Don't use a bulb in series with the supply though if the supply is a SMPSU - it could burn out. SMPSUs have negative resistance inputs so bringing up the supply slowly can be fatal.
No worries - I wasn't trying to catch you out - you were speaking about transformers and for them its a great way to test. Just SMPSUs are becoming more and more common so I wanted to head off problems before they occurred so to speak 🙂
I'm not sure I get this. If a circuit has negative impedance, then it is better to put it in series with a larger resistance than with a smaller one. It will oscillate when the resistance it is lower than the absolute value of its own negative impedance.
In other words, if it oscillates with the higher resistance of a light bulb, it will also oscillate when connected directly to the mains which have a much lower impedance.
Is it something I'm missing ?
Yeah, you're missing that its not the oscillation that kills the PSU - its the running at under the minimum voltage. At too low a voltage it draws current beyond the rating of the switching element. For more details here's the link where I learned about it -
Low-Line Overstress of Constant-Power Loads
Low-Line Overstress of Constant-Power Loads
Ok, but then it's not the negative impedance of a SMPS that makes it fail when put in series with a light bulb, it's the fact that it draws more current. Even if the SMPS had positive impedance, it could still fail the same way.
What you said about the negative impedance had me worried for a little while, because I might test a small SMPS in the future using the light bulb method.
But now it's clear that this method can be applied provided that the current remains within the specs.
The SMPSU draws higher current at lower voltage - that's what negative resistance means. If it had positive resistance (meaning higher current draw at higher voltage) it wouldn't be an SMPSU - an SMPSU keeps the output power constant while operating within its envelope.
This method can be applied provided you have an undervoltage lockout - but then you'll get oscillation which brings us back to where you came in 🙂
This method can be applied provided you have an undervoltage lockout - but then you'll get oscillation which brings us back to where you came in 🙂
Well it seems it was the multimeter, I borrowed a mates and it is producing the voltages as it should so I'll be hooking it up to the PSU boards next 🙂
And btw I think you all mean negative differential resistance.
And btw I think you all mean negative differential resistance.
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Afternoon chaps, new possible problem. The PSU boards produce +37V at +V with respect to +PG and -37V at -V with respect to -PG. However there is 26.6V at -PG with respect to +PG. Ie, the mnius power ground is running ~27V above the positive power ground. Hence the potential difference between -V and +V is not the 2x 37V intended, but 2x37 - 27.
Btw this is the kit from chipamp.com and the connections I've made are as follows:
0V (black&yellow) to NOT AC1 & NOT AC2 (these have lines above them, the NOT logic symbol)
25V (red) to AC1
-25V (orange) to AC2
Could this problem resolve it's self when the PG's are connected together? (my understanding is that they should be connected together at the ground star) Just don't want to connect them on the off chance and cause a bang... thanks for all the help, I'll get there in the end 🙂
Btw this is the kit from chipamp.com and the connections I've made are as follows:
0V (black&yellow) to NOT AC1 & NOT AC2 (these have lines above them, the NOT logic symbol)
25V (red) to AC1
-25V (orange) to AC2
Could this problem resolve it's self when the PG's are connected together? (my understanding is that they should be connected together at the ground star) Just don't want to connect them on the off chance and cause a bang... thanks for all the help, I'll get there in the end 🙂
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Sounds like you have the secondaries connected anti-phase. Try reversing the connections on just one set of secondary leads, I suspect that will fix the problem.
Mike
Mike
That didn't have any effect on the output voltages 🙁 I did it on one of the boards so I could compare to the second and they are the same.
OK, you reversed just ONE set of secondary leads and nothing changed? Can you post a schematic?
Mike
Mike
OK, took a look at "Chipamp.com" and I think I know what's going on here. It looks like that power supply kit is a dual independent type, basically two separate rectifier/filters, each running off of one transformer secondary. So it won't matter which way the secondaries are connected (in this case, none of the transformer secondary leads should be connected together at any point). Do you have the PG+ and PG- at the power supply outputs connected together? If not, that's where the problem lies. Take a look at the schematic below.
By the way, the first meter you used was blown when you tried to take voltage measurements with it set for ammeter mode, it should always be set for voltage mode when reading voltages.
Mike
By the way, the first meter you used was blown when you tried to take voltage measurements with it set for ammeter mode, it should always be set for voltage mode when reading voltages.
Mike
Cool that worked, cheers. I do have 0.1 v still between the grounds but should this be ok once there are connected. I was thinking it could be some residual ripple and the multimeter is reading it as dc.
You can promote headroom instead of clipping, at the expense of control.
Work Arounds:
sink output noise by Lenard-Elliot Variable Current drive (output)
multiply resistor values by 3 to 5 (sample/feedback)
increase gain to max surge potential (sample/feedback)
the different options available at inverting mode (sample/feedback)
use a rail to rail cap (amp board power circuit filter)
use lightspeed attenuator (decreased dampening at input)
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