overvoltage to a circuit board lamp

[Cornboats]

I'm not sure if this thread belongs here, apologies if it doesn't. I have a small circuit board on my bench that is causing much head scratching. There is an on board 12 v DC 21 Watt lamp that is burnt out. The voltage across it measures 49V so its little wonder. The ac supply is from a centre tapped transformer.
with the control board disconnected the tappings read blue 1.8v, purple 3.7v and black 9v but connected they are blue 19v, purple 11v , black 22v
These are all ac.
When I change my multimeter to read dc all the tappings read 25v
I don't understand why the difference in voltages connected and then unconnected and why the reading of 25vdc on each tapping.
I'm trying to find out why there's 50v across the bulb when it should only be 12v.

Can someone please illuminate me

[eccerr0r]

Re: overvoltage to a circuit board lamp

Would be interesting what the actual circuit that we're talking about here. One aspect is the output impedance of the supply. If it's relatively high, the voltage will decrease as load is put upon it.

Also what the waveform of the output, when dealing with AC, the RMS voltage is what's important when trying to feed the lamp. The peak voltage may be quite high but the RMS voltage may be low enough to not burn the light bulb.

Then there's the other possibility the regulation of the output simply is completely broken, causing the voltage to rise out of spec.

[Cornboats]

Re: overvoltage to a circuit board lamp

Thanks for taking an interest.
Its a control board from a garage door operator. There is an ac transformer with 3 tappings feeding the board. There is a bridge rectifier but as I understand it is only possible to connect 2 of the tapping to it. There is switch mode rectifier U1520 in the build circuit but if I'm reading correctly this is in the ground line!
There are a couple of large caps but I think thats ist regarding regulation.
I'm puzzled because I would have expected to find 12v at the lamp maybe a bit higher but 50v?

High impedence / low impedendce-not sure

The main control fuse is on 1 amp as the load is a small 24v dc motor.

[reycan68]

Re: overvoltage to a circuit board lamp

Hi i'm new here.But i think your transformer has a partial shorted turns at the primary that is if its transformer type power supply.If its switchmode supply the feedback circuit has a problem

[R_J]

Re: overvoltage to a circuit board lamp

If you measure 50 volts accross where the 12 volt lamp goes its likely because there is a resistor in series with the lamp to drop the voltage. because there is no current being drawn there will be no voltage drop so full voltage is measured.
post a couple pictures of what you have so we can see what you are working with

[Cornboats]

Re: overvoltage to a circuit board lamp

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.

Heres some pics

[R_J]

Re: overvoltage to a circuit board lamp

The 20v 3 watt led blew up because it did not draw as much current so the voltage drop across the resistor was very low and was likely closer to the 50 volt supply. likely around 40 volts after the resistor, still too much for the led lamp.
Someone likely put the wrong lamp in, Is there somewhere on the unit that lists what the replacemnt lamp should be?

Are you sure its not a 24 volt 15 watt (E14) lamp

Like this one https://www.eclats-antivols.fr/20-x-...m_english.html

[eccerr0r]

Re: overvoltage to a circuit board lamp

My current thought is likewise, it seems that a higher voltage lamp is required despite it seemingly matches a "1156" car lamp because that socket looks really weird - a bayonet base...

What's the back side of the board like?

The transformer appears to be 32 volt (RMS), when rectified and filtered, it will look like 45+ volts DC. Possibly a higher voltage bulb is needed if it's directly controlled by one of the relays.

[R_J]

Re: overvoltage to a circuit board lamp

horman, Bernal Rallye Brand circuit boards use bayonet base. 24 volt 10W If it is a bayonet type base I suspect the lamp burned out and someone replaced it with an automotive lamp

[eccerr0r]

Re: overvoltage to a circuit board lamp

http://www.bulbtown.com/7512_Miniatu...ase_p/7512.htm ?

Hmm no, this one is dual contact at the bottom... but anyway it's likely a higher voltage filament and not 12V.

[budm]

Re: overvoltage to a circuit board lamp

I expect 24V lamp too since the relay coils and motor are also rated at 24VDC.

