Why does a 74HC595N work in LED matrices?












1














I have checked a lot of LED matrices, and mostly a 74HC595N shift register is used. In some cases a TPIC595B like below.



Example: Driving LED arrays with Arduino



Circuit from above example:



enter image description here



I understand that for sinking a TPIC is needed, because when all columns are given 25 mA, it would result in a total current of 200 mA. The 74HC595N can only handle 70 mA.



As I understand, the 74HC595 controls the rows one by one. However, if all columns of a row are getting a current of 25 mA, will there be 200 mA through a single source pin of the 74HC595? Or should there be max. 70 / 8 = 8.8 mA per LED? The TPIC can handle 150 mA per pin, so that more than enough.



Can the 74HC595 handle 8 LEDs at 20 mA in the above circuit?










share|improve this question


















  • 1




    Bit of a duplicate of this question?
    – Finbarr
    1 hour ago










  • @Finbarr not a duplicate, but a nice bit of extra information, thanks for the link.
    – Michel Keijzers
    1 hour ago
















1














I have checked a lot of LED matrices, and mostly a 74HC595N shift register is used. In some cases a TPIC595B like below.



Example: Driving LED arrays with Arduino



Circuit from above example:



enter image description here



I understand that for sinking a TPIC is needed, because when all columns are given 25 mA, it would result in a total current of 200 mA. The 74HC595N can only handle 70 mA.



As I understand, the 74HC595 controls the rows one by one. However, if all columns of a row are getting a current of 25 mA, will there be 200 mA through a single source pin of the 74HC595? Or should there be max. 70 / 8 = 8.8 mA per LED? The TPIC can handle 150 mA per pin, so that more than enough.



Can the 74HC595 handle 8 LEDs at 20 mA in the above circuit?










share|improve this question


















  • 1




    Bit of a duplicate of this question?
    – Finbarr
    1 hour ago










  • @Finbarr not a duplicate, but a nice bit of extra information, thanks for the link.
    – Michel Keijzers
    1 hour ago














1












1








1







I have checked a lot of LED matrices, and mostly a 74HC595N shift register is used. In some cases a TPIC595B like below.



Example: Driving LED arrays with Arduino



Circuit from above example:



enter image description here



I understand that for sinking a TPIC is needed, because when all columns are given 25 mA, it would result in a total current of 200 mA. The 74HC595N can only handle 70 mA.



As I understand, the 74HC595 controls the rows one by one. However, if all columns of a row are getting a current of 25 mA, will there be 200 mA through a single source pin of the 74HC595? Or should there be max. 70 / 8 = 8.8 mA per LED? The TPIC can handle 150 mA per pin, so that more than enough.



Can the 74HC595 handle 8 LEDs at 20 mA in the above circuit?










share|improve this question













I have checked a lot of LED matrices, and mostly a 74HC595N shift register is used. In some cases a TPIC595B like below.



Example: Driving LED arrays with Arduino



Circuit from above example:



enter image description here



I understand that for sinking a TPIC is needed, because when all columns are given 25 mA, it would result in a total current of 200 mA. The 74HC595N can only handle 70 mA.



As I understand, the 74HC595 controls the rows one by one. However, if all columns of a row are getting a current of 25 mA, will there be 200 mA through a single source pin of the 74HC595? Or should there be max. 70 / 8 = 8.8 mA per LED? The TPIC can handle 150 mA per pin, so that more than enough.



Can the 74HC595 handle 8 LEDs at 20 mA in the above circuit?







led led-matrix 74hc595






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 2 hours ago









Michel KeijzersMichel Keijzers

5,75992562




5,75992562








  • 1




    Bit of a duplicate of this question?
    – Finbarr
    1 hour ago










  • @Finbarr not a duplicate, but a nice bit of extra information, thanks for the link.
    – Michel Keijzers
    1 hour ago














  • 1




    Bit of a duplicate of this question?
    – Finbarr
    1 hour ago










  • @Finbarr not a duplicate, but a nice bit of extra information, thanks for the link.
    – Michel Keijzers
    1 hour ago








1




1




Bit of a duplicate of this question?
– Finbarr
1 hour ago




Bit of a duplicate of this question?
– Finbarr
1 hour ago












@Finbarr not a duplicate, but a nice bit of extra information, thanks for the link.
– Michel Keijzers
1 hour ago




@Finbarr not a duplicate, but a nice bit of extra information, thanks for the link.
– Michel Keijzers
1 hour ago










2 Answers
2






active

oldest

votes


















2














All the LED current flows through the Vcc pin of the 74HC595 (and the GND pin of the TIPxxx). The absolute maximum rated current through Vcc (or GND) of a typical 74HC595 is indeed 70mA.



