Strange observation but in a good way. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 06, 2021, 07:38:15 am
content: Recently I have noticed something in a power supply I had modded a while back. What this is, is power supply adapters the Dell, HP and one of two others. When I had modded these, in use I couldn't pull large currents with out them shutting down. About 5 Amps maximum, over that and it would shut down. Recently I tested a circuit, linear regulator with 4 X pass transistors with one of the HP PSU's. I manage 12 Amps current draw with it showing no signs of shutting down. The HP power brick has a maximum current rating of 16 Amps. Why I was able to pull that much current through this linear power supply circuit I have no idea. If I try that directly from the HP power brick, it would just shut down. Any thoughts why this is possible through the linear with pass transistors, but not possible straight from the HP adapter.Thanks for reading.I'm really curious what's going on with this.

author: moffy
author_rank: Super Contributor
timestamp: May 07, 2021, 03:10:13 am
content: The only thing I can think of is that somehow the linear supply is limiting the initial surge current.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 07, 2021, 07:16:51 am
content: I thought that as well, but wasn't sure. There is a short delay before the load is fully powered up ( halogen lamps) I started with 50 watts just testing the linear circuit, and went up to 200 watts 4 X lamps. But there is a tiny, probably milliseconds delay. I've only got a 60 MHz old Hameg scope, so don't think I would see the delay on that. If I load 200 watts straight to the HP power brick, it just shuts down.

author: moffy
author_rank: Super Contributor
timestamp: May 07, 2021, 11:14:06 am
content: The halogen cold resistance is much lower than when hot. You are definitely getting a large start up surge, until the elements heat up.

author: moffy
author_rank: Super Contributor
timestamp: May 07, 2021, 02:26:57 pm
content: I think the current pass transistors of your linear supply are current saturating during startup, thereby limiting the initial surge current, and allowing the PSU to drive a higher load.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 08, 2021, 12:24:11 pm
content: The circuit is just a linear 7812 with 4 X 36C PNP transistors. I just grabbed the HP power brick to test this circuit, before putting in a PSU build that has 2 X toroidal transformers 16 Volts each 160 VA units. It's complete now, I'm quite please with its performance as a linear high current power supply. I need to get a new lab PSU for testing circuits, I'm just not ecstatic about the Amazon offerings, and on a budget most electronic retailers want silly money for a lab PSU for the hobbyist. I was just supprised the HP power brick won't start up with a 200 watt load directly, but put it's output through a linear regulator with pass transistors circuit, and you can power 200 watt load and possibly more.

author: mikerj
author_rank: Super Contributor
timestamp: May 09, 2021, 10:11:20 am
content: Quote from: davelectronic on May 08, 2021, 12:24:11 pm4x50W halogen lamps are not a 200W load when first powered. The cold resistance of the filaments will be around ten times lower when cold than when running at rated power, so your PSU is effectively trying to start a 2000W load.Your linear regulator design is either current limiting or simply dropping sufficient voltage through mediocre regulation to provide a soft start. The supplies (either transformer or SMPS) intended for driving these lamps in domestic installations are designed to cope with this surge current, but a PC PSU will simply shutdown as soon as some overcurrent threshold is hit.

author: davelectronic
author_rank: Frequent Contributor
timestamp: May 09, 2021, 01:01:59 pm
content: Yes I remember that about halogen lamps cold resistance. It's not really a very good load for testing power supplys. And I'm familiar with the PC / IT power supply hardware shuts down if it's loaded with the same halogen lamps. I wouldn't mind building an adjustable electronic load, but it might be beyond my abilities. The linear circuit isn't for precision regulation, it only needs to supply power to a HF linear amplifier, and be clean power with very little electrical noise, oh, and stay between 11.40 Volts and 14.40 Volts. It's wired in to the case now, and putting out 12.64 Volts, dropping to 12.53 Volts with a 12 Amps load. Tested with a DC clamp meter and multimeter. I'm not going to use this power supply yet until I've put together a separate crowbar circuit, also built in a suitable project box. Just incase it ever went over voltage, it won't brick any HF radio gear.