tristan.

 

Sorry, forgot to ask about some specifics of the circuit.

What are the pins used on the 555, and there seems to be a resistor between the 555 and the MOSFET, what value would this be?

Also the variable resistor, what would be a suitable value range?

Cheers,

 

... there seems to be a resistor between the 555 and the MOSFET, what value would this be?

The purpose is to limit current, to protect the 555. So set the value as if the timer is driving a short. The gate of the MOSFET is like a capacitor, so the bigger that resistor, the longer it takes to charge it. But at high frequency, the gate capacitor will take a lot of current to charge and discharge, and that current could overwhelm the 555. So it's a balance.

Your 555 questions are all answered on this site in the 555 references.

 

Also the variable resistor, what would be a suitable value range?

The VR is a 100K one.

The circuit diagram is attached below. Circuit simulation indicates that the possible range of control is from 5% to 95%. The graph shows the average power dissipated in the heater element with various variable resistor settings.

You can use any available N-Ch MOSFET with rating 100V or higher and amperage 10A or more.