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Hi all
I am trying to build a variable power supply using a transformer I have (out put: 24V, 2A)
I have basic knowledge on how to control the voltage, but I have no idea how to control the current! the transformer output current is 2 Amps, how can I make it variable for example ( from 100mA to 2000mA )
Also I need advice on controlling the voltage.
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Hello, Azhar.
Knowing how to control the voltage output of a power supply, your problem is already almost* solved because the output current can easily be converter into another voltage quantity (that can additionally control the power supply output if it exceeds a certain predetermined or adjustable current threshold) by using a shunt resistor which, according to Ohm's Law, produces a voltage linearly analogous to the electric current that passes through it.
So, you will be having two different voltages to monitor and deal with (by the use of error amplifiers), in order to construct a voltage & current regulated power supply.
-George
[ * ] The real challenge in this endeavor (even for a seasoned designer) will be the frequency compensation of the PSU voltage and current regulation; but this is another subject matter with a life of its own... -
Try this one I designed for constant voltage or constant current and it goes to almost zero out with the 555 timer circuit to shift the min. output voltage that is usually 1.2V.
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LM317 datasheets have some application hints, showing how to to do voltage and current regulators.
For example, here's an example from the LM317 datasheet, which involves a LM301 opamp and a MJ4502 pnp power transistor (which you can replace with something else that's similar, probably TIP36c, mj1501-mj1504 etc )
*Lights in constant current mode
Figure 46. 5A Constant Voltage/Constant Current Regulator
Note the opamp that requires -6v and +6v probably but may tolerate up to 24-30v.
The "linear psu.rar" has the schematic of a very simple linear power supply
There's a transformer with several windings , two separate windings (can be a separate transformer) to power the opamps and other things with +/- 12v , two separate windings or a separate transformer for the multimeters that display the voltage and current on the front panel.
Depending on how much the voltage is set, the opamp triggers relays to couple to link windings of the transformer together so that the voltage on those power transistors is always just a bit more than the output. Those transistors dissipate the voltage difference as heat so it makes sense to use several steps.
Then depending on the current set another opamp limits the voltage and current relying on the sense resistor.
The last two designs attached are two commercial power supplies which follow the same principles
- a transformer with multiple windings which are linked together as needed using relays based on output voltage and current desired
- a transformer or separate transformer windings for +12 (for the relays) and +8v , -8v , +5 and -5v for opams and other integrated circuits used
- opamps that use the voltage drop on a current sense resistor and the current set by user through potentiometer or keyboard buttons to adjust and limit current/voltage
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Sorry for rebooting an old thread. But is there changes that can be made to make the current adjust pot a more common value. Wanted to use a 10 turn and a 250k is over $100. Thanks.
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make the current adjust pot a more common value.
as the potentiometer is being used as a resistor divider, you can swap it out for a 220K 10 turn pot, to keep the ratio the same would mean replacing the 330K resistor on the same net to a 290.4K resistor (or 270K + 22K for something close enough)