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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 18th Feb 2020, 12:17 am   #21
jamesperrett
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Default Re: Replacement bulbs with LED question

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Originally Posted by ajgriff View Post
There's something not quite right about all of this. The bulbs look like they're connected to an output of the PSU rated at 18V and .28A. Four series connected bulbs marked 6.3V at .3A each doesn't make much sense unless I'm completely misinterpreting the arrangement.
I'd guess that the idea is to under run the bulbs to improve their lifespan. However, the current still seems a bit high unless the rest of the circuitry takes very little power (or it was designed for bulbs that take a lower current).
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Old 18th Feb 2020, 12:35 am   #22
broadgage
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Default Re: Replacement bulbs with LED question

If all four bulbs are run in series from an 18 volt supply, then a supply current of about 280ma is reasonable.
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Old 18th Feb 2020, 12:42 am   #23
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Default Re: Replacement bulbs with LED question

I'll take your word for it 'broadgage' as I'm not prepared to try and do the maths but it seems an odd way to go about things. Also I'm not entirely sure from the pictures whether or not the bulbs are connected across the DC or AC output which will make a difference if switching to LEDs.

Alan

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Old 18th Feb 2020, 1:23 am   #24
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Default Re: Replacement bulbs with LED question

The pictures clearly show the lamps connected across one of the two 18V transformer secondaries. The other secondary feeds a bridge for the electronics dc supply. It may be "odd" but it looks deliberate. The underrun bulbs would give a fairly gentle yellowish light which would be quite in keeping for the application. Lower current 6V bulbs could of course just be fitted if required.

If LEDs are seriously favoured, it looks like with a little modification another bridge and capacitor could be fitted to provide a suitable dc supply!
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Old 18th Feb 2020, 1:30 am   #25
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Default Re: Replacement bulbs with LED question

Makes sense Chris although it wasn't clear to me. Must be getting tired as it's late - that's my excuse anyway.

Alan
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Old 18th Feb 2020, 3:37 am   #26
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Default Re: Replacement bulbs with LED question

If you are doing the LED fitting yourself (as opposed to buying LED replacement units), there is no need to to rectify the AC.

All you need to do is use two LEDS per each bulb location, connected in anti parallel to each other with a capacitor dropper.

You can even use the MES globe bases by carefully breaking out the glass and soldering the LEDS in.
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Old 18th Feb 2020, 10:59 am   #27
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Default Re: Replacement bulbs with LED question

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Originally Posted by ajgriff View Post
There's something not quite right about all of this. The bulbs look like they're connected to an output of the PSU rated at 18V and .28A. Four series connected bulbs marked 6.3V at .3A each doesn't make much sense unless I'm completely misinterpreting the arrangement. As the unit has an unknown history I'm wondering whether or not someone's previously fitted the wrong bulbs. Does it otherwise work correctly? I know the bulbs don't work but have you tested each of them for continuity or can you see that the filaments have broken?

Alan
Hi Alan.

That could be true I suppose. I've never seen the lamps work you see as it came to me a few weeks ago.

Definately all of the bulbs are blown.
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Old 18th Feb 2020, 11:01 am   #28
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Default Re: Replacement bulbs with LED question

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Originally Posted by Terry_VK5TM View Post
If you are doing the LED fitting yourself (as opposed to buying LED replacement units), there is no need to to rectify the AC.

All you need to do is use two LEDS per each bulb location, connected in anti parallel to each other with a capacitor dropper.

You can even use the MES globe bases by carefully breaking out the glass and soldering the LEDS in.
I wish I knew what that diagram meant. LOL - Could it be drawn in physical terms?
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Old 18th Feb 2020, 11:26 am   #29
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Default Re: Replacement bulbs with LED question

It's rather diificult to imagine four bulbs wired in series all failing simultaneously. I would think it more likely that one has failed causing all four to extinguish?

Surprise has been expressed at the lamp power, but they would have be running at well under a watt each. In a car application the illumination of the smallest dashboard instrument would have used a 2W bulb or sometimes more.

I think it'll be difficult to match the pleasant warm glow of an under-run filament using LEDs. I'd simply replace all four bulbs and move on. They'll last for many, many years running at 4.5v.