[R_J]

Re: overvoltage to a circuit board lamp

If he would have posted Make and Model it would easy to find the correct lamp.

[Cornboats]

Re: overvoltage to a circuit board lamp

Many thanks for all your replies. Even though there is no definitive answer talking thro a problem helps.
Its a door operator made by Nice model Spin control brd SNA2
the spec for the lamp is 12v 21w type b15
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.

I know I'm being incredibly thick here but can someone please explain to me why the voltages vary;
The ac supply is from a centre tapped transformer.
with the control board disconnected the tappings read blue 1.8v, purple 3.7v and black 9v but connected they go up to- blue 19v, purple 11v , black 22v. (readings are a.c) With load on shouldn't these go down?
How is it that when I change my multimeter to read dc, none of the tapping voltages vary like they do on a.c, all the tappings read 25v dc even though the rectification is on the board not at the transformer.

[budm]

Re: overvoltage to a circuit board lamp

"The ac supply is from a centre tapped transformer.
with the control board disconnected the tappings read blue 1.8v, purple 3.7v and black 9v"
What did you use for the reference pin to get those THREE reading?
What are the AC Voltage reading between Black and Purple?
What are the AC Voltage reading between Purple and Blue?
BTW, what did you use for GND ref, when you reported those Voltage readings in other posts? I think there is error in measurement.
Can we see the straight shot pictures of the transformer label?
Can we see the resistor "green-purple-silver....gold"? That is <1 Ohm resistor if those are the correct color as reported.
https://www.manomano.co.uk/control-u...sna2-2-2006898
Please also remove the lamp and take the straight shot pictures of the Top and Bottom side of the board?

[Cornboats]

Re: overvoltage to a circuit board lamp

I used the earth lug attached to the transformer as reference.
I also took voltages between pins
between blue & purple = 10V
between purple & black = 26V
between blue and black = 37v

heres a pic of the resistor- looks like a 5 Watt to me.

Many thanks for taking an interest- I'm really struggling with this one.

[budm]

Re: overvoltage to a circuit board lamp

OK, so it is not center tap transformer.
Since we do not know at this point if the GND lug on the transformer is connected to the cold side circuit GND or not so we need the pictures of the board as I requested to find the correct ref. GND point first to get the correct Voltage measurement result so the numbers will make more sense.

That is 0.47 Ohms resistor (Yellow :Violet: Silver)

[R_J]

Re: overvoltage to a circuit board lamp

I just checked the manual and it says it takes a 12 volt 21 watt lamp with BA15 fitting

In post #6 you said that there was a burnt trace and a blackened resistor, is the 0.47 ohm resistor that was blackened? or was it another one? If it was a different one what value did you replace it with?

[eccerr0r]

Re: overvoltage to a circuit board lamp

I hope after all's done and through it's not as something simple as someone reconnecting the wires to the transformer wrong, causing severe grief...

[Cornboats]

Re: overvoltage to a circuit board lamp

In answer to the last 3 replies. The connections to the transformer have not been disturbed. To my knowledge that is.
The resistor I replaced was a 470R I put in a 3 watt ceramic 470R. It is situated directly in front of the lamp holder.
The earth lug is connected to the mains incoming supply and no 1 pin of the board connector.
I was originally called by the customer in Dec last year. The operator was not working. The control board electrolytics all had esr readings out of spec so I changed them all -about 8 - 2 large ones as well as the 470r resistor capacitor nearest the lamp was bulging and leaking, heat I assumed.
I uploaded pics previously but if it helps here's some more. I do really appreciate everyones input-thks

[budm]

Re: overvoltage to a circuit board lamp

https://www.badcaps.net/forum/attach...4&d=1519836953
So did you replace this resistor? I can see it on the board with one leg lifted off the board.
Does it show < 1 Ohm on your meter, you also indicated that you replaced it with 10 Ohms resistor: "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" It is totally confusing.
"The resistor I replaced was a 470R I put in a 3 watt ceramic 470R"
What was printed on the old resistor?