Sinking Current With Darlington Array - Page 1
Forum section: electronics / beginners

author: ed_reardon
author_rank: Regular Contributor
timestamp: July 20, 2016, 07:31:51 am
content: Hello all,Numpty question of the week! Forgive me if this sounds daft, I'm learning.I'm attempting to repair a unit with four pistons driven by a microcontroller, the pistons are permanently at 12V and switched to ground by a Darlington Package (ULN2064B)Can I just ask:- What does 'K' refer to on the pin-out? This is fed by 12V to the IC.- By my reckoning to sink the current the collector of each Darlington should be connected to the negative side of the solenoid, so when the M/C sends the base high current can flow from collector -> emitter.Now the rub, and I know this sounds daft, please, forgive me, I'm a nurse by training!As this is running in the sinking configuration the emitter (where-ever that's gone on the I/C) should connect to ground?Sorry I sound like a numpty!Cheers,Ed

author: CJay
author_rank: Super Contributor
timestamp: July 20, 2016, 07:44:08 am
content: Quote from: ed_reardon on July 20, 2016, 07:31:51 amlooking at the block diagram for the chip, K is the cathode of the flyback diode, it's there to protect the darlington from the induced current caused by the collapsing magnetic field when the coil is de-energised.The block diagram isn't great, if you look further down the data sheet you'll find a much nicer partial schematic on page three that makes it much more clear.

author: ed_reardon
author_rank: Regular Contributor
timestamp: July 20, 2016, 07:54:59 am
content: Quote from: CJay on July 20, 2016, 07:44:08 amAh, that's a mystery solved! Thank you for that, I understand the fly-back principal, in this configuration it looks like this is permanently powered.So now having had a look at the more relevant diagram I think I now get it, base switches the collector (which is common across each 'pair') to ground!Thanks a ton for your help CJay, and your patient understanding!

author: Brutte
author_rank: Frequent Contributor
timestamp: July 20, 2016, 02:32:05 pm
content: QuoteWhen you switch off a charged inductor, a huge negative voltage appears across it and consequently across the switch (a darlington pair). The transistor has its Uce (voltage in between emitter and collector) limit and that diode gets positively biased during such discharging situations. Tie those to Vcc so that collector voltage never exceeds Vcc+diode drop.QuoteMind these internal diodes also have their limits - if you plan to discharge huge energies, provide some bigger external diodes. Preferably schottky ones.QuoteThe emitters are all wired to GND plane inside of the package.

author: ed_reardon
author_rank: Regular Contributor
timestamp: July 20, 2016, 08:49:03 pm
content: Quote from: Brutte on July 20, 2016, 02:32:05 pmCheers Brutte, the flyback principal I'm fine with (I'm an electrican by trade) now I've had a better look at the exploded diagram I get the principal much more clearly.Is there any benefit to having these in a sourcing or sinking configuration or does it make precious little difference?

author: CJay
author_rank: Super Contributor
timestamp: July 20, 2016, 09:15:30 pm
content: Quote from: ed_reardon on July 20, 2016, 08:49:03 pmThey're designed to sink current, common ground.

author: ed_reardon
author_rank: Regular Contributor
timestamp: July 21, 2016, 07:10:13 am
content: Quote from: CJay on July 20, 2016, 09:15:30 pmBut is there are common-collector type package available?Cheers,Ed

author: Brutte
author_rank: Frequent Contributor
timestamp: July 21, 2016, 08:01:52 am
content: Quote from: ed_reardon on July 21, 2016, 07:10:13 amWith bipolar there is no difference in between PNP or NPN - same performance. It is a matter of popularity, not technology. CheckSMA4020.With unipolar the N has the obvious advantage over P. If you want P then it is cheaper to do it N + charge pump. Only the smallest drivers use P.

author: CJay
author_rank: Super Contributor
timestamp: July 21, 2016, 01:43:48 pm
content: Quote from: ed_reardon on July 21, 2016, 07:10:13 amProbably but I'm not sure I could name one, I've not seen many or any at all and I'm not sure there's any advantage to using them that way.