Circuit smoking, need help 211007

[allenpitts]

Hello Dallas Makerspace forum,

Have built 12 volt circuit that uses a motion detector, HC-SR501, to activate a 555 timer in monostable mode to turn on lights. The idea is to set the unit in a closet overhead so that when the closet is entered the lights stay on for a minute.

I bread boarded the schematic and tested it and it worked as expected.

A PCB was designed and manufactured.
The PCB was tested successfully using six 5mm thru hole
LEDs as a load for several days.
This a graphic of the 3 layer PCB

A fixture was 3D printed and four automobile back up lights
were substituted for the six 5mm LEDs, OXILAM Backup Reverse Lights 7440 7443 LED Bulbs, 6.2 watts each. (2.06 amps)

The Oxilam LEDs were substituted and tested for about a week and then it smoked
and stopped working.

Figuring some anomaly, weak component or construction mistake, the PCB was
rebuilt and replaced. This time it blew up immediately.

So it was conjectured that the load transistor, Q2 (AUIRF540Z) was overloaded. But I checked the specs and that transistor is (if I read the datasheet correctly.) is good to 36 amps.

So cant figure out why it runs for weeks using the six LED load (20mA each, 120 mA total) but when the system is hooked up to the larger load it blows up.

Have tried simulating with LTSpice but not able to input the schematic.

Any help figuring this out would be most appreciated. In fact, reimbursement is available on a flat fee or hourly basis for competent adult supervision.

Thanks.

Allen Pitts, Dallas MakerSpace since 2018
469 713 4147
[email protected]

[Raymond]

Recalculate the gate resistor… I think you’re way too high with 470 ohms, putting the MOSFET into linear instead of switching and the heat build up is destroying it.

[malcolmputer]

In the other thread they started on this 11 days ago I suggested the same. I think it’s either not fully open or it’s spending too long in the not open state (too high of a switching freq or the like).

Also, there’s no heatsinking.

See comments in this thread:

12 Volt PIR to Timer 210927

Hello MakerSpace Electronics, Have developed a circuit, PCB and 3D-printed object that turns on four 12 volt LED lights when activated by a PIR sensor that controls the on-time of a 555 timer in monostable mode. This is the schematic [image] This a picture of the fixture with the lens slid back partially so the inside electronics can be seen. [image] After designing the schematic, breadboarding t…

[artg_dms]

Schematic needs to be updated.
So what smoked? Components? PCB etch?
Remove R3 and D1 - not needed for this app and may interfere with driving pwr mosfet.
Replace R4 w/ 10 ohm or just short - let LM555 drive gate directly.
Is your pcb etch capable of handling 2 amps?
Is your 12Vdc pwr source greater than 2 amps?
Recommend placing a tantalum or electrolytic cap across the pwr rails on the pcb.
Assuming Q2 pwr mosfet is a TO-220 case.
Looking at IRF540Z datasheet -
https://www.infineon.com/cms/en/product/power/mosfet/n-channel/irf540z/
RDSon ~27mOhm
Guestimated total current is ~2A
Dissipated Pwr = I^2R = 2^2 x 0.027 = 4 x 0.027 = 0.108 watts or about 100 milliwatts.
Given the app and the load duty cycle - heatsink may not be needed - but can’t hurt.

I may have overlooked some things - assuming that the front end of the ckt works, these are the things I would check and / or modify.

Some other references -
https://www.electronics-tutorials.ws/waveforms/555_timer.html
https://www.electronicdesign.com/power-management/article/21795203/lm555-makes-inexpensive-power-driver

[allenpitts]

Hello Raymond, Malcomputer, artg_dms, and the Dallasmaker Space electronics forum,

Sincere apology for double posting. I guess when I posted ‘Circuit Smoking 211007’ the post,
‘12Volt PIR to Timer 210927’, slipped my mind.

1. So both Raymond and Malcomputer indicate that R4 from the 555 output to the base of Q2
   is too high at 470 ohms. artg_dms says 10 ohms or even eliminate. Will try 10 ohms and advise.

2. artg_dms, please sir, why does the schematic need to be revised?

Thank you gentlemen for you time and courtesy.

Allen Pitts

PS
If any of you will be at the Makerspace next weekend (Oct. 16-17)
please let me know. ([email protected] 469 713 4147) I live
ten minutes from MakeSpace and perhaps could demo the
circuit operation for you.

