Power supply problems for CNC rotary table

[Steve CrowParticipant @stevecrow46066]

I recently bought a Sherline CNC rotary table and controller. It came with a USA spec 120V -24V transformer and a UK spec 240V – 120V step down converter.

After about an hours use, the whole set up stopped working.

I got in touch with Mill Hill Supplies who sent me one of each of the units so I could determine it was the UK – USA converter at fault.

After another hour and half use, the new one packed in!

I'm reluctant to get yet another converter as there must be something fundamentally at fault here.

The first unit is labelled as follows:-

Input: 120V AC 38W

Output: 24V DC 1000mA

The second unit:-

Input: 230V

Output: AC 110V 45VA

I have a huge blind spot when it comes to all things electrical, so apart from the volts, the numbers mean nothing to me.

Can anyone advise me on what might be going wrong? Or a possible alternative method of power supply?

Many thanks,

Steve

[Steve CrowParticipant @stevecrow46066]

[EmgeeParticipant @emgee]

Hi Steve

You could use a 230v ac to 24v 1A DC power supply. (or go for a 2A unit)

Do you have any test meters to confirm the load taken by the rotary table, it may need more than the 1A supply is capable of on a continuous operation.

Emgee

[Michael GilliganParticipant @michaelgilligan61133]

Steve,

There seems to be something seriously wrong in the 'Supply Chain' communication !!

If the markings are as you indicate then:

The first unit is for use on American mains, and it supplies DC to the controller

The second unit lowers UK mains voltage to American level AC

Therefore … You need to connect them in series

And it certainly should not be possible to do what you appear to have done.

Hopefully I have mis-understood what you wrote.

MichaelG.

.

For peace of mind … could you please post a photo or two ?

Edit: On further reflection, I'm sure I must have mis-understood

That said: I find the low current of the DC supply rather surprising

https://www.sherline.com/product/8700-cnc-4-rotary-table-indexer/#specifications

Edited By Michael Gilligan on 09/07/2019 19:24:07

[JasonBModerator @jasonb]

Not sure how Steve could have connected them up or what he "appears to have done" but they just come with a US 2 pin wall wart that puts out 24V DC so you just plug that into a stepdown converter UK3pin/ US 2 pin 240 to 110.

I'd do a s Emgee says and try to get a wall wart that you can just plug in at home ther gives 24V DC, Shereline suggest this as an option.

 

 

Edited By JasonB on 09/07/2019 19:24:24

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 09/07/2019 19:21:35:

Not sure how Steve could have connected them up or what he "appears to have done" but they just come with a US 2 pin wall wart that puts out 24V DC so you just plug that into a stepdown converter UK3pin/ US 2 pin 240 to 110.

.

My mis-understanding, I'm sure, Jason

Confused by the reference to 'first unit' and 'second unit' whilst discussing substitutions.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Following yesteday's confusion …

I remain astonished that the device is powered by a mere 24W supply

The performance of these little tables is legendary.

MichaelG.

.

https://sherline.com/wp-content/uploads/2015/12/8700inst.pdf

https://www.sherline.com/product/8700-cnc-4-rotary-table-indexer/#video

Edited By Michael Gilligan on 10/07/2019 10:05:05

[SillyOldDufferModerator @sillyoldduffer]

Strikes me the finger of suspicion is pointing at:

The second unit:-

Input: 230V

Output: AC 110V 45VA

The table, motor and keypad controller are OK because they started working again after replacement power supplies were received.

Steve: to eliminate any doubt did you replace both power units or just the 230 to 110V converter? (Ideally just the 230 to 110v unit!)

That the unit ran for up to 1½ hours suggests something is overheating. As Michael remarked, it's feels odd that Sherline are running a 2A stepper and controller from a 24W DC supply, but they probably know what they're doing! Anyway the web isn't full of complaints that the unit is unreliable, and there's no mention in Michael's pdf instructions about allowing cooling-off time.

I'm suspicious of the 45VA unit, which is also on the weedy side. It probably contains a small transformer protected by a thermal fuse. If the transformer gets too hot, or the fuse is oversensitive, it will cut out. These things are usually sold as travel accessories, for example so a 240V UK electric shaver can be used in a 110V US Hotel Room. Maybe you've been given a unit designed for 10 or 15 minutes shaving, and it can't cope with an hour's solid work? (How hard are you working the table – roughing out spokes on a big steel flywheel or delicately putting teeth on a thin brass gear? If the former, don't forget the Sherline is a dinky little machine, not a chop saw!)

