How fast should a switching PSU startup? - Page 1
Forum section: electronics / beginners

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 27, 2025, 03:42:03 pm
content: I am working on a broken Tektronix TDS 754D scope, it stopped "booting" on the previous owner. I am not ready to go all in on a repair yet but I had an interesting question to ask the forum.The Tek troubleshooting flowchart told me that the front panel assembly, which has its own 68HC05B microcontroller, is not communicating with the main digital board. After some checking with a scope, sure enough the lights are on but nobody is home, there is absolutely no processor activity. The clock is present and the MCU is not in reset.My observation was that all the front panel lights take a portion of a second to increase brightness after turning on the unit. Measuring with a scope, all the power rails will jump to about 1/2 normal voltage at the instant of power-on, and then slowly ramp to normal voltage over 200 - 300 milliseconds. Is this normal? I have not seen a switching power supply startup like this before, perhaps some kind of soft start?My concern was that the voltage ramping like this would mess with digital circuits if they were not in reset. I find it very unlikely that this is THE problem, but I want to make sure it is not A problem.Thanks!

author: SuperFlyEDSguy
author_rank: Contributor
timestamp: August 30, 2025, 05:16:12 am
content: The good news is that actually sounds like normal operation!Switching power supplies take at least some nominal amount of time (in µs) up to a few hundred ms (just like yours is doing) to complete their startup by design. If it doesn’t ramp-up and then stabilize, you could end up seeing too much initial current on start and blow a fuse, trip a breaker, etc. It could also take quite a toll on components within the power supply damaging its components secondary to such a sudden current spike, but even if it doesn’t cause immediate damage you’ll still likely reduce the overall life of the unit.So, to prevent that all from happening, you will generally seeone or more of the three following things,such as:Big capsthat take time to charge thereby causing a ramp-up.AnInrush Current Limiter (ICL)circuit that starts off with a high resistance and subsequently goes to low resistance. Basic Ohm’s Law shows that R=E/I, so with enough of a decrease in resistance, we would see a proportional increase in current.There may even be adigital solution (IC-based)to precisely control the ramp-up!Now, as far as it causing a problem with the digital circuits,I would actually think just the opposite as we are preventing overcurrent at startup from potentially destroying such circuitry.It’s also likely starting at a high enough voltage to allow the digital circuit(s) to initialize without any issue and subsequently ramp-up while still being within spec for them, whereas at least VDCis concerned anyhow. I’ve added digital monitoring circuits to analog power supplies similar to this way before to ensure that they get controlled voltage using a voltage regulator, they won’t be as finicky, but they do get enough voltage to normally start while my big caps are taking their initial charge. That’s also preventing overcurrent globally. So, it just works.The funny thing is that we will see the opposite occur with a basic power supply that has digital monitoring circuits because those initial big caps take a couple of seconds to discharge upon power down.So, you’ll actually see the voltage drop on the LED voltage meter once you shut off the power supply until it gets to a point that it no longer gets enough power to maintain the display and “flickers out.” Of course you could prevent that by using discharge resistors parallel to those big caps, which will result in a controlled rapid discharge. That’s more for aesthetics though as older devices with digital displays like VCRs did the very same thing all the timeand it never broke their LED readouts!Basically, if curious, you’d use something like a 1KΩ 2W resistor in parallel to drain capacitors between 2200µF and 4700µF. At least that should work in theory, but I usually don’t bother even taking the time to do this with home built power-supplies. I’d rather take that time and utilize the space on the board to ensure that I have bypass caps to suppress HF noise where needed and protection diodes to prevent reverse voltage from inductive loads potentially frying my voltage regulators, transistors, etc. (also as needed for that particular application.)Hopefully, that all makes sense, it’s getting kinda late and I probably could have explained this a little better.The main takeaway is simple, the switching power supply sounds like it’s working as it should.Anything beyond that I would need to think about, but power supplies are one of my strong areas, so I feel confident telling you this much.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 30, 2025, 04:35:47 pm
content: Thank you very much for your response!I figured that this would be the case, but I wanted to make sure. The Tektronix TDS 754 does not have schematics available so I have a long road of troubleshooting ahead of me.

