Bench Power Supply - Current Question

[alteredsounds]

Just picked up a big old Farnell bench power supply that does 0 - 30vdc at 5A.  For some reason I have it in my head that I should bring that current down to 1 or 2 amps as it could potentially destroy some parts at initial power-ups.  I realise the the circuits will only use what they need but it's the power-up thing that I'm wondering about.

Cheers, 

[GibsonGM]

For stomp box/low draw circuits, using a fuse in-line (on the hot line) before the circuit will provide some good protection.  I'd try something like a 50 or 100 mA fast-acting, up to 500mA depending on what you expect the circuit to draw.  Even a 9V battery has enough current-delivering ability to easily fry poor little stomp circuits!  ;o)

Simply using a 1 or 2A fuse will "lower' the supply to that level in a virtual way - but no fuse can guarantee total protection of the circuit!

Remember that the 5A at 30V potential can pose a safety risk, as well...a fuse can't really trip fast enough to protect YOU.   I wonder if using an isolation transformer that can only put out .5A would be a better way to go....(?)   At any rate, respect that current level!  I'm sure someone with more transformer knowledge will respond with more about an isolation trafo...

[alteredsounds]

Thanks for the reply Mike, I did wonder about some sort of isolation transformer protection to bring it down to a 'friendly' level.  Shame there isnt anyway to limit the current on the unit but its a nice big solid unit and 0-30 is a great range for what I would need.

[drewl]

If it doesn't have a current limiter use a fuse in line.

[alteredsounds]

Just been googling it but I guess it would be fairly simple circuit to add variable current control? The fuse would add protection but I'd like to just drop the current to 1A I think.

[Minion]

If you are useign a Variable PSU then just set the PSU at 0v when you power it up and then slowly ramp up the power to the desired Voltage....You can also make an adjustable Regulated PSU with a LM317 regulator and a couple Caps resistors and a Trim pot which should solve any current inrush problems...You might also consider a soft start curcuit ,Or put a switch on the Secondary side of the Xformer so you can turn the secondary on after the Primary and aviod the inrush current....You can also put a 0.1ohm 5w power resistors on the secondary side and to will absorb a lot of the inrush current without any real Voltage drop...

Just a few suggestions....

Cheers

[alteredsounds]

Good idea, starting with the voltage at 0v and then raising it to the desired amount, if I were to put a 0.1 resistor inline with the positive output would that function to absorb the voltage drop?

[Minion]

The 0.1R Power resistor will help absorb some of the inrush current without haveing any major Voltage drop ,The Voltage drop will of cource increase as the Current demand increases but if you are going to be useing it for small signal curcuits like pedals and such then that should have any real effect....

Cheers

[alteredsounds]

thanks for your help minion

[George Giblet]

What I would do is open the unit up and find a low valued resistor in the 0.1 ohm to 1ohm range (I suspect it will be something like 0.22ohm).  This resistor will set the maximum current.

What you can do is change the this current limit resistor to a larger value so the current limit circuit limits at a lower current.  If the power supply is 5A and you want 1A you simply increase that resistor by a factor of 5.

If you want to go a bit further you can add a switch to switch between the two values.  This is an easy mod and is one way a professional power supply would do it (there are others).   Adding a variable current control is usually a much more involved exercise.

There is a correct way to do this.   What you do is permanantly wire the 1A resistor value to the board then switch the low valued resistor in parallel with it - unfortunately that means you will need to changed the existing low valued, otherwise the high current range will limit to 5+1A = 6A which is a little too high.  You should *not* connect the two resistors to an SPDT switch and switch between them, you can blow something up if the switch contact has an intermediate open state.

If you want to salvage the existing low valued resistor you wire the resistors in series and reduce the 1A resistor by the subtrating off the seried 5A value.  You wire the switch such that it shorts the 1A resistor (switch shorted = 5A, switch open =1A).