Does the rectified voltage have the same ground as the other part of the circuit?
Also, try to connect the load between the drain and the 15V supply and see if it chops it.

 

the peak value of dc is 30volts

i connected the MOS as you said with 10 ohm between pin 3 and gate

10k across the drain and source and got a saw thooth shaped wave

the rectified power source is from variac to bridge

the circuit has its own transformer and power supply

 

For the circuit to work properly you need to connect the ground of the rectified voltage to the ground of the other circuit. Also, you get a sawtooth wave because the MOS turns on slow. You will need to put a BJT after the 555 to decrease the turn on time. Also, replace the 10K resistor with a 1K to decrease the turn off time. Also, disconnect the load from the rectified voltage and connect it to the 15V and observe the waveform.

 

mik3 should i put a resistor in the ground between the rectified dc and the circuit

 

mik3 should i put a resistor in the ground between the rectified dc and the circuit

No you shouldn't.

 

mik3 your the greatest it started to work finally


i still have to find a bjt and put it in to make the front of the wave square


but once i hooked the two grounds together it started to chop the signal


it is making the variac buzz what can i do to stop this

 

Ok that's good. I thought this about ground from the beginning but on your diagrams you had grounds to both supplies and I assumed that they are connected in reality too. The buzz is usually due to loose windings or a loose piece of metal on the variac or near it. How does the waveform looks like?

 

the wave form looks like a series of straight up spikes

that climb from base to a peak and start to decend after the peak

the center base line voltage also rises a little

the buzzing is decently loud sounds amost like when it is direct sort it but not quite as loud it dosent make this sound when its connected directly with out pulsing

what do i need to setup a bjt

the irfp450 gets hot real quick

thanks alot mik3 this was driving me nuts for a while

 

Disconnect the 10 ohm resistor from the 555, connect the emitter of the BJT to the 10 ohm resistor, its base to 555 and its collector to 15V. If you want decrease the 1K resistor to 500 ohm to decrease the turn off time.

 

If it is possible take a photo of the output wave form and attach it here to see how it is.

 

i can attach a picture tommorow my girlfreind has my camera

i will also have to get some bjts from radio shack

and new 555 as it seems to have finally went after all the messing with it

thank you very much mik3 i will talk to you tommorow

 

mik3 do you have a part number to a bjt i can use ive went to 2 places and couldnt find any neither place even knew what i was talking about

 

Thanks for the photo but isn't the wane very flat on the top?

 

i changed the mosfet to this new igbt i got today now have ixsh24n60 series and wave looks like this

100k resistor across the gate and collector


i had the power on only 3 or 4 volts could be why its so flat also

just relized when i had turned the nob on scope i hit the toggle from dc to center which is why it look like that

 

This looks better and the flat portion has almost disappear.

 

with this igbt what resistor should i use between 555 and gate

also is their any way to protect the igbt from emf blowing it up

 

You place a diode in parallel with the load but I don't if you can do it in your case because you want to make something to resonate. Another solution is to put a transient suppressor in parallel with the IGBT with a break down voltage less than the IGBT can withstand.

 

could i use a transient suppresion diode or maybe an isolation transformer

also if you look at the last wave form i posted using the igbt the waves are just straight up peaks their is no width to them can i widen the width up by changing the resistor from collector to gate to a higher resistance or is this pretty much how an igbt works

thank alot mik3 for your help i really appreciate it