Just to say it's sorted. All the guff about the overvoltage protection
with the SG 6105 proved a red herring in this case. As the spec covers the
range 10.1 - 14.5v (typical) without mods.

All that was needed was to alter the voltage on the 'IN' pin. But with the
fan plugged in. Without it any alteration to that voltage caused the PS to
shut down. With it, I've got my 13.8v (approx) without problems. 270K was
the value (original 120K) needed in this case. But probably won't help
others since this is a generic PS with no maker's name.

--
*Acupuncture is a jab well done*

Dave Plowman London SW
To e-mail, change noise into sound.

On Wed, 05 Aug 2009 14:48:07 +0100, "Dave Plowman (News)"
put finger to keyboard and composed:

Just to say it's sorted. All the guff about the overvoltage protection
with the SG 6105 proved a red herring in this case. As the spec covers the
range 10.1 - 14.5v (typical) without mods.

All that was needed was to alter the voltage on the 'IN' pin.

You are not actually altering the IN voltage. It remains at +2.5V due
to the action of the error amp. The other input to the amp is internal
to the IC and is fixed at +2.5V.

But with the
fan plugged in. Without it any alteration to that voltage caused the PS to
shut down.

Were you testing the supply without a load?

With it, I've got my 13.8v (approx) without problems. 270K was
the value (original 120K) needed in this case. But probably won't help
others since this is a generic PS with no maker's name.

If you did not remove the +5V sense resistor, then the output may
fluctuate as the load varies.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

In article ,
Franc Zabkar wrote:
On Wed, 05 Aug 2009 14:48:07 +0100, "Dave Plowman (News)"
put finger to keyboard and composed:

Just to say it's sorted. All the guff about the overvoltage protection
with the SG 6105 proved a red herring in this case. As the spec covers the
range 10.1 - 14.5v (typical) without mods.

All that was needed was to alter the voltage on the 'IN' pin.

You are not actually altering the IN voltage. It remains at +2.5V due
to the action of the error amp. The other input to the amp is internal
to the IC and is fixed at +2.5V.

Right - so what is happening to alter the output voltage?

But with the fan plugged in. Without it any alteration to that voltage
caused the PS to shut down.

Were you testing the supply without a load?

No - I was loading it with a 55 watt car headlamp bulb.

With it, I've got my 13.8v (approx) without problems. 270K was
the value (original 120K) needed in this case. But probably won't help
others since this is a generic PS with no maker's name.

If you did not remove the +5V sense resistor, then the output may
fluctuate as the load varies.

Seems pretty stable - at least for my purposes.

- Franc Zabkar

--
*Age is a very high price to pay for maturity.

Dave Plowman London SW
To e-mail, change noise into sound.

On Thu, 06 Aug 2009 10:57:08 +0100, "Dave Plowman (News)"
put finger to keyboard and composed:

In article ,
Franc Zabkar wrote:
On Wed, 05 Aug 2009 14:48:07 +0100, "Dave Plowman (News)"
put finger to keyboard and composed:

Just to say it's sorted. All the guff about the overvoltage protection
with the SG 6105 proved a red herring in this case. As the spec covers the
range 10.1 - 14.5v (typical) without mods.

All that was needed was to alter the voltage on the 'IN' pin.

You are not actually altering the IN voltage. It remains at +2.5V due
to the action of the error amp. The other input to the amp is internal
to the IC and is fixed at +2.5V.

Right - so what is happening to alter the output voltage?

I don't know much more about the black art of switchmode PSUs than the
average tech, but AFAICS in this particular case all one needs to
understand is that the error amp inputs must be equal when the supply
is regulating correctly. If the output is too high, then the error amp
reduces the drive to the chopper (by narrowing the pulse), thus
bringing the output back down. Conversely, if the output is too low,
then the error amp increases the drive to the chopper.

But with the fan plugged in. Without it any alteration to that voltage
caused the PS to shut down.

Were you testing the supply without a load?

No - I was loading it with a 55 watt car headlamp bulb.

With it, I've got my 13.8v (approx) without problems. 270K was
the value (original 120K) needed in this case. But probably won't help
others since this is a generic PS with no maker's name.

If you did not remove the +5V sense resistor, then the output may
fluctuate as the load varies.

Seems pretty stable - at least for my purposes.

- Franc Zabkar

I mentioned this possibility because I had a fake 400W PSU (really
only 200W) where the +12V rail moved about in response to changes in
the +5V load. Also ISTR that some HDD enclosures include a dummy load
resistor for load balancing.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.