Cheers
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Old 18th Feb 2020, 12:14 pm   #30
David G4EBT
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Default Re: Replacement bulbs with LED question

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Quote:
Originally Posted by Terry_VK5TM View Post
If you are doing the LED fitting yourself (as opposed to buying LED replacement units), there is no need to to rectify the AC.

All you need to do is use two LEDS per each bulb location, connected in anti parallel to each other with a capacitor dropper.

You can even use the MES globe bases by carefully breaking out the glass and soldering the LEDS in.
I wish I knew what that diagram meant. LOL - Could it be drawn in physical terms?
It just shows pairs of back to back LEDs in series.

The attached sketch shows the physical appearance of an LED and the theoretical symbol.

LEDs have one leg longer than the other - that is the positive (anode) connection.

(The negative connection also has a small 'flat' on the rim of the lens, but not very obvious.)

Hope that helps a bit.
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Old 18th Feb 2020, 2:06 pm   #31
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Default Re: Replacement bulbs with LED question

Thanks David. And many thanks to all with this so far. This place is brilliant.

Thanks, Tirgon. I think they must have blown over time. No idea if all blew at the same time. They might even be the original bulbs.

The unit looks like it's from the 80's.

I think I'll just get some normal bulbs first and see if they work. Who knows, they might have been replaced with the wrong type and they all went at the same time and never replaced.

https://internationallamps.co.uk/pro...0-6-5v-300ma/]

But do you think these would work as a replacement? once I've established the ones above work properly?

https://internationallamps.co.uk/pro...e10-mes-white/

...Or do LED bulbs need to be wired up differently?

BTW. I do have a multimeter. I could check the voltage of the bulb fittings but which setting would be best to use on the multimeter?
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Old 18th Feb 2020, 4:23 pm   #32
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Default Re: Replacement bulbs with LED question

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I think they must have blown over time. No idea if all blew at the same time.
Actually I meant that because they're in series, once one goes open circut, there immediately ceases to be any voltage across the other three to cause them to fail as well. The first bulb acts as a 'fuse'. (- Think Xmas lights!)

Cheers
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Old 18th Feb 2020, 5:06 pm   #33
Strange Brew
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Default Re: Replacement bulbs with LED question

Ahh... okay.

Well, just checked all the bulbs again and they are definitely dead. So maybe someone kept swapping them round, maybe?

I've ordered some identical bulbs so I'll update when I get them.
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Old 18th Feb 2020, 5:09 pm   #34
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Default Re: Replacement bulbs with LED question

Just occasionally, I've noticed series arrangements of bulbs all being open circuit- I wondered if the first to go open circuit goes briefly into low-impedance metal-vapour arc mode, giving the remaining old and weak filaments a final kick-up that fractures them. This particular case is a low-voltage, low-energy situation though, so that seems unlikely.

More prosaically, a string of weary and time-served filaments might simply fracture if the device is dropped or heavily knocked.
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Old 18th Feb 2020, 7:15 pm   #35
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Default Re: Replacement bulbs with LED question

Quote:
Originally Posted by Strange Brew View Post
Thanks David. And many thanks to all with this so far. This place is brilliant.

Thanks, Tirgon. I think they must have blown over time. No idea if all blew at the same time. They might even be the original bulbs.

The unit looks like it's from the 80's.

I think I'll just get some normal bulbs first and see if they work. Who knows, they might have been replaced with the wrong type and they all went at the same time and never replaced.

https://internationallamps.co.uk/pro...0-6-5v-300ma/]

But do you think these would work as a replacement? once I've established the ones above work properly?

https://internationallamps.co.uk/pro...e10-mes-white/

...Or do LED bulbs need to be wired up differently?
I'd be wary of wiring those LED bulbs in series. They might work. They'd probably be OK if you wired a 27 ohm 1W resistor across each bulb holder to swamp out any current variations in the LEDs.
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Old 18th Feb 2020, 8:26 pm   #36
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Default Re: Replacement bulbs with LED question

18V AC unloaded may go up to 24V, ideal for 4 x 6.3V bulbs. And as it is raw AC I would stick to filament lamps. As to the 0.28A output being overloaded I bet the other side has very little taken from it, all will be well.
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