PSU Voltage seems high.

Hi
I've just built up the hobbycnc 3-axis kit. Driver board is working fine off a bench PSU. I've built up the Psu with capacitor and rectifier supplied in kit and a 300VA transformer (30V,10A). Before I connected it up to the driver board I checked the output voltage and i'm getting 47vDC. I've checked it on a scope and i'm getting no ripple. This seems to be a bit high for the driver board. I've checked the input to the recrifier and it 29.7vAC so the transformer's giving the correct voltage out. Has anyone got any ideas why i'm getting such a high output and can the driver board take it or will it fry the chips?.

Thanks in advance

Matt

This is because it rectifies the peak AC voltage which normal AC outlets peak ac voltage is around 170v. After it is rectified and filtered, typical output is 1.414x the transofmer output voltage.

Jon

As Jon said, (apart from UK voltage being 220 :) ) rectified AC to DC increases the voltage by around 1.4. If you're after 30vdc you need a transformer output of ~21vac before rectifier. Don't connect your driver to 47vdc because it will blow it.

Andrew

Quote:

---

Originally Posted by mnewsholme

Hi
I've just built up the hobbycnc 3-axis kit. Driver board is working fine off a bench PSU. I've built up the Psu with capacitor and rectifier supplied in kit and a 300VA transformer (30V,10A). Before I connected it up to the driver board I checked the output voltage and i'm getting 47vDC. I've checked it on a scope and i'm getting no ripple. This seems to be a bit high for the driver board. I've checked the input to the recrifier and it 29.7vAC so the transformer's giving the correct voltage out. Has anyone got any ideas why i'm getting such a high output and can the driver board take it or will it fry the chips?.

Thanks in advance

Matt

---

30VAC(notDC) rectified makes @ 1.41 times DC volts. 30 times 1.42 equals 42.6VDC NO load. It will drop a volt or two under load. Your line voltage is over the input voltage. DO not use above 46VDC.

Dave Rigotti

Ok I guess I'll swap out the 30v,10a transformer for a 24v,12a one. That should drop voltage down to a safe level but still have plenty of current in reserve.
cheers :cheers:
Matt

You could always take some turns off, lowering the voltage will up the Current capacity, providing the wire size is OK, but 2amps extra coverage should be possible.
Rough guide for a full Wave Bridge.
VAC = 0.8 x (VDC + 2)
IAC = 1.8 x IDC
Al

ok changed the transformer for 25vAC, 12A . getting 38.5vDC now.
absolutely no ripple :cool:
Matt

Quote:

---

Originally Posted by mnewsholme

ok changed the transformer for 25vAC, 12A . getting 38.5vDC now.
absolutely no ripple :cool:
Matt

---

Remember the ripple has to be measured at the maximum load current to obtain the degree of ripple.
Al

Quote:

---

Originally Posted by mnewsholme

ok changed the transformer for 25vAC, 12A . getting 38.5vDC now.
absolutely no ripple :cool:
Matt

---

Matt, I have just finished my 3 axis board which works great off a bench psu (great job Dave :cheers: ), where did you get your transformer from?

Steve

got it from rswww.co.uk. bit expensive but it was exactly the right rating i wanted. Part number is 223-3207. 25v, 6a toroidal. I think it was about 20 quid but then another tenner delivery :frown:

Thanks Matt, I'll get one on order

Steve