The absolute maximum peak current per LED is thus 8.75mA, or an average current of about 1mA per LED (1/8 duty cycle per LED).



In practice you should stay WELL away from the absolute maximum value.



To put it explicitly, this is a hobbyist-level circuit, designed by someone who doesn't care or doesn't know much about reliability (assuming they actually recommended anything like the currents you stated). Using the 74HC595 to drive a high-side driver array or prebiased PNP transistor duals would be much better. They're designed as logic shift registers, not as load drivers.



Using such drivers you could also get a much higher brightness. An average current of 10mA per LED requires a total current of 640mA, obviously, which means that the source drivers need to handle 80mA each (with all potentially on at once) and the sink drivers need to hand 640mA each (with each one seeing a 1/8 duty cycle).






share|improve this answer























  • Thanks, so it would help to use a TPIC for sourcing too (than I can use 150 / 8 = 18.75 mA per LED, in one row) ?
    – Michel Keijzers
    2 hours ago






  • 1




    The TPIC has open-drain DMOS transistors so it wouldn't work. You need a high-side (source) driver.
    – Spehro Pefhany
    2 hours ago










  • Ah great, than I could use a 74HC595 to drive multiple 2N7000 transistors for sourcing and a TPIC595B for sinking. I think the TPIC can do not 640 mA, but I will scan through the rows anyway.
    – Michel Keijzers
    2 hours ago






  • 1




    You need p-channel transistors, eg. AO3401 and to invert the logic (low = on). The TPIC can handle about 250mA/output with only one on (see figure 10).
    – Spehro Pefhany
    1 hour ago












  • Thanks again for that tip ... however I will search for some throughole similar P channel transistor since I never used SMD so far.
    – Michel Keijzers
    1 hour ago



















1














In this example, the 74HC595 sources say 8-10mA per output as determined by R1-R8. One output of the TPICx595 is turned on a time to sink 64-80mA of current if all 8 LEDs are on. That's it. The 640mA number is not correct.
Each column is turned on for 2-3mS, then it is turned off, data for the next column is turned on, and the next column drive is turned. Repeat for all 8 columns.
That's the basis of multiplexing - cycle thru 8 columns quickly and fool the eye into thinking all 64 LEDs can be on at once, when in reality only 8 are ever turned on.






share|improve this answer





















  • Than I will check if 8-10 mA is good enough (probably it is), and use a 74HC595 and TPICB595 (as you proposed earlier). At least I now have a better understanding. I will do first some tests before I ask more (for most experts here) trivial questions.
    – Michel Keijzers
    19 mins ago











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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









2














All the LED current flows through the Vcc pin of the 74HC595 (and the GND pin of the TIPxxx). The absolute maximum rated current through Vcc (or GND) of a typical 74HC595 is indeed 70mA.



The absolute maximum peak current per LED is thus 8.75mA, or an average current of about 1mA per LED (1/8 duty cycle per LED).



In practice you should stay WELL away from the absolute maximum value.



To put it explicitly, this is a hobbyist-level circuit, designed by someone who doesn't care or doesn't know much about reliability (assuming they actually recommended anything like the currents you stated). Using the 74HC595 to drive a high-side driver array or prebiased PNP transistor duals would be much better. They're designed as logic shift registers, not as load drivers.



