Probing transient response of a current limiting circuit - Page 1
Forum section: electronics / projects

author: Alex30
author_rank: Regular Contributor
timestamp: September 24, 2014, 12:17:58 pm
content: So I have made a circuit in LTSpice and am building it on my breadboard. The circuit is a power supply that uses the usual method of a shunt resistor and a comparitor to sense the current and then just dump the voltage adjust on the regulator when the comparitor turns on.I was just wondering whats the best way to probe the transient response of the current limit using my analog oscilloscope? I currently do not have a frequency generator BTW. Is it possible to use the probe adj on my oscilloscope (0.5Vpp @ 1kHz) to drive a transistor and do it this way or would this be very bad for the oscilloscope?Cheers

author: Andy Watson
author_rank: Super Contributor
timestamp: September 24, 2014, 12:23:42 pm
content: 0.5V pp is insufficient to turn on a silicon transistor and it probably has very limited current drive too. How about using a 555 driving a chunky mosfet?

author: Alex30
author_rank: Regular Contributor
timestamp: September 24, 2014, 12:54:53 pm
content: Yeah thats true, probably would be more trouble than it's worth and could damage the oscilloscope, just a crazy idea thats all. 555 + mosfet is a good idea. I found thishttp://www.electronicecircuits.com/electronic-circuits/accurate-50hz-oscillator-circuit-using-555-and-7490Might give this a go, looks like a reasonable design.Probably need a mosfet driver though as 16mA is the highest current draw the decade dividers can supply. I wish there was a cheap module I could just buy for this purpose

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 07:59:48 pm
content: There is no difference as far as safety between the compensation output on an oscilloscope and a built in arbitrary waveform generator or function generator. Some older oscilloscopes used a BNC or other connector for the compensation output and included multiple frequency and voltage selections. A BNC is especially useful because then a coaxial BNC to probe tip adapter may be used during compensation for a very clean compensation waveform.The old Tektronix 7904 for instance has a BNC output which produces 4 millivolts, 40 millivolts, 0.4 volts, 4 volts, or 40 volts at DC, 1 kHz, or 1/2 the repetition rate of the B sweep gate which allows one of the timebases to control the output frequency or more complex things to be done.As far as the transient response measurement, you can just AC couple the square wave into the reference level for the current limit feedback loop. A function generator is more convenient though since you may want to look at the difference between small signal and large signal response and run the tests at different operating points.

author: Alex30
author_rank: Regular Contributor
timestamp: September 25, 2014, 03:12:29 am
content: Quote from: David Hess on September 24, 2014, 07:59:48 pmOkay, will keep that in mind. Just realized though that 1khz is too fast anyway. Doing some tests in LTSpice I will need something in the 20-50Hz range so I will still need to build something, going to go with a 555 I think, just need to find the time.Also thanks for the idea of just coupling the wave to the current limit loop, makes more sense as I won't need a mosfet (or the associated driver) with that setup so it will make things easier for me. I would get a function generator but not willing to spend the money atm, especially as all I will likely use it for is probing transient response and if I can get away with a simple pulse generator using a 555 or similar that should just do me for the while

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2014, 11:15:50 am
content: Quote from: Alex30 on September 25, 2014, 03:12:29 amOnce tuned a fast design will get away with a 1 kHz source but I start out in the 50 to 100 Hz range.QuoteLast time I did this it was with a big fixed output regulator, 28 volts at 10 amps, where I was testing the load response and I just used a MOSFET with an array of big power resistors on the output. I drove the MOSFET directly from my function generator through a low value gate resistor.I am not fond of the 555 but have built similar little square wave oscillators with comparators as test sources when needed.

author: dannyf
author_rank: Super Contributor
timestamp: September 25, 2014, 11:21:36 am
content: QuoteAbsolutely yes.QuoteIt can be.