# DC Variable power supply

I would like some advice on a particular circuit design for a variable DC power supply.

Here is some information about what I want to do:

I want to do open and short circuit tests on an alternator to find it's internal impedance (Note this is a pure alternator with no regulator/rectifier. Similar to an alternator used on a 3 phase generator and NOT a vehicle). This involves me increasing the output voltage from a DC power supply across the rotor (which is just a copper coil with fixed dc resistance, so increasing DC voltage increases current and increases magnetic field strength) and then measuring the AC generator emf/currents (Also note I don't have access to the alternator yet so don't know what rotor field voltage/current is required).

I plan on using the 240v mains which will be rectified to give my DC power supply. I will start with being able to supply 2 amps at the DC output to start with for my tests and I intend to use the full AC peak voltage of 320V, so around 300vdc. I have used a 1:1 isolating transformer in case I need to probe my oscilloscope in the circuit. My questions are based on this and are listed below:

1. If I use a 240vac variac to supply my full bridge rectifier, I can simply vary the AC peak voltage and hence vary the DC output voltage. This will supply a constant voltage to the load regardless of the current drawn, as the AC current can increase and should work very well?

1. Is it better to remove the variac from the circuit and rectify the full AC peak voltage and use a variable resistor (hand potentiometer) on the DC side to vary the voltage? There will be a big power dissipation through the pot is the obvious disadvantage.

1. Or can I replace the series potentiometer in the circuit with a voltage regulator IC and use a potentiometer to adjust the output voltage? I doubt I will get IC to work from 0 - 300Vdc say for example?

• Any one of those may work. There isn't a linear regulator IC that'll do the job, but a circuit could be designed. The variac + rectifier is pretty old school. It will be big and clunky, there will be some ripple unless you use more filtering (or an absurdly large capacitance), and the output won't be truly regulated. But if you have the variac, and you can live with the downsides, it'll certainly be the easiest and most direct way to get what you're looking for. – TimWescott Dec 26 '18 at 21:22
• And the obligatory safety warning: read up on safety around high voltages, and don't kill yourself, please. – TimWescott Dec 26 '18 at 21:23
• And the obligatory safety warning: read up on safety around high voltages, and don't kill yourself, please. – TimWescott Dec 26 '18 at 21:23
• Your variac diagram is the best of your stated options – Autistic Dec 26 '18 at 22:13
• @TimWescott Thanks Tim. I think the variac may produce good enough results for my application. Don't worry, I will be careful, thanks. – David777 Dec 26 '18 at 22:57