Using such drivers you could also get a much higher brightness. An average current of 10mA per LED requires a total current of 640mA, obviously, which means that the source drivers need to handle 80mA each (with all potentially on at once) and the sink drivers need to hand 640mA each (with each one seeing a 1/8 duty cycle).






share|improve this answer























  • Thanks, so it would help to use a TPIC for sourcing too (than I can use 150 / 8 = 18.75 mA per LED, in one row) ?
    – Michel Keijzers
    2 hours ago






  • 1




    The TPIC has open-drain DMOS transistors so it wouldn't work. You need a high-side (source) driver.
    – Spehro Pefhany
    2 hours ago










  • Ah great, than I could use a 74HC595 to drive multiple 2N7000 transistors for sourcing and a TPIC595B for sinking. I think the TPIC can do not 640 mA, but I will scan through the rows anyway.
    – Michel Keijzers
    2 hours ago






  • 1




    You need p-channel transistors, eg. AO3401 and to invert the logic (low = on). The TPIC can handle about 250mA/output with only one on (see figure 10).
    – Spehro Pefhany
    1 hour ago












  • Thanks again for that tip ... however I will search for some throughole similar P channel transistor since I never used SMD so far.
    – Michel Keijzers
    1 hour ago
















2














All the LED current flows through the Vcc pin of the 74HC595 (and the GND pin of the TIPxxx). The absolute maximum rated current through Vcc (or GND) of a typical 74HC595 is indeed 70mA.



The absolute maximum peak current per LED is thus 8.75mA, or an average current of about 1mA per LED (1/8 duty cycle per LED).



In practice you should stay WELL away from the absolute maximum value.



To put it explicitly, this is a hobbyist-level circuit, designed by someone who doesn't care or doesn't know much about reliability (assuming they actually recommended anything like the currents you stated). Using the 74HC595 to drive a high-side driver array or prebiased PNP transistor duals would be much better. They're designed as logic shift registers, not as load drivers.



Using such drivers you could also get a much higher brightness. An average current of 10mA per LED requires a total current of 640mA, obviously, which means that the source drivers need to handle 80mA each (with all potentially on at once) and the sink drivers need to hand 640mA each (with each one seeing a 1/8 duty cycle).






share|improve this answer























  • Thanks, so it would help to use a TPIC for sourcing too (than I can use 150 / 8 = 18.75 mA per LED, in one row) ?
    – Michel Keijzers
    2 hours ago






  • 1




    The TPIC has open-drain DMOS transistors so it wouldn't work. You need a high-side (source) driver.
    – Spehro Pefhany
    2 hours ago










  • Ah great, than I could use a 74HC595 to drive multiple 2N7000 transistors for sourcing and a TPIC595B for sinking. I think the TPIC can do not 640 mA, but I will scan through the rows anyway.
    – Michel Keijzers
    2 hours ago






  • 1




    You need p-channel transistors, eg. AO3401 and to invert the logic (low = on). The TPIC can handle about 250mA/output with only one on (see figure 10).
    – Spehro Pefhany
    1 hour ago












  • Thanks again for that tip ... however I will search for some throughole similar P channel transistor since I never used SMD so far.
    – Michel Keijzers
    1 hour ago














2












2








2






All the LED current flows through the Vcc pin of the 74HC595 (and the GND pin of the TIPxxx). The absolute maximum rated current through Vcc (or GND) of a typical 74HC595 is indeed 70mA.



The absolute maximum peak current per LED is thus 8.75mA, or an average current of about 1mA per LED (1/8 duty cycle per LED).



In practice you should stay WELL away from the absolute maximum value.



To put it explicitly, this is a hobbyist-level circuit, designed by someone who doesn't care or doesn't know much about reliability (assuming they actually recommended anything like the currents you stated). Using the 74HC595 to drive a high-side driver array or prebiased PNP transistor duals would be much better. They're designed as logic shift registers, not as load drivers.



Using such drivers you could also get a much higher brightness. An average current of 10mA per LED requires a total current of 640mA, obviously, which means that the source drivers need to handle 80mA each (with all potentially on at once) and the sink drivers need to hand 640mA each (with each one seeing a 1/8 duty cycle).






share|improve this answer














All the LED current flows through the Vcc pin of the 74HC595 (and the GND pin of the TIPxxx). The absolute maximum rated current through Vcc (or GND) of a typical 74HC595 is indeed 70mA.



The absolute maximum peak current per LED is thus 8.75mA, or an average current of about 1mA per LED (1/8 duty cycle per LED).



In practice you should stay WELL away from the absolute maximum value.



