Maker Pro
Maker Pro

Boost converter output voltage being limited by led current draw?

alex ezorsky-lie

Aug 27, 2015
27
Joined
Aug 27, 2015
Messages
27
I am new to boost converters and high power led’s but I have a 1meter led strip containing 240 2835-led’s and the strip is rated for 24v. The assumed necessary watts to power this strip fully is 36watts.

My goal was to see if I could power it using a single sony NPF970 battery which outputs 8v and a max of 5amps (40watts).

I purchased this 250w boost converter https://www.amazon.com/dp/B076J8734G/ref=cm_sw_r_cp_api_i_ImLBEbDZRCQZQ

I set it up and connected my multimeter and was able to output up to 30v and used the pot on the converter to trim down to 25v. I started with the current pot turned all the way down. I connected my strip and it did indeed light but the output voltage dropped to 14v. Increasing the voltage pot did nothing but increasing the current pot did allow the voltage to rise. I could crank the current pot up until it was putting out 1.6 amps and the voltage was at 22v. Any higher and the battery cut out (I was over the batteries 5a max current draw).

I’m sure this is obvious to you but I can’t find a clear explanation as to why I couldn’t output 24v with lower current. Is the LED strip determining a minimum current draw that is high enough that they themselves are limiting potential output voltage given a fixed input wattage?

Any clarification much appreciated!
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,770
Joined
Nov 17, 2011
Messages
12,770
I can’t find a clear explanation as to why I couldn’t output 24v with lower current.
The current is determined by the LED strip'S construction. Typically resisstors are used in this kind of LED strip to limit the current for a given operating voltage (24 V in your case).
The current limiting in the converter senses the actual current drawn and reduces the output voltage if necessary to prevent a current higher than the one that was set using the trimmer. When you connect a load that would draw a higher current than set (e.g your LED strip) The converter has to reduce the output voltage to limit the current.
This behavior is to be expected and normal.
Is the LED strip determining a minimum current draw that is high enough that they themselves are limiting potential output voltage given a fixed input wattage?
The LED strips do not limit the voltage. Nor do they limit the current. As explained above they are designed to draw a (more or less) defined current at the nominal operating voltage. When you increase the voltage, current rises, too. When you decrease the voltage, current falls, too. but you cannot decrease the current and keep the voltage constant.

Apart from the LEDs with their nonlinear characteristic (read our resource on driving LEDs if you are interested in more details) you can approximate the behavior of the strip as resistive. The relation between voltage and current can the be described as
I = V/R
where I = current, V = voltage, R = resistance. With R = (24 V)²/36 W = 16 Ω (note: this is an approximation for input voltages around 24 V, in detail this becomes more complex due to the characteristics of the LEDs, but I will not go into more detail here).
So with I set by the current limiter of the converter, you get V = I × R which cannot give 24 V for a current lower than 1.5 A.
 

Bluejets

Oct 5, 2014
5,995
Joined
Oct 5, 2014
Messages
5,995
Op's battery spec, could not see any maximum current draw listed.

Name:Digital battery Model:NP-F970
Capacity:7.2 V 7200 mAh
Size:70 mm x 58 mm x 37 mm
Weight:291.8 g

A rather expensive special purpose battery to be used for playing with LED's I would imagine.
 

alex ezorsky-lie

Aug 27, 2015
27
Joined
Aug 27, 2015
Messages
27
This is really helpful thanks!

I have moore’s law in my head but I guess I thought a converter changed the balance of the output’s I/V but really with a fixed R you are balancing the input IxV with the output IxV
 

alex ezorsky-lie

Aug 27, 2015
27
Joined
Aug 27, 2015
Messages
27
Op's battery spec, could not see any maximum current draw listed.

Name:Digital battery Model:NP-F970
Capacity:7.2 V 7200 mAh
Size:70 mm x 58 mm x 37 mm
Weight:291.8 g

A rather expensive special purpose battery to be used for playing with LED's I would imagine.

here is a link to one website with a max current spec that matches what I’ve experienced
https://cinequipt.com/products/zz-fv-va1223/

And no I didn’t spend $56 I spent $16 ;)
 

Bluejets

Oct 5, 2014
5,995
Joined
Oct 5, 2014
Messages
5,995
I'd be giving these a wide berth and spend the money on the driver designed for the LED strip.

Too many unknowns in your specs.
 
Top