Convert master psu to bench psu

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Elminster
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Convert master psu to bench psu

Post by Elminster » Sat Jun 23, 2018 9:32 pm

Had a new psu for the master sitting around for a couple of years, SimonI project, next on my todo list.

I was thinking I might reuse the old psu as a bench psu, should provide enough amps for what I need and already had caps replacement done. This is often on ATX PSUs. The only issue with the master is I assume it doesn’t have a 3.3v rail hiding in there.

So if I want have outputs for 3.3, 5 and 12volts I am going to need to take a feed from the 5volt and level shift it.

There are several ways I could do that, resistors, diodes or by a ready made level shifter. Any one got any opinions on the best way to do it?

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danielj
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Re: Convert master psu to bench psu

Post by danielj » Sat Jun 23, 2018 9:42 pm

I wouldn't lash something up to give 3.3V - it won't be particularly accurate. There are plenty of devices that will do the job for you though, regulators as you say, and these little wonders too: https://uk.rs-online.com/web/p/switchin ... s/9068478/

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Re: Convert master psu to bench psu

Post by cmorley » Sat Jun 23, 2018 9:49 pm

Just use a linear regulator from the 5v output.
e.g.
https://www.rapidonline.com/st-ld1117v3 ... or-82-3066

50p

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Elminster
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Re: Convert master psu to bench psu

Post by Elminster » Sat Jun 23, 2018 10:30 pm

I have a load of 7805s for 12 to 5 but nothing for 5 to 3.3. would I need a couple of electrolytic caps?

I admit the 50p over £12 option is pretty tempting, will look at data sheets later.

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Re: Convert master psu to bench psu

Post by MartinB » Sat Jun 23, 2018 10:58 pm

Duncan wrote:...would I need a couple of electrolytic caps?
Yep, if you’re going to do the 5v regulated to 3.3v as Chris suggested, just use a 10uf Tantalum (hence electrolytic) from the regulator input to ground and a second from the output to ground.

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Elminster
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Re: Convert master psu to bench psu

Post by Elminster » Sat Jun 23, 2018 11:04 pm

MartinB wrote:
Sat Jun 23, 2018 10:58 pm
Duncan wrote:...would I need a couple of electrolytic caps?
Yep, if you’re going to do the 5v regulated to 3.3v as Chris suggested, just use a 10uf Tantalum (hence electrolytic) from the regulator input to ground and a second from the output to ground.
Don’t think I have those in the parts bin, will have to order those as well.

Thanks all.

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Re: Convert master psu to bench psu

Post by hoglet » Sun Jun 24, 2018 1:48 pm

What are you looking to use the power supply for?

I would generally advise against using a Master/Beeb (or PC) power supply as a bench power supply.

One of the most useful features of a bench power supply is the current limit. You set it just above what you expect your project will need. Then, if there are any mishaps like slipped scope probes, you are much less likely to cause damage (or destroy your nice scope).

Another reason is the Master power supply has an internal fuse, which is a pain to access to change.

It's definitely worth searching eBay for a good used bench power supply. This is a decent one:
https://www.ebay.co.uk/itm/Thandar-TS30 ... 3776727680

Look for one with a separate output on/off switch, as well as the main switch. That lets you switch on and check the voltage / current limit value before applying power to your circuit.

Why not sell the Master power supply and put the funds towards a dedicated unit?

Dave

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Re: Convert master psu to bench psu

Post by flaxcottage » Sun Jun 24, 2018 2:28 pm

I second that - from experience. I have an old Farnell 0-30v, 0-2A variable PSU which is similar to the Thandar one.
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Re: Convert master psu to bench psu

Post by CMcDougall » Sun Jun 24, 2018 3:01 pm

Just use a PC desktop ATX PSU.
Depending on age, it has -5 & -3.3 already.
plenty YouTube vids on how to do it, but it's a no brainer anyways .
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Re: Convert master psu to bench psu

Post by hoglet » Sun Jun 24, 2018 4:14 pm

CMcDougall wrote:
Sun Jun 24, 2018 3:01 pm
Just use a PC desktop ATX PSU.
Depending on age, it has -5 & -3.3 already.
plenty YouTube vids on how to do it, but it's a no brainer anyways .
A typical 300W ATX Power Supply has 5V at 20 amps and 3.3V at 16 amps.

