Can you, or should you not, run a 24 VDC LED off of a 30 VDC power supply?

Thread Starter

Riskinit

Joined Jan 28, 2022
65
I have a "cheap" power supply adapter from China that is supposed to generate 24VDC but is actually generating 30VDC. It is almost like someone meant to put a 30VDC sticker on the unit (I have 2 of them with the same problem).

Except that I explicitly ordered 24VDC, both units have a sticker mentioning 24VDC.

What I want to know is what is the worst that can happen if I plug in a 24VDC LED into these power supplies that are generating 30VDC. Is this a bad idea?
 

ericgibbs

Joined Jan 29, 2010
19,120
hi Risk.
Often low cost power supplies will be higher without any load and only be at their stated voltage on the rated load current.

Measure the voltage when you have a load attached, use a Test load of say 240R or so, post what you measure
E
 

Papabravo

Joined Feb 24, 2006
21,314
You can always use a supply with a higher voltage as long as you limit the current with a series resistor. This is for a LED or a string of LED's that do not require a series resistor when connected to a +24V supply. When using a +30V supply you want a resistor of an appropriate power rating that will drop +6V at the current required by the LED or LED string when connected to a +24V supply.
 
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Thread Starter

Riskinit

Joined Jan 28, 2022
65
hi Risk.
Often low cost power supplies will be higher without any load and only be at their stated voltage on the rated load current.

Measure the voltage when you have a load attached, use a Test load of say 240R or so, post what you measure
E
Hmmmmmmm. I don't have a resistor on hand so I took an old 24VDC LED (part #: 800T-QAH24R) to see what I would measure. The LED did come on by the way. With no load on I measure 31.61 VDC and with the LED attached I measure 29.65 VDC.

The reason I bought these particular units (WDU24-300 by TRIAD) was because I wanted to limit my current to 300 mA.

I have another unit (WDU24-1500) which is correctly putting out 24 VDC but my current is now limited to 1.5 A.

I guess the question is whether or not I can run a system that requires 150 mA on a power supply that may or may not be generating the voltage I expect.
 

ericgibbs

Joined Jan 29, 2010
19,120
hi,
Say you want to drop from 28V down to 24V ie: 4V drop at 0.3Amp = 1.2Watts minimum,
I would choose a 5Watt. Rvalue = 4V/0.3A = 13R say a 15R or 18R would be OK.

E
Corrected Typo.!!!
 
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BobTPH

Joined Jun 5, 2013
9,287
A power supply rated fir 300 mA does not mean it limits the current to 300 mA. It means that is the most current you can draw without damaging it. When you use a constant voltage supply, the load determines how much current is drawn.

Which brings us to your LED. There is no such thing as a 24V LED. This is, presumably a module with LEDs in series, possibly with a resistor or more sophisticated electronics so that it can operate at a constant voltage if 24V.

please supply specs for rhe LED module you are using.

Bob
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
A power supply rated fir 300 mA does not mean it limits the current to 300 mA. It means that is the most current you can draw without damaging it. When you use a constant voltage supply, the load determines how much current is drawn.

Which brings us to your LED. There is no such thing as a 24V LED. This is, presumably a module with LEDs in series, possibly with a resistor or more sophisticated electronics so that it can operate at a constant voltage if 24V.

please supply specs for rhe LED module you are using.

Bob
Ahhhhhh, and here I thought i was being clever and not needing to buy a fuse.

The LED is this one but red.

https://www.rexelusa.com/p/76950/al...n-20-ma-discontinued-/662073930754/800t-n319g
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
That lamp is rated at 20mA, not 300.

Bob
Yes, my system has multiple LEDs and other equipment totalling between 100 mA and 200 mA currently.

But honestly just sounds like I should purchase a 1.5 A power supply, add a small fuse or zener diode, since the 300 mA power adapter isn't actually doing what I think it does.
 

Ya’akov

Joined Jan 27, 2019
9,265
For LEDs you want a constant current supply. You can buy LED drivers which are purpose built for this.
The supply you have is just a transformer, a bridge rectifier, and a capacitor. The 300mA rating is just the maximum guaranteed output not a promise to provide 300mA and no more.

A constant current supply will adjust the voltage to maintain the current at the set level.

But an LED driver, it’s the right thing.
 

Ya’akov

Joined Jan 27, 2019
9,265
He is not driving an LED, he is driving a lamo designed to operate at 24V. A constant current source would be redundant.

Bob
I somehow missed the post with the specification. I wonder if AC/DC mean they are just running it for half the cycle on AC or if there is a rectifier in there. Probably the latter.
 

Jon Chandler

Joined Jun 12, 2008
1,098
The LED is question is an LED indicator bulb, with an internal resistor to provide the LED 20mA when powered by 24 volts.

So, to address one statement, yes there is such a thing as a 24v LED, 12v LED, and perhaps many others - they just include an integral resistor for the specified voltage.

This LED bulb can safely be operated with a 30v supply with the addition of an additional series resistor.

Let's pick 2.2v as the typical forward voltage for a red LED. Specified current is 20mA at 24 volts.

R = V / I = (24 -2.2) / 0.020 = ~ 1k <--- internal resistor

R = V / I = (30 -2.2) / 0.020 = ~ 1.4k <--- total resistance for 30v supply

Therefore, the needed additional series resistor = 1.4k - 1k = 400 ohms. A standard value of 390 ohms will do the job, or even a little higher to prolong LED life.
 

Tonyr1084

Joined Sep 24, 2015
7,999
I have another unit (WDU24-1500) which is correctly putting out 24 VDC but my current is now limited to 1.5 A.
All this means is that the supply is capable of delivering 1.5A if needed. Doesn't mean that if you hook up a device that only draws 300mA it will be blown. It just means the supply has head room for more components to be added.
Will a resistor have trouble running 300 mA through it? (Heat dissipation mainly)
Multiply the voltage and the current to get the wattage. 24V x 0.3A = 7.2 watts. That would require a resistor rated for 10 watts. A 5 watt resistor would burn up. Smoke, heat and excitement will ensue. You must use components sufficiently rated above the anticipated load to operate safely and without failure.
 

BobTPH

Joined Jun 5, 2013
9,287
Except the LED lamp takes 20 mA, not 300, look at the link he posted The only mention of 300 mA I recall was the rating of a power supply, which seems to be powering more than just that LED.

Bob
 
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