I've got the power

by Ivan Hamilton 3/31/2008 1:50:00 PM

My power supply arrived today. I picked it up relatively cheaply ($243.56 AUD to my door) off eBay to run the 36V motors I have.

Problem is... it's a 48V supply, not 36V. They had 12V, 24V & 48V models, and I thought I'd better get a larger one and tune it down, rather than a smaller one tuned up. I've seen a number of electronic components are rated at around 30V, and if I took a 24V model with 30V rated components and ran them at 36V, I might be pushing the envelope a bit. I think it's safer to get a 48V unit, and try to run it at 36V. The unit was "user adjustable +/-10%", and being a switched mode power supply I assumed that changing the output voltage a bit more (25%) should be easy enough.

How difficult could it be?

On opening the case, I found a lot more than I expected. I guess the extra componentry is for the extra features (Auto on/off cooling fan, Built-in EMI Filter, Over Load and Short Circuit Protection, Over Voltage Protection, Auto-recovery after protection, etc). Luckily the user voltage adjustment was easily located, and tracing that back along the PCB resulted in a fairly straight forward couple of components before the signal hit a riser card full of epoxy and SMDs. I'm taking a guess that the riser is the control smarts.

 

 

Tweaking the pots changed the output voltage from 44-52V but the voltage to the riser was always 2.49V according to my DMM. Based on this, it uses a simple voltage divider and the control circuit attempts to maintain 2.5V on the sense line.

Assuming the pots are centered, 2.5V at 2K85 needs 48.11V at 54K85. For 36V to give 2.5V a total of 41K04 is required. Replacing R2 with 37K19 would give 36V. 39K is the closest E24 value, and that would give 37.6V. With a little adjustment, 36V should be possible. I fired up my trusty soldering iron & swapped it in. Hold breath... switch on...  tweaked the pots... voilà! 36V :-) I've put one of the motors on the supply, and it kicks like a mule during take-off while voltage holds at a rock steady 36.0V. I don't know what effect dropping the voltage has on its ability to deliver current, but if it could give 20A at 48V it should do at least as much at 36V. If it can maintain the same power output, 27A should be possible.

Without knowing too much about the workings of this power supply, I appear to have changed it from 48V to 36V. I don't underestimate the complexity and there's probably more to it... but for the moment, it appears to work.

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"My inner nerd can beat up your inner nerd."

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