LED Driver - Page 1
Forum section: electronics / beginners

author: Tnixsw
author_rank: Contributor
timestamp: March 09, 2023, 03:23:06 am
content: Hi all,I am trying to refurbish an old piece of equipment that has damaged multiplexed LED display driver.The attached circuit is similar to that used in the original which had purpose designed IC's that are not available anymore.The upper half is a PNP constant current circuit.The middle NPN is to turn on the LED anode, 1 per anode - 8 required.The lower NPN is to activate the common cathode - 1 per digit.The power supply is about 4V, the LED current is about 40mA (pulsed).I think I can use a modern current source like ALS5809-40 instead of the PNP transistors.LED ON time per segment is about 40uS - 320uS update per digit.I am only a hobbyist, so just hoping to check if this would work.Edit: Sorry didn't have LED segment in drawing - updated.cheersTony

author: pqass
author_rank: Super Contributor
timestamp: March 09, 2023, 04:42:44 am
content: Quote from: Tnixsw on March 09, 2023, 03:23:06 amIt MAY work but only if the Vf of the LED is low (red or green; 1.9V-2.2V).Why? Because if the lower cathode driver NPN is on, the upper anode driver NPN emitter would be raised by that LED Vf and the base would be 0.7V higher still. But will your MCU (I presume) source enough voltage to produce a current in the base?For example, 2.2V LED Vf + 0.7 Vbe = 2.9V If your MCU produces 3.3V (logic H), then 3.3V - 2.9V = 0.4V across a 4K7 resistor = 85uA, then multiply by 100 Hfe = 8.5mA Ic at most allowed making your CC source useless. If instead you used a 1K base resistor, Ib=400uA, and thus 40mA Ic.This is why you'd normally put a PNP for the upper anode driver. But then would need a NPN to drive its base.Play with it here:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKABcLiUQUAWW3rx78oIJlGiE8ZMgl6YMmYr3IMYhMIqUYwJFFwyFkVACZ0AZgEMArgBs27CoJ4IjCLi6NVxMXstzYGLzYxNgo2HiG2GLq0hiy8lhKKsYgZlZ2DgBOnNx8tB6aXuCELADuTkIofJXghlDltUW5dV4sAEotze7cYCh4olQiQlSj0AgsOXJVrrXYCNxUYPAsAOZzC3NSgyyBBXmKLcG9OwAOIISzzZdG+eA7FTfC+88Nj1f1Pa0NHNMgggMvgDRBAYHIuMs8Hg5ERINVoupiOg0HgwLxCNhCJAjMtTBYbPZJrVgbw8ENSSCVusvghPh4vqMWOdCId0UIWdw-qC3hdWYQhED+TyObUac5IB1-rp-hTMQNgaN-lRsNAEYNxkzedxjlr-kLuRLHqywkcTRLfs5XC8rSCJHIofIEHgMYIkaQJCRIBEWRh8NJMCgkLi0vjMpLeNKbXKKFjBkqQCq1WMJlNLbHSbRY0NfRKAM5StzpskxxVUKy2XN0Fj5iNGEnFhWicuVzUiv5t+oGxodtz0+oS87OoQ6ocUTsPEAYPAC+o9nmjnUinUDyfTvVCKcznETzfr3Vs+dC2lGXfHhq6TkeA9iqohjL2Bi2Ogme6l2Csc5-YF-BB9V8NalLT-H9+h2IA

author: Tnixsw
author_rank: Contributor
timestamp: March 09, 2023, 07:00:35 am
content: Thanks for the reply.The LED VF = 1.7V. The micro can source/sink up to 25mA from any IO pin.The data sheet specifies Voh = Vcc-0.6V max @ 6mA, but is for guidance only.With a 10K load on an IO pin, about 300uA, Vcc = 3.3V, I measure Voh = 3.29V.I'll have to measure what 10K pull down + base drive (say 1.2mA) does to Voh, but I would think not much.Then...1.7V LED Vf + 0.7 Vbe = 2.4V Voh = 3.2V (say), then 3.2V - 2.4V = 0.8V across 1K2 resistor = 667uA. Ic around 66mA. (53mA @ 1K5)Thanks for the link, I'll try it out.cheersTony

author: pqass
author_rank: Super Contributor
timestamp: March 09, 2023, 07:18:12 am
content: Quote from: Tnixsw on March 09, 2023, 07:00:35 amWhy are you bothering with the CC source and anode driver?Can you drive the anodes directly from the MCU through a series resistor?Are the LEDs so old (and dim) that you need to push 40mA through them?

author: Tnixsw
author_rank: Contributor
timestamp: March 09, 2023, 07:33:50 am
content: 40mA was the original design spec. The constant current circuit in the image is the original one used.The LEDs are multiplexed and most segment are on for about 40uS, some others for a shorter time - 91 LEDs in totalIt takes roughly 3.6mS for a full refresh, one LED at a time, so the duty cycle is very low, even so, the display in normal operation is quite bright.cheersTony

author: Tnixsw
author_rank: Contributor
timestamp: March 09, 2023, 11:20:05 pm
content: Would using a constant current source IC be a better option than the 2 transistor approach. I have not used these devices before.Something likehttps://www.digikey.com.au/en/products/detail/diodes-incorporated/AL5809-40S1-7/5030202?s=N4IgjCBcpgbFoDGUBmBDANgZwKYBoQB7KAbRABYAGcgDgCZyQBdAgBwBcoQBldgJwCWAOwDmIAL4FyATkbQQySOmz4ipEHTDkA7DWnM2nSD37CxkkAFo6CBVH4BXVcUhkArM3EWbrkJjc0lPpM4kAcheersTony

author: pqass
author_rank: Super Contributor
timestamp: March 10, 2023, 02:17:37 am
content: I can't think of a reason why the following isn't a good idea... Please someone let me know.That is, drop the 2-transistor or proposed specialized CC IC entirely and just use an emitter resistor on the anode NPN.The way I see it, you can rely on a consistent voltage (MCU GPIO output) on the base of the anode NPN. The only other variable is temperature (-2.2mV/°C) on the LED, and NPNs (everything really), which seems to affect the CC by +/-4 mA (41 mA to 33 mA) for +/- 100mV change in Vf+2Vce represending a +/- 15°C change. This might be acceptable for this application.Note: I crudely simulated the temp change by creating two new LED models with different Vfs; one with 2.1V another with 2.3V, the default is 2.2V.Play with it here:http://www.falstad.com/circuit/circuitjs.html?ctz=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

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2023, 02:20:35 pm
content: It would be better to put the two BJTs in series and put the LED in series with the collector of the top one.