Why are some power supplies full of resin?

[Okto1984]

Some computers, such as my C64, have a power supply which has been filled with resin. I've tried, but failed to figure out why. Many other power supplies of similar age which appear to do much the same thing don't contain resin. It makes the PSU very heavy and virtually impossible to repair. Probably also makes them run hotter? I suppose it does reduce the chances of cracked solder joints, but I don't know that for sure. Any ideas why it's there would be very welcome?

[Herald1360]

Possibly to make them safer/more robust, but the cynic in me reckons it's more likely just to make "No user serviceable parts inside" that wee bit more of a challenge

Along with D-type connectors with unheard of pin configurations and other similaf treats

[Nickthedentist]

It's called "potting compound".

Amongst other things, it keeps moisture out and can prevent "tracking" in very high voltage circuits (think of triplers in old tellies).

Sometimes, it's deliberately used when the designers want to keep their design secret and/or prevent its repair or modification.

Other advantages listed at the bottom of this page: http://uk.rs-online.com/web/p/pottin...ounds/1991468/

N.

[SiriusHardware]

I once did laboriously chip a C64 PSU out of its resin tomb, repair it and put it (minus resin) into a normal, ventilated PSU housing. The owner (an unemployed friend) could not afford to replace it - this was when the C64 was still current, and expensive.

[Andrew B]

I remember ages ago that I was given a C64 without a power supply. Not to be defeated I made one using a large 9 VAC transformer, I fitted rectifier, smoothing caps and a couple of the 78XX regulators for the 5v & 9v, these were bolted to the aluminium case, worked very well. A lot of the C64 faults were heat related, especially the PSU going "bricked"

I agree with the comment "There are no user serviceable parts inside", "and to make damn sure that you can't fix it, we filled it full of epoxy" should have been added.

[TonyDuell]

I thought the 9V input to a C64 was AC, straight off the transformer secondary.

Certainly that's the case for the C128, the PSU for that has a 9V AC output from a transformer secondary and a 5V DC output from a switching regulator circuit running off another secondary. Fortunately the one I have is not potted the only problem in getting inside was silly caps over the fixing screws.

Incidentally, I believe 'no user serviceable parts inside'. After all, if I am servicing it, it's not working, so I can't be a user.

[SiriusHardware]

The (C64) one I disembowelled had at least one regulator on a large U-shaped aluminium heatsink - I remember it because it was, in fact, the regulator which had failed in this particular case.

If Atari are typical, then manufacturers tended to have PSU units made to a particular specification by a number of different manufacturers - there were at least four or five distinctly different PSUs used in the Atari ST, for example, although all had the same physical dimensions, fixing points and output connections just as PC power supplies do.

Some manufacturers may have chosen to 'pot' their version of a power supply, others not: All versions of the Amiga A500 power supply that I have seen were not potted.

[TonyDuell]

I am not surprised about the regulator, from memory the C64 PSU is (unregualted) 9V AC and regulated +5V DC.

The Amiga 500 supply looks very similar to the C128 one, but it is not interchangeable. The Amiga one is a normal SMPSU giving +5V and +/-12V I think. And (fortunately) I've never seen a potted SMPSU.

Often microcomputer SMPSUs were made by a company specialising in such devices to the specification of the computer manufactuer. Astec are the well-known SMPSU manufuacturer along with Boschert in larger machines, but there are many, many others.

[turretslug]

As well as the advantages (and disadvantages!) listed above, there might have been an element of sound-deadening- when there's both ripply mains and high AF chopping going on, some chokes and HF transformers can produce intensely irritating shrieks and buzzes that are otherwise difficult to suppress. Perhaps particularly when switchers were relatively novel and things were low on the learning curve.

[TonyDuell]

The C64 supply that is potted is a normal mains-frequency transformer. As I said, I have never seen a potted SMPSU.

[Okto1984]

I hadn't noticed before, but it does seem the switching PSUs were never potted. Other manufacturers like Acorn around that time were using these switch mode supplies. I've never had any noise issues with these however. Faint buzzing is the most they seem to generate.

[robin coleman]

We used to pot all of the transformers at quad to stop them being copied and noise
Regards
Robin

[Oldcodger]

Originally Posted by TonyDuell

The C64 supply that is potted is a normal mains-frequency transformer. As I said, I have never seen a potted SMPSU.

If you venture into military radio and get inside a Clansman RT353, you'll find the output stages are potted in silicone. All the control circuits are on a PCB, but after that it's all inside the potted section. After all , you wouldn't want the 800v line getting on to the logic 5v rail ,and end up with cooked chips. .
Might be more pleasant than slopjockey dishes up, but only just.

[McMurdo]

could also be somethng to do with creepage and clearance to get some approval or another. Most of the C64 psu is transformer and heatsink, and is double-insulated. It might have needed potting to conform to its flash test etc

[Mad Mal]

Some machines were shipped with a Black rectangular block PSU with ridges that were supposed to act as cooling fins.... Some chance. These things could get so hot you could have cooked an egg on some of them. I did see one third party PSU that was switch mode, but shortly after that CBM released the Amiga... The first home computer to TRULY multi-task. (And it used an eighth of the resources that a PC did at the time). The PSU's for these were Switch mode on later models too. Some rare early examples were linear, but weren't potted.

[TonyDuell]

Did the Amiga pre-date (Microware) OS-9 Level 1 for the TRS-80 colour computer and Dragon 32? I ran it on the former, and it was certainly multi-tasking and even multi-user.

[Skywave]

Anyone here remember the Roband 'red brick' regulators with gold-coloured pins emanating from underneath? They were an all-resin encapsulated PSU regulator block. They date from the late 1960s / early 1970s.

Al.

[Top Cap]

Oh Yes! Awful things that put your MTBF (mean time between failure) figures in just double hour figures. I opened one up once (5V unit), not an easy task, like breaking into a diamond! It was heinous, a birds nest without a pcb and appeared to be a BFY51 driving a 2N3055. It looked like a lash up without a heat sink, then drop into a potting box, then pot with a material so nobody will ever see what they are paying $$ for. Never liked their stuff after that, would never ever use any of their supplies in designs, but came across the same sins on other stuff they made which came my way for repair. Urgh!

[Skywave]

Yes: driven by curiosity, I opened up one too - and found what you did.

Al.

[cheerfulcharlie]

Presumably those old computers found themselves in allsorts of places as the technology was introduced? those resin filled supplies got them more brownie points (probably ensured they were 'intrinsically safe') if say they were in an area of petrochemicals.