overvoltage to a circuit board lamp

[eccerr0r]

Re: overvoltage to a circuit board lamp

The MOSFET gate drives are level shifted by the 4504 IC. I would hope the gate voltages be either 0 and 13V, at least before (and ideally after) the 100R resistors R2 and R5. If the voltage difference across R2/R5 are significant, likely the IRF540 that it's controlling has shorted out and failed.

A hack is to replace the 0R47 R3 that you've been messing with with a much higher value though it will dissipate more energy, though make it high enough and the lamp can't draw enough current to light full brightness and it starts pulling less... but based on the voltages you say you're seeing, this resistor is going to get even more toasty unless you get it to draw less through the lamp (or a low power LED...)

I still don't know exactly why they built it this way. The first guess was current regulation, which is ludicrous for an incandescent lamp. The second guess I have is burn out detection - if it has this feature and it pointed out that it needed a lamp repaired, then that would explain things.

[CapLeaker]

Re: overvoltage to a circuit board lamp

much easier than that. look at an LED driver / Constant Current regulator and convert it over to LED. The CC will only allow a certain current to be drawn from the LED. Depending of the LED driver and the LED (does a regular small LED work?) you could use an AL5809 from Digikey. Part #: AL5809-20P1-7DICT-ND
This will regulate up to 60V down to that what a LED needs. Of course there are others too.

[Cornboats]

Re: overvoltage to a circuit board lamp

Thanks for your suggestions. I did already replace 0R47 R3 with a 10R 5watt WR5S. This started smoking with a couple of secs. What size do you think would avoid this?

Is the led I've already tried not a regular led 3Wac/dc 10-20V?

Could I use this one by increasing the size of R3 if so to what?

I'll check voltages pre and post R3/R5 and post results later today.

[Cornboats]

Re: overvoltage to a circuit board lamp

So I've tested the gate voltages pre and post R3 & R5 r100. When mains is turned on all readings are 0
with motor on the gates start at 1.70v and slowly goes down every time the motor is stopped and started again to 1.13V . The readings pre resistor @ the processor is .20v higher, no more.
The fet for the flashing lamp only differs in that the voltage drops to 0 and then back up again as fast as the blink of your eye during motor operation.

Constant current regulator- I found this on a suppliers site I use.
http://cpc.farnell.com/on-semiconduc...220/dp/SC08256
Does anyone know what size resistors to use with it to drive the led I've mentioned previously.

[eccerr0r]

Re: overvoltage to a circuit board lamp

Well, ideally the correct value is 0R47 for R3 if you want to stick with the 21W bulb and the microcontroller is working correctly. However you really want to cheese it, try that 3W 20V LED and swap R3 with something like 220 ohms 3W. But something doesn't seem right here...

BTW are you measuring the voltage around R3/R5 versus the source pin of the MOSFET? Those voltages of 1.7V are way too low to even turn the mosfet on, and if they're not on (or shorted-defective), the lamp can't burn out...so we have a measurement inconsistency here. That 2200uF 50V capacitor's GND pin should be okay as a reference, as well as pin 8 of the 4504 IC.

[R_J]

Re: overvoltage to a circuit board lamp

You can't just measure the dc voltages on the gates, you need a scope to look at the waveform. The correct resistor is a 0.47 ohm 3 watt, changing it will also effect the MOS1 circuit and its Lamp connected to M1. Stop over thinking it and replace the lamp with the correct one.

[eccerr0r]

Re: overvoltage to a circuit board lamp

It seems there was an experiment to stick in a proper bulb, and it measured 15V across it, which is a bit high for a 12V bulb - that's almost 50% over power which will reduce the lifetime like 1/30 of rated bulb lifetime.

Or was this misinterpretation of post 6?

[R_J]

Re: overvoltage to a circuit board lamp

[QUOTE=Cornboats;808241]I put the bulb in and checked the voltage 15V. The previous lamp burnt the track and blackened a resistor so I suggested to the customer he use an led. He put one in max working voltage 20v 3 watt. It blew up./QUOTE]

he also said : There was a resistor in the feed line of the lamp that wasn't measuring its coloured banding. green-purple-silver....gold. Removed it measures 17.29K so I put in a wp5s 10R 5W. Powered on , light was over bright and wp5s started smoking with a couple of secs


Looks like he put the bulb in and checked the voltage, but that voltage may have mean misleading depending on where he measured it, that mosfet might be pulsed and not simply turned on like a switch. He also installed the wrong resistor 10 ohms vs 0.47 which would likely cause some problems.

[Cornboats]

Re: overvoltage to a circuit board lamp

I took the gate voltages using the mains/board earth as ground.

I'll scope them when I get back from work today.
I did put the right spec bulb in 12v 21w. With on there was 15v across it and the r10 resistor I replaced the r0.47 with was smoking with couple of secs.

