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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 15th Sep 2019, 4:18 pm   #21
turretslug
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Default Re: Adjustable voltage 15 amp power supply

I think that the ceramic button on the base is simply to stop the package being forced too close to the PCB (say a rigid GRP board with tight and slightly-imprecisely-placed holes) and possibly fracturing/impairing the insulating seals and resulting in die contamination, or causing excess heat/cool stress fatigue. A bit like the hairline cracks that could occur from stressing a glass-base valve's pins.

A good design shouldn't cause the 723 package to get overly warm- but a squiggly TO-5-type heatsink will do no harm, I'd fit one and have done as a hobbyist with a few in the bits drawer, rather than an OEM worried about the bottom line.
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Old 15th Sep 2019, 5:14 pm   #22
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Default Re: Adjustable voltage 15 amp power supply

All the circuits shown are "linear regulator" types; these can work well when the load-delivery is within narrow margins [ let's say 2 or 3:1]

If the load-demand varies between 5:1 10:1 or 20:1 the requirements are a bit different - and in such circumstances a 'closed-loop- SMPS makes a lot more sense.

If you want to supply 10A at 6V and also 20V at 10A 'linear' regulator is going to be dissipating horrible amounts of waste-power.

This sort of thing is really, really the kind of instance where a switching PSU is going to be the way ahead. Spending 50 on a proper SMPS can easily be recovered in terms of the electricity-bill over the next couple of years!
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Old 15th Sep 2019, 5:42 pm   #23
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Default Re: Adjustable voltage 15 amp power supply

I have considered commercial SMPS and even bought a 20 amp one form Watson & Stanton.
But the problem with them is they all seem to have the mains earth connected to the negative DC output.
If anyone can recommend one which does not have an earthed output I am happy to try it.

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Old 15th Sep 2019, 7:40 pm   #24
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Default Re: Adjustable voltage 15 amp power supply

I have also considered getting one of those LED SMPS, they are cheap enough to not worry about invalidating its warranty by modifying the output to isolate it.
Do you think it would be possible to isolate the negative OP from earth.

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Old 15th Sep 2019, 7:55 pm   #25
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Default Re: Adjustable voltage 15 amp power supply

Quote:
Originally Posted by turretslug View Post
I think that the ceramic button on the base is simply to stop the package being forced too close to the PCB (say a rigid GRP board with tight and slightly-imprecisely-placed holes) and possibly fracturing/impairing the insulating seals and resulting in die contamination, or causing excess heat/cool stress fatigue. A bit like the hairline cracks that could occur from stressing a glass-base valve's pins.

A good design shouldn't cause the 723 package to get overly warm- but a squiggly TO-5-type heatsink will do no harm, I'd fit one and have done as a hobbyist with a few in the bits drawer, rather than an OEM worried about the bottom line.
THe can is electrically connected, usually to the substrate
The ceramic pill prevents the package sitting down onto the board and the can touching any topside tracks or groundplane.

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Old 15th Sep 2019, 11:58 pm   #26
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Default Re: Adjustable voltage 15 amp power supply

Aha yes, that makes good sense- obvious now that you mention it! Especially if there's things like close-by, slightly-protrusive vias and the like.
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Old 16th Sep 2019, 1:21 am   #27
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Default Re: Adjustable voltage 15 amp power supply

If used within spec, the metal can '723's don't need a heatsink (and if it's getting that warm, somethings wrong).

What they do need however (also applies to DIL version) is good spike suppression of the incoming supply.

Had a lot of trouble with them destroying themselves until we worked out it was the spike caused by the on/off switch that was the cause of the problem.

Fitting the line filters after the switch cured it.
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Old 16th Sep 2019, 8:59 am   #28
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Default Re: Adjustable voltage 15 amp power supply

723s are also fiendishly sensitive to any RF around them, so power supplies for transmitters and receivers have to be very well filtered to stop RF getting in via mains or output connections. Treated properly, they work very well as fixed voltage supplies.

If you want to make a variable supply to go up to a chosen voltage, and down to damn all, then the 723 isn't for you, and also schemes using 3-terminal regs run into trouble.

