24V supply leaps to 35V when driver board connected!

[HankMcSpank]

I'm puzzled here.

I have a Mechatronics 3 axis driver board.

I have a 24V PSU (whichwith no load, outputs about 35V as measured with my Voltmeter)

I didn't want to connect it up to the driver board without putting a test load across the 24V first (to check)- so I connected up a 10K resistor - I then measured the supply voltage with my Voltmeter ....it was 24V.

Ok, so now I connect up my Mechatronics stepper driver board....at this point the voltage goes back to 35V (& this is with the 10k load resistor still in place). For the life of me I can't figure out why - anyone got a top tip or two?

(PS Not sure if it's significant...but when I connect my stepper board to the PSU, my stepper board also is connected back via a 25W ribbon cable to my laptop.

[Al_The_Man]

A 10k resistor across a 35v PS is not really a test load, It will hardly see the 3.5ma that would be drawn.
To provide a decent test load the resistor should be sized close enough to cause near the rated current to flow.
BTW that is a fairly high output for a supposedly 24vdc supply, even off load.
What is the rated current or VA of the PS?
Al.

[HankMcSpank]

Originally Posted by Al_The_Man A 10k resistor across a 35v PS is not really a test load, It will hardly see the 3.5ma that would be drawn. To provide a decent test load the resistor should be sized close enough to cause near the rated current to flow. BTW that is a fairly high output for a supposedly 24vdc supply, even off load. What is the rated current or VA of the PS? Al.

You make a fair point about the 10K resistor (but it's better than the PSU floating with zero load). I used this resistance as that's what I measured when I put my Voltmeter probes across the +ve & -ve terminals of the mechatronics stepper driver board.

The 10k resistor actually did it's job & the PSU output came down to 24V. (The PSU is quite chunky - its rated output it 15A @24V). I should add the purpose wasn't to load it (because even at 1K across 24V it's only going to draw 24mA), just to give the PSU a 'load' to see.

Just done one more test - I isolated the stepper driver board from the PC (disconnected the 25 way), so the only external connection to the stepper board is the 24V. I then put a 1k resistor across the PSU 24V DC output - tested fine again at 24V ....however as soon as I connect it up to the stepper board, once again I measure 35V?!!!!

This one has got me beat!

[Al_The_Man]

Originally Posted by HankMcSpank You make a fair point about the 10K resistor (but it's better than the PSU floating with zero load). I used this resistance as that's what I measured when I put my Voltmeter probes across the +ve & -ve terminals of the mechatronics stepper driver board. The 10k resistor actually did it's job & the PSU output came down to 24V. (The PSU is quite chunky - its rated output it 15A @24V). I should add the purpose wasn't to load it (because even at 1K across 24V it's only going to draw 24mA), just to give the PSU a 'load' to see. ....however as soon as I connect it up to the stepper board, once again I measure 35V?!!!!

From what you are saying, I am assuming it is a regulated switching supply?
A linear supply of the correct sizing, either regulated or unregulated should not perform that way with a minimum load such as that.
1/ It is doubtful your resistance reading across the input to the stepper board is going to be realistic, as a digital meter on ohms range will not measure the effect of semiconductor devices in the board.
I am guessing that the 'Real' load of the stepper board is causing an effect on the output of the switching supply.
Al.

[HankMcSpank]

Just to give some conclusion here - it looks like it's the actual PSU.

I had a 16V laptop PSU lying around so tested that with my stepper driver board- 16V rock solid both disconnected & connected to the stepper board. I put an oscilloscope on the 24V PSU ....very weird - certainly not DC ....if fact more like an odd shaped sawtooth 24V peak to peak (whereas scoping the 16V showed the +ve rail to be rock solid ripple free)

[armstrong360]

I may be that you are not loading the supply enough. Some switching power supplies don't work with less than 10 % load. I would put at least a 2 amp load and remeasure the output.

[irving2008]

I agree with armstrong - you need to put a decent load on the power supply. Also it may be that the supply doesnt like 'reactive' loads and goes into some form of self oscillation when connected to the driver. A good resistive damping load, drawing at least a couple of hundred mA (something like a 100ohm 10W resistor) across the supply line should help.

Personally for motor drives I prefer a simple transformer/bridge/big C supply - less to go wrong and less susceptible to the back EMF from the motor as well.