What does this do? Solenoid driver LDO - Page 1
Forum section: electronics / projects

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 01:54:34 pm
content: Hi Folks,Can you help me please understand the what and the why here?This is the solenoid driver board from an HP 22GHz RF step attenuator.It runs on 22-24V. The input 22V is connected to the common of all 4 coils, while each end is driven to ground on command by a channel on one of the ULN2003As. The pulsed switching logic (solenoids are mechanically bi-stable) is implemented via the 4 MM74HC123AM multivibrators. These run off of 5V, so there is an LM78 5V LDO to create the supply for them.There is a diode on the input of the LDO in series. If I deciphered the marking right (not sure) it's a 7.5V vreg diode, basically a zener.My question is if you think I got the part right, (Z61 BZX84-B6V8 Phi C SOT23 0.3W zener 6.8V ±2%)and if yes why exactly is it there, and how is it doing whatever it is doing.Is it just to off load the LDO? Why? It's rated for 35V input.Is it back EMF protection?Is it ripple suppression, in conjunction with the two caps?What when and how?Thanks!p.s. if anyone's interested in how these are built I uploaded some videos of me taking it apart then putting it back together. It's a pretty fascinating construction.This was only intended for myself for reference, so mild swearing, barking and bowel noises might be audible.https://photos.app.goo.gl/HiDYBwDeJag5umWA9

author: wasedadoc
author_rank: Super Contributor
timestamp: March 18, 2023, 02:02:41 pm
content: The regulator may be rated for the voltage but what about the power? Voltage drop multiplied by the current. The Zener reduces the voltage drop across the regulator and hence the power dissipated.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 02:11:57 pm
content: Quote from: wasedadoc on March 18, 2023, 02:02:41 pmMaybe. The worst case combined current draw of the 4 logic chips is a total of 20mA, and they won't be operating neither continuously nor simultaneously in real life. But to be fair if run from 24V that's 19V*.02A=380mW dissipated in the LDO which is not insignificant.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 18, 2023, 03:28:36 pm
content: BTW The regulator is not an LDO.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 04:19:04 pm
content: Quote from: wasedadoc on March 18, 2023, 03:28:36 pmThank you, ...I guess? Feel free to point out typos too!#butthurt

author: wraper
author_rank: Unknown
timestamp: March 18, 2023, 04:28:19 pm
content: LDO is a type of voltage regulator with a low (minimal) voltage drop. So LDO are a subset of voltage regulators. Although almost anything modern is LDO. 78XX are very old voltage regulators with high voltage drop.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 04:36:19 pm
content: Yes, thank you. It is correct and I understand that there is legit information here and good intentions. My mind didn't bother making a connection that 7805 is not an LDO. I'm only moody because I feel like it's nitpicking considering it has very little relevance to the problem. Anyway I shall be more precise with my reverse engineering and wording in the future.

author: tooki
author_rank: Super Contributor
timestamp: March 18, 2023, 05:01:16 pm
content: LDO is a feature, and a feature that has a nameprecisely to differentiate from traditional linear regulators that aren’t LDO. It’s like calling a paper letter an email: totally missing the point of the word!I get it, to people who didn’t experience the emergence of LDOs, LDO linear regulators may make up the majority of linear regulators you see or use. Nonetheless, LDO isn’t a synonym for “linear” regulator.And it’snottotally irrelevant: the dropout voltage is precisely the reason for the zener. (Granted, the minimum dropout isn’t the issue here, but understanding dropout voltage is the key concept in your question.)

author: james_s
author_rank: Super Contributor
timestamp: March 18, 2023, 05:26:35 pm
content: Yes this drives me nuts too, it's like calling any vehicle a sports car, or any airplane a light sport aircraft. LDO is a specific subclass of linear voltage regulator and in many cases it leads to confusion.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 05:27:34 pm
content: Quote from: tooki on March 18, 2023, 05:01:16 pmYes, that makes it totally irrelevant.An LDO or non-LDO would dissipate precisely the same amount here. If the zener is indeed intended as a relief for that, which I am still not 100% convinced, it doesn't matter which kind the regulator is. Maybe there is something I'm missing here, e.g. an architectural side effect that makes LDOs more effective at conducting heat away from the critical junctions, but I haven't yet been offered such an explanation.Quote from: tooki on March 18, 2023, 05:01:16 pmThat doesn't make sense. The diode can only lower the input voltage to the regulator, so if anything, it makes it worse. Assuming 7V drop on the diode, the input is still far above what the 7805 needs to properly regulate.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 18, 2023, 05:36:42 pm
content: @MarsupilamiThat it not being an LDO may not be relevant here is why I took the trouble to write "BTW". But if you were to continue to refer to any regulator as an LDO there would be circumstances where your error was relevant.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 05:41:04 pm
content: Quote from: wasedadoc on March 18, 2023, 05:36:42 pmFair enough, thank you. I'll do better.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 05:46:00 pm
content: Quote from: james_s on March 18, 2023, 05:26:35 pmSo if I asked you directions to the closest gas station to refuel my sports car, but I was sitting in a Geo Metro, the only thing you'd be inclined to tell me is that wait a second this is not a sports car? Duuude.

author: tooki
author_rank: Super Contributor
timestamp: March 18, 2023, 07:15:12 pm
content: Quote from: Marsupilami on March 18, 2023, 05:27:34 pmSorry, hunger-induced brain fart on my part, was confounding dropout voltage and the voltage drop across the regulator.

author: james_s
author_rank: Super Contributor
timestamp: March 19, 2023, 01:29:36 am
content: Quote from: Marsupilami on March 18, 2023, 05:46:00 pmNo, I'd direct you to a gas station, but I might also point out that you have an unusual definition of sports car. If somebody else had already given you directions to the nearest gas station I might only point out the sports car thing, because it doesn't make much sense to give you directions if somebody else already has.