LED dimmer without flicker ... why not variable resistor?

Hi, I have a 12Volt 20 Amp Power supply running 7meter strip LEDs (5630 SMD 24watt per meter) and it works fine.

When adding 12 ohm (Brown,red,black,gold) resistor it dims leds without flikker. I can add up to about 8 resistors in series and lights dim.

Now - I want to buy a 0 - 100 ohm variable resistor to control this and elactronic shops tells me they do not recomend me doing it like that. I must use puls dimmer. I cannot use puls couse this is for photography and you see the flicker clearly in photos.

Am I missing something?

Thank you for your help.

This is what the shop say "I'm sorry but your power requirements make the use of an adjustable resistor too impractical! The only way around this is to use some electronics, unfortunately we do not have something readily available on our shelves to suit your needs. In our case we will have to custom make this for you "

The problem is that high power variable resistors are expensive and hard to find. If you can get the one you need, and don't mind paying for it and don't care about the efficiency issues, it's a very reasonable solution.

The only drawback of your idea is that it is quite inefficient. Whether or not this is a problem is a separate question, and one that only you can answer.

I don't know if you can get a variable resistor rated for that much power... especially one that is easy to use. However, a quick search for "power rheostat" at www.digikey.com produces quite a few options. For $25, you can get a rheostat (adjustable resistor) that can handle 55W. That's a lot cheaper than I was expecting. You'll have to crunch the numbers and figure out what resistance value and power rating you need, but it's likely you'll find something that works.
edit: $25?? did I say $25? Sorry, I was looking at the wrong column. It's actually over $200. I'd still recommend crunching the numbers and seeing what power rating you need. The bench power supply I mention below might be a fairly cost effective solution after all <end of edit>

If you feel compelled to throw more money at the problem and improve the efficiency of the solution, you could buy a nice bench power supply that uses an internal switching regulator. It'll let you dial in the current that you want to feed to the LEDs. Here's one from B&K Precision that lists for $339. :
http://www.bkprecision.com/products/power-supplies/1688B-1-18v-0-20a-dc-power-supply.html

Another option is to use a switching driver, then filter the pulses out to eliminate flicker. Be advised, though, that any method that doesn't pulse the LEDs will change the color as you change the brightness. It's not obvious to the untrained eye, but as a photographer you may find this important.

But wait a minute. I hadn't paid attention to the numbers. 7 meters at 24W/m is 168W. At 12V that's 14A. If you put even 1A through a 12 ohm resistor, it will drop the full 12V of your power supply. What gives?

By my estimation you want more like a 0-10 ohm resistor. One rated at 14A will be a 2000W resistor, which will be massive and capable of frying eggs (the burners on my stove and most hair dryers are around 1500W), though in your application it would never get particularly hot, because only a tiny section of it would see high currents.

I'm starting to like Steve K's idea of a bench supply. You could save yourself $100 and get one like http://www.ebay.com/itm/MASTECH-HY3...251?pt=LH_DefaultDomain_0&hash=item20d67184c3. The B&K is a very good one, the Mastech is decent. Both of them are switching supplies with the pulses filtered out.

Plan B: just turn the LEDs on for a short bit of time, using it effectively like a flash. You'd have to synchronise it with your shutter, of course, and be able to control the on time. It's a bit more complicated, but could be quite a bit cheaper in terms of parts cost. Perhaps there are plans on the web somewhere??

Doesn't make sense to use a resistor when you can use a constant current supply to do the same thing.

That said, with 12V strips, using current control you could be all over the map with output of the individual LED groupings. Does it matter?

Custom high speed PWM could be used that works with your photography is an option.

DIWdiver said:

The problem is that high power variable resistors are expensive and hard to find. If you can get the one you need, and don't mind paying for it and don't care about the efficiency issues, it's a very reasonable solution.

Click to expand...

