Seven segment current requirements and dongle sucking power greater than 1Amp

Chuck Rice

.
The datasheet for the LTC-4627JR 7-segment display (cheap $3 part) says that it is rated at 25mA per segment.
There are 7 segments times 4 digits + three extra segments for the colon. That would be 775 mA.
But the display is multiplexed, so there is only one digit displayed at a time,
but displayed really fast so they look like they are all being displayed at once.
So the max current should really be 175 mA. I think.

Am I calculating this correctly?

I ask because I have a 9V 1A supply powering the proto board.
This works fine if I have the USB dongle plugged into the computer and into the protoboard.
It also works fine if the dongle is not plugged in.

But if the dongle is connected to the proto board and not to the computer, the proto board
will reboot over and over until I completely plug or unplug the dongle. I think I remember
seeing that the dongle will suck power from the proto board if it is not powered by the
USB. But since I have a 1 Amp supply, it bothers me that it cannot provide enough power
to boot the proto board when the dongle is plugged in, but not connected to the computer.
.

Post Edited (Chuck Rice) : 6/25/2008 4:57:40 PM GMT

Leon

You don't want to put the maximum current through the LEDs. 20 mA should be fine. Worst case with all segments on for one digit is then 7x20 = 140 mA, plus the colon. Your 175 mA isn't far off that.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 6/25/2008 4:59:50 PM GMT

Mike Green

The problem you're seeing with rebooting has nothing to do with the drain of the display. This is a known problem with the PropClip / PropPlug and has to do with "sneak paths" through the FTDI chip when the board is powered, but the FTDI chip is not (because it's powered by USB). The solution is to remove the PropClip / PropPlug when there's no USB connection. There was also something mentioned about putting a pulldown resistor on the Protoboard to suppress the problem, but I don't have that link.

With multiplexed displays, you're correct that only one digit is turned on at a time, but it's common to drive that digit harder (with a higher current) so the average current may be the same as the unmultiplexed total current. On the other hand, 75mA seems a bit high for a normal per segment current and may represent a maximum allowed current rather than a recommended current. The 75mA value would be the peak current allowed for a multiplexed (or any other) display. The average current would be more like 20mA or less which is more commonly used for most LEDs. The total average current would be something like 250mA or less depending on the brightness desired.

Chuck Rice

Thanks. The 74mA was a type-o. It is really 25mA in the datasheet. The max current per segment is 90mA.

So the dongle (propPlug in this case) is not really drawing any current to speak of? Just enough to bring the reset pin to the reset state?

OwenS

What happens is your Prop outputs a high voltage on it's TX pin, which enters the FTDI chip in the dongle. Since the dongle is powered off, this causes the protection diode to Vdd to activate, thus powering the chip via it's RX pin. The chip at power up outputs the Propeller's reset signal, which causes the transmit pin to become an input, turning off the dongle, which allows the whole process to repeat.