I have a battery charger that uses a 24VDC 300mA wall transformer. The transformer died, and the closest replacement I could find was 24VDC @500 mA. The new transformer did seem to charge the battery pack just fine, but the next morning it too was dead. Is it possible that the higher amp rating caused it to burn out? Or should I rather suspect that the battery charging circuitry has become defective?


No, the higher amp rating would just mean it can supply more current if the circuit "asks" for it. The voltage rating is the main thing here, and that matches okay.

The fact that the 500mA power pack blew too suggests something is pretty wrong somewhere. How did you confirm it blew? Have you tested the output with a multimeter?

What model charger is it? Are the batteries you are using the correct type for the charger?

If you have a bench supply you could plug it (current limit it to say 400mA) in and see how much current it is drawing. I would test both with no batteries in and batteries on charge. If you don't have a bench supply with current display you could rig a 24V source up and use a multimeter to test current.
Also, other clues you can check for - see if any components are getting very hot (especially if not loaded with batteries) With circuit off, test resistance between power and ground to see if it is low. I would be suspicious of anything under 1k or so.

  • \$\begingroup\$ The wall wart measured 30V when I first plugged it in. After it stopped working, it put out only 10V (with no load), and slowly decreased to about 4V, which is where it stayed. The charger is built in to a power tool, so I can't provide information about the brand. All I can say is that the batteries are ni-cad cells adding up to 24V total. \$\endgroup\$ – Dave Oct 24 '11 at 18:57
  • \$\begingroup\$ I don't have another working ower source, so I can't test the current draw. \$\endgroup\$ – Dave Oct 24 '11 at 18:57
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    \$\begingroup\$ Did you wait a while and retest the output of the power pack? It may have a protection device which may take some time to reset after overload. Test it from cold (before plugging into the charger) if it still reads low voltage then something else is wrong. \$\endgroup\$ – Oli Glaser Oct 24 '11 at 19:04
  • \$\begingroup\$ It's been a few days already and it's still low. \$\endgroup\$ – Dave Oct 24 '11 at 20:12
  • \$\begingroup\$ @Dave - Ah, sounds like it's not very well then :-) Check the battery is not faulty, if it's a cheap charging circuit it may not have any proper charge control (e.g. I have a cheap Dremel copy that simply applies the DC straight to the battery, so I have to time the charge manually) \$\endgroup\$ – Oli Glaser Oct 24 '11 at 22:09

You say "transformer", but a transformer doesn't put out DC. The wall wart must be more than just a transformer.

The current rating of a power supply only indicate what it can deliver. The load decides how much to actually draw. A 24V 500mA power supply is therefore a superset of a 24V 300mA power supply. Unless the power supply has deliberate current limiting and the particular load makes use of that, there will be no harm using the 500mA supply instead of the 300mA supply.

It sounds like the original was just a ordinary off the shelf power supply, so most likely did not have particularly accurate or specific current limiting such that the load could rely on it. That would be unlikely design anyway. Therefore, no, most likely no harm was done using the 500mA supply.

It is strange that this supply also died. These supplies are usually short-circuit protected. Even if the load were a dead short, they therefore should not be damaged. However, there are a lot of cheap supplies optimized mostly for low cost out there. Some even outright lie and don't do what the nameplate says, or only in the best possible conditions you're not likely to duplicate.

Does the new supply have a fuse? If so, the load could have a short and the fuse blew like it should. In that case, replace the fuse and the supply will be fine again, but the charger is dead.

Test the new supply with a meter without the charger connected. Is it really dead, or just act dead with the (probably broken) charger connected?

  • \$\begingroup\$ For all practical purposes, it's dead... putting out only 4V now. Even if it has a fuse, I don't think I could could open the plastic case to replace it without destroying the thing. \$\endgroup\$ – Dave Oct 24 '11 at 19:00
  • \$\begingroup\$ @Dave: There might possibly be some kind of thermal overload sensor inside. Let it sit completely disconntect from the world at room temperature for one hour and then measure the output voltage just with a meter, not with the charger connected. If it's still 4V then, yes, it's probably dead. In that case you might was well open it and see what king of interesting stuff is inside and maybe learn what went wrong. \$\endgroup\$ – Olin Lathrop Oct 25 '11 at 23:01
  • \$\begingroup\$ I've basically concluded that the primary wiring has burned out (the plug prongs measure infinite resistance). Is it possible that the batteries became internally shorted, and therefore are drawing too much current from the power source, causing it to burn out? \$\endgroup\$ – Dave Oct 26 '11 at 14:18
  • \$\begingroup\$ @Dave: Are you sure the input to this power supply is just a transformer primary? That's not how modern power supplies generally work anymore. More likely it is a full wave bridge into a cap. A switcher then drives the primary of a much smaller transformer at much higher frequency than the power line. If this is the case, it would require about 1.4V on the input just to turn on the bridge diodes. Your ohmmeter may not put out that much, giving the appearance of a open circuit. Also, if the internal cap is charged then there won't be any current until much higher voltages. \$\endgroup\$ – Olin Lathrop Oct 26 '11 at 15:36
  • \$\begingroup\$ This is a very basic wall wart, I doubt there is any of that extra stuff in the input circuit. (I cracked open the old power supply, and the prongs were soldered directly to the transformer wiring. Presumably the new supply is similar.) \$\endgroup\$ – Dave Oct 27 '11 at 2:42

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