I have two sets of LED-7-segs.
Each with 4 Digits, all 4 inch tall.
One set CC, the other CA.

Combined they consume some smooth 1.68A / 1.4A according to the data sheets.

That is manifold the current the driver can provide, let alone that the driver is fulled by an Arduino (I'll change that as well).

My question is: What components could I use to supply additional current to the digits?

Basically it seems like I am looking for a certain type of transistor:
Controlled by the driver, it forwards current from a seperate 'unlimited' supply.
At the same time it has to be quite fast, to keep pace with the driver.
I don't want to build this myself, as time matters more than cost,
so out of the box solutions are highly appreciated.

Also I am pretty sure the thing I'm looking for exists, but I can neither find an example part nor the name of such a component.

Datasheets: Drivers: ICM7218 (A Version for CA LEDs, B Version for CC)
CC LED: Kingbright SA40-18EWA
CA LED: Kingbright SA40-19SGWA

  • \$\begingroup\$ How much current per segment? Are the displays common anode or common cathode? \$\endgroup\$ – Majenko Oct 21 '14 at 9:28
  • \$\begingroup\$ @Majenko 60 / 50 mA per segment. I actually got a mixed set. Added datasheets and details in question. \$\endgroup\$ – Mark Oct 21 '14 at 9:47

That kind of large digit display uses many LEDs in series-parallel. You need more voltage than the ICM7218 can provide- in particular the forward voltage for each segment is 8V typical, 10V maximum. Thus you'd probably want at least 12V, and preferably 15V to drive the LEDs (note carefully that the decimal point has fewer LEDs and series and does not have two parallel strings so it has less forward voltage for the same brightness). Higher voltage than the absolute minimum is preferably to allow the use of simpler source and sink drivers and to allow a bit more resistance (or compliance for a constant-current circuit, as the 7218 uses) so the brightness variations between segments and digits would not be a problem.

While it would be possible to add boosting and external current control to the 7218, you could run into problems with ghosting and it would not be a simple circuit.

I think you're better off to start from scratch with a 15V or higher supply and darlington array or MOSFET drivers.

Here is a much better datasheet direct from the manufacturer.

enter image description here

One simple way to drive four large digits like this would be to use 74HC595s (four of them daisy-chained to drive four digits) and four ULN2803 darlington drivers (and 32 series resistors- one for each segment and a higher-value one for each decimal point). That is what is called "static drive"- the segments are on continuously, not multiplexed. The ULN2803 can handle up to 50V and can drive up to 50mA with all 8 outputs on in a fairly hot environment (based on the datasheet, it looks like they're intended to be used at about 20mA/segment). Total current would be maximum 20mA * 32 = 640mA @ 15V for four digits.

enter image description here

That will work with the common-anode displays (as shown in the above display schematic). For the common-cathode ones, you'd have to use a 8-channel similar source driver UDNxxxx in place of the ULN2803.


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