edm wirecut

okay i have converted a standard edm into a edm wirecutter ,,But the pulsed dc power supply is different for a wirecutter,,This is what i have is 1uS on time and 1uS off time and from 1A to 50A from the original machine ,,And what i need is from 1~3 uS on time and approx double for the off time and adjustable current from 1A to 6A but i cannot get the times from the control ,The current i can deal with within the original control,, So i am looking to make the off time longer ,,My questions to you guys is this

1- Is there anything i can do with the wave form after it is generated, Or should i try and alter the driver signal to the power board, I can get some more info from the boards if needed ie part numbers , pics,

Regards
Bernie

There likely is something you can do about that, but to be more specific, one would need the schematic of the supply you are working with. I suspect what ever generates the 500 KHz signal may be modified to give a lower % on period (i.e., lower duty cycle).

John

John

I dont have the schematic unfortunately , It is more the off time that is the problem needs to be longer,,I thought maybe i could change the off time after it is generated that way i dont play around with the original electronics

Regards
Bernie

What you have is called Pulse Width Modulation (PWM). It is conceivable you could take the signal (i.e., the low current PWM) and modify it, for example by dropping every other pulse, but you will still need some idea of how it is designed. At 500 KHz, there is no way to do it mechanically. At a minimum you need to find the low-current signal that is probably being used to control the on/off periods of the power side.

John

There is a bank of mosfets on a circuit board ,I will take a picture and post it up

B

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Gate drive pins for the mosfets are usually the far left pin, if you look at the side of the mosfet with the part number on it. It appears that R32, R34, R36, R38 are gate resistors. I can't tell whether the color code is Red,Black,Red (i.e., 2000Ω) or Brown,Black, Brown (100Ω). Both are possible, but 100Ω is more likely. Please confirm the colors.

If you trace the gate circuit back, you should find what is driving it. A typical circuit might consist of PWM generator --> gate driver --> gate resistors --> gates. Sometimes the PWM generator and gate drive are integrated into a single chip. A gate drive is needed to generate the current needed to turn on the paralleled mosfets quickly.

If I were to change the drive for a unit, I would attack it at the PWM generator. The chip labeled TC4011BP is a quad NAND gate. It may or may not be involved in the gate drive. Anyway, if you can locate and identify the PWM generator/gate drive chips, there may be a way to change a resistor to change the duty cycle.

For example, the TPIC2101 and LM3524 are older PWM generator chips. The SG3525 is another common generator chip. Their cycle can be controlled with resistors and or capacitors. If you can locate the chips doing the same function on your board, then you might be able to reverse engineer at least the PWM generation part of the design and change its duty cycle.

Failing that, I would strongly recommend against trying to design a standalone drive for the mosfets, disconnecting the current gate drive resistors and inserting the new drive and appropriate resistors. The most likely outcome from doing that would be to destroy the device. There is almost certainly some sort of feedback from the mosfets to their controller.

John

I found the incoming signal with my scope coming from the front control panel so i hooked up my signal generator to the twin plug named ca1 in the fourth pic down and it worked so now i can control the on ,off , freq times, So should work now ,Just need to find some time to play around some more ,,I alway chomp of more than i can chew but thats how you learn hey,,Thanks for your help with this much appreciated

bernie

If it works, then the only thing I can suggest is to keep fiddling with it until it doesn't.

More seriously, when you have time, it would be interesting to find the PWM generator on the front panel that feeds CA1. If it is one of the common chips, it may be quite simple to make an adjustable duty cycle without needing your signal generator. I am envious of you having an EDM.

John

Haha fired it up this morning and it didnt work thought my s/ gen was kaput but no played around and it worked again so ??
I will investigate the front control for the pulse circuit as that would be the smart thing to do,I might as well pull it all to bits hey , I have a small toolroom in australia and i have 3 edms 2 working and one in bits hehe

B

Okay i have followed the signal back as far as a hex buffer HD7407P after that i am not sure i think it goes to a programable device a PEEL18cv8p seems to have the pulse coming from it dont want to play with this chippy ,, Can i place a rc type settup at the buffer to give me a longer off time or slow the frequency down ,Then i could speed up the on time on the machine,, Just not quite sure how to configure this
Any help appreciated

Bernie

I would look at doing it digitally. If you have access to Horowitz and Hill (The Art of Electronics) go to the section on sequential logic, page 510 and later. One approach is to divide the frequency with a flip-flop, then AND that output to the input (see page 557). There are various ways to skin that cat. H&H show a slightly more complex circuit to generate a quadrature signal. Unfortunately, I have an early meeting today, so didn't have the time to get any real circuits for you. Try looking up D-flip flop and frequency dividers.

John

I have been giving this some thought in my sleep and what i need is to change the ratio between on and off times,As the off time is a percentage of the on time and thats the problem ,I need to alter the times seperately not just the frequency my mistake,,Still looking at the flip flop info maybe could still do something there

B

While I think the digital solution is still by far the best, your original question about using an RC delay has been bugging me. Today I had some time to simulate a circuit I dreamed up:



And here is the simulation with Multisim:



The circuit is quite sensitive to the component values, particularly the two resistors. These values essentially drop one of the cycles from the input. That is, it takes two cycles to charge C1 sufficiently to trigger the inverter. (The existing buffer IC on your board could probably be used instead of two inverters.) I do not believe you can get the output on-time to equal the input on-time of a single cycle with this circuit, but you can get close. Alternatively, a small change in the relative values will give the same frequency as the input, but a shorter duty cycle. That is, the on-time during a cycle is considerably shortened.

Anyway, at least I got this off my mind.

John

PWM generator

using 555 ic with comparator can do the PWM generator work and is simple.

Charles