Do you know how much damage an accidentally touching a scope ground on the +5V line would do?

So please don't just use an ATX power supply!

Dave

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Re: Convert master psu to bench psu

Post by CMcDougall » Sun Jun 24, 2018 5:07 pm

/\ he wanted -5 & -3.3 so hence use a ATX :roll:

they only throw out .5amp'ish so perfect for other stuff to attach.

New ones don't have -v anymore :evil:
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Re: Convert master psu to bench psu

Post by CMcDougall » Sun Jun 24, 2018 5:15 pm

plus why anciently touch the wrong colours?? unless colour blind :shock:
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Elminster
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Re: Convert master psu to bench psu

Post by Elminster » Sun Jun 24, 2018 8:48 pm

I think you are all missing the point. :D The idea is not to get a great bench psu but just to do it because I can! So no I don’t want to buy a bench psu or an atx psu as that would defeat the purpose.

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Re: Convert master psu to bench psu

Post by CMcDougall » Sun Jun 24, 2018 9:26 pm

still think its a bit of a waste & curfuffle 8)

unless you took the broken M128 apart & used the key switches for an electron! =D>

If it's in good condition , ie Not rusted to bits as they usually are, Retro clinic /MarkH was after them a while ago, so must be scarce compared to B Micro. :shock:
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Elminster
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Re: Convert master psu to bench psu

Post by Elminster » Sun Jun 24, 2018 11:02 pm

CMcDougall wrote:
Sun Jun 24, 2018 9:26 pm
still think its a bit of a waste & curfuffle 8)

unless you took the broken M128 apart & used the key switches for an electron! =D>

If it's in good condition , ie Not rusted to bits as they usually are, Retro clinic /MarkH was after them a while ago, so must be scarce compared to B Micro. :shock:
Broken? Waste? Nothing broken or being wasted, I just have a spare psu I want to make use of, and I never said it wouldn’t be reversible. I don’t need to remove any connectors, just put it in a box so I don’t electrocute myself.

Edit: And add the circuitry for 3.3v, it was more for something to do than any real need to do it.

Edit2: And remember it loses its metal shielding to the new psu, and no there is no rust on either of my masters.
Last edited by Elminster on Sun Jun 24, 2018 11:16 pm, edited 2 times in total.

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Re: Convert master psu to bench psu

Post by 1024MAK » Mon Jun 25, 2018 10:32 am

The selection of the regulator or DC/DC converter for the extra circuitry to provide the 3.3V depends on your requirements. If you only want a relatively low current output, use a low voltage drop series pass voltage regulator. This will suit most requirements. Otherwise go for a DC/DC converter.

Although tantalum capacitors are often recommended, it's often possible to use a low ESR electrolytic in parallel with a 100nF multilayer ceramic capacitor.

There is a relatively simple circuit that can add an adjustable current trip function. This is NOT capable of regulating the current. Instead it is the electronic equivalent of a MCB. If you are interested, I'll dig up a schematic.

Mark
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Elminster
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Re: Convert master psu to bench psu

Post by Elminster » Mon Jun 25, 2018 12:28 pm

1024MAK wrote:
Mon Jun 25, 2018 10:32 am

There is a relatively simple circuit that can add an adjustable current trip function. This is NOT capable of regulating the current. Instead it is the electronic equivalent of a MCB. If you are interested, I'll dig up a schematic.

Mark
Any info/ideas you have would be great. I remember watching an EEVBlog episode where he said eveyone should build their own bench psu, this is kind of a halfway house.

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Re: Convert master psu to bench psu

Post by Elminster » Mon Jun 25, 2018 3:34 pm

For the 12v/5v/3v side of the PSU. Either the Beeb or the Mean Well PT-65A for that matter. Would the following be up to the job. (Marine wiring 11Amps at 12/24Volts).

