Arc ignition?

[Javert]

I need an arc lighter.
Parameters:
Input voltage 12 maybe 24V
Output voltage typically 6kV - 10kV, short pulses say 2 micro seconds with a repetition ratio of 50-100Hz
Control by MCU, GPIO or direct PWM from MCU.
Can you advise any proven design?
Nothing HV transformer from an old TV, something small

I need to limit the upper voltage to max 12kV.
Is an MOV battery enough or is another solution more appropriate?

 

[carpenter]

For similar purposes, small HV flyback transformers are often used as examples FBT-17
Take the picture of the connection for guidance, it is from the Internet, the author used what he had on hand, so 500V MOSFET at 12V
MOV together with spark gap is commonly used in similar design,
the energy from the secondary must be destroyed somewhere if there is nowhere to jump the spark.

 

[KlausST]

Hi,

driving the IRF840 with less than 4.5V will not make them completely ON.

Better use a logic_level_MOSFET or a suitable driver circuit.

Klaus

 

[igeorge]

@ KlausST
Do you have a model /part number for logic_level_MOSFET

 

[danadakk]

Not expert here but logic level MOSFETs at 500V like 840 ? Not sure there is much
offering here, especially at 3.3V kinds of drive. More like 100V is area of offerings ?

Question is what does your circuit experience for Vd during transients ?


Regards, Dana.

 

[KlausST]

Hi

Do you have a model /part number for logic_level_MOSFET

No, I´d use a search tool provided by distributors and manufacturers.

Klaus

 

[carpenter]

A small note that will hopefully correct the errors and inaccuracies in the previous post.

In the picture we see how the FB transformer works. the transistor open, the current passes through the primary, the current increases, the energy is stored in the core of the transformer and at the moment of switching off the current begins to flow through the secondary and the energy from the core is discharged.
tcharge = tdischarge
there is a requirement for tdischrge =2us (2000 ns)
The switching times of the transistor should be significantly shorter than the time for which the transistor is open.
Reason?
1. During switching, the transistor heats up, its resistance is higher than RdsOn
2. This affects the current through the primary and secondary of the transformer. Ultimately, this affects the quality of the spark.

Why am I writing this?
Larger MOSFETS have some significant gate capacity that needs to be charged and discharged when opening and closing transistors.
A typical GPIO MCU has a maximum current of 25mA.

Example , AOD2610 60V, RDsON about 10mOhm. If we use for driving MCP1416 1,5A Mosfet Driver will be switching time about 10ns. If we control the transistor directly GPIO MCU , the switching time is extended to approx. 160 ns. I remind you, the pulse lasts 2000ns, switching takes 2x 160ns.
Yes, at a pulse frequency of 100Hz and used for ignition, it may be necessary to consider whether it bothers or does not matter and how much, but in general we should always be aware of it.
Even a MOSFET that switches at 2.5V is sometimes not good to control the MCU directly

 

[Javert]

Thank you all.
I tried to find a datasheet for FBT-17, without success.
So at least I looked at professional Chinese designs with these transformers.

They usually use direct AC voltage AC 24, AC 110, AC230, AC380, which switch to the transformer with either a thyristor or a relay.
On HV secondary they have, 1-4 pcs 2.2nF 15kV capacitor, spark gap and resistor 2-15W 1k for example.

The spark gap is quite clear. The voltage on the secondary increases until the energy has no way to flow away, if he can't jump where he has, he will jump on the spark gap.
High voltage capacitor. It limits the voltage peak and spreads the energy in time, it probably has a wave to the length of the spark. Do you see it the same way?
Load resistance Load resistance. Why?

What is it all for?
During welding, a spark that jumps between the electrodes ionizes the gas (e.g. argon) and the ionizing gas then better conducts a lower voltage arc, which begins to burn between the electrodes. The second thing that helps is the radioactive elements in one of the electrodes, the radiation again helping to ionize the environment.

I measured the parameters of high voltage ignition in an expensive professional welder of non-Chinese design and there were pulses of 6 kV, length 2us and frequency 65Hz. measured via a 100M resistor divider.
From the logic of the matter, I believe that the amount of voltage and the slope of the leading edge of the pulse and thus the sparks have the main influence on the spark quality. but then I don't know why the Chinese have 2.2nF 15kV capacitors on the output?

 

[betwixt]

Possibly the missing link in the explanation is that the transformer output is DC.
Flyback transformers are slightly different to conventional ones, firstly, they are often autotransformers and secondly, they are constructed in 'pancake' layers with a diode between each layer or sometimes a single high voltage diode and layers directly linked. The idea is to induce the voltage in stages with an insulating gap between them so no parts are so close together that they internally arc.

The 15KV capacitors are almost certainly there to charge up with the DC until the spark gap flashes over. The transformer output impedance will be relatively high because of the thin wire, spaced windings and the rectifiers so if it was used without the capacitors, the gap would have to be smaller.

Brian.

 

[FvM]

I agree that the transformer is most likely a "diode-split" transformer with built-in rectifier diodes. Another unclear point is the connection to the arc welder. The ignition devices that I know connect to the high current circuit by a series transformer. Its properties define the requirements for the igitor circuit..

 

[Javert]

That makes sense.
I tried to find the jump distance for the spark gap, but I didn't succeed.
More precisely, I found for tens, hundreds and mega volts, but for 5 - 15kV I did not find anything accurate

>The ignition devices that I know connect to the high current circuit by a series transformer.
Yes , high current side 18 turn 3x10mm Cu HV side 10 turk with ignition HV.

This is probably related to the part of the circuit that is visible on the first ignition board.
he voltage on the DC24V connector is controlled by relays SMI-24VD qnd SLA-24V 30A.
switching on a smaller relay brings AC230V (P1) to the primary transformer and this generates HV for ignition
What makes a big relay?
Shorts or disconnects the circuit C6,C7,R3,D1 and any on P2 F-PUT
Maybe just protecting the inverter's current source from high ignition voltage?