Characterising the 7805

As the LED strips are going to be mobile, I needed to find a way of powering them. After some investigation, the 7.2 volt NiMH battery packs used by radio controlled cars seemed like a good choice - they are high capacity, relatively cheap and readily available. Although the Arduino Pro Mini has an on-board regulator, the HL1606 strips don't and according to the datasheet they need between 3V and 5.5V, with 5V being the preferred voltage. That meant dropping the 7.2V from the batteries down to 5V. The obvious first choice was the venerable (and cheap) 7805, specifically the 2A version - STElectronics L78S05CV. I'd need 2 regs for each LED set to give the required 4A. The regs are £0.62 from BitsBox, with heatsinks and the recommended caps the total cost was about £3.00 per LED set. There was only one problem though, the 7805 datasheet says the minimum input voltage is 8V, the batteries only output 7.2, so it seemed they wouldn't work.

The next option was a LDO regulator such as the LM2940 but although they have a dropout voltage of only 0.5V, they are only rated at 1A and are £1.44 each, which would mean that the parts for each LED set would be over £9.00 - ouch. Other options included a homebrew switching regulator such as this one - not ideal because of the increased build and testing time, or a off-the-shelf switching regulator such as the PTH08T231W - again, not ideal because of cost - £11.00 each, plus £12.00 delivery from Mouser.

Bearing in mind this project is cost-constrained, none of the alternatives to the 7805 really looked viable. I decided to buy a couple of 7805s and see if they would actually work, despite what the datasheet said. What I needed to do was to use a 7805 to power my 2-strip prototype with a range of different input voltages, and that required something I didn't have, a variable output bench power supply - and at about £100, it wasn't something I was going to rush out and buy. Bob from Hacman pointed me in the direction of FabLab, and an email confirmed that they had a suitable bench power supply. As an aside, FabLab is a wonderful (and free!) resource that has all sorts of toys such as a laser cutter, a milling machine, a 3D printer and a CNC router - highly recommended!

The first test measured max/min output voltage against input voltage, using my standard test pattern set. The graph is below:


Things of note:

  • Vout never got above 4.85V. That's obviously less than 5V, but still within the spec for the 7805, which is 4.8V to 5.2V. I compared a couple of regs, they were all much the same.
  • As Vin drops, Vout and quality of the regulation also drops. At Vin of 5.5V Vout swings by about 0.6V.
  • Above Vin of 7V the load-induced Vout swing is around 0.1V.
  • Below Vout of around 5.5V the operation of the strips became unstable. The datasheet says they will work down to 3V, but like much of the information in there, it's incorrect.
  • There was little perceived change in LED brightness across the usable voltage range.

The second test measured input and output current against input voltage, with all 20 LEDs turned fully on (R+G+B on 100%), representing the maximum load the LED strips can generate. The graph is below:


Things of note:

  • Maximum load was at 7.2V and above, with Ain of 1.7A and Aout of 1.67A.
  • Below Vin of 7.2V Ain & Aout dropped fairly linearly with Vin.
  • Below Vin of 6V the LED strips became unstable.
  • The heatsink on the 7805 got very hot with voltages above 7V, increasing as the Vin increased. That's expected as the 7805 is a linear regulator and it dissipates the difference between Vin and Vout as heat, but a heatsink is absolutely necessary.

Much to my surprise, it appears that the 7805 will be OK for my application. The information I've found for NiMH battery performance suggests they have a flat output voltage of around 1.1V per cell until around 80% discharge, at which point the output voltage drops rapidly, That means I'll be getting about 6.6V from each pack, which whilst not ideal is certainly workable. The best solution would be a switched mode regulator, but as cost is a major constraint for this project, that isn't really an option.

Categories : Tech, AVR

Re: Characterising the 7805

These regulators do need a bit of a dropout voltage, and that will also change based on the load and the particular sillicon involved. Have a look in the datasheet to see if they given any general or specific characteristics.