voltage divider for a power source

[vaughandy]

Hi,

Probably a stupid question, but I was wondering if it's wise to make a voltage divider out of high-watt rated power resistors in order to provide 175V from a 220V source. I've ordered a new transformer with a 175V secondary, but it's backordered so I'm wondering if I could use something like this in the meantime.

[Enzo]

That would not be the way. Use a variac to dial down your excess voltage source while you wait for the real part.

[vaughandy]

That would not be the way. Use a variac to dial down your excess voltage source while you wait for the real part.

figured. but what exactly is the danger?

[R.G.]

figured. but what exactly is the danger?

It's not a danger exactly, but it is impractical and could be dangerous depending on how you do it.

The problem is - how much current do you need? If the current is trivially small, then doing this is probably OK. If you're trying to do more than a few milliamperes from the "175" source, it's a power nightmare. The power resistors eat a huge amount of power compared to what they provide if you get any sort of consistent voltage at the "175V" output.

[vaughandy]

Hmm, i see.

Unfortunately I don't have a variac, but I do have an adjustable
DC power supply I can set to whatever. If I hook that up with the right polarity to
The circuit that wants to see 175v AC, should I expect that to work?

[pdf64]

Can you provide more detail on what this is - it's all but impossible to give an accurate answer from this info?
On the face of it, you can't get 175 Vac out of a DC supply (without complex converters).
Also, is the 220Vac your line supply (ie from all outlet)?Pete.

[km6xz]

I am pretty sure Vaughandy meant connecting the variable DC supply to the circuit that needs to be powered, not the main input.
That would work fine.
Just connect it on the output of the rectifier so the transformer is isolated by the rectifiers.
As a design prototyping tool and a bench source I used(it is back home in storage, darn it) an old tube regulated Lambda 72a variable supply that had metered(current and voltage) for B+ to 500V, Screen, bias and two heater supplies, 6.3 and 5vac. It made prototyping a new amp or analog processor a snap. It also worked on the bench as a known good, breaker protected and metered patch-in replacement for the a damaged power supply section, that allowed getting the rest of the unit working while waiting for power supply or power transformer to arrive. Small variacs are cheap surplus, 1-2 amp range so building up a couple supplies that are isolated, one that could be for bias voltage ranges, another for screens and a anode supply would be a great experimenter's tool. It would not have to be regulated, just metered on each leg.

[vaughandy]

I am pretty sure Vaughandy meant connecting the variable DC supply to the circuit that needs to be powered, not the main input.
That would work fine.
Just connect it on the output of the rectifier so the transformer is isolated by the rectifiers.
As a design prototyping tool and a bench source I used(it is back home in storage, darn it) an old tube regulated Lambda 72a variable supply that had metered(current and voltage) for B+ to 500V, Screen, bias and two heater supplies, 6.3 and 5vac. It made prototyping a new amp or analog processor a snap. It also worked on the bench as a known good, breaker protected and metered patch-in replacement for the a damaged power supply section, that allowed getting the rest of the unit working while waiting for power supply or power transformer to arrive. Small variacs are cheap surplus, 1-2 amp range so building up a couple supplies that are isolated, one that could be for bias voltage ranges, another for screens and a anode supply would be a great experimenter's tool. It would not have to be regulated, just metered on each leg.

thanks km! I'll give that a try tonight.

Yeah, I should definitely look into a variac. I'm pretty new to all this but am learning quickly and imagine I'll working on this stuff in the future. I'm actually a molecular biologist so I have easy access to the variable DC power supply, hence why I wanted to know if that would work.

thanks again!

[vaughandy]

thanks km! I'll give that a try tonight.

Yeah, I should definitely look into a variac. I'm pretty new to all this but am learning quickly and imagine I'll working on this stuff in the future. I'm actually a molecular biologist so I have easy access to the variable DC power supply, hence why I wanted to know if that would work.

thanks again!

As I think about it, will I need to set the DC voltage higher than 175? The circuit is designed to rectify 175V AC into DC, but isn't the DC voltage usually higher once rectified? I'm starting with DC and running it into a circuit with rectifers, which should essentially do nothing given that it's already DC... how should I be thinking about this?