High current, low frequency function generator output - Page 1
Forum section: electronics / beginners

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: March 29, 2018, 11:12:37 pm
content: Hi,I need to apply a low frequency square or sine wave across a load with an impedance between 1R and 10K at voltages between 1 and 10V and frequencies between 0.5 and 100Hz. I'm able to do that with my proof-of-concept device by using a cheapFeelTech unit gain buffer amplifierdesigned to go with theFeelTech function generatorI use.I'm running into problems at the low impedance end of the range that I need to measure (my real-world loads are salt solutions) and it shows up with a simple test load such as a 10R resistor. My measurement is very simple: acquire the voltage being applied and the voltage output from a uCurrent Gold connected in series in order to measure current. From that I can do a complex impedance analysis. The attached waveforms show the problem (this was for 10V across a 10R resistor.) The negative part of the current (blue) is clearly distorted compared to the source voltage (red.) When I drive this hard enough I get something resembling third harmonic distortion. So, I assume I'm simply running up against the design limits of the power amp.Question 1: does anyone know of an inexpense (<USD150) off-the-shelf unit gain amplifier that can provide up to 3A at up to 10V at low frequency (I only need DC to 100Hz)?Alternatively, I may try building something aroundthis BJT current-buffering circuit. The problem is I can't find a suitable power supply to provide +/-15VDC (or whatever voltage I choose.) This seems quite improbableWhat few I've come across have a minimum current load that exceed what would be my typical loads.Question 2: what options are there for such a simple power supply need. I'm truly stumped that my hours of searching haven't turned up anything and I must be missing something really obvious. I do have a bench top supply but I need something to integrate with the rest of my equipment in a 1U enclosure.Any guidance will be very much appreciated.Thanks.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 30, 2018, 08:29:29 am
content: if you dont care much about distortion, you can make a simple push pull amplfier for it, could worth $10 at most (for low fewer 10's watt demand)...for lower distortion, it will require more components, you maybe can learn from audio amplifier design (google is friend). for power supply you can use two batteries in series, if this is a one-off project. for more permanent use you can build voltage doubler from a simple step down mains transformer or two floating smps in series, another $10-$1000 and beyond depending on wattage rate and design you required. ymmv.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 30, 2018, 08:39:31 am
content: Quote from: JohnnyMalaria on March 29, 2018, 11:12:37 pmYes, you are missing something really obvious.You can usetwo AC-DC bricks.Example:Meanwell IRM-45-15

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: March 30, 2018, 05:36:48 pm
content: Thank you - definitely the obviousI assume that I would just use two of these the same way I use two 9V batteries connected in series for a +/-9V supply - assuming the V- outputs of the bricks are isolated from the mains input?

author: MasterT
author_rank: Frequent Contributor
timestamp: March 30, 2018, 06:32:30 pm
content: 3A is not much current, almost any decent audio amp. IC would satisfy requirements. You may check TDA7293/7294 modules on aliexpress. There is BTL configuration, mostly used for automoto audio equipment, that 'd simplify power supply to single rail +12V. If you do research on ebay/ali you may get complete stereo audio amp. for less than $20. 20 Hz lower end limit easy to extend down to DC.