To put it explicitly, this is a hobbyist-level circuit, designed by someone who doesn't care or doesn't know much about reliability (assuming they actually recommended anything like the currents you stated). Using the 74HC595 to drive a high-side driver array or prebiased PNP transistor duals would be much better. They're designed as logic shift registers, not as load drivers.



Using such drivers you could also get a much higher brightness. An average current of 10mA per LED requires a total current of 640mA, obviously, which means that the source drivers need to handle 80mA each (with all potentially on at once) and the sink drivers need to hand 640mA each (with each one seeing a 1/8 duty cycle).







share|improve this answer














share|improve this answer



share|improve this answer








edited 2 hours ago

























answered 2 hours ago









Spehro PefhanySpehro Pefhany

204k4150408




204k4150408












  • Thanks, so it would help to use a TPIC for sourcing too (than I can use 150 / 8 = 18.75 mA per LED, in one row) ?
    – Michel Keijzers
    2 hours ago






  • 1




    The TPIC has open-drain DMOS transistors so it wouldn't work. You need a high-side (source) driver.
    – Spehro Pefhany
    2 hours ago










  • Ah great, than I could use a 74HC595 to drive multiple 2N7000 transistors for sourcing and a TPIC595B for sinking. I think the TPIC can do not 640 mA, but I will scan through the rows anyway.
    – Michel Keijzers
    2 hours ago






  • 1




    You need p-channel transistors, eg. AO3401 and to invert the logic (low = on). The TPIC can handle about 250mA/output with only one on (see figure 10).
    – Spehro Pefhany
    1 hour ago












  • Thanks again for that tip ... however I will search for some throughole similar P channel transistor since I never used SMD so far.
    – Michel Keijzers
    1 hour ago


















  • Thanks, so it would help to use a TPIC for sourcing too (than I can use 150 / 8 = 18.75 mA per LED, in one row) ?
    – Michel Keijzers
    2 hours ago






  • 1




    The TPIC has open-drain DMOS transistors so it wouldn't work. You need a high-side (source) driver.
    – Spehro Pefhany
    2 hours ago










  • Ah great, than I could use a 74HC595 to drive multiple 2N7000 transistors for sourcing and a TPIC595B for sinking. I think the TPIC can do not 640 mA, but I will scan through the rows anyway.
    – Michel Keijzers
    2 hours ago






  • 1




    You need p-channel transistors, eg. AO3401 and to invert the logic (low = on). The TPIC can handle about 250mA/output with only one on (see figure 10).
    – Spehro Pefhany
    1 hour ago












  • Thanks again for that tip ... however I will search for some throughole similar P channel transistor since I never used SMD so far.
    – Michel Keijzers
    1 hour ago
















Thanks, so it would help to use a TPIC for sourcing too (than I can use 150 / 8 = 18.75 mA per LED, in one row) ?
– Michel Keijzers
2 hours ago




Thanks, so it would help to use a TPIC for sourcing too (than I can use 150 / 8 = 18.75 mA per LED, in one row) ?
– Michel Keijzers
2 hours ago




1




1




The TPIC has open-drain DMOS transistors so it wouldn't work. You need a high-side (source) driver.
– Spehro Pefhany
2 hours ago




The TPIC has open-drain DMOS transistors so it wouldn't work. You need a high-side (source) driver.
– Spehro Pefhany
2 hours ago












Ah great, than I could use a 74HC595 to drive multiple 2N7000 transistors for sourcing and a TPIC595B for sinking. I think the TPIC can do not 640 mA, but I will scan through the rows anyway.
– Michel Keijzers
2 hours ago




Ah great, than I could use a 74HC595 to drive multiple 2N7000 transistors for sourcing and a TPIC595B for sinking. I think the TPIC can do not 640 mA, but I will scan through the rows anyway.
– Michel Keijzers
2 hours ago




1




1




You need p-channel transistors, eg. AO3401 and to invert the logic (low = on). The TPIC can handle about 250mA/output with only one on (see figure 10).
– Spehro Pefhany
1 hour ago






You need p-channel transistors, eg. AO3401 and to invert the logic (low = on). The TPIC can handle about 250mA/output with only one on (see figure 10).
– Spehro Pefhany
1 hour ago














