Help on power supply

bowdown

Member
2004-02-10 5:59 am
Perth
Hi Guys just wandering if someone can help me with this situation.

I have a 2KVA toroidal that produces 71 V AC without a load which is about +/- 100V DC but i can only get 100V caps so i wanted to try and drop the voltage to +/-90V DC or maybe even lower. How can i go about this and does someone have a schematic to do it?? PLease please please this thing cost me a fortune, I asked for 65VAC but they said its more because they have almost 10% regulation on it.:bawling:

Please help

Cheers

Bowdown
 

sek

Member
2003-05-21 2:51 pm
Berlin
Hi,

do you know how a (toroidal) transformer works and how it is constructed?

It is possible to achieve your goal without wasting power (heat) or paying complicated electronics. Unwinding some rounds of the secondaries, that is. But you'd have to be sure that you know what you are doing. ;)

Of course, you could also regulate the voltage down a bit, but at 2kVA this won't work without a switching regulator. And then you could have bought an expesnive switching supply in the first place...

So, the forum can surely explain what to do, but I first want to be sure you know what you're doing. Because doing it wrong would deliver some hundres volts at some hundred amperes (peak). :hot:

Cheers,
Sebastian.
 
Put a small load on it and see what the DC drops to. Often a few extra no-load volts can be due to garbage on the mains charging up the caps to a potential that's higher than the actual peak of the mains sinewave.
As far as cap ratings go, it really is best to keep within ratings. Measure your mains at different times of the day to determine the maximum value and decide whether your caps are acceptably rated.
If the caps are NG, consider using 63v caps in series.
 
Reducing voltage.

Bowdown, I have had this situation before, the easiest way to lower the voltage is to wind several turns of wire in buck (reverse to existing windings) This seems a little wastefull but it is easy to do and does not add any appreciable load to the transformer.
This also saves you from having to unwrap and unwind the item. Regards, Steve
 
Hey thanx guys for all the replies so quick.

Ok i dont know how to unwind the toroidal and its core is full of resin for a mounting screw, so i dont think i can add or remove any windings. I think i might just have to try some sort of regulated power supply, not sure how to design one though, i have had heaps of experience with raw power supplies but thats really all.

cheers

Bowdown
 
Bowdown... Wait a second....

In your first post you mention that your supplier gave you a higher voltage rating because of the regulation.... Maybe he was right

I can't read your mind, but if i'm right, your 100V is based on the calculation of 71VAC * 1.4 = 100V right? The basic square root of two rule.

Now, the AC rated voltages for trannies are usually given under full AC load. When you apply rectification, you would tend to believe that you are dealing with peak voltage. This is only true in an unloaded situation.

For a DC load no higher than 0.63 times the rated AC current, you will end up due to regulation, diode losses and so on, with a voltage about 1.25 * the AC rated voltage. So 71 * 1.25 = 88.7V and you are where you want.

I got these figures from my own toroid supplier (Amplimo in the Netherlands), and they advised me the same as your supplier.
For 12V secondaries, you should apply a factor of 1.1, 30V is 1.2, 100V is 1.25 and 230V is 1.35 the rated AC voltage.

My suggestion is: Build your psu in a test-setup, apply the expected load as a DC resistor, and measure the voltage. Safest way to go. Don't worry about the caps; they will hold the high voltage for a while (but don't exaggerate).

Good luck
Bouke
 

alexl

Member
2005-03-20 5:39 am
canada
Similar issue

Hi,

I'm building an amp using the Tripath chipset and I have a similar issue with the rail voltage being a bit higher than expected. I found that the main lines voltage at my place varies from 120 to 123,5V. Honestly, I expected to get 115V... I didn't checked before ordering the transformer. My mistake. It is a 1kVA, 45V dual secondary.

With the transformer I have at 123,5V and under no load, I get 69V on the rails. I need to get something around 65V as the Tripath chipset must run under 70V and preferably not much higher than 65V. I found that I could lower the voltage a bit using a couple of diodes in series after the rectifier bridge to somewhere around 64-67 volts. Is this a good way to lower the voltage?

I can't unwind the transformer as it is wrapped and filled with resin.
 

Kenshin

Member
2005-01-09 9:51 am
China
Re: Similar issue

diodes have a forward voltage drop ~ 0.7V at big current.
but the voltage drop at very low current is smaller, it can be less than 0.2V.
So the method is not reliable.

try some regulation at DC output ? the voltage drop is low, so a linear regulation is enough. ;)

alexl said:
Hi,

I'm building an amp using the Tripath chipset and I have a similar issue with the rail voltage being a bit higher than expected. I found that the main lines voltage at my place varies from 120 to 123,5V. Honestly, I expected to get 115V... I didn't checked before ordering the transformer. My mistake. It is a 1kVA, 45V dual secondary.

With the transformer I have at 123,5V and under no load, I get 69V on the rails. I need to get something around 65V as the Tripath chipset must run under 70V and preferably not much higher than 65V. I found that I could lower the voltage a bit using a couple of diodes in series after the rectifier bridge to somewhere around 64-67 volts. Is this a good way to lower the voltage?

I can't unwind the transformer as it is wrapped and filled with resin.