Another excellent read from gw1 @ austech
After criticising poor quality of clone DM500 power supplies for a while I figured it was time I looked at them more closely to understand how they work (or why they don't).
Those who try repairing faulty ones by replacing one or more electrolytic capacitors will sometimes find that's not enough: sometimes the replacements sizzle and burst too. And you'll find resistors overheated and burned out. What's usually happened in these cases is the regulator and/or PWM controller IC has failed, resulting in the output rising above 20V which causes a bunch of parts to burn out.
Having learned that people like Fernbay have replacements available quite cheaply I've formed the view that it's not really worth the time it takes to dismantle, repair and reglue them - certainly not if it's more than just one or two capacitors. Not everyone will agree of course, and to help those who decide to persevere I'm posting info about the couple of DM500 PSU versions I've seen so far. There may be a errors, I don't guarantee everything is 100% correct. But it gives you an idea what you're dealing with, how they work, and how to troubleshoot if you want to persevere with a repair.
If you come across a new version you might like to take some snaps and add your observations.
I've also seen cases of intermittent failure. This can be caused by hairline fractures in the PCB, or bad solder joints on heavier components (increasingly common with lead-free solder nowadays).
If you compare the schematics to reference designs from component manufacturers (eg this or this) you'll notice several short cuts taken in the DM500 supplies. For example,
After criticising poor quality of clone DM500 power supplies for a while I figured it was time I looked at them more closely to understand how they work (or why they don't).
Those who try repairing faulty ones by replacing one or more electrolytic capacitors will sometimes find that's not enough: sometimes the replacements sizzle and burst too. And you'll find resistors overheated and burned out. What's usually happened in these cases is the regulator and/or PWM controller IC has failed, resulting in the output rising above 20V which causes a bunch of parts to burn out.
Having learned that people like Fernbay have replacements available quite cheaply I've formed the view that it's not really worth the time it takes to dismantle, repair and reglue them - certainly not if it's more than just one or two capacitors. Not everyone will agree of course, and to help those who decide to persevere I'm posting info about the couple of DM500 PSU versions I've seen so far. There may be a errors, I don't guarantee everything is 100% correct. But it gives you an idea what you're dealing with, how they work, and how to troubleshoot if you want to persevere with a repair.
If you come across a new version you might like to take some snaps and add your observations.
I've also seen cases of intermittent failure. This can be caused by hairline fractures in the PCB, or bad solder joints on heavier components (increasingly common with lead-free solder nowadays).
If you compare the schematics to reference designs from component manufacturers (eg this or this) you'll notice several short cuts taken in the DM500 supplies. For example,
- Better designs use two 470uF capacitors after the schottky diodes rather than a single 1000uF because the lower capacity parts cope better with the high ripple current.
- Better designs use 25V output capacitors instead of 16V. In fact you'll see a number of reference designs use quite small output electrolytics, eg only 100uF. This illustrates that you're better off using a smaller capacitor with a safe voltage rating than a larger one with minimal safety margin.
- Better designs specify higher rated schottky diodes to cope with transients (eg 16A instead of 3A, and higher voltage rating too)
- Better designs sometimes specify higher rated resistors (1W instead of 0.5W etc)
- Better designs specify high ripple current type for electrolytic on the output of the schottky diode
- Some designs use two schottky diodes in parallel rather than just one; some PCBs were designed for two but only one is fitted.
- Better designs specify higher voltage rating on feedback diodes to cope with transients.
- Better designs specify higher rated drive MOSFET.
- There are in some cases discrepancies between suppression capacitor grades (X1, X2, Y1, Y3) marked on the PCB and the parts actually fitted. If you're not familiar with these parts it's worth taking the time to learn about them, for example here.
- Many switchmode designs take feedback from the supply side of the output filter (10uH) rather than the output side.
- The yellow kevlar (or whatever it is) tape around the transformers and heatsink is important for safety; don't remove it.
- The glue on some components is to reduce buzzing which is a common problem in switchmodes.
- You need to be careful when removing PCBs from the cases because adhesive on the rubber standoff beneath the board can be strong; you might fracture PCB tracks if you're not careful when removing it.