Voltcraft FSP1132 13.8v £18 power supply teardown

[MrSlack]

I'm building a power supply that doesn't take up quite as much space as my HP 6236B or Thurlby PL330 so I can do some stuff in front of the television on the sofa (I'm lazy; sorry!). Priced all the components up on RS including hardware and I was down £50 already. Pah, screw that. After some searching, I found a linear supply on Rapid Rapid Electronics at the moment for £18.25 inc VAT which is just insanely cheap if you ask me (considering I put down £200 for my PL330 years ago)

http://www.rapidonline.com/Electrical-Power/Voltcraft-FSP-1132-Fixed-Voltage-Power-Supply-51-7496

This has a chassis, transformer rated high enough, binding posts, heatsink, mains switching and fuses which I need so I decided to nab it for evisceration. It's a 13.8v 2A continuous 4A peak LINEAR power supply. Output is floating so you can chain these if you need to. At that price, if it was crap I wouldn't care.

FACE:



Ammeter is crap. The power switch is lit however which is nice. The binding posts are cheap but reasonable and have 4mm banana sockets built in.

BUM:



Fused! IEC socket! Nice heatsink (will see what is hiding under there in a minute)! more than I expected for the money.

Exposed BUM:



Ubiquitous and old friend the 2N3055 power transistor hiding behind there. I will warn you that screwing the protective cover back onto the thing was a complete sod due to how the screws are recessed.

INSIDES:



Pretty standard affair. Lots of nice heatshrink over everything, braided sleeves for the power transistor, board mounted on edge. 18-0-18 center tapped transformer. Now there's a thought; this thing can push 36v over the taps. Wonder if I can kick out 1.2-30v from my modifications at 1A which is enough for this.

BINDING POSTS:



Was surprised that they actually bothered to use crimps on these and screw them on rather than just bodge solder them. Good show.

EARTHING:



Crimped earth strap, straight to IEC socket, nice and tight fitting with shakeproof under it. Good show again. Probably not going to die using this, well not until I've modified it anyway.

BOARD:



Board is mounted on two bits of right angle metal screwed to the base. Was easy to remove. Pretty standard affair. Two diodes + center tap = full wave rectifier. Cheap filter cap, although it is marked with 105 degrees but I'm not sure I believe it. Fully discrete voltage regulator circuit. Rectifier diodes are about 1cm off the board I assume for airflow. I haven't bothered to trace this as I'm yanking it out and replacing it.

ANOTHER BOARD SHOT:



Different angle. 2S transistors. Easy enough to repair if anything blew up if I'm honest.

OPERATING:



Quick reassembly and test. Works. Output is a little high so could be adjusted down but I only want it for it's body and some organs so I haven't bothered.

Quite remarkable for £18.25. I couldn't have built it myself for less than twice that.

[rich]

Thanks for the nice teardown pics 

General construction looks better than expected and plenty of space to customise. I may have to follow your lead now , but with a different end goal in mind.

The silk screen says 11 - 22V next to the adjust pot - any idea if it really is adjustable over that range?

[MrSlack]

I didn't try it to be honest. Will have a go later when the kids are in bed and post back.

[rdl]

Power switches that operate side to side instead of up and down are a pet peeve of mine, but other than that it doesn't look too bad. I much prefer analog ammeters on power supplies, so that's a plus.

[KJDS]

I had a pile of Voltcraft 1525 power supplies a while back. I've sold almost all of them, kept a couple for my own use. Great things, 3-15V, 25A switchmodes, never had a failure on them. It's not often I see any voltcraft stuff about, but when I see more I'll buy it.

[Vgkid]

Thanks for the teardown, I was expecting that the common lm723 would have been used.

[MrSlack]

Thanks for the teardown, I was expecting that the common lm723 would have been used.

Probably would have been a significant cost item against the rest of it!

At least it can be fixed if it goes wrong! I'm reverse engineering it this evening. Will swap transistors out for Euro stock equivalents (BCxxx) and will see if I can just mod it for what I want. Looks like a simple regulator with hefty pass transistor and not much else. Will check ripple and dropout voltages as well when I do this. Might be a very reusable design!

[Vgkid]

^^^ Good point.
Then again at 13.8v for amateur radio, good stability is not required .

[MrSlack]

Indeed. Appears to be pretty stable on load changes from a couple of experiments I just did (switching between two load profiles).

Reverse engineering is under way slightly scuppered by a child that doesn't want to sleep...

Found a couple of turds so far. C7 is a dead short on the PCB. This appears to be a design/routing error! It's soldered in but goes round an inch loop of PCB and shorts both cap terminals out. Doesn't do a thing. Also there is a diode labeled with a capacitor prefix (C) instead of (D). Schoolboy errors. Not impressed!

Further progress after child dealt with and coffee necked. It's 22:15 here and I'm finishing this tonight if it kills me.

So far I'm going to make a new board for it using an LM338K as I've got one lying around and the thing is the same TO3 footprint as the ancestral 2N3055.

[MrSlack]

Ok so I got half way through the reverse engineering effort this evening before my eyes gave in. I'll pick this up in a few days again as I've got to concentrate on work (boo!)

Some findings...

1/ Voltage can be adjusted between 12.3v and 15.5v for the 13.8v model. I reckon you could change the feedback divider top and tail resistors to change this range a bit.

2/ C7 doesn't do anything at all.

3/ I've cranked mine up to 15.00v and it's stable there. I'm going to add a slave LM317L on the breadboard to control output between 3-12v which is good enough for regular work for me. No modification required then i.e. path of least resistance!

[Vgkid]

You could keep the 3055, and drive it from the lm317. Just need to add some current limiting.

[Ian.M]

As ST still have the LM723 in production, it would make more sense to use that, which would give you the current limiting you need.     If you wanted to get fancy, you could rewire it with a bridge and a 15V,2A/30V,1A switch.  It could be rather nice with a digital dual display panel meter (e.g. http://www.ebay.com/itm/DC-Combo-Meter-Dual-Voltmeter-Ammeter-0-33V-0-999-9mA-1A-2A-3A-High-Precision-GZ-/171258623046) replacing that cheezy ammeter and a couple of pots for voltage and current limit.

[MrSlack]

Not a bad plan. Will definitely consider that.