When building a variable power supply, where is the ideal place to put the variac? before or after the power transformer?
Thanks in advance.
Thanks in advance.
If it's a switching supply it's best to not use a variac at all.
If it's 'linear', before the transformer.
If it's 'linear', before the transformer.
I'd suggest variac before transformer. Variac will then see nominal input Vac, and provide indicated Vac, which can be above mains Vac. Your Vdc will move about with mains Vac fluctuations. You can start with variac output at 0Vac, to minimise inrush of transformer and cap filter, and can bring load up whilst checking all is ok - so especially good for fault-finding and initial checking of operation at lowered stress, and forming e-caps, etc. Lot's of bench-top uses. But need to be aware of max current rating of variac output, and not exceed that.
If the power transformer is placed before the variac, isn't the secondary winding of the power transformer isolated from the mains?🤔
Variac Transformer 0-250V 110V Input
I think this is depends on the maximum output voltage of the variac and the input and output voltage ratings of the power transformer.
Variac Transformer 0-250V 110V Input
I think this is depends on the maximum output voltage of the variac and the input and output voltage ratings of the power transformer.
I think thimios was was more about reminding OP that the output of the variac is not mains isolated (such as when being followed by a transformer), even though the rectifier and DC output would be isolated (assuming the power transformer was safety compliant, and was installed in a safe manner such as with a protective earth connection). An electric shock is still a risk no matter what, so the OP should really be aware of safety concerns, and not pursue this activity without experience and oversight from a competent person imho.
when testing the variable power supply, is it necessary to put a load on the circuit? (You can assume that I have a bleeder resistor on the last cap). And if so, without it, will there be any damage anywhere?
I ask only because on preliminary low voltage tests without any load, I watch the voltage go up (normally) as well as the current! This doesn't make sense to me except that without a load, the circuit is operating under an infinite load. Either that or I mis wired it.
Any ideas?
Any ideas?
No, it is operating with a zero load, which is an infinite impedance. They are inverses.
If the current is going up there is either a load or a short circuit.
I am using this schematic which is easily available everywhere. The only difference is that I am using two chokes instead of the 7805s. And while I would readily admit that I make mistakes in my wiring, I can say that having checked my wiring 4 times, I am pretty sure that it is wired up correctly. I also changed the bridge rectifier 2x. Other than than that, I cannot understand where/why there is current draw without a load. It is very frustrating.
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