Why am I able to use a dc wall wart instead of an ac one to charge my lawn mower?

[Tom lakovic]

Hi all.

Moderately knowledgable electronics wise and yet I am completely stumped by something that happened today.

My 24v black and decker (aka craftsman) cordless mower has not been charging and so I opened it up to look around and try to diagnose. Noticed a couple 12v lead acid batteries wired I'min

Not sure how smart the charging circuit is but the board is fairly "big" (I know that's meaningless. I'm just saying there's got to be something it's doing .

1st thing I thought to test was the wall wart but I assume,ed wrongly that it was outputting dc and saw very little mV on my meter so diagnosed (wrongly) that it burned out and promptly spiced its proprietary connector to a led over 24v dc wall wart I had if slightly lower amperage. (600ma vs 1A)

Plugged it in and saw the mowers charging green light come on (hadn't seen THAT in a while) left it charging for about 8 hrs and then checked the OLD wall wart the original proper one and noticed (to my shock) that it said 24v AC output (1A). Omg. I ran out to he garage and pulled off my spliced 24v DC (600ma) wall wart (showing green barge light and not at all warm) and tested the mower and

Kapow! It started right up seemingly 3/4 charged sounding. I unplugged everything and scratched my head.

So after all that preamble I have one question:
HOW CAN THIS BE?

Do you think the charging circuit can smartly handle either ac or dc and I lucked out to not fry anything?
Why would they ship it with an ac wall wart if the batteries are dc? Should I continue using my splicerific friend or not?

Cheers!
Tom
Portland OR

 

[KrisBlueNZ]

Hi Tom and welcome to Electronics Point

The board inside the mower will include a bridge rectifier on it. When you plug in an AC plugpack, this bridge rectifier will convert the AC to DC so it can charge the batteries. (There will probably also be a smoothing capacitor after the bridge rectifier.) When you plug DC into the mower, the bridge rectifier will pass the DC on to the charger circuit as well, and it may appear to charge.

Bridge rectifying 24V AC produces a DC voltage that's higher than 24V. Typically aound 30V depending on how much current the charging circuit draws from it. This is because the voltage of the positive and negative peaks from a 24V AC supply are higher than 24V (by a factor of the square root of 2, which is 1.414) (and also because AC plugpacks are not regulated and the 24V AC specification applies under maximum load; under less load, the voltage will be higher).

When you feed 24V DC into the bridge rectifier, its output will be only around 22V because of voltage drop in the diodes. This probably won't be enough to charge the batteries.

So don't use your DC circuit. Use the adapter that came with the mower.

Also, the socket on the mower should be labelled 24V AC. That should make it clear that you're supposed to feed AC into it.

For more information, Google some of those keywords - bridge rectifier; AC RMS and peak voltage; diode voltage drop.

 

[Tom lakovic]

Kris. Wow what a great response. Thank you!
I will read up about bridge rectifiers for sure.
And I will stop using the dc pack I cobbled together.
---

However a couple points remain (seemingly) unaccounted for by that explanation :
1. The dc plug pack DID charge he mower. From flat batteries to about 3/4 after 8 hrs. Mower was running. When I realized my mixup. It was totally flat earlier.
2. The green charging light came on after I put on dc power. But wasn't doing it with the ac plug pack.

 

[KrisBlueNZ]

It's possible that the DC supply would charge the batteries. I doubt it would charge them fully. Whatever reported that they were 3/4 full may have been wrong.

I don't know why the charging indicator didn't light up when you used the AC supply. Do you think it isn't charging at all? Check the AC voltage coming out of it. Ideally you should check that it remains above 24V when it's plugged into the mower, but it may not be practical to get access to the connection points.

 

[shrtrnd]

Hi Tom and welcome to Electronics Point

The board inside the mower will include a bridge rectifier on it. When you plug in an AC plugpack, this bridge rectifier will convert the AC to DC so it can charge the batteries. (There will probably also be a smoothing capacitor after the bridge rectifier.) When you plug DC into the mower, the bridge rectifier will pass the DC on to the charger circuit as well, and it may appear to charge.

Bridge rectifying 24