If the item is under warranty, speak to the supplier. Otherwise, 240 to 110V converters are inexpensive. You could try one rated for 100W, or even 200W like this example.

Dave

[Ian S CParticipant @iansc]

I maybe wrong(quite likely) could it (the 230v to 110v power supply), be frequency dependent, I think transformers for 60 hz have less iron than a 50 hz one, this could cause over heating if the load were to get near the transformer rating, although I suppose it's an electronic rather than transformer supply.

Ian S C

[not done it yetParticipant @notdoneityet]

VA is not Watts. Likely a Cos Theta of 0.8?

The US Spec DC supply will be rated at 60Hz.

Yes, you can get away with mixing VA, Watts, and 50 or 60 Hz but you do need to know what you are doing.

I would suggest the step down transformer is likely only 36W

Step down transformers with diode bridge (with smoothing capacitor?) are not that efficient and may well require rather more power than the 240-120 transformer can supply, especially with the mixed power factors and Hz.

What a mismatch of bits!

[Robert Atkinson 2Participant @robertatkinson2]

First thing first,

It is ILLEGAL for a UK company to supply a US only specification power supply for use in the UK.

The best option is to get a proper UK approved power supply. The 120V converter is rated at 45 VA (Volt Amps) this means only resistive (or PF=1) loads can be at the full 45 watts. The 120-24V supply is only 24VA output an input is likely to be 1.1 times this. BUT its an electronic supply and these have very poor power factor so VA at the input is likely to be well above 45VA. Add in the fact that we are 50Hz and the mains voltage is likely to be nearer 240V than 230 The little converter is over stressed. It is also probably an autotransformer and the 24V supply is only rated at 120V there is a shock hazard if the neutral of the transformer or it's connection fails.
I'd be less worried about a 24V 1A supply on a notionally 2A stepper motor because the controller is likely to limit the current and may even have a switch mode regulator that will increase the current available. What plug is on the controller end of the 24V supply? I almost certainly have a supply that will fit this and is properly designed and approved.

Additionally, is the controller CE marked? No disrespect to Bryan Mumford but I'll bet it's not. Again it's illegal to sell this in the UK unless it meets the required regulations and is CE marked. In this case at least the EMI regs apply to the controller and LVD to the power supply.

Robert G8RPI.

Robert G8RPI.

[EmgeeParticipant @emgee]

This or similar rating would be my choice of unit,

**LINK**

Emgee

[Clive FosterParticipant @clivefoster55965]

Those step down wall warts are spectacularly inefficient. I have ones of the same rating to run the 110 V solenoid output valves on my Bijur Spraymist systems. They get very warm indeed. Too hot to hold for a while, OK to touch. But haven't popped (yet). If I forget to switch them off they get quite warm to the touch.

Mine say Cos Theta is 0.73.

About thruppence ha-penny a pop on E-Bay so wasn't expecting much more than just good enough. No way on earth would I send one out if selling new tools and similar equipment was part of my a business tho'.

As Emgee suggests a proper 240 volt PSU can be got at modest cost.

Robert is correct concerning legality. But good luck in finding anything that really meets both the spirit and letter of the regs at this end of the market. Whatever the sticker says.

Clive

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Robert Atkinson 2 on 10/07/2019 13:00:48:

… I'd be less worried about a 24V 1A supply on a notionally 2A stepper motor because [ … ]

.

Just for the sake of pedantic clarification, Robert …

I wrote that I was 'astonished' by the 24W output rating of the DC supply; not that I was 'worried'.

The wider concerns that you express, I believe merit discussion/debate.

MichaelG.

[Steve CrowParticipant @stevecrow46066]

Many thanks for all the replies.

It is definitely the 240V step down at fault. It is too hot to handle after an hour. It is also only rated for an hour over 30 V/A (Which I assume is the same as Watts?).

I never considered the legality of the set up. The controller is not CE marked but does it need to as it's a 24V device?

I would like to use a proper 240V to 24V power supply as Emgee suggested but that would involve a knowledge of things electrical that I don't possess. I have neither the tools or the inclination to start "wiring" things.

I have seen plug-in 240V to 24V 1amp convertors on ebay and RS components sell dozens of different types but I can't be sure that the plug for the controller will be right. I've been told that it's a non standard size.

