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eyeofjake

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First, I will say that electronics is not my main specialty. I learned what I know from college physics and from my dad being an Electrical Engineer. I'm a chemist by trade. I'm building an high voltage capacitor discharge array circuit for an experiment at work. Being that we are on a budget, I decided to do it myself.

I opted for a boost converter to keep things simple as it's DC to DC. So I wired up a 555 timer and calculated the frequency and duty cycle to bring 12 volts up to 350 max. Wired the circuit and the first time was successful on a protoboard. Wanting to make it into a cleaner circuit, I made a clone to fit a sealed chassis with firing switch. But this time, it doesn't work. So far, I have either cooked the 555 chip or the MOSFET into submission. The capacitor is getting the 12 volts and all the pins are registering voltage. From what I can see, the MOSFET isn't switching.

Despite my attempts to troubleshoot this, I can't figure out what is going on. I could really use some input.
 

alec_t

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Welcome to ETO!
Can you post a schematic of your exact circuit?
 

eyeofjake

New Member
Sorry, 7 pin is connected to R1 between 6 and 8.

pin 3 is output to the mosfet gate

pin 5 isn't connected
 

ronsimpson

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You need a diode, (fast diode)
You have L1 from Batter (+) to 14V(+). You can not have DC voltage across a inductor.
Insert a diode and it should work.
upload_2017-4-3_19-59-10.png
 

eyeofjake

New Member
Sorry I forgot to draw that in as well. I have a 600 volt diode. I'm my part time job doing EMS. Let me put in a better schematic drawing
 

eyeofjake

New Member
What would cause the 555 timer to get wildly hot? My first circuit that worked, the circuit ran relatively cool and charged the cap bank to 400 volts no problem in about 30 seconds. My new one just gets to 12 volts and the timer chip gets really hot or the mosfet gets hot as well.
 

ronsimpson

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timer chip gets really hot
Please cut the GATE from the 555. Connect Gate to ground so the MOSFET is not working.
Now run the 555. Does it get hot?
The plan is to divide the circuit into pieces to see what is wrong.
 

Tony Stewart

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Start by defining your specs, V+ C, thus Energy 1/2CV^2 , Discharge Current , thus ESR, Irms on cap rating etc etc
 

eyeofjake

New Member
Input current is 12 Volts DC via a bank of 18650 batteries.
Capacitors are 120 uF rated at 450 in parallel for a total of 240 uF
Discharge current is going to be through the roof as it's going to be dumped through a 5 mm long piece of nichrome wire in a blasting cap to ignite a mixture of lead styphnate and Potassium Perchlorate

Goal voltage is 300-350 volts. No more than 400 as I'm not trying to approach the threshold of the caps max.

I'm shooting for a minimum of 5 joules output and obviously 240uF at 300 volts will satisfy that. Will allow me to fire several nichrome bridges at once if necessary.
 

Pommie

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If you are able to program pic chips then you maybe interested in a project I did a while ago. In this thread I posted details of a power supply for nixie tubes. The final design was adjustable from 150V to 400V. It was at much lower current as >5 joules can be lethal at 400V. The circuit is basically the same as yours only using a pic chip to provide the pulses and to measure the voltage.

Mike.
 

Tony Stewart

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Input current is 12 Volts DC via a bank of 18650 batteries.
Capacitors are 120 uF rated at 450 in parallel for a total of 240 uF
Discharge current is going to be through the roof as it's going to be dumped through a 5 mm long piece of nichrome wire in a blasting cap to ignite a mixture of lead styphnate and Potassium Perchlorate

Goal voltage is 300-350 volts. No more than 400 as I'm not trying to approach the threshold of the caps max.

I'm shooting for a minimum of 5 joules output and obviously 240uF at 300 volts will satisfy that. Will allow me to fire several nichrome bridges at once if necessary.
So this is just an electric match
 

Pommie

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Out of curiosity, do you have details of the nichrome bridges? Diameter and length would be nice.

Mike.
 

dr pepper

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You need a resistor in series with the fet gate to limit current from the 555 and so as not to try and switch it at too high a rate, 10 ohm ought to be ok.
Also a 100nf decoupling vap across the power pins of the 555 might help, and a 10nf from pin 5 to ground, also route the wires to the inductor and output ground direct to the power source and put a 100uf or 470u cap across the source, and route the power supply lines to the 555 separately back to the power source, basically everything to the right of the 555 with its own power lines, and everything to the left with its own power lines back to the supply/battery.
If you wound the inductor its windings must be tight and close to the core, its leads to power and the fet drain must be short, to avoid parasitic oscillations.
 

eyeofjake

New Member
Bingo. We are building a custom electric match ignitor to kick off our blasting caps. This is also our detonator for several other prototype weapon systems we are using. Unfortunately, we are still a new company and don't have the funds to buy high end blasting machines that run 2-3k a piece. Pretty ridiculous.

The nichrome bridges will be 4-5 mm in length of 28 or 30 gauge wire connected to 22 gauge copper wire placed into the initiator compound.

OK, so put a resistor in line to the gate of the MOSFET to drop the current, cap across the source to the 555, and a cap to ground out pin 5. I did solder a pins 2 and 6 together with a piece of wire as they are pulling the same voltage from the resistor. I wonder if they are getting shorted.

I did wind my own toroid. Ran the math and got an inductance of about 2 mH give or take. I'll swap it out for an inductor I extracted from a compact fluorescent bulb. Perhaps that should help.
 

Willen

Well-Known Member
Don't the 555 need a high voltage spike protection came from the inductor?
 
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