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| Power supply with arbitrary voltage waveform and variable series resistance |
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| nihial:
Hi, I'm trying to build a power supply that has the following characteristics : * Arbitrary voltage waveform * Variable series resistor * Highest output voltage 30V * Maximum current 20A For the arbitrary voltage waveform generation, I was thinking about a arbitrary waveform generator driving a power op amp. The variable serie resistor however is giving me problems. The idea is to use a fet as a variable resistor but I have issues with the feedback network. R_mos is found to be : R_mos = (Vcc-V_Ru)/I = (Vcc-V_Ru)/(V_Ru-V_Rl)*R Thus since R is a constant R_mos is a ratio of two voltages, and I don't know how I can translate that into a voltage to compare with a reference voltage that would be an image of the wanted resistance value. Bandwith doesn't need to be very high I think a few kHzcan do the trick. Any ideas are welcome I don't see an analog solution with my current knowledge |
| james_s:
Think of what a series resistor is actually doing, it's limiting the current through the load by dropping the excess voltage. You might look at the schematic for the vector monitors used in some of the old arcade games like Asteroids or Tempest, since beam deflection is proportional to current through the yoke, they use a deflection amplifier that takes its negative feedback from a current sense resistor between the bottom end of the yoke winding and ground. If you put something in the feedback path to vary the gain on this signal it should be possible to control the current limit. Maybe you can provide a few more details on what exactly you're trying to achieve? |
| duak:
I've attached a schematic of a generalized circuit that should do what you want. Please excuse the utter crudity. It doesn't show parts needed for stability or for protection. Ignoring U2 for now, opamp U1 and FET M1 form a classical voltage regulator where the output voltage is defined by the setting of R1 - Vset. My idea is that adding a resistor in series with its output is the same as reducing its output voltage as a function of the output current. This is done by U2, an instrumentation amplifier (INA) that senses the output current and produces a voltage to the Error Amplifier of -Iout x R1 x AV + Vset. The output resistance of the power supply will be R1 x AV and is independant of Vset. Please note that this is a feedback loop that's enmeshed within another one and it may be difficult to stabilize with high values of AV. |
| ivaylo:
The 20A/30V requirement makes it a 600W device. The Kepco BOP series PSUs meet all other requirements but this (including modulating the output). Highest power class is 400W and those weigh 100lbs+... |
| TheUnnamedNewbie:
Does it have to be a linear supply? What bandwidth do you need? It could be possible to do this with a switching supply to make it not need 50 kg of heatsinks. |
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