Looks reasonably priced compared to RS/Farnell etc. Wondered if I was missing something.

https://www.ebay.co.uk/itm/11-AMP-Rated ... 66cabddade

Obviously the mains side (for power switches) would need something more along the lines of

https://www.ebay.co.uk/itm/Single-Core- ... ZFwsTVzJXg

Surprising how epensive wire is, especially as the componet companies often sell it in big reels.

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Re: Convert master psu to bench psu

Post by Elminster » Thu Aug 09, 2018 10:09 pm

Made progress on this in that I ordered a front end in the form of a DPS3005 with the USB and Bluetooth modules.

i.e. of the sort found here

https://www.aliexpress.com/item/RD-DPS3 ... 16119.html

I also looking at a 220v powerswitch, IEC kettle panel recepticle and a plastic case

Although I am now tempted to power it with a Meanwell 36V 150W DC PSU (LRS-150-36).

https://www.meanwell.co.uk/lrs-150-36.html

Which would nearly be able to deliver the 30V 5A the front end can do.

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Re: Convert master psu to bench psu

Post by Elminster » Sun Aug 12, 2018 3:38 pm

hoglet wrote:
Sun Jun 24, 2018 1:48 pm
One of the most useful features of a bench power supply is the current limit. You set it just above what you expect your project will need. Then, if there are any mishaps like slipped scope probes, you are much less likely to cause damage (or destroy your nice scope).
My variable 0-30v & 0-3a bit should be sorted

But might add a constant 5v/2.5a (or 3a) posts. So taking a beeb Master psu you could build in the protection. But because of the voltage drop using the regulator I assume would need to drive this from the 12v supply. A guesstimate is would give 2.5 Amps, unless you actually chucked to a new meanwell and that would give the 3amps. You could also, wheels spinnning in head, use the unregulated 5v and -5v to still drive a real master or Beeb, if it had a psu issue, I.e. have some sort of harness.

You could also then have a switch that switched over the regulated 5v/3a to a standard unregulated Beeb external connector. Which you could then drive external devices from, like Simon’s example above.

So this sort of merges, the variable bench psu, protected fixed voltage/current idea and simons external box connector idea into one.

Wheels still spinning. Could have a switch to switch the 5v between, 5 and 3.3v.

All I then need is some extra temperature sensors, a fan control and some large fans. And a case to put it all in.

I should never have started watching the eevblog series of usurpply build your own power supply videos ....

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Re: Convert master psu to bench psu

Post by Elminster » Fri Aug 17, 2018 5:20 pm

Got the 200w PSU and the digital regular.

Just need a case, a switch, some wire and a plug
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Re: Convert master psu to bench psu

Post by Elminster » Mon Aug 20, 2018 10:30 am

Was thinking about Earth strapping (aka grounding) and such like, and wodnered whther it is worth adding an Earth (green) binding post to bench psu. Anyone ever use this? I know you can do things like use it for putting psu in series or creating. negatie voltage, but not sure how useful that would be.

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Re: Convert master psu to bench psu

Post by 1024MAK » Mon Aug 20, 2018 2:53 pm

Is it worth having a ground/earth terminal post on a bench PSU? Yes.

However, how it is used is another matter...

When it comes to ground/earth, there are two types of power supply arrangement. Grounded / earthed and floating.

With a grounded / earthed arrangement, one of the DC output conductors is linked electricity to the ground / earth terminal / connection on the incoming supply cable. Usually (but not always) this is the 0V connection on the output. If the PSU only has one output, this is sometimes called the negative output. This type of PSU cannot be connected in a series arrangement to increase the output voltage of the system.

The PSU used in the BBC B is this type.

Floating PSUs have (an) output(s) that have/has no intentional connection to the ground / earth terminal / connection on the incoming supply cable. So if you connect a load between any output connection and ground / earth, no current should flow. The advantage is that you can connect the same type of PSUs in series to produce an overall greater output voltage.

