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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