Thanks again for that tip ... however I will search for some throughole similar P channel transistor since I never used SMD so far.
– Michel Keijzers
1 hour ago




Thanks again for that tip ... however I will search for some throughole similar P channel transistor since I never used SMD so far.
– Michel Keijzers
1 hour ago













1














In this example, the 74HC595 sources say 8-10mA per output as determined by R1-R8. One output of the TPICx595 is turned on a time to sink 64-80mA of current if all 8 LEDs are on. That's it. The 640mA number is not correct.
Each column is turned on for 2-3mS, then it is turned off, data for the next column is turned on, and the next column drive is turned. Repeat for all 8 columns.
That's the basis of multiplexing - cycle thru 8 columns quickly and fool the eye into thinking all 64 LEDs can be on at once, when in reality only 8 are ever turned on.






share|improve this answer





















  • Than I will check if 8-10 mA is good enough (probably it is), and use a 74HC595 and TPICB595 (as you proposed earlier). At least I now have a better understanding. I will do first some tests before I ask more (for most experts here) trivial questions.
    – Michel Keijzers
    19 mins ago
















1














In this example, the 74HC595 sources say 8-10mA per output as determined by R1-R8. One output of the TPICx595 is turned on a time to sink 64-80mA of current if all 8 LEDs are on. That's it. The 640mA number is not correct.
Each column is turned on for 2-3mS, then it is turned off, data for the next column is turned on, and the next column drive is turned. Repeat for all 8 columns.
That's the basis of multiplexing - cycle thru 8 columns quickly and fool the eye into thinking all 64 LEDs can be on at once, when in reality only 8 are ever turned on.






share|improve this answer





















  • Than I will check if 8-10 mA is good enough (probably it is), and use a 74HC595 and TPICB595 (as you proposed earlier). At least I now have a better understanding. I will do first some tests before I ask more (for most experts here) trivial questions.
    – Michel Keijzers
    19 mins ago














1












1








1






In this example, the 74HC595 sources say 8-10mA per output as determined by R1-R8. One output of the TPICx595 is turned on a time to sink 64-80mA of current if all 8 LEDs are on. That's it. The 640mA number is not correct.
Each column is turned on for 2-3mS, then it is turned off, data for the next column is turned on, and the next column drive is turned. Repeat for all 8 columns.
That's the basis of multiplexing - cycle thru 8 columns quickly and fool the eye into thinking all 64 LEDs can be on at once, when in reality only 8 are ever turned on.






share|improve this answer












In this example, the 74HC595 sources say 8-10mA per output as determined by R1-R8. One output of the TPICx595 is turned on a time to sink 64-80mA of current if all 8 LEDs are on. That's it. The 640mA number is not correct.
Each column is turned on for 2-3mS, then it is turned off, data for the next column is turned on, and the next column drive is turned. Repeat for all 8 columns.
That's the basis of multiplexing - cycle thru 8 columns quickly and fool the eye into thinking all 64 LEDs can be on at once, when in reality only 8 are ever turned on.







share|improve this answer












share|improve this answer



share|improve this answer










answered 21 mins ago









CrossRoadsCrossRoads

1,3008




1,3008












  • Than I will check if 8-10 mA is good enough (probably it is), and use a 74HC595 and TPICB595 (as you proposed earlier). At least I now have a better understanding. I will do first some tests before I ask more (for most experts here) trivial questions.
    – Michel Keijzers
    19 mins ago


















  • Than I will check if 8-10 mA is good enough (probably it is), and use a 74HC595 and TPICB595 (as you proposed earlier). At least I now have a better understanding. I will do first some tests before I ask more (for most experts here) trivial questions.
    – Michel Keijzers
    19 mins ago
















Than I will check if 8-10 mA is good enough (probably it is), and use a 74HC595 and TPICB595 (as you proposed earlier). At least I now have a better understanding. I will do first some tests before I ask more (for most experts here) trivial questions.
– Michel Keijzers
19 mins ago




Than I will check if 8-10 mA is good enough (probably it is), and use a 74HC595 and TPICB595 (as you proposed earlier). At least I now have a better understanding. I will do first some tests before I ask more (for most experts here) trivial questions.
– Michel Keijzers
19 mins ago


















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