PSS Some other questions in posts

1. The voltage outpit of the 555 timer at the gate of Q2 is 10.9 volts
2. The power source is a regulated 12 volt, 4 amp ac/dc/adapter.
3. Not sure if the PCB trace will handle 2 amps but will query the manufacturer.
4. The IRF540Z is a TO-220 case.

[michaelb]

I suspect the trace leading back from the source of Q2 to pin1 (ground) of the 555 is too small. The increased current causes a significant voltage drop which effectively lowers the Vgs of the transistor putting it into saturation. I don’t believe this is due to the gate resistor, since gate current in a MOSFET will be zero after a few mS of being on, and the resistor only effects the switching performance.

If you want some notes on your PCB layout…

• Use a copper plane on the bottom layer for ground. This is common practice for signal integrity (i.e. it would have prevented the issue I suspect here), and also eases routing (since ground goes almost everywhere on a PCB)
• Don’t use the default trace width. Most software defaults to 10mil traces which are difficult to manufacture (if you’re routing/etching your own) and difficult to repair if you lift a trace while prototyping.
• Carefully lay out your components to keep the routing as simple as possible. This will absolutely fit on a 2 layer PCB. For example, the output pins could be flipped 180deg to avoid crossing the green and orange layers.

For trace widths, I like to stick to 24 mils as a default, but it’s not a magic number. Just something that’s large enough to rework (and make, back when I was milling boards) and not so large that I can’t route them anywhere. If I have a circuit that’s going to require more than a few hundred mA’s, I’ll try and go wider, or best use a site like this one to get a good idea of what trace widths I should use for the current.

[artg_dms]

@allenpitts

artg_dms, please sir, why does the schematic need to be revised?

Q2 was a low current bipolar transistor. Upgraded to pwr mosfet. Your driving a gate, not a base. Schematic symbol is different. So are driving requirements.

@allenpitts

• The voltage outpit of the 555 timer at the gate of Q2 is 10.9 volts
• The power source is a regulated 12 volt, 4 amp ac/dc/adapter.
• Not sure if the PCB trace will handle 2 amps but will query the manufacturer.
• The IRF540Z is a TO-220 case.

All of this looks good.
If trace that handles the higher current.path needs to be beefed up, you can solder hookup wire on top of the trace. Not elegant but will get the job done.

[allenpitts]

Hello Raymond, Malcomputer, artg_dms, michaelb and the Dallas MakerSpace Electronics forum,

Your input is most appreciated. It is written: the gods will smile on one who helps a lost stranger.

I think the reason why artg_dms said the schematic needed to be updated was because the schematic showed a BJT transistor at Q2 instead of a MOSFET.
Revised schematic:

Reducing the value of R4, the transistor between the 555 and the MOSFET did improve the operation of the circuit.

But to show respect for the time, energy, knowledge and expertise of the post respondents it must be admitted that the failure was caused by a construction blunder
that was picked up by visual inspection done by Jerry Elderidge, an electronics engineer at Harmon, who happened to be at Dallas MakekerSpace on Saturday.

Mr. Elderidge, who was working on a model boat, pointed out that on the power distribution board that was created to disseminate
the twelve volt output to four auto back up lights, the stranded wire was not carefully partitioned. One of the strands in the plus voltage
slots had strayed onto one of the ground strands and thus was causing a short.

The sockets came from the vendor with stranded wire which is always avoided because stranded wire is harder to use in construction.
In the picture above a short piece of of solid 22 guage has been soldered to the stranded pig tail. The soldered joint was then
covered with heat shrink tubing.

Thanks.

Allen Pitts
[email protected]

[kbraby]

That’s a good catch that is tough to find over photos. The other option would have been a crimp sleeve and pruning any strands that missed the sleeve. But kudos for not just tinning the stranded wire and clamping, as that can be a high creep solution eventually leading to loose contact.

[Raymond]

Glad you found the issue… All we had to go on was the original post above so finding a short is something we couldn’t do based on the pictures.

[HankCowdog]

@kbraby

But kudos for not just tinning the stranded wire and clamping, as that can be a high creep solution eventually leading to loose contact.

Good to know: that would have been my solution to the stranded wire issue.