DIY PSU build query...

[joe.ninety]

I should explain...

Probably like most people who are new to CNC'ing, once the bug bites I just want to get on with something ASAP. Unfortunately, I've come to discover that every decision seems to rely on another until you've gone full circle and are back at the start ;-) Well that's how it feels anyway.

I was hoping that the PSU would be something that it was possible to get started on, and that it would be possible to 'spec' one up that could work across a variety of motor choices, and also still be usable in the future on possibly a larger machine. So essentially an over engineered/spec'ed one.

My curiosity was piqued by Jazz saying that a 6A PSU didn't allow much leeway for expansion. So as I stated, I just wondered what people were using if they wanted 45V but at a higher VA output. Was the reason that 45V was recommended over 50V simply because the 50V version has a lower current output... 5A on the 50V compared with 5.56A on the 45V?

The simple answer might be to just buy another brand as Andy suggested.

Cheers

[Kitwn]

I should explain...

Probably like most people who are new to CNC'ing, once the bug bites I just want to get on with something ASAP. Unfortunately, I've come to discover that every decision seems to rely on another until you've gone full circle and are back at the start ;-) Well that's how it feels anyway.

Every answer raises two new questions. You'll have to get used to it I'm afraid!

There are plenty of sources of transformers. Have a look at RS Components for example. Even if you don't buy from them, you'll get an idea of the range available. Note that the DC voltage (that's appalling English, but everyone knows what it means) you will get from a finished supply is higher than the RMS voltage rating of the AC transformer, if a DC supply is what your drivers require. I've linked to the site linked below before but it's a good intro to DC PSU design. A supply for a CNC machine can be allowed to have quite a high ripple voltage and thus less capacitance than would be needed for, say, a hi-fi audio amplifier.

http://www.skillbank.co.uk/psu/

[dazp1976]

This one matey. 2*50v 1000va. Bosh.
https://uk.rs-online.com/web/p/toroi...rmers/1234006/

[JAZZCNC]

Anyway, I'm not going to ask any more questions until I've put more flesh on the bones, but that in itself is difficult if i don't know what I've got in mind is possible. I'm just trying to gather some underlying info and knowledge that means I don't end up wasting either my time or others in coming up with designs that don't have a hope of being good.

No don't stop asking questions. Start a Build thread and ask all your questions in there. Don't worry about asking what may feel like dumb questions or offering up weird and wonderful suggestions. This way everything related to your machine is in one place and others coming to the party late can look back and see what's been said before.

This one matey. 2*50v 1000va. Bosh.
https://uk.rs-online.com/web/p/toroi...rmers/1234006/

Not sure who that's aimed at but 1000Va is on the large side and not really needed for anything but larger motors on typical 4 axis machine. Also for anyone with 70Vdc drives then 50Vac Secondaries are too large because they will be right at the 70Vdc mark so not enough safety margin. Always leave roughly 10% safety margin from the drives Max volts. Only use 50Vac secondaries with 80Vdc drives.

[Desertboy]

No don't stop asking questions. Start a Build thread and ask all your questions in there. Don't worry about asking what may feel like dumb questions or offering up weird and wonderful suggestions. This way everything related to your machine is in one place and others coming to the party late can look back and see what's been said before.



Not sure who that's aimed at but 1000Va is on the large side and not really needed for anything but larger motors on typical 4 axis machine. Also for anyone with 70Vdc drives then 50Vac Secondaries are too large because they will be right at the 70Vdc mark so not enough safety margin. Always leave roughly 10% safety margin from the drives Max volts. Only use 50Vac secondaries with 80Vdc drives.

I use 45v secondary's when wired in serial and rectified gives me 72v without load and 68v with load.

I also have a 50v one I don't use which gives 78v without load and 73v with load, I bought 2 because when Maplins went bankrupt they sold them 1/2 price and that's when I built the PSU.

Also I blew up the first transformer I bought which I paid full whack from RS for, wired the rectifier the wrong way round I seem to remember. So buying 2 was my insurance policy lol.

It's also the reason I got 1Kva transformers there's was a limited choice of what was left, was in the voltage range and was cheap.

[Doddy]

I use 45v secondary's when wired in serial and rectified gives me 72v without load and 68v with load.

I also have a 50v one I don't use which gives 78v without load and 73v with load, I bought 2 because when Maplins went bankrupt they sold them 1/2 price and that's when I built the PSU.

Also I blew up the first transformer I bought which I paid full whack from RS for, wired the rectifier the wrong way round I seem to remember. So buying 2 was my insurance policy lol.