The solution that appeals at the moment is the 200W 240 to 110V as suggested by Dave. Can anyone see any pitfalls with taking this route?

My main concern is somehow frying the controller!

Any further suggestions or observations are more than welcome.

Steve

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Steve Crow on 10/07/2019 18:06:24:

Many thanks for all the replies.

1. It is definitely the 240V step down at fault. It is too hot to handle after an hour. …

2. I have seen plug-in 240V to 24V 1amp convertors on ebay and RS components sell dozens of different types but I can't be sure that the plug for the controller will be right. I've been told that it's a non standard size.

.

Responding to just those two points, Steve [my numbering]

1. That clearly means that your issue is with Millhill Supplies Ltd. … Who it appears have so far behaved very honourably, and would probably welcome the 'learning opportunity'.

2. So far as I can see from various illustrations; the non standard connector is a DIN Plug [as used on some audio equipment, and not to my personal taste for power supplies] … I will dig a little further.

MichaelG.

.

Interconnecting cable is shown here:

https://www.mscdirect.com/product/details/67274613

… Is that what you have, Steve ?

Edited By Michael Gilligan on 10/07/2019 18:29:22

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Emgee on 10/07/2019 13:50:45:

This or similar rating would be my choice of unit,

**LINK**

Emgee

That, on it's own is not suitable. It is a component, not a consumer power supply. To be safe it needs a housing, power inlet connector (or properly strain relief on the cable), fuse and output connector as a minimum. IT probably needs n EMI filter as well. Just sticking a mains lead on this is down right dangerous.

Robert G8RPI.

[Steve CrowParticipant @stevecrow46066]

Michael, Millhill have been very helpful with this but I will inform them about this problem.

The DIN cable is from the controller to the stepper motor. The power plug at the controller end is simple hollow cylinder type found on many low voltage devices. It is the size of this that is (allegedly) non-standard. It's 5.5mm dia and 10mm long.

Steve

[Robert Atkinson 2Participant @robertatkinson2]

The DIN connector is for the controller to stepper motor connection. The power seems to be a standard cylindrical power plug. These come in several size especally the centre hole. If @Steve Crow can tell me the OD and ID (gaugig with a pin or drill shank is best) of the plug and if it is positive or negative centre (should be marked on label) I can sen you a suitable NOS quality supply (Laptop "soap on a rope" style) for the cost of postage.

The controller does have to be CE marked for sale (or use unless personally inported) in the UK / EU. As it has a microprocessor it needs to meet EMC directive and even if it does not need to meet machinery, LVD or LVD it still needs the mark to confirm this. Unfortunatly most small importer in niche markes either don't know or don't bother.
The controller without PSU is $50 cheaper so that would buy a proper supply like
https://uk.farnell.com/xp-power/vet30us240c2-ja/adaptor-ac-dc-24v-1-25a/dp/2708291
even without the cost of the 120V adaptor.
I don't recommend buying mains powersupplies on ebay/amazon/banggood/aliexpress as you have no idea of the quality.

Robert G8RPI

(This stuff is part of my day job I'm a Chartered Engineer and have done CE/UL/CSA compliance)

Edited By Robert Atkinson 2 on 10/07/2019 19:32:29

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Steve Crow on 10/07/2019 19:05:21:

Michael, Millhill have been very helpful with this but I will inform them about this problem.

The DIN cable is from the controller to the stepper motor. The power plug at the controller end is simple hollow cylinder type found on many low voltage devices. It is the size of this that is (allegedly) non-standard. It's 5.5mm dia and 10mm long.

Steve

.

Thanks for the clarification, Steve

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Steve Crow on 10/07/2019 19:05:21:

… The power plug at the controller end is simple hollow cylinder type found on many low voltage devices. It is the size of this that is (allegedly) non-standard. It's 5.5mm dia and 10mm long.

.

I forgot to mention, earlier … those 'barrel connectors' come in a bewildering range of 'standard' sizes:

**LINK**

https://en.m.wikipedia.org/wiki/Coaxial_power_connector

MichaelG.

[Robert Atkinson 2Participant @robertatkinson2]

I can confirm that the correct power plug for Sherline / Mumford controller (at least the one Steve has) is a 2.5mm x 5mm coaxial DC power plug. Fairly standard part.

Robert G8RPI.

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