----------One possibility could be the WW variable resistors commonly available from auto accessory stores that do car Hi-Fi, they're used for rear speaker balance. - they sometimes turn up in tarted up but cheapo anyway domestic speaker cabinets. - A pulse controller shouldn't be ruled out, out of hand, there's nothing to prevent a rectifier & smoothing cap being added to the output - some of the many off the shelf LED switcher chips have example circuits in the datasheet with capacitor smoothed output.

SemiMan said:

That said, with 12V strips, using current control you could be all over the map with output of the individual LED groupings. Does it matter?

Click to expand...

I did some crude dimming using a pot on ~9 12V modules in a strip with no perceptible changes in module-to-module output. The main weakness with the arrangement was my use of a linear pot.

idleprocess said:

I did some crude dimming using a pot on ~9 12V modules in a strip with no perceptible changes in module-to-module output. The main weakness with the arrangement was my use of a linear pot.

Click to expand...

Single strips are often okay as most or all LEDs will be from the same reel and binned together. It is when you have different strips that things get dicey.

It would work, but pointless. A high current handling, fast responding variable resistor is the gist of the linear dropout power supply topology. The resistance is varied either through manual input or feedback loop. It's a very robust setup and can last decades. In recent days, it is used where exceptionally stable, low noise power is needed, but usually where power level is insignificant like the power for pre-amp.


Unfortunately, the conversion efficiency is 100% minus (proportion of total voltage on resistor). The higher the proportion of resistor, the better the control characteristics, but worse the efficiency. Older linear power supplies commonly got efficiencies of 20-30%. The efficiency is still better than incandescent in that the lamp itself doesn't become less and less efficient as it gets dimmed, but linear control is so inefficient that you overall lumen per watt would drop so low that its a joke.

Phase control dimming is one of the worst control method. It adds harmonics to the line, it sings, it creates RFI, etc. The only reason for their existence is to utilize existing behind-the-wall wires in retrofit applications.

It's still difficult to get smooth dimming across the full available range and dimming range is quite restrictive. You'll find that CREE CR6 dims very nicely down to a few percent. Dimmers that use neutral and do not depend on the load to power itself don't experience triggering issues the same way as load powered device. The constraint is the lack of neutral in some switch boxes.

The possible dimming range is from 0deg delay (direct wire) to 180 degree delay (off). The differences between different types of dimmers is the range of delay and calibration. (i.e. if the load reaches minimum at 50 degree delay and locks to maximum at 15 degree delay, the upper and lower would be something like 55 degree to 10 degree of delay range and the slider is calibrated such that the entire range of the potentiometer is expanded to control within those these limits. When you use the wrong dimmer that delays beyond permissible specs you experience malfunctions and/or limited resolution. (imagine a gas pedal that must be pushed in half way to keep the engine on, then the next 25% in position controls from minimum to maximum throttle, and the last 25% that just stays in max throttle. You could lock out the bottom half, but you still lose that portion of resolution.

The best dimming and least adverse impact on power quality is achieved when each device receives direct power and additional inputs(wires, optical, RF, etc) are used to control it. Lutron proprietary design uses phase control dimmer on the third wire for control purpose only, which is used as analog input telling the ballasts where the internal circuitry should dim down to.

In many battery powered flashlights, its fairly common to find parallel LEDs with series resistor for each string. It's very cheap to make, but the consequence is that about 30% of power delivered from battery is dissipated directly in the resistor. When you compare the total efficiency, the power consumed by resistor can't be excluded. It is directly penalized against it.

As the voltage is decreased, the wavelength of the blue LED can shift by as much as 20nm. This can result in a slight alteration of the color. I am rather sensitive to things like flicker, and would much rather have to deal with a small amount of color shift.

OK, I am lost on how these are set up.
Are these set up five in series?
what about the scr type dimmer in my dining room?
so when you decide that you need three meters long, do you think that you need 1 amp. So use 12 volts per meter,
or stick with 12 volts, 24 watts divided by three, a little less.