mosfet gets "stuck on" when using project volatage but not external voltage for dc-ac inverter

Thread Starter

.frank.

Joined Feb 21, 2016
46
It's in the programming for the PIC. Did the program tell the PIC to toggle the mosfet gate at 1 MHz? I doubt it.
Another huge weakness of mine - I DONT CODE. I used Flowcode to hurdle that obstacle. But I did not add anything special about clock speed besides the base speed of the internal oscillator.
 

Thread Starter

.frank.

Joined Feb 21, 2016
46
What clock speed is the pic running at, is it set to 8mhz internal?
Odd, now that I look - the speed is set to 8mhz in flowcode for the chip using the internal oscillator. How in the world then am I seeing 1mhz on pin 3 which is the clock out speed? Also I see 1mhz with the first picture from the scope with the circuit on, but at rest.
 

#12

Joined Nov 30, 2010
18,217
You have a circuit that switches a light on at maybe once per second. It is absolutely stupid to switch the LEDs or the inverter on and off at megahertz speeds in order to get them to change once per second.
 

Thread Starter

.frank.

Joined Feb 21, 2016
46
You have a circuit that switches a light on at maybe once per second. It is absolutely stupid to switch the LEDs or the inverter on and off at megahertz speeds in order to get them to change once per second.
I thought that too, but I DO need the rising AND falling edge of when the sound is detected to work in the < 50 ms range. Can I just back down the speed of the pic and try again as is? Or should I try to use the 1uf capacitor across the D/S terminals as Dodgydave suggested?
 

Dodgydave

Joined Jun 22, 2012
9,910
Might be a good idea to post your pic code, maybe you're not turning the pic output off on RC5, or use another output, try the capacitor if you want.
 

Thread Starter

.frank.

Joined Feb 21, 2016
46
Might be a good idea to post your pic code, maybe you're not turning the pic output off on RC5, or use another output, try the capacitor if you want.
I thought that at one point too and made the gpio pin change but to no avail - also still cant be the issue since I can swap out the inverter for an led and it works just fine.
 

Thread Starter

.frank.

Joined Feb 21, 2016
46
ive had a large TL494 power supply the MOSFET would latch if I cranked up output voltage to 60 volts under load (free runnining voltage 105 volts).

Essentially no regulation at all.

Ive swapped with a large BJT problem was cured.
I am only switching 3 volts here at ~250 ma - I chose a fet for this, should I switch over to a bjt for this job?
 

#12

Joined Nov 30, 2010
18,217
I DO need the rising AND falling edge of when the sound is detected to work in the < 50 ms range.
50 milliseconds is a lot slower than one microsecond. Adding a 100 ohm resistor to the gate of the mosfet is not going to interfere with 20 Hz.
This entire line of inquiry is absurd. You are not switching the loads at a megahertz. You are switching them about once per second.
 

takao21203

Joined Apr 28, 2012
3,695
when I didnt have equipment I was quite good isolating issues so I made switching circuits, used high frequency PICs and I2C.

Only using LEDs and piezo buzzer and voltmeter.

These problems are common so try to isolate them.

make sure theres no unintended reset.
 

Thread Starter

.frank.

Joined Feb 21, 2016
46
First I would like to thank everyone for such efforts to help a total stranger - I mean this sincerely - thank you all

I will try swap out the fet for a bjt and I will post back in a few days (since I don't even own a single bjt)

any part# suggestions for me to order on the bjt for this project?
 

Thread Starter

.frank.

Joined Feb 21, 2016
46
50 milliseconds is a lot slower than one microsecond. Adding a 100 ohm resistor to the gate of the mosfet is not going to interfere with 20 Hz.
This entire line of inquiry is absurd. You are not switching the loads at a megahertz. You are switching them about once per second.
I will try the 100 ohm resister as well

"This entire line of inquiry is absurd" <---only because you are at a much higher level of knowledge and experience and are getting frustrated with me since I am at such a low level. It might be absurd from your point of view, but from mine, I am learning a great deal. Either way, thank you for your input...it's valuable.
 

takao21203

Joined Apr 28, 2012
3,695
OMG

I only visit this forum for recreational purposes, I enjoy communication about the topics here.

It doesnt matter if you follow instructions here but in order to become good or even recognizeably more skillful than before, you simply need a lot of experience.

If your problems are commonplace try a forum.

Id recommend buy some 2n2007 + SMD adapters there are also DPAK kinds for switching you can parallel BJTs and some can handle in excess of 1 Amps.

Checking on unintended reset well you could add a piezo and make a short beep at startup. If you hear any beep then youve got the culprit.
 

#12

Joined Nov 30, 2010
18,217
you are at a much higher level of knowledge and experience and are getting frustrated with me
I am getting frustrated with you because I defined the switching speed in post #20, told you to read it in post #29, explained it again in post #44, and you are still asking about speed in post #45, so I explained it again in post #50.
 

Thread Starter

.frank.

Joined Feb 21, 2016
46
I educated myself on the oscillator control of the pic and the chip is actually running at 8MHZ

I adding a 100ohm resister in line with the gate of the fet which slowed the response time by a time of 1ms - no big deal, but did not fix the problem.

Now that I understand how to control the speed of the chip, I lowered the frequency to as slow as the onboard oscillator would allow to 31 kHz. Now nothing works - the gate on the sound detector triggers fine, but the pic gpio's will no longer send any voltages out to any of the leds any more.

Still waiting for bjt's in the mail to try and swap it out for fet.

Again thank you all.
 
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