Soft start for low voltage applications.

[davelectronic]

I'm trying to find a solution to a converted Dell  AC ADAPTER D220P-01 shutdown with anything over 4 Amps load (resistive halogen lamp) I have a few of these and want to draw more than 4 Amps from these dell power bricks psu units. Not sure if I've asked this question before. But when i can't find a solution, i just shelf things like this in hope i can find a solution later down the road.

As the psu shuts the secondary low voltage down, I'm re visiting these units in hope of the possibility of finding a softstart circuit to limit this protection control activation coming on at start up medium to heavier loads. The units are 12 Volts and rated at 18Amps 220 watts, for a maximum ball park figure on current load, 10 to 12 Amps would be fine. I don't expect the full 18 Amps, and have no use for current that high.

A typical application for these units is using one to power a boost converter, this in turn powers my rotary tool (dremel) at much reduced speeds for intricate use. The boost converter gives me 19 to 60 Volts DC. Tjis is perfect for my needs and removed the voltage potentiometer on the boost converter,  and replaced with same value panel mount potentiometer for greater control. I'm currently using a HP 203 watt power adaptor brick to run the boost converter. This HP unit doesn't shut down under load. The rotary tool has a universal wound motor i believe,  so its happy to run on main AC or for my needs reduced DC voltage.
Any help with the dell shut down problem would be greatly appreciated.
Couple of pictures of psu units and boost converter I'm trying to get put in an enclosure.

[Siwastaja]

Find the controller IC. It likely has a softstart feature configured with an external capacitor, which ramps up the reference voltage. Typically this capacitor is quite small, like 10nF. You can try substituting it with something ridiculously large, like 2.2 uF. Look if you can find the controller datasheet though so you don't need to randomly guess which capacitor it is. (It's typically just connected between one device pin (which isn't connected anywhere else) and ground.)

This is a bit hacky, but could work.

[davelectronic]

Thank you for the tip, i will try and find a schematic for the psu board. As far as reverse engineering goes, I'm just a hobbyist. I'm uncertain how easy a circuit diagram will be to find. I might google some IC's that are in the region of the output protection area, but I'd be winging it if I'm being honest.

[Siwastaja]

If you have no schematic, the controller IC typically is a chip with approx. 20 leads, with a small bunch of small SMD passives (resistors and capacitors) around it. One of the leads goes to a small capacitor, other end of which goes to ground. If you measure the voltage at this capacitor during operation, it should be at least the reference voltage of the IC (typically 0.8V to 1.2V), or more in some ICs, but definitely no less. If you have an oscillosscope, you should see a stable DC voltage; even better, scope it during turn-on, and you should see it ramp from zero, during a few milliseconds. This is your softstart capacitor and you can try replacing it with a larger value like 1uF and see if the output ramps more slowly during power-up.

Remember to be very careful when probing or modifying mains stuff, see the usual advice elsewhere.

[davelectronic]

Yes i will find this IC,the only thing that might be tricky is if this capacitor is an smd component. I have tried using this dell unit today, and added the load by means of a dc to dc converter. It managed nearly 6 Amps at 24 volts powering a couple of halogen lamps. Then i pushed it closer to 30 Volts just to see if it would shut the psu down. One of two things could of happened, blown lamps, or psu shutdown. the strange thing is if i connect the load before powering it on. It powered up the lamps, if i try and add the load to an already powered up psu, it instantly shuts the psu down. I'm definitely going to try and track that protection IC down.