DC PS with strict current limiting on short circuit, DIY schematic - Page 1
Forum section: electronics / testgear

author: electr_peter
author_rank: Unknown
timestamp: December 11, 2014, 10:00:40 pm
content: I noticed that my PS (SMPS type with CC/CV modes) has weak current limiting when load is changing to almost short circuit (current severely overshoots for a brief moment). Overshoot occurs mostly due to capacitance across output and possibly due to limited loop speed.I am looking for low power supply:6V or more, regulation to 0V, CV mode1A or more, regulation to 0A, CC mode6W or morestrict current limiting on short circuit condition (max +20%)other features - not important. OV/OC and sense wires would be nice, but not necessarycheapDo you know of such new/used DC PS?Am I better off with DIY approach?

author: babysitter
author_rank: Frequent Contributor
timestamp: December 12, 2014, 10:21:00 am
content: Hi, look at explosion-protected power supplies if you want to go big. My grandcousin worked at a factory of those and was impressed that even using a file to the contacts to generate a nasty connect-disconnect scheme, there was not a single spark or spike on the connected o-scope.To go small, have a look at Bob Peases productions, he has a current-limited, output-capacitor-less PSU but only specced to 300mA in "Analog troubleshooting".

author: electr_peter
author_rank: Unknown
timestamp: December 12, 2014, 05:54:12 pm
content: Quote from: babysitter on December 12, 2014, 10:21:00 amThese seem to be specialised pieces of equipment used either for explosion work or for work near explosion site. Very expensive, hard to source and not really that useful as general adjustable DC PS. No sparking though. At least I have not found adjustable type yet.Quote from: babysitter on December 12, 2014, 10:21:00 amI found that circuit. It employs zener reference, LM741, two transistors on output and some resistors for biasing. Only two caps are in circuit, they are not directly connected to output.However, current/voltage does not go to 0 and current is not that easy to adjust. Maybe some modifications could help.I will search for small power DIY PS designs then - maybe something adequate will pop up.

author: macboy
author_rank: Super Contributor
timestamp: December 12, 2014, 07:40:23 pm
content: The current spike is the fault of the output capacitor. To reduce the spike, you can try to add series resistance to the capacitor. If you want 6 V and current spike hard limited to 1.2 A, then you need to add 5 ohm of series resistance. The power supply will likely still work fine with this series resistor, although load regulation (response to changes in load resistance) will suffer. Do not simply remove the output capacitor; that will almost certainly make the power supply unstable.

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2014, 07:42:50 pm
content: I have been thinking about easy to adjust discrete current limit circuits ... could something like this be made to work? The control loop for CV mode not shown, base voltage for Q5 sets the current limit and it's going to need some capacitance to make it not oscillate at the very least.

author: electr_peter
author_rank: Unknown
timestamp: December 12, 2014, 08:00:13 pm
content: Quote from: macboy on December 12, 2014, 07:40:23 pmIf supply is set to 6V and 1A, then short 1.2A burst on short circuit would not be that bad. However, if supply is set to 6V and 10mA, overshoot is massive (I would like max + 20% - 12mA in this case).5 Ohm would not help too much, as cap is charged to 6V. 6V/5 Ohm = 1.2A burst in any case independent of current limit.Quote from: macboy on December 12, 2014, 07:40:23 pmI would not try that as it would probably drive SMPS out of balance.I do not recall exactly, but somewhere I read that with some HP/Agilent bench supplies you can set 20-30V, 10-20mA and connect the LED across the output. The LED does not explode because current is regulated very well.On a similar note, I read that it is not recommended to connect LED across 20-30V with 10mA on typical DC PS if you do not like sharp glass bullets flying around.

author: AG6QR
author_rank: Frequent Contributor
timestamp: December 12, 2014, 08:34:23 pm
content: Quote from: electr_peter on December 12, 2014, 08:00:13 pmI've also blown my multimeter's 400mA fuse, when testing the current draw of a newly built circuit that should have required 12V and about 300 mA. I thought I was smart to set the power supply current limit to 350mA, which should have protected my meter's fuse. But I neglected to account for the fact that my circuit had some capacitance on its input, and my power supply had some capacitance on its output. When I connected them through my meter, the brief inrush wasn't stopped by the power supply's current limiting circuitry, but was instead stopped by my meter's HRC fuse. Oops.One strategy to use to avoid brief current spikes at startup is to first short the power supply out, thus putting it in constant current mode. Then attach the load in parallel to the short, and finally power up the load by removing the short. The supply should be current limited from the very beginning, starting at 0 volts, and will only go into constant voltage mode if and when the voltage exceeds its limit.

author: electr_peter
author_rank: Unknown
timestamp: December 12, 2014, 10:06:00 pm
content: I found a video about HP E3632A and current limiting. It looks pretty goodDoes anyone now cheaper, lower power DC PS options with superb current limiting?

author: Marco
author_rank: Super Contributor
timestamp: December 13, 2014, 02:45:34 pm
content: Another option is using a buck pre-regulator with current limit and a opamp+nmos linear regulator. Load the buck down a bit with a resistor and a TVS (which will be the only capacitance for your output). Bucks inherently are rather good at CC.