author: SuperFlyEDSguy
author_rank: Contributor
timestamp: August 31, 2025, 02:45:17 am
content: Yeah, no problem at all, I’m just glad that you were able to find it useful (or at least useful enough)!A few thoughts. This is just some quick brainstorming, nothing very granular, but try any recommendations, especially the isolation ones towards the end, and hit me back up with the answers to any other questions. I’ll see if any other ideas come to mind.Would the previous owner have told you if it took a fall before the trouble started?What about any signs of physical damage or possibly even over-heating?Does the fan work?Likewise on the PCBs, any indication of burning, discoloration, etc.?What about the solder joints throughout the PCBs, what story do they tell? Do they look decent?Can you find a date code, manufacturer date, etc.? When was it made?I’m not familiar with the internals of this scope, LMK if it uses a CMOS battery to help save settings?Any beeps or other noises coming from this thing, especially at the moment of startup and just after?Do you have a logic probe available?Can you try disconnecting the front panel from digital board and see if the rebooting process goes further?Do you see any other points of isolation as I would try separating functionality by removing any cables to/from the main PCB or digital board?Ultamitly, I think your PS is fine, that means now isolating what could be ruled down to the true culprit!This shouldn’t take you long and will hopefully shed some light on the true issue!Maybe PM me the results to ensure that I see them.Hopefully you’ll be able to get it working, what other staples do you have on your bench to use for troubleshooting?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 31, 2025, 04:00:27 am
content: I have some history on this TDS 754.The person I got it from bought it from an auction a number of years ago, the screen did not work when he got it, so he just used an external VGA monitor and never fixed it. From what I understand the scope worked fine before it stopped booting.The internals of the scope are very clean, it looks like it was well cared for in its previous homes.The fan woks fine.No signs of stress on the PCBs or solder joints, it is mostly surface mount components.The scope was made around 2000.This scope uses two "DALLAS" devices, a DS1486 timekeeper with ram, and a DS1250Y nonvolatile SRAM.There are no boot beeps.I do have a logic analyzer (kind of).I have tried isolating some components, no changes.The DALLAS chips are very likely dead, however I found no reports of this causing the scope not to boot, just loosing calibration data. Although now that I think of it I don't know if there are any other ROMS on the CPU board, I will have to take a closer look. There are solutions to replacing these devices since they all fail, I do have a tool to read and write ROMs.I have a decent amount of tools at my disposal, power supplies, scopes, signal generators. I have a Digelent Analog discovery 2 with a logic analyzer, it is not fast and can only store 4096 samples, but it works.Hopefully this provides some information.

author: MrAl
author_rank: Super Contributor
timestamp: August 31, 2025, 05:01:17 am
content: Quote from: Alex_Baker on August 27, 2025, 03:42:03 pmHi,A lot of modern switching power supplies have a slow start mechanism. This is to prevent a huge startup current that can happen when the control first starts up due to it being at 100 percent when there is no output yet and the output cap is not charged yet. It takes a lot of current to charge up the cap quickly but by ramping it up slowly it takes a lot less current. That helps to protect a lot of the parts in the circuit as well as prevent a huge inrush current.The exact mechanism and exact ramp up time depends highly on the design goal. Some may ramp up faster than others.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 31, 2025, 08:23:35 pm
content: This thread is a little off-topic at this point put I figured I would post this here anyway. I found this thread:https://www.eevblog.com/forum/repair/dallas-ds1486-and-ds1250-replacement-designs/50/Which says the internal display will alternate 6 and 8, which my scope was doing and I forgot to mention it. I decided to cut open the two dallas ICs and attach a new battery.The surgery went well, however now I need to reinstall the scope's firmware. This appears to be best done through the GPIB port using a GPIB to USB adapter. I don't currently have an adapter at the moment and they are very expensive on ebay. I will purchase one eventually but I may be able to borrow one from a local group. For now I am waiting on that before I can proceed.

author: SuperFlyEDSguy
author_rank: Contributor
timestamp: September 01, 2025, 12:40:57 am
content: Quote from: Alex_Baker on August 31, 2025, 08:23:35 pmOr, maybe you could just build one for the price of an Arduino, parallel cable, and a couple of extra items! See below, is this what you’re looking for?!https://sdfa3.org/david/ar488/build.htmlLMK if that will work. It should save you significant money, and it looks pretty easy to throw together.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: September 01, 2025, 04:39:04 am
content: I did look at some of these diy projects, but I don't have a gpib compatible connector laying around. I am also unsure is these devices would work with the old tektronix program used to load firmware.If I find a gpib cable or something I may give it a try anyways.

author: SuperFlyEDSguy
author_rank: Contributor
timestamp: September 01, 2025, 09:15:24 am
content: I would try it if it was me as I’m pretty certain that it would work, however the cable is where you’ll run into an issue if not careful. I was going to send you a link to a parallel cable with a Centronics connector that I found on Amazon for $10. That’s when I realized something,that specific Centronics connector is IEEE 1284, although it looks like the one, it has a 36 pin count on the Centronics-side!SoDON’Tgo with a standard parallel printer cable!GBIP also uses a Centronics connector, however that one would be IEEE 488, and therefore has a 24 pin count!Both Centronics cables appear virtually the same, but it goes without saying that you’ll need the one with the correct pin count for your specific application.So, I subsequently tried using“IEEE 488 Extension cable”as my search term in Amazon and found them starting at around $25 with next day and/or second day shipping! That’s the one you’ll need.I intentionally keep a few extra Arduinos around for purposes like this as well as a couple of extra Raspberry Pi Pico 2 controllers. They make great interfaces in general. You could even debug one controller with a second! So, if you have an extra, I would just put it to good use and try it. You have far more to gain if it works out for you. Besides they’ve come down so much in price overall that you could get one cheap even if you don’t have a spare on hand.I would recommend using a board with the controller that will give you screw down terminals or something along those lines to make the wiring setup easier. I definitely wouldn’t be soldering anything until I was absolutely certain that I got the pins numbered correctly. I’ve done projects where I was trying so hard to not mess up something like this and still ended up doing it backwards, so I’m far more careful now. Using a board with screw down terminals will be a perfect way to go.LMK your thoughts. I hope this helps!