Voltage sensitivity of loads

Estragon · April 2017

Thinking of led lights in particular, but could apply to any DC load?

Using a 12v system as an example, voltage could vary from about 10.5 to nearly 17v or so. Even assuming loads could be turned off for eq, normal Vabs on cold batteries could be >16v.

I'm wondering if there's any way to be reasonably sure an led light, for example, will take the range?

I know marine led fixtures do (or at least should) because although they don't often specify a range they're intended to be powered directly from a house bank that will see a range like that. The marine ones also have a huge price range. Is this just because, like household fixtures, much of the price reflects the fashion element, or would price also indicate anything about ability to take wide voltage range?

I don't know about the type fed by 120vac. Most seem to be 12v. Some(most, all?) take 12vac from the 120vac transformer. Do they care if it's 12vdc? Would they be less likely to take the voltage range?

I assume voltage range would be at least partly determined by conductor size (for low voltage) and insulation (for higher voltage), but without a published spec., is there any way to tell?

BB. · April 2017

You pretty much need to know the details of the circuit design to be sure... But, some AC/DC capable LED lights may have a pretty wide range. If you have a 12 VDC light, it has to take something like srt(2) * 12 vdc ~ 17.0 peak nominal voltage (peak of an AC sine wave).

You can find LED "bulbs" that are designed for 12/24 VDC vehicles:

https://www.amazon.com/s/ref=nb_sb_ss_i_1_17?url=search-alias=aps&field-keywords=12+24+volt+led+bulbs&sprefix=12+/+24+volt+led+,aps,289&crid=1KYELWXWKEX3H

12/24 VAC/VDC secuirty light:

https://www.amazon.com/QUANS-Security-Floodlight-Waterproof-Superbright/dp/B00RCSAQJS/ref=sr_1_4?ie=UTF8&qid=1491617213&sr=8-4&keywords=led+flood+lights+12+24

And there are some lights that run from 10-80-90 VDC too:

https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=12+48+volt+led+bulbs&rh=i:aps,k:12+48+volt+led+bulbs

The electronics are getting pretty standardized and not too difficult to implement to cover a wide variety of input voltages/ac/dc options. However, for high volume items like lighting--The mfg. tend towards the least expensive solution as possible--Which can put limitations on what the light/fixture will support.

-Bill

mike95490 · April 2017

I like this replacement bulb AC/DC 8-30V
https://www.wiredco.com/10_LED_SMT_SMD_Warm_White_G4_Disc_LED_Module_12VAC_p/10ledwarmwhiteled.htm

Estragon · April 2017

Thanks for the info and links. It sounds like if there's no explicit specification for voltage range, I should assume it won't handle much of one.

Bill - does it follow that a bulb that takes 12vac and would see ~17vac peak should be able to take ~16vdc?

BB. · April 2017

sUnfortunately, it depends on the "ballast" circuitry of the bulb (ballasts are used in LEDs to limit current to the rated ability of the LED die. More or less, just like the ballast of a florescent bulb works). The Current/Voltage curve of the LEDs (and florescent lights) are very "step". A slight change in operating voltage (and/or temperature of the LED/Florescent tube) will dramatically change its "resistance" if fed with a battery/voltage source and the current flow. Buy regulating the current, aka a "current source", you have stable operation).

For example, if the LED bulb has a simple resistor as the ballast, then 12 Volt AC RMS (what the meter reads) and 12 Volt DC have the same "average power" available. And feeding 17 volts DC to the bulb could also supply too much current to the LED and overheat it.

Generally, a simple LED bulb will have a full wave (2 or four diode) rectifier on the input and use the DC output as an input to the "current" regulator. So, the electronics have to be able to "manage" the 17 volt peak input.

But you also need to look at the circuitry/operation of the regulator too... Many of the LED bulbs, to save money, size, and have a longer circuit life, do not have a "filter capacitor" on the DC output of the regulator--You will see a "pulsing" (or strobing) output of the LED (120 Hz)--So feeding DC at 17 volts, may still push too much "average" current through the bulb--You have constant voltage on the input, and no 120 Hz "rectified sine wave" through the LED--higher average current.

I guess, if you wanted to do an experiment (and possibly sacrifice the bulb)--You can measure the current and voltage through the bulb. Measure 12 VDC and current. Then run 17 volts through the bulb. If the bulb has constant power load (i.e. 9 watts at 10.5, 12, and 17 volts; Power = Voltage * Current) then you have a ballast circuit that feeds the same current regardless of the input voltage (within the operating range). If you see the power vary a lot over the input voltage range, then I would suggest that the bulb is not a good candidate for running from 12 VDC solar supply.

-Bill

animatt · April 2017

I used to have some cheap led "bulbs" for 12v usage. I I had them running from a 24v to 12v converter. A converter in theory would provide a constant voltage situation. A good thing.

But converter died and all loads setup to it saw 24v. Not a very good failure mode.

I noticed 24 volts on 12v side of things first in a sure flow rv pump running very fast.
When bulb turned on was much brighter.
Worked for a short time like 20s until the circuitry melted its traces and fell to pieces. And also melted some plastic housings.

jonr · April 2017

I noticed that a Cree LED bulb designed for 120VAC worked well even with low DC voltages.

Sometimes you can look at the circuitry and guess whether it can handle slightly higher voltages. For example, a LED+driver designed for 12V that has 16V electrolytic capacitors would not be a good candidate for use in a system using lead acid batteries. 25V electrolytics - maybe.

Estragon · April 2017

Thanks, very informative.

I thought about using a 48-12v converter to run 12v loads when setting up my system. I'm glad I didn't if the converter can fail in a way that lets 12v gear see 48v.

For the cabin I might use a constant voltage output 12v-12v converter if I can't be sure of voltage sensitivity of bulbs/fixtures. If that failed, loads would only see a more reasonable overvoltage. For the boat, I'll just make sure there's a spec for voltage range.

I'm leaning towards using bulbs rather than integrated fixtures, at least in the cabin. Most of the cabin ones will be built into bulkheads, cabinets, etc., and changing out whole fixtures when they fail might be a problem. Even without the variable voltage issue, I don't trust the 50,000hr lifetime claimed for these things.

Voltage sensitivity of loads

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