author: electr_peter
author_rank: Unknown
timestamp: December 13, 2014, 03:51:07 pm
content: Quote from: Marco on December 13, 2014, 02:45:34 pmWould circuit like this (LT note, page 3, figure 5) work? I would like 0-1A, 0-6V regulation.I think it may work. I just don't see an obvious way to insert current regulator/limiter between buck and linear reg at this moment. Pre-regulator should take into account voltage drops from current and voltage regulators.

author: electr_peter
author_rank: Unknown
timestamp: December 26, 2014, 02:31:46 pm
content: I am designing a DIY PSU with low output capacitance and good current regulation as good name brands are a bit expensive.So I am drawing preliminary schematic for PSU with low output capacitance. See attached image for general circuit idea.@Marco suggested to use op-amp and N-MOSFET transistor for lower output capacitance requirement.Power transition sequence:Mains, switch, mains filter, fuse, transformer, diode bridge, fuse, filter->buck pre-regulator with N-MOSFET and comparator, N-MOSFET and voltage/current loop with zener separator->current source for a load, TVS and simple diode for protection, remote sense wire connection, output switchCurrent and voltage loops are connected via zener diodes. Output voltage is read at remote sense terminals.This circuit is a draft with no values fixed at the moment, but I am aiming for 0-12V 0-2A output (or lower). I would like to have little current overshoot during short circuit situation, so I added no capacitance at first on the output.I have few questions about PSU that forum members may have some answers to:I chose to combine current and voltage loops with a single transistor. Another approach would be to use two MOSFETS for current and voltage control in cascade. Is this combined approach with one MOSFET and two zeners reasonable?Could such circuit be stable without a capacitor on output at all? Without testing it is hard to say, but in principle? How much capacitance I could need - 10uF, 1uF, 0.1uF, 0.01uF?Is there significant differences between bipolar transistor and N-MOSFET for main power regulator? Would one or the other be more stable with no or very little capacitance?Could buck pre-regulator start on first power on? Comparator non-inverting/inverting pins are at the same potential during initial start. Any tricks to make it start?Anything else strange about this circuit?

author: electr_peter
author_rank: Unknown
timestamp: December 29, 2014, 09:03:06 am
content: I cleaned up schematic a bit.Remote sense wiring needs some attention.

author: timb
author_rank: Super Contributor
timestamp: December 29, 2014, 10:45:48 am
content: Couldn't you just use a low value series current sense resistor in series with the output terminal, which is hooked to a biased SCR? The biasing of the SCR determines your current clamp set point (which could be, say, 5% over the CC set point). The SCR is placed across the output terminals, acting as a crowbar to catch the current spike. Then, once the constant current circuitry catches up, a comparator (or suitable circuit) will disconnect the SCR (by toggling an inline FET or something).This is similar to how some old 60's/70's precision lab supplies did it.Sent from my Tablet

author: electr_peter
author_rank: Unknown
timestamp: December 29, 2014, 11:23:28 am
content: Quote from: timb on December 29, 2014, 10:45:48 amI need to look into SCR and find a better solution for sense and remote wires with design above. Does SCR work at low voltages (0.1-2V) and high currents (2-3A)?My PSU will have +/-9V rails, so biasing should not be a problem.Thinking about it, fuse near the output terminals (as weel as big diode) with SCR should be included just in case for protection.

author: timb
author_rank: Super Contributor
timestamp: December 29, 2014, 11:51:12 am
content: They can normally handle from an amp to hundreds of amps depending on the model. Essentially, once you've applied enough voltage to the gate to trip an SCR, it will stay tripped until the gate voltage is removed *and* the current across it is removed (like a triac). The exception to that is with special types of "gate turn off" SCRs that allow you to switch them off with a negative pulse.So yes, so long as the current is above the holding point, the SCR will latch irregardless of the current. You just need to pick one with a suitable holding point for your needs.In all the supplies I've ever designed, I always implement a diode, fuse and overvoltage crowbar SCR (set for 110% of the supply voltage if it's non adjustable). If something goes wrong and the crowbar activates, the fuse will blow if the current limiting doesn't kick in.Implementing a simple adjustable voltage crowbar alone has saved many a fried board!Sent from my Tablet

author: nowlan
author_rank: Frequent Contributor
timestamp: December 29, 2014, 12:12:59 pm
content: Might be worth reading if you havent already.DC POWER SUPPLY HANDBOOKApplication Note 90Bhttp://cp.literature.agilent.com/litweb/pdf/5952-4020.pdf

author: electr_peter
author_rank: Unknown
timestamp: January 12, 2015, 09:11:04 pm
content: I updated schematic for PSU. As mentioned, PSU will have very little output capacitance at first to see if it works. SCR is not included as I not see absolute necessity to it at this point.Sense wire INT position is meant to directly connect internal voltage sense to output and connect +S and -S to output via very high resistance.Sense wire EXT position is meant to connect internal voltage sense to output via low resistance and provide zero resistance +S and -S option.Some protection is still lacking for sense circuitry against wrong connections.If you have ideas how to do things better, please share.Next step - finalise component values.