Variable Inrush Current Limiting by Using Electronic Load in Series? - Page 1
Forum section: electronics / beginners

author: byte
author_rank: Contributor
timestamp: December 22, 2021, 03:02:37 pm
content: Hello,I know there was a similarly posted question a while ago, but it didn't answer my question:https://www.eevblog.com/forum/beginners/electronic-load-as-current-limiter/I'm working with various DC voltages on my bench while troubleshooting faulty equipment for repair. I was checking if a relay was mechanically working by using test probes connected to my bench power supply. I calculated the voltage based on the resistance and current-draw from the relay's datasheet. I set the current and OCP limits to the datasheet current draw, but like an idiot I forgot that there are output caps on the power supply (and that the OCP on my supply is software based and not instantaneous). When I touched the relay with the probes, it exploded the flyback diode for the relay, probably because I wasn't paying close enough attention and had forward biased the diode (diode was rated for 60v, and relay was 24v).If I had simply added a series resistor of the right value I could have prevented exploding the flyback diode (was a 1A diode, but the relay only needed 30ma to which my power supply was set). So this got me thinking, I should always use series resisters for this type of testing if I want to truly prevent overcurrent given the caps on the output stage of my bench supply. I'm wondering if there is an easy way to achieve this without having to mess around with loose resistors all the time.A couple ideas came to mind:1. Look for some kind of banana plug adapter that I can swap resistors. Still a pain with loose resistors but simplifies the setup. Not sure if such a thing exists.2. Use one of those decade resistance boxes I've seen on people's benches (not sure what they are called). Problem is I am unsure if they are meant to dissipate higher wattages - I probably only need 5w max3. I'm getting a Matrix programmable load for xmas that has a constant resistance mode. From what I have seen of programmable loads, they can react very fast (instantaneously?) to changing voltages and currents - but I'm unsure if it will be fast enough. I can certainly test once I have it in my hands. Pros for this is the easy setup, higher wattages, but downside is it may not work for inrush, and won't work with AC (not that I need that often)All of these have one downside I see, is that I won't always get the voltage I expect to the device because of the voltage drop across the resistor/load. So some extra calculation steps are required to ensure I get the voltage I want.I suppose the more general question is: how do the experts handle this kind of need (precise voltage with a precise current limit)? Do you have a better power supply than I do with no caps on the output stage? Or maybe you just do the extra work of plugging in loose resistors and doing the math to make sure you don't deliver too much current to devices under test?Thanks!

author: ledtester
author_rank: Super Contributor
timestamp: December 22, 2021, 04:46:49 pm
content: How about a simple constant-current circuit? Design it so that it will never supply more than, say, 100 mA. I imagine that should be enough to activate most relays.Here's one based on an LM317:http://bristolwatch.com/ccs/LM317.htmThe resistor R1 can be a pot with a resistor in series so the resistance has a minimum value -- that will set the maximum amount of current.Or... just have a bunch of fixed resistors you can switch between them a jumper.The LM317 can handle input voltages up to 37V so you could just power it from a fixed 30V supply, for instance.

author: byte
author_rank: Contributor
timestamp: December 22, 2021, 05:52:32 pm
content: Quote from: ledtester on December 22, 2021, 04:46:49 pmIs this what the professionals do? Seems redundant when I already have a constant current power supply. Obviously, there would be no cap on the LM317 output so I suppose that achieves the goal. But why don't bench supplies already do this?Btw, I'm not looking to test only relays. Could be testing any device/component under power like a relay, an led, a diode, or just plain protecting against a dead-short on when applying power.Quote from: ledtester on December 22, 2021, 04:46:49 pmI'm working with up to 48v DC in some cases repairing a broken UPS.I had another thought too... I suppose the simplest option may be to just change my testing procedure. If I disable the output of my power supply, let the caps discharge, then apply the test leads before enabling the power supply that might be enough to prevent the caps from charging and delivering a huge inrush current compared to enabling the output before applying the test leads. It's just a slower and more clumsy without 3 hands when testing several relays.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 22, 2021, 06:17:09 pm
content: I have never managed to have this issue.Maybe your particular PSU has totally excessive amount of output capacitance?With HP 6632B, I have never succeeded in destroying a diode or an LED.I am careful not to set the voltage to some ridiculously high maximum value, turn output on and then probe on things, though. If I need to probe with output already set on, I set the voltage as close as possible to the expected voltage drop over the devices. Sometimes use a series resistor.If this was a trivial problem to solve, the power supplies would not have output capacitance in the first place. Adding a secondary layer of current limiting circuitry only duplicates what's already inside the supply, and if you then lack the output capacitance, you would have voltage regulation issues, in worst case oscillating output; in best case, just poor transient response.

author: ledtester
author_rank: Super Contributor
timestamp: December 22, 2021, 07:54:51 pm
content: Quote from: byte on December 22, 2021, 05:52:32 pmfwiw, there is a high-voltage version of the LM317:https://www.ti.com/lit/ds/symlink/lm317hv.pdf

author: byte
author_rank: Contributor
timestamp: December 22, 2021, 08:01:29 pm
content: Quote from: Siwastaja on December 22, 2021, 06:17:09 pmI suspect my cheap power supplies just have a lot of capacitance to try to make up for noise/ripple. Though I have both a linear and switching power supply on hand, this particular incident happened on the switching one (Riden RD6012). I should test my (also cheap) linear power supply for the same problem. But as you implied, I assume all power supplies are going to have some degree of capacitance on the output stage.Quote from: Siwastaja on December 22, 2021, 06:17:09 pmYou probably didn't mean it this way, but I didn't think 24v was ridiculously high for a 24v relay or a 60v diode. It's the lack of instantaneous current limiting that was the problem.Quote from: Siwastaja on December 22, 2021, 06:17:09 pmThanks, this is what I was thinking too - but you articulated it much better than me.So it sounding like a series load is the way to go. But this brings me back to my initial question, can I use my electronic load constant current mode to do that?Bonus question: I'll want to test any assumption I make - but what is the best way to measure fast current transients? Are oscilloscope current clamps or current probes fast enough for something like this?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 23, 2021, 09:41:39 am
content: Quote from: byte on December 22, 2021, 08:01:29 pmYeah, well, the 60V for a diode is the reverse breakdown value. In forward direction, it's a <1V device, so there is 23V "in excess". The point is, charge delivered to the load is linearly dependent on the capacitance (which you can't affect except by replacing the supply with something else or modifying it), and voltage difference (here, 23V) - this you can control. Also, peak current is I = U/R where R is the ESR of the output cap, any wiring, connectors, etc. Here again avoiding excess U is the key.So in a nutshell, do not connect an output enabled power supply, set to more than a few volts, into a diode in forward direction. I know, we make mistakes. My point is, it might be easier to just go on and avoid the same mistake (and replace the diode) instead of trying to engineer a better/faster current limiting circuit.Hotplugging a live DC source to a circuit is a real problem (two points: inrush current, and voltage peaking due to LC ringing); I have had that sneak in one of my designs only to be caught when already a few units delivered to customers! Actually, a lab supply with more output capacitance would have helped me to find the problem earlier.It's a good use pattern to connect the test leads, then use the "output enable" function of the PSU to produce controlled voltage rise.