The second hand market has plenty of nice high power power supplies by Farnell and HP. I'd just buy one of these If I needed that sort of output. Telford Electronics and Stewart of Reading are worth checking. The latter may sound like a hair salon, but it's a long established and reputable surplus test equipment dealers.
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Old 17th Sep 2019, 7:55 am   #29
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Default Re: Adjustable voltage 15 amp power supply

Agreed. One of the killers isn’t actually the circuit which is quite easy but the thermal design. Consider shorting out the supply at 15A with an output voltage of 2V and input of 24V, both reasonable values. You’ve got to lose 360W somewhere which is quite difficult. Big supplies don’t do that - they use preregulation.

This is a problem with LM317 designs you see everywhere. A typical specification is 1.2-20V at 1.5A. Which is fine until you ask 3.3V at 1.5A of it. Min spec that’s 23V in and 3V out so 30 watts going out in that little LM317 on a tiny chinesium heatsink. Bang it goes! Not immediately either as they sort of work outside their SOA for a bit then go floppy and then explode.

I’ve got a meaty HP 6286A here that can shift 0-20V at 10A which is less than prescribed. It’s large, heavy and complicated!
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Old 17th Sep 2019, 8:42 am   #30
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Default Re: Adjustable voltage 15 amp power supply

Also, failing regulators prefer to fail shorted, so whatever 3.3v circuitry you're powering gets the thrill of the full unregulated >24v

In the design of some big piece of equipment, it's traditional to assign the power supply to the most junior designer, if a student isn't available Yet, power supply design is quite subtle and a power supply fault can usually take out the whole of the rest of the equipment.

Radio amateurs spend thousands of pounds on a transceiver, going for the one with the most features bells and whistles and then power it from the cheapest '30 amp' power supply they can find.

Spend time on designing a good power supply. Add a crowbar if possible.. Some professional variable supplies have tracking crowbars.

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Old 17th Sep 2019, 9:48 am   #31
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Default Re: Adjustable voltage 15 amp power supply

In the light of the helpful comments I am reassessing how I tackle this. I am thinking if I am going to make a linear regulated supply that it needs to be far less adaptable and more specific to its main purpose.
Additionally I am thinking of buying a cheap LED SMPS, they are only about 15.
Are these sort of units likely to fail to high voltage, I would think if they fail they would go to zero output. They are supposed to have short circuit protection.

"Input: AC110-220V 50/60Hz
Out put: DC 12V
Protections: overload/ over voltage/ short circuit
Cooling by free air convection
(Not suggest for use in bathroom)
Connection:
L, N: AC power input
GND: DC power output "-"
V: DC power output "+"
+V / ADJ: Adjust the output voltage
Safety Compliance: CCC/ FCC / CE"

Has any one experience of these.
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Old 17th Sep 2019, 6:33 pm   #32
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Default Re: Adjustable voltage 15 amp power supply

I'm not sure how the OVP works in those (a crowbar would prevent the following condition), but a common failure mode in cheaper switch mode supplies is the feedback loop going open. The output voltage may more than double under such conditions.
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Old 17th Sep 2019, 8:06 pm   #33
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Default Re: Adjustable voltage 15 amp power supply

That picture looks like a Mean-Well. A strange choice of name I always thought. Much better to be Done-Well.
Just my thoughts over some years...

The thing I don't like about most of them - for LED use or otherwise - is in-rush current. There is usually plenty of it. I wouldn't put such on a lighting switch since early contact burn is likely.

Also as an SMPSU it will probably generate RF noise - if that matters to you.
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Old 18th Sep 2019, 1:10 pm   #34
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Default Re: Adjustable voltage 15 amp power supply

There's a parallel thread going on Golborne about that supply. It's definitely not a Meanwell, although it does look awfully like one. Meanwell make extremely good power supplies, whereas this one is a cheap and nasty supply that bears a(n illegal) CE mark, despite not having PFC and very limited input filtering and no output filtering. No crowbar either. More details of that supply can be found here: http://www.imajeenyus.com/electronic...ge/index.shtml (complete with schematic and video).
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Old 18th Sep 2019, 3:54 pm   #35
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Default Re: Adjustable voltage 15 amp power supply

Thanks for the warning. Yes I should have added Meanwell have never given me any problems. I just smile at the choice of name and how they decided on it.
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