Hi, I am looking for some help from an electronic engineer. I am not an electronic engineer, but very long time ago when I was at a university, we did touch the subject.
I am looking for a power circuit step down. It is to be used with pyrography pen to burn feathers on the bird woodcarvings I do. Commercial models are available but are out of my price range and cannot afford to buy these.
I am prepared to build one if I could get help to design a circuit from one of you clever guys.
The power circuit I need is variable AC SUPPLY CIRCUIT.The circuit has to be 230/240volt mains supply. The step down supply is also AC.
A very old circuit in my friends machine is as follows:
The mains AC is connected to a switchable potentiometer 220kOhm 250v. The live output of potentiometer switch is then connected to a 250mA fuse and then the fuse is connected to a step down transformer.
A connection before the fuse is connected to a capacitor 0.1mF 250v X2. Which is connected to the other side of potentiometer switch and also to a Phase control Regulator 250W 230v @50hz.
It is PC1R. This regulator is old and can not get one.The middle connection of regulator is connected to return from transformer and also to a 15 ohms and 560k ohms resistors. The middle of the resistors is connected to centre of potentiometer. The end of potentiometer is connected to the end of 560 k ohms resistor. The end of the 560k ohm resistor and potentiometer is connected to the third pin marked 1, of the regulator.
There is also a light across the switch to show power is on.
The transformer is a bit vague but on another machine I think it is 130watt. Input is 230v live side and zero blue wire side. The output is 0v to 2.7 volts 10 amps nominal. The pyrography pen is connected to output side of transformer. The pen works on both.
So when you turn the potentiometer, the power is switched on and the red light comes on. When you turn the potentiometer clockwise it starts to heat the nib, the more you turn the hotter it gets and it can actually glow. The nib is made of special material to withstand heat.

Has anybody got a modern circuit to do this duty or can some body help me to get one made. I shall be very obliged for you help. Basically I need a circuit diagram and the spec of components I need to use. I will build it myself from the diagram. Perhaps any safety advise will also be welcome.
Much obliged.

 

You could probably use a light dimmer switch rated at 250 watts or greater. This is one example.

Les.

 

You could probably use a light dimmer switch rated at 250 watts or greater. This is one example.

Les.

Thanks Les, but I need a more robust circuit, where power is all smoothed out and most importantly it is finely controlled. I have heard of cases where dimmer switch on its own will burn out.

 

There is an improved version of that..
CSR1540A.


http://uk.farnell.com/united-automation/csr1504a/ic-power-regulator-15a/dp/1213084

I would use an ordinary 500W mains lamp dimmer.

 

Are you planning to make or buy your stylus?
I ask because that will determine what you are going to need RE power supply.

To do a good job you are going to need something that maintains a constant temperature or at the very least has a maximum temperature.
That suggests a constant current circuit as opposed to a constant voltage or a stylus with a high positive temperature coefficiant.
So hotter results in less current and with a little care the system becomes self limiting.
Have a look at PTC heating elements, used in all sorts of things now.

I would also strongly recomend using an isolation transformer, because of the safety aspect, and keeping your working voltage below 30V, ideally less.

Have you looked at cheap temperature controlled soldering stations? What is your budget?

You should also check out vapeing device circuits. These utilise vasriois resistive wires and variable voltage, but typically read the coil temperature by calculating its resistance, using voltage and current, whilst it is on.
The simpler ones dont even vary the voltage they simply switch the coil off when the current falls to some prefefined point, indicative of a temperature, and then monitor the resistance with a small current and switch it in again when the temperature has fallen a little.
Effectivly you would be building a free runuing oscilator with a PTC element, your stylus, as its timing element.

Al

 

Thanks Les, but I need a more robust circuit, where power is all smoothed out and most importantly it is finely controlled. I have heard of cases where dimmer switch on its own will burn out.

What are the actual specifications of the stylus?

The controller could probably be redesigned from the ground up to eliminate expensive and possibly dubious mains side control.

 

Are you planning to make or buy your stylus?
I ask because that will determine what you are going to need RE power supply.

To do a good job you are going to need something that maintains a constant temperature or at the very least has a maximum temperature.
That suggests a constant current circuit as opposed to a constant voltage or a stylus with a high positive temperature coefficiant.
So hotter results in less current and with a little care the system becomes self limiting.
Have a look at PTC heating elements, used in all sorts of things now.

I would also strongly recomend using an isolation transformer, because of the safety aspect, and keeping your working voltage below 30V, ideally less.

Have you looked at cheap temperature controlled soldering stations? What is your budget?

You should also check out vapeing device circuits. These utilise vasriois resistive wires and variable voltage, but typically read the coil temperature by calculating its resistance, using voltage and current, whilst it is on.
The simpler ones dont even vary the voltage they simply switch the coil off when the current falls to some prefefined point, indicative of a temperature, and then monitor the resistance with a small current and switch it in again when the temperature has fallen a little.
Effectivly you would be building a free runuing oscilator with a PTC element, your stylus, as its timing element.

