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Quad 303 LED issue

John_73

John_73

pfm Member
Hi folks.

Having issues getting a 5mm ultra bright orange LED to work properly in a Quad 303. The neon lamp is pretty much kaput, and doing some Googling around it seems most recommend placing with an LED coming straight off the psu cap. Dada offer this in their kits too.

The measured DC voltage right off the caps is 78V. Forward voltage of the LED is approx. 2.2V. Forward current is 20mA.

Using ohms law I end up needing approximately a 3.7k current limiting resistor, at 1.5W. I used a 3.3k 2W metal oxide PR02 type I had to hand.

The bulb lights up perfectly fine, but the current limiting resistor gets hot extremely quickly, to the point you can smell the paint starting to burn. I’m sure if I’d left it it would have started smoking.

Have checked all wiring and all’s fine, and the amp was, and still is, working beautifully.

Do I need a higher wattage resistor, or use a different (non ultra bright) led type? I notice Dada use a standard red led, with a 12k resistor, and only use a 0.5W type. So not sure why I’m getting this over-heated resistor yet it’s specced for 2W?

Cheers,

John.
 
Can you not just find another bulb thingy? They last 50 years or more as far as I can tell! I bet Quad still have them.
 
Tony L said:
Can you not just find another bulb thingy? They last 50 years or more as far as I can tell! I bet Quad still have them.
Click to expand...

That’s my plan B, and I sent a message to Quad earlier today. The neon bulb apparently adds a little noise to the input. To be perfectly honest I’ve never heard any noise, but the led seemed like a good and much longer lasting compromise. A new neon would make things easier though.
 
John_73 said:
Hi folks.

Having issues getting a 5mm ultra bright orange LED to work properly in a Quad 303. The neon lamp is pretty much kaput, and doing some Googling around it seems most recommend placing with an LED coming straight off the psu cap. Dada offer this in their kits too.

The measured DC voltage right off the caps is 78V. Forward voltage of the LED is approx. 2.2V. Forward current is 20mA.

Using ohms law I end up needing approximately a 3.7k current limiting resistor, at 1.5W. I used a 3.3k 2W metal oxide PR02 type I had to hand.

The bulb lights up perfectly fine, but the current limiting resistor gets hot extremely quickly, to the point you can smell the paint starting to burn. I’m sure if I’d left it it would have started smoking.

Have checked all wiring and all’s fine, and the amp was, and still is, working beautifully.

Do I need a higher wattage resistor, or use a different (non ultra bright) led type? I notice Dada use a standard red led, with a 12k resistor, and only use a 0.5W type. So not sure why I’m getting this over-heated resistor yet it’s specced for 2W?

Cheers,

John.
Click to expand...

I'd expect an ultra bright LED at 20mA to be virtually lighting the room up! I'd also expect the resistor to get that hot.... power ratings are a "commercial rating" generally and the max they can take and still last for say 5 years at temps where they discolour and initially smell of burning... Your 2W resistor is dissipating 1.75W there! Try a 5K6 or 6K8 3W wirewound. It will dissipate around 1W and get hot but be very reliable.
 
Yes, no need to go anywhere near 20mA. 10k / 1W would also be fine and probably bright enough. As you bought 3k3 2W resistors, just use 2 in series. 1/2 the current and 1/4 the power in each.
AP
 
The original is a neon. Plentiful and cheap still (like https://cpc.farnell.com/w/c/electrical-lighting/light-bulbs-tubes/neon-bulbs/prl/results?st=neon or eBay) Change the 100k ohm resistor too if you go that way.

If you go LED, then as Jez says wow that will be bright! A standard red or orange LED is way better suited. Try a 12k or 15k ohm 1 watt resistor that gets you 6 or 5mA current. And 1/2 watt of dissipation. You could just try that with the LED you have first...
 
Thanks for the replies everyone, much appreciated.

Strangely the light from the ‘ultra bright’ orange led was not blinding at all. Rather normal in fact. Still wondering how DaDa get away with fitting only a 0.5W resistor when there’s little difference between the specs for their standard red led and the orange one I have.

I’ve ordered a standard set of orange leds now anyway, and will keep those ready to use if Quad can’t provide the neons.

@snowman_al Thanks for that link, but I haven’t much of a clue what to look for other than the physical dimensions. Looking at the circuit the neon comes off the 120V feed, via the 100K resistor you mention. If I bought a replacement I’d prefer it was as close to the original stock item as possible. Any suggestions from that list? Thanks.
 
Driving an LED from a theoretically correct 390ohms (for 20mA) from a 12V supply (IIRC) on a project of mine was way too much for me - finished up using 10k.
 
Neon indicators are hard to find these days and the available parts, usually found in trailing mains sockets, seem short lived
 
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