A lot of PSUs are of this type due to them being class II and therefore having no ground / earth wire in the mains lead.

So if the SMPSU you are using has floating outputs (and no internal connection to ground / earth), then if you need to use the bench PSU in place of a PSU that normally has a ground / earth referenced output, just link the appropriate (often 0V) connection to the earth terminal on your bench PSU.

The earth terminal is also handy for connecting an anti-static wrist strap to...

Mark
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Re: Convert master psu to bench psu

Post by Elminster » Mon Aug 20, 2018 3:12 pm

The intial SMPSU (36V 6A) has the earth terminal, this then feeds into the DC-DC Buck type 30A/5A unit, that unit only take +/- and has no earth connection. From this you then have you feed to your circuit.

The case i have ordered is metal so I planned to earth the case and the main input PSU to mains earth. I can connect another binding post to the front, which would then conncted to the case/mains earth.

So I end up with both types of psu in my bench supply but the one feeding the circuit is flaoting.

If I understand correctly.

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Re: Convert master psu to bench psu

Post by 1024MAK » Mon Aug 20, 2018 5:34 pm

If the SMPSU (switch mode power supply unit) internally has earth (ground) connected to any of the DC output connections, it's not a floating output. Some SMPSU have earthed outputs, but most do not (and their outputs are floating). The documentation should say which type it is.

Or you can test it. With it not connected to anything, you can test it using the resistance range on a multimeter. Test between the earth connection point and each output terminal in turn.

The metalwork should always be earthed unless it is a class II appliance with insulation that meets the relevant standards.

Mark
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Re: Convert master psu to bench psu

Post by Elminster » Mon Aug 20, 2018 9:17 pm

1024MAK wrote:
Mon Aug 20, 2018 5:34 pm
If the SMPSU (switch mode power supply unit) internally has earth (ground) connected to any of the DC output connections, it's not a floating output. Some SMPSU have earthed outputs, but most do not (and their outputs are floating). The documentation should say which type it is.

Or you can test it. With it not connected to anything, you can test it using the resistance range on a multimeter. Test between the earth connection point and each output terminal in turn.

The metalwork should always be earthed unless it is a class II appliance with insulation that meets the relevant standards.

Mark
Both PSUs fitted internal. One is AC to DC, the second is DC to DC (a DPS3005). The documentation will be in uneliable pseudo English so I would test it any way.

You can run the DC to DC off battles.
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Re: Convert master psu to bench psu

Post by Elminster » Mon Sep 17, 2018 9:17 am

The case is made of 4 metal sections. I am thinking of going a bit strap crazy and strap them all.

Wish I had a laser cutter!

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Re: Convert master psu to bench psu

Post by Elminster » Fri Sep 21, 2018 8:18 pm

Slowly getting there. I use the word 'prototype' to negate wonky holes. Not actuall a master psu in sight of course. That idea went away.

Has 2 PSUs. 1 for the 30v/5A supply and the other drives the fans and eventually the 5V/3.3V 1.5A supply. (Hence the extra holes, the final extra hole is for Earth). Still have a couple of switch to fit on the front before giving it a clean. Power and Kettle receticle on the back. Probably a few wires as well.
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Re: Convert master psu (or not) to a bench psu

Post by Elminster » Mon Sep 24, 2018 3:21 pm

Part one of project complete. Channel 1 (32v, label is incorrect) power working. Fan controller with 4 fans is working (despite initally I reversed +/- and had smoke coming out of the fan controller).

Channel 2 is something I need to design a custom pcb that specifically fits what space I have left in case, this is part 2. It will be a switchable 5v/3.3v only supply, running of the same PSU as the fan controller. The whole is for the switch, misplaced the nuts for the moment.

There are 3 earth connection on case, and both psu's are Earth, and the Earth Terminall at the front is also Earthed.

No expense spared on the labelling. Next I need to see if I can PC to talk to the Bluetooth module on the psu.
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Re: Convert master psu to bench psu

Post by 1024MAK » Mon Sep 24, 2018 5:46 pm

Powerful stuff :D

Mark
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