Al

Thanks for reply. I will be making my own pens from a nichrome wire. It depends on the thickness of the wire. Thicker wires require higher current to heat up. The temperature can vary between 400 Dec to 1000 deg C. The max current need for the thickest wire to 1000 deg c is about 5 amps. I got the figures from suppliers.
The transformer needs to supply that and is varied with potentiometer. I see the voltage fixed and current variable.
Transformer is definitely used. All I need is circuit between the switc, potentiometer and the transformer which regulates the supply.

Many thanks

 

OK 101...
You cant fix voltage and vary current, it simply doesnt work like that.
Current is the result of a voltage applied to a resistance, if we are talking DC, there are other factors invlolved if the voltage has some freequency component.

Either way the power disipated will be I²R ant this can just as easily be the average I (Current)

When we talk about current limmited or fixed current supplies, even ones with a variable setpoint what we atre actually talkking aboutr is a system that measures the current and adjusts the voltage to modify it as necessarry.

In your case you need to work out the resistance of your stylus at 1000C, which will be much higher than its cold resistance and can be calculated from the cold resistance of a known length of wire and the coefficiant of that wire which will b e in ohms/C

Your power supply needs to have sufficiant voltage to drive at least 5A into that load.
Now you have to think about control...
if you vary the voltage the without any clever electronics you will get a reasonable degree of control because nichrome increeses its resiatance significantly as it heats up. It is often called resistance wire for this reason and is used directly connected to a voltage source.
As it heats its resistance increases which reduces current and thus heat... net result a given voltage produces a given temperature.
However this only works if you have a reasonable length, you will need to do some simple maths...

I suggest you start with a step down transformer for isolation, rectify and smooth its output and than add a buck converter module with a current limit circuit on board. Doing that you will be safe and have control of voltage or current depending on your adjustment.

To get slightly cleverer you would need the same control but add a method of sensing or estimating the temperature and making adjustments to the voltage to maintain it.

Remember that nichrome changes resistance with temperature... well if you know the voltage, current and cold resistance you can work out the approximate temperature.

There are many ways to do what you need but buck modules are cheap and readily available and modifying them for external control is not too chalenging either.

I have in the past used basic buck boards and controlled them with the PWM output from an arduino, it takes only a few resisters, a capacitor and a diode. If you added a current sensing module, again very cheap, your Arduino could actually manage the temperature.

There are probably circuits out there but if not building the eliments one by one and then combining them to make a system wouldnt be gtoo hard.

Al

 

OK 101...
You cant fix voltage and vary current, it simply doesnt work like that.
Current is the result of a voltage applied to a resistance, if we are talking DC, there are other factors invlolved if the voltage has some freequency component.

Either way the power disipated will be I²R ant this can just as easily be the average I (Current)

When we talk about current limmited or fixed current supplies, even ones with a variable setpoint what we atre actually talkking aboutr is a system that measures the current and adjusts the voltage to modify it as necessarry.

In your case you need to work out the resistance of your stylus at 1000C, which will be much higher than its cold resistance and can be calculated from the cold resistance of a known length of wire and the coefficiant of that wire which will b e in ohms/C

Your power supply needs to have sufficiant voltage to drive at least 5A into that load.
Now you have to think about control...
if you vary the voltage the without any clever electronics you will get a reasonable degree of control because nichrome increeses its resiatance significantly as it heats up. It is often called resistance wire for this reason and is used directly connected to a voltage source.
As it heats its resistance increases which reduces current and thus heat... net result a given voltage produces a given temperature.
However this only works if you have a reasonable length, you will need to do some simple maths...

I suggest you start with a step down transformer for isolation, rectify and smooth its output and than add a buck converter module with a current limit circuit on board. Doing that you will be safe and have control of voltage or current depending on your adjustment.

To get slightly cleverer you would need the same control but add a method of sensing or estimating the temperature and making adjustments to the voltage to maintain it.

Remember that nichrome changes resistance with temperature... well if you know the voltage, current and cold resistance you can work out the approximate temperature.

There are many ways to do what you need but buck modules are cheap and readily available and modifying them for external control is not too chalenging either.

I have in the past used basic buck boards and controlled them with the PWM output from an arduino, it takes only a few resisters, a capacitor and a diode. If you added a current sensing module, again very cheap, your Arduino could actually manage the temperature.

There are probably circuits out there but if not building the eliments one by one and then combining them to make a system wouldnt be gtoo hard.

Al

Al,
Thanks for your kind responce. As I wrote in my very first request, that I am not an electronic engineer but have very limited knowledge when I was at Uni many years ago. That is why I am seeking some help.
Can I send you a circuit which I have drawn. This is from very old machine.
I just want to replicate it but may need to put a modern regulator , transformer and capacitor. Or a new circuit with modern components. I am looking for an electronic engineer who can check and recommend changes with modern components. An engineer who can caculate the necessary voltages etc and ensure it will be safe.
If you are prepared to help/reccomend, I will be much obliged.
If not then I think I will need to stop this blog and go looking for an engineer who will help and I can pay for an hour or two of his time. I can not pay too much as I am a retired man.
Please let me know if you would like to see the circuit?