Sorry, pedantic mode (it might be important for someone blindly following this to avoid damaging drivers etc.):

Wired two 45V windings in series and you get 72V DC off load?, there's something amiss there. Perhaps in parallel? - I'd expect a 45Vrms winding to present around 64Vpk-pk, probably 62V after rectification and smoothing. Add 10% for the 220VAC rated primary and the nominal 240VAC UK supply and you're getting within a gnats of the 72V.

Two 45V windings in series you should expect 90VACrms, leading to around 128Vpk-pk, around 126V after rectification and smoothing. And then you can play games with the 220VAC primary versus nominal UK supply.

For reference, I also bought a toroidal tx from Maplin on closure...



That pokes out 72V off-load, rectified and smoothed, measured with cheap Chinese current/voltage DMM panel meters. Two secondaries driving two separate circuits - but this should give the same voltage as two secondaries in parallel (not series). The maths for mine = 50VAC * SQRT(2) - 2 * Vf(rect) = 69V - within about 5% of measured - I'm happy with that.

[Desertboy]

Sorry, pedantic mode (it might be important for someone blindly following this to avoid damaging drivers etc.):

Wired two 45V windings in series and you get 72V DC off load?, there's something amiss there. Perhaps in parallel? - I'd expect a 45Vrms winding to present around 64Vpk-pk, probably 62V after rectification and smoothing. Add 10% for the 220VAC rated primary and the nominal 240VAC UK supply and you're getting within a gnats of the 72V.

Two 45V windings in series you should expect 90VACrms, leading to around 128Vpk-pk, around 126V after rectification and smoothing. And then you can play games with the 220VAC primary versus nominal UK supply.

For reference, I also bought a toroidal tx from Maplin on closure...



That pokes out 72V off-load, rectified and smoothed, measured with cheap Chinese current/voltage DMM panel meters. Two secondaries driving two separate circuits - but this should give the same voltage as two secondaries in parallel (not series). The maths for mine = 50VAC * SQRT(2) - 2 * Vf(rect) = 69V - within about 5% of measured - I'm happy with that.

The bridge rectifier drops the voltage, it converts AC to DC you connect both outputs in series run it through the rectifier and 72v out the other side in DC.

You need capacitors to clean up the DC.

In center tapped full wave rectifier, only one diode conducts during each half cycle. So the voltage drop in the circuit is 0.7 volts. But in the bridge rectifier, two diodes which are connected in series conduct during each half cycle. So the voltage drop occurs due to two diodes which is equal to 1.4 volts (0.7 + 0.7 = 1.4 volts). However, the power loss due to this voltage drop is very small.

https://www.physics-and-radio-electr...rectifier.html

[Clive S]

I use 45v secondary's when wired in serial and rectified gives me 72v without load and 68v with load.

Are you sure you about the serial or do you mean parallel ?

The bridge rectifier drops the voltage, it converts AC to DC you connect both outputs in series run it through the rectifier and 72v out the other side in DC.

You need capacitors to clean up the DC.

I think you need to re visit this as I don't think you understand it. It is also misleading for others.

Doddy is correct.

[JAZZCNC]

My curiosity was piqued by Jazz saying that a 6A PSU didn't allow much leeway for expansion.

This here is exactly why you shouldn't buy anything until you fully understand what's been said and why.!

My suggestion to that person to use a toroidal over a the 6A PSU would have been based on them using both the secondary coils.

Building a PSU isn't difficult and the best choice for a CNC machine but you can't just guess at it.

My strong advice is DONT buy anything until you have researched exactly what your building and what components your going to use. Even then the electronics are best left until the end of the build.

If your not sure what you need then ASK don't read posts taking bits from here and there because often you'll put 2 & 2 and end up at 6 because there are so many different ways of doing this it's easy to misunderstand, even if you think you do.! So Unless your 100% sure it's correct for your needs then just ASK for a second opinion before buying.

[Desertboy]

This here is exactly why you shouldn't buy anything until you fully understand what's been said and why.!

My suggestion to that person to use a toroidal over a the 6A PSU would have been based on them using both the secondary coils.

Building a PSU isn't difficult and the best choice for a CNC machine but you can't just guess at it.

My strong advice is DONT buy anything until you have researched exactly what your building and what components your going to use. Even then the electronics are best left until the end of the build.

If your not sure what you need then ASK don't read posts taking bits from here and there because often you'll put 2 & 2 and end up at 6 because there are so many different ways of doing this it's easy to misunderstand, even if you think you do.! So Unless your 100% sure it's correct for your needs then just ASK for a second opinion before buying.

Wise man that Jazz you should listen to him.