Another dumb PSU question

[DaxLiniere]

Hey guys,
I have a piece of gear that requires 12vDC@3A(max) and 5vDC@1A(max). The internal existing switching PSU is broken, the unit is fine.

Due to multi-pin plug arrangements (I have USB data too), I am considering replacing the dual voltage PSU with a single 12vDC 5A PSU.
I would then mount a 5vDC regulator inside the unit.

Is there a problem with using the 5vDC reg to obtain the dual rails?

 

[DaxLiniere]

Bump? Please, someone?

 

[gyraf]

If the +12V and the +5V already share a common 0V reference, I don't see any structural problem in this - other than that you will dissipate 7W of heat from your 5V regulator (use good heatsink) - but not knowing the circuit, it's hard to say for sure.

If the 5V and 12V are not sharing common ground, the scenery is much more complicated.

Jakob E.

 

[DaxLiniere]

Yes, common ground.

7watts, eh? Is there a better option than 7805? I was planning on bolting the vREG to the metal chassis.

 

[gyraf]

No better option - a 7V drop at 1Ampere gives you 7W. Only alternative is going switch-mode, and you probably don't want that.

Jakob E.

 

[Nishmaster]

What is the purpose of the 5V rail?

USB can provide you with that. Pin 1 of the USB spec is VCC(Red): +5V. I think most hardware can provide 1A.

 

[Andy Peters]

What is the purpose of the 5V rail?

USB can provide you with that. Pin 1 of the USB spec is VCC(Red): +5V. I think most hardware can provide 1A.

USB sources 500 mA max for the VBUS (5V) if the device enumerates as a high-power device. Otherwise, it sources only 100 mA. And if the USB device goes into suspend mode (no transactions for more than 3 ms), the current source is reduced to something like 100 uA.

-a

 

[DaxLiniere]

Thanks for all the replies everyone.

Some thoughts:
The device is CM Labs MotorMix control surface, which has MIDI in & out. It originally had an internal switching PSU, but 3 days after I got it, a client kocked it to the floor.
The PSU board inside is rather flexible and the impact made the underside short-out with the steel chassis. Fried.

No worries, I wanted it to have a single lead instead of 3 anyways.

So I bought a USB->MIDI lead. A very cheap one, mind you, and it works flawlessly! (us$10??) This was dissected and mounted inside the MotorMix and a USB-B socket mounted on the chassis.
In the interim, I've been running the MotorMix from my PC tower's PSU, working fine for almost a year.
I used a 5-pin plug in the hope of jamming +5, +12, gnd, USB data+ & USB data- down the one cable. No luck, USB device detected as faulty. Separate connections: everything works fine again.

If I stop using the PC's PSU, I'm worried about sharing ground between USB (powered by PC) and 5v/12v ground (powered by external PSU). I suspect some damage could be done to the PC if potential difference existed.

So I'm thinking I'll just use a FireWire400 lead and sockets. They're 6 pin PLUS shield. USB likes shielding and my previous 5-pin incarnation wasn't. (maybe part of the problem)

So...
1SU 12v
2SU 0v
3:USB 5v
4:USB 0v
5:USB data+
6:USB data-
Shield:Shield

I wanted to use FW800 and maintain the dual-rail supply, but I can't find the sockets in Australia (at a reasonable price for 2 units.)
Also, dual-rail PSUs with enough current can't be found cheap in Australia. Boo!

One end of this lead will obviously plug into the MotorMix and the other end into the PSU, which will have a USB-B socket fitted to it, acting as a break-out-box of sorts.

 

[pstamler]

Well, you could drop the 12V down to 7V with a nice hefty 10W dropping resistor. That way the reg would only be dropping 2V and dissipating 2W.

Peace,
Paul

 

[DaxLiniere]

Is that really the best idea? I just have no idea on that.

 

[Harpo]

what about a center tapped transformer and regulating to +7V/-5V.
-5V rail used as gnd, may be confusing if somebody other than you has to fix it one day, ..
or dual secondary with stacked +5V and +7V.

 

[mwkeene]

Well, you could drop the 12V down to 7V with a nice hefty 10W dropping resistor. That way the reg would only be dropping 2V and dissipating 2W.

Agreed. 7W is a lot of power. You'll want at least a TO-220 package for the vreg, and you'll probably need to attach it to a heatsink if you don't use a series dropping resistor.  Even 1W is enough to make a "naked" TO-220 run quite warm, so transferring the power that would be dissipated in the IC to a 10W or even a 20W resistor (10W wirewound would get pretty warm) would help the vreg run cooler.  Just be sure to bypass the vreg input with enough capacitance to keep it stable.

-Mike

 

[DaxLiniere]

Hmm, but if I run a resistor in series before the vreg, won't I still be dissapating 7W total, making the net heat output of the resitor and vreg combined the same as vreg on it's own?

Also, how much capacitance would be required for vreg bypass 1000uF? 2200uF?


Thankyou very much everyone for getting involved to help me out.