(sorry for the wall of text, I tried to give as much Info as possible)
I am pretty new to electronics and am working on a project regarding controlling the flow of a pump (BLDC based). End goal: Apply a sinusoidal supply around a variable DC offset with variable amplitude (at least +-4V) and frequency (0-10 Hz, low Freq). One big hurdle is definetly the pump... The manufacturer only provides a very short explaination of the electrical aspects of the pump. Unfortunately, I HAVE to use this model.. Datasheet (I am using M400-S180): https://docs.rs-online.com/9836/0900766b8162656f.pdf. Currently I am using the XR2206 kit for the modulated sine wave. It has a DC Bias of 6V with an Amplitude of +-4V (8Vpp). But it only gives out 20 mA which is not enough to power the pump (I measured the internal Resistance of the pump: 2V - 0,09 A - 22 Ohms, so I guess it needs at least 0,1 A to start). So I wanted to use an bjt common collector amplifier (PNP) to boost the current. I watched some videos and tried to calculate the right resistor values. Basically I assumed Beta=10 and wanted Ie=200mA so Ib=20mA.I am not sure how to calculate Resistances with AC so I assumed to use Vrms? Vmax=10V -> Vrms=7V, R1=7V/20mA = 350 Ohms (used 330). R2=7V/200mA= 35 Ohms. At the time I did not have 35 Ohms at hand so I used the next smallest, 17.5 Ohms so double the current.
It worked okayish. The pump started and amp/freq modulation was possible via the XR2206. But the max Vpp changed from 8 Vpp (+-4V) to 3 Vpp (+-1.5V) and also the 17,5 Ohm resistor got pretty hot because of the large current. Also, the Vrms of the Output was only 5V. I am not quite sure why the amp dropped but maybe its because of the difference in output impedance of circuit and input impedance of the pump? (I am honestly a quite a bit confused by impedances so this could be complete rubbish). Also the circuit was not so good designed as I found out through more videos.
I now tried on using a two stage amplifier: Common Emitter (NPN 2N3904) + Common Collector (PNP BC640). I used a better more stable circuit and calculated the Common Emitter circuit values. C1=10uF (AC coupling, I tested with an Osci so no calculation here). According to the tutorials I_R1 = I_R2 >>I_B and I_C = I_E. I wanted V_E = 0.6V. V_B = V_BE+V_E = 1.2V. I_E = 50mA and Beta = 100 according to the Datasheet of the NPN, so I_B = I_E/100. I_R1 should be some 50 times higher than I_B (so we have a stable amplification), I_R1 = 0,012A. Lastly R1 = 1000Ohms, R2 = 100Ohms. V_E=1/2 Supply = 600mV, and with I_E = 50mA; R_E = 10Ohms.
Vpp_out=10V from Vpp_in=8V. I could not test it with the pump because the current was not amplified yet. So I now tried combining both amplifier with a new Common Collector circuit. Spoilers: It did not work. So I left the Common Emitter as was and said I wanted I_E2 to be at least 120mA. R7 = 12V/120mA = 100Ohms. I_B2 = I_E2/100 = 1,2mA. I_R3 = I_B2*50 = 60mA. R_3+R_4 = 12v/60mA = 200Ohms. Oh.. pretty low, so I settled for I_R3 = 48mA, R3+R4 = 250Ohms so R3 = 220 Ohms R4 = 47 Ohms.
Well as I already teasered it did not work. Amp Modulation was reduced to +-1V (2Vpp). The pump did turn on but the whole circuit drew more than the 120mA calculated, It was around 300mA. Since then I have tried numerous simulations on LTSpice so I dont have to build up/down circuits every 10 mins but until now no success. Can anyone suggest me a direction where I can head to or point out the flaws in my circuit/calc? Does anyone have a clue why the amplitude of the sine wave dropped? Also: Sorry for the Variable-formatting, pretty messy!