I have a friend that wants me to convert One of these over to white LED's. The issue is that the red LED's are spec'd at 2 volts, and white is 3 to 3.3 volts. So a 5 LED segment would be 15-16.5 volts.
Would this be safe? What value resistor should be used, if any?
You can use a small NPN resistor to safely switch the ground (-) side of the LED's. You'll need a voltage source a few volts higher than the expected voltage drop, so maybe 18 or 20 volts. The current limiting resistor is calculated as normal: (supply voltage - total voltage drop) / current. Something like (18 - 15) / 0.02 = 150 ohms. That assumes an 18V supply and 20 mA current through the LEDs.
johnwasser:
You can use a small NPN resistor to safely switch the ground (-) side of the LED's. You'll need a voltage source a few volts higher than the expected voltage drop, so maybe 18 or 20 volts. The current limiting resistor is calculated as normal: (supply voltage - total voltage drop) / current. Something like (18 - 15) / 0.02 = 150 ohms. That assumes an 18V supply and 20 mA current through the LEDs.
guess i didn't specify enough.
Voltage source is 11.1 volt LiPo battery. so each LED would get 2.22 volts with resistor removed where 3.0-3.3 is optimal. But I still need the resistor, right?
The PCB is in series, so I can't make them series-parallel or anything fancy.
so each LED would get 2.22 volts
Which is not enough to turn it on, so no it would not work.
But I still need the resistor, right?
Right.
There are circuits that can boost the voltage to LEDs, however they are not simple. Assuming you want to switch the segments from a 5v signal, and you want good current control, it needs about 3 transistors (or a switching regulator IC) and one inductor per display segment. Better to increase the battery voltage if you can, or else rewire the LEDs as 2 chains in parallel (each with its own series resistor).
Use a boost regulator like this one to get higher voltage and the 140mA per digit that is needed.
input voltage: 1.5 V to 16 V
output adjustable from 4 V to 25 V
2 A switch (and input) limit
I use this in my main fencing scoring machine to convert 5V up to 12V to:
light up 10 strings of 5 LEDs each, and 2 strings of 3 LEDs
send 12V power to two remote boxes, each containing:
their own sets of 10 strings of 5 LEDs
7805 regulator that powers a promini
6 other LEDs that run on 5V.
So it can certainly handle one 7-segment display.
This one might be all you need even for a couple dollars less
Don't forget to order some free back issues of elector magazine, the July/August issues are the Project issues, all kinds of interesting things written up.
CrossRoads:
Use a boost regulator like this one to get higher voltage and the 140mA per digit that is needed.Pololu Adjustable Boost Regulator 4-25V
input voltage: 1.5 V to 16 V
output adjustable from 4 V to 25 V
2 A switch (and input) limitI use this in my main fencing scoring machine to convert 5V up to 12V to:
light up 10 strings of 5 LEDs each, and 2 strings of 3 LEDs
send 12V power to two remote boxes, each containing:
their own sets of 10 strings of 5 LEDs
7805 regulator that powers a promini
6 other LEDs that run on 5V.So it can certainly handle one 7-segment display.
This one might be all you need even for a couple dollars less
Pololu Adjustable Step-Up/Step-Down Voltage Regulator S8V3A
Don't forget to order some free back issues of elector magazine, the July/August issues are the Project issues, all kinds of interesting things written up.
CR, I was looking at using a booster indeed. I'm going to have a 12 volt regulator to bring down the ~18VDC transient spikes (14.8RMS) in the electrical system, so I figured I can use a 12->17.5V booster. The 5 volt regulator is rated for a 25Vin and only will be powering the 328 and shift registers so it should be fine with the extra power dissipation.
The 12V LDO is on a small 8.5C/W heatsink in a vented enclosure (possibly with a 60mm computer fan) so up to 2 amps in 120 degree texas heat shouldn't be a problem on a 19 Vmax system (IE,a 16 volt system with similar transients).
So normally it's Vin-> 12V LDO -> 5V LDO
on custom boards, it'll be Vin -> 12V LDO -> pololu board -> 5V LDO.
CrossRoads:
Use a boost regulator like this one to get higher voltage and the 140mA per digit that is needed.
I've been looking for a 2A boost IC that works down to 3V (to drive LED matrix displays) I looked at the link, but there is no technical info about that board...do you know what Boost IC they're using?
Nope, can't tell what it is. Will see if I can read anything on my board tomorrow.