Let's say I've got a transformer with independent dual 120V primaries (to allow for for 240V operation) and a 300V secondary. What would be the considerations for wiring the unused primary in series with the secondary to achieve 420V?
Obviously there's a concern that the insulation might not withstand the higher voltage.
What happens to the current rating? It seems to me that it would be unchanged, right? The existing secondary is already capable of handling that current, and the added primary would ordinarily be seeing *more* current than that if it were wired as half of a 240V primary.
Or am I way off base here?
Obviously there's a concern that the insulation might not withstand the higher voltage.
What happens to the current rating? It seems to me that it would be unchanged, right? The existing secondary is already capable of handling that current, and the added primary would ordinarily be seeing *more* current than that if it were wired as half of a 240V primary.
Or am I way off base here?
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Okay, instead of a generic theoretical, let me just describe the actual use case. I'd like to get 400V at around 195mA maximum load. I've got the transformer pictured below - you'll note there's no 5V secondary, so I'll be using solid state rectification.
I get 273VAC unloaded with an estimated 58R source resistance. After a CLC filter PSUD2 says that's about 325VDC @ 195mA.
If I connect the unused 115V primary to the 243V secondary, I get 395VAC unloaded, with an estimated 102R source resistance. With the same 195mA load, PSUD2 says a CLC filter yields 460VDC, and an LC yields 360VDC. With judicious choice of a small input cap, I can fine-tune that voltage to 400.
But at what cost/risk? Shorting through the insulation is a possibility, since the secondary now peaks at 533V, almost 150% of the 357V PWV rating. I can fuse both primary and secondary to protect from that. I'm well below the 325mA current rating, but does that change with this topology?
I get 273VAC unloaded with an estimated 58R source resistance. After a CLC filter PSUD2 says that's about 325VDC @ 195mA.
If I connect the unused 115V primary to the 243V secondary, I get 395VAC unloaded, with an estimated 102R source resistance. With the same 195mA load, PSUD2 says a CLC filter yields 460VDC, and an LC yields 360VDC. With judicious choice of a small input cap, I can fine-tune that voltage to 400.
But at what cost/risk? Shorting through the insulation is a possibility, since the secondary now peaks at 533V, almost 150% of the 357V PWV rating. I can fuse both primary and secondary to protect from that. I'm well below the 325mA current rating, but does that change with this topology?
Recommend mods shut you down as this is unsafe. You are bypassing the isolation of the transformer, and relying on wire enameling to isolate primary from secondary. Unsafe.
If I am picking you up right ?-----the voltage boosting aspect of the transformer will not function as the secondary discharges an induced voltage relying on the collapsing magnetic field --but maybe somebody can "correct me " here?
It works, but is unsafe, you are essentially bypassing the isolation provided by separate bobbins and relying on wire enamel for mains isolation.
Got it, thank you. Won't do it.
Is this different than bucking the primary with an unused 5V or 6.3V winding? Or is that just as dangerous? I see people talking about that all the time.
Is this different than bucking the primary with an unused 5V or 6.3V winding? Or is that just as dangerous? I see people talking about that all the time.
Bucking or boosting is safe, if you look at the schematics carefully you will see isolation is not compromised.
OH NO NO. You caught me, I read your post too quickly. Bucking or boosting with a SEPARATE transformer is OK, bucking or boosting the primary with an usused secondary is unsafe, it is exactly the same issue. If people are discussing that "all the time" on this forum, then it should be dealt with by the mods and concerned members.
Okay, totally understood. So from a safety perspective, the basic rule is that it's never okay to connect windings between the primary and secondary sides, is that right?
This is the same reason why a variac isn't a safe isolation device, right?
This is the same reason why a variac isn't a safe isolation device, right?
Yes, insulation from primary to secondary is not the same as between windings. For an EI the primaries and secondaries are typically on different bobbins. For a toroid, there is typically a layer of insulating wrap between them.