Failing SPS705 power supply (68vdc Leadshine)

[dsc]

Just a quick question for all the electronic brainiacs out there who know a lot about power supplies.

I'm currently using a Leadshine SPS705 (68vdc) unregulated power supply which seems to be on it's last legs. Output voltage is fluctuating really badly, going from zero to 50ishV and back down, often failing to even reach 50vdc and slumping down to barely 20ish vdc. No idea what failed or why it failed, it is only used 2-3 times a day and only for 10-15sec, then off all the time, has a single stepper motor on it with a DM856 controller.

Plan of action is to check all the diodes in the bridge and possibly try to repair or get a new unit, but it will take around 20min to get the current unit out of the machine so I wanted to double check what it might be before trying to fix.

Rgds,
T.

[john swift]

seeing its an unregulated switch mode supply
it could be a fault with the stepper motor or driver is causing the supply to protect its self

if you do open the power supply
you will find the bridge rectifier is connected across live & neutral watch out - the large capacitor may take a while to discharge the 340V DC it has across it when the mains supply is on

John

PS
do you have a 60 or 100W GLS bulb you can use as a dummy load to test the power supply ???

*GLS bulbs are old fashioned incandescent light bulbs with the tungsten filament the greens don't like

[dsc]

Thanks for your reply John and for the tips on the cap charge!

Indeed it might be a faulty controller / stepper, I simply jumped to the conclusion it's the PSU as the voltage was fluctuating so much, first only moving between 40-60VDC, then slowly drifting more and more towards lower voltages, now only moving between 0VDC and 20ish VDC. Oddly enough the stepper still works, not straight away after switching the power on, but if you leave it for a few minutes the voltage must creep above 20VDC as this is the absolute minimum the DM856 can work.

The PSU is overspec'd for the stepper set up as it stands, I'm using a SY57STH56-3008B Nema 23 stepper and all it does is move a few steps each way, a few times a day, then the whole lot is powered off. There's also an additional circuit added on the output of the PSU which is a Zener diode used to drop the voltage from 68VDC to 24VDC which can then be used to drive a panel mounted indicator, see here:

http://www.mycncuk.com/threads/7948-daft-question-on-LED-indicators-and-high-VDC

No idea if this is somehow affecting the PSU over time and causing a failure? Another thing which happens fairly often is powering the kit on / off, typically 2-3 times a day, sometimes more often. There is a servo with a controller mounted on the same AC supply (it's got an AC filter mounted to reduce noise when it comes on / off), but it has it's own MCB, the SPS705 is powered via an additional switch, so even if there was some back current from the servo controller startup, the additional switch should protect the SPS705 (or so I think).

Regards,
Tom

EDIT: not got any GLS bulbs at the moment but I can order some I reckon (assuming they are still available)

[john swift]

the last GLS bulbs I bought was from a local discount store
https://www.bmstores.co.uk/search?q=light+bulb
but i see the nearest the have now are 240v Halogen Bulbs
https://www.bmstores.co.uk/products/eveready-100w-b22-eco-halogen-bulb-3pk-322405

ebay & amazon have them

RS Components have these
https://uk.rs-online.com/web/p/gls-incandescent-light-bulbs/0604624/

I guess Rough Service Lamps for industry are excluded from the ban

the first time I used a GLS bulb as a dummy load was when I built the Forgestone 500 TV in 1982

24460

the 143V DC switch mode power supply had to be set up correctly before it was connected to the rest of the receiver
to avoid the EHT going over 24KV and the CRT giving you an X-ray over dose



John

[cropwell]

You can still get the 100W Halogen filament strip lights they use in work lights at places like Toolstation. Would these be OK? It is only a resistive load you need. John - What do you think ?

[john swift]

I am not sure how the cold resistance of a Halogen bulb compares with that of a GLS bulb

As long as the cold resistance of the halogen lamp is not too low for the power supply to start
the Halogen lamp should be OK
cold the 240V 100W GLS bulb about 43 ohms


to setup the Forgstone TV Kit
the GLS bulb was used as a low cost alternative to a
600 ohm 100w resistor

since then I have use the bulbs to test other things like the speed controls used in a Clarke CL300M lathe

https://www.model-engineer.co.uk/forums/postings.asp?th=38809

John

[cropwell]

I am not sure how the cold resistance of a Halogen bulb compares with that of a GLS bulb
cold the 240V 100W GLS bulb about 43 ohms

The only halogen security light I have is 500W and that measures up at 8 ohms cold, so 40 for 100W seems a reasonable guess !

[dsc]

I'll get one from ebay if I can't find any locally.

Is there anything to pay attention to when hooking it up to the PSU? was thinking of using a lamp holder with two wires connected to the PSU to make sure all the connections are proper.

T.

[dsc]

Quick update to this thread, I've had another look at the way everything is wired and it turns out both this PSU and a servo controller is on the same AC supply, fired off the same main switch. There's an AC filter on the AC supply to stop anything flowing back into the AC line, but there isn't really anything protecting the PSU on the AC side. No idea whether any damage can happen this way when the servo controller is switched on / off, but I thought I'd check.

Also, just noticed that if I leave the entire rig powered for a long time (ie. over 10minutes), the PSU eventually produces around 40VDC which is fairly steady. Could this be related to a dead capacitor perhaps which is simply taking a very long time to charge? again, no idea if this even happens in a PSU, so just throwing an idea out there.

T.

[cropwell]

Could this be related to a dead capacitor perhaps which is simply taking a very long time to charge? again, no idea if this even happens in a PSU, so just throwing an idea out there.

I have not known a capacitor to fail in this way, normally they bulge and short circuit (BANG!) if bad enough.

[Clive S]

Also, just noticed that if I leave the entire rig powered for a long time (ie. over 10minutes), the PSU eventually produces around 40VDC which is fairly steady. Could this be related to a dead capacitor perhaps which is simply taking a very long time to charge? again,

Have you measured it with a load on. You could have a very high resistance (dry joint) to account for the long charge time.