From what your saying I should replace the r.47 with like and try lamp and test voltage again.

[R_J]

Re: overvoltage to a circuit board lamp

10 ohms is way too high a resistance for that circuit. The circuit also monitors the voltage drop across R3 (VsLM) which sends that signal back to the microprocessor pin 25 via R20. I suspect the micro controls the mosfet MOS2 and MOS1 based on the feedback it gets acrosss R3

[Cornboats]

Re: overvoltage to a circuit board lamp

Many thanks for your imparting your knowledge and experience. So I'll do 2 things.
I'll put in a R.47 and see what the voltage across the lamp is.
I'll then put my spare led in after changing the resistor to R220 as suggested above and see what the effect is Or do you think this isn't such a good idea as the voltage drop across the new resistor is going to be different.

[eccerr0r]

Re: overvoltage to a circuit board lamp

As I don't know the intent of measuring the voltage across R3 it's best not to muck with it. Due to the much lower current flowing with the LED, it's safer to go to that 220R resistor, though I also specced it to be 3W - it will be dissipating quite a lot of heat (first guess would be around 1.8W or so, depending on the actual drop of your LED.)

Again I made some quite big assumptions because I don't know the exact specs of your LED so take with a grain of salt and perhaps put in a fuse to make sure you don't fry it... Just don't try to power the light bulb with the 220R.

[R_J]

Re: overvoltage to a circuit board lamp

The circuit will likely fry the led, If you change R3 to 220 ohm you could risk damaging the processor input. If the original parts are not reinstalled I do not see this ending well.
You seem to think that the mosfet is just used as a simple switch. IT IS NOT.
Please look more closely at the schematic

[eccerr0r]

Re: overvoltage to a circuit board lamp

Oh yeah forgot about that. Yes that R47 needs to be there. The 220R needs to be in series with the LED separately on the other side. Dammit I killed an AVR this way, just barely over VCC, why did I forget... grr...

Ultimately it is still a saturated switch as there's no way to go between on and off. Alas it can only do a mathematical average based on the sampling frequency; no guarantees of how the lamp will actually behave...

[R_J]

Re: overvoltage to a circuit board lamp

By R47 do you mean R3 0.47ohm. installing a led and a 220 ohm resistor will not draw much current and throw out the current feedback to the pic, which drives the level translator ic, I bet it uses a pulse and not just a straight high or low level.

[budm]

Re: overvoltage to a circuit board lamp

I wonder what the purpose of the second lamp socket M1 is for and it also has snubber diode D2 (inductive load?).
I think the 0.47 resistor burnt out is due to the MOSFET is not being PWM due to the drive is stuck on high instead of pulsing.

[Cornboats]

Re: overvoltage to a circuit board lamp

Thanks for all your replies- You've given me a lot to think about. Need time after work to digest before I can reply.

yes I meant R3 0.47R

[eccerr0r]

Re: overvoltage to a circuit board lamp

I wonder what the purpose of the second lamp socket M1 is for and it also has snubber diode D2 (inductive load?).

I suspect it's for an external relay control, but this once again is a guess.

I think the 0.47 resistor burnt out is due to the MOSFET is not being PWM due to the drive is stuck on high instead of pulsing.

I'm still baffled by this "feature."

The only way to ensure the 12V 21W lamp (~7 ohms when hot) does not burn out is to PWM it. Assuming the software was working, and Cornboats swapped it for a 10R resistor, the microcontroller input likely has already been damaged due to the issue R_J pointed out. However, even if it were not damaged, it should have incorrectly calculated it was over current well in advance and shut off the lamp - PWM or not...

So something either still doesn't make sense or doesn't matter anymore...

[budm]

Re: overvoltage to a circuit board lamp

Yes, it does not make sense to have current sensing resistor for Both MOSFET's for lighting up the light and for the relay driver on the second MOSFET.

[Cornboats]

Re: overvoltage to a circuit board lamp

Apologies for taking time to get back. I've scoped the fets. The gates both look similar. 5 divs set at .5ms with 1/2 div gap between square waves. Can't see the bottom of the square waves unfortunately so can't read the height but the multimeter says 1.6v.
The source is 0. Is this because there's no bulb fitted?

Looking thro the threads Budm was querying wether board was connected to cold earth. The large caps negatives are & ditto the 540 fet sources.

The 540r resistor I changed for a ceramic I changed back just to see . This gets really hot regardless of wether the motor is running or not in as little as 5 mins. So is there any problem with me changing it back? According to the drawing its fed from a diode and purple of the transformer.

It doesn't look like I'm going to get a result, other than leave out the bulb holder to prevent the board overheating the caps again.

I'd welcome your final comments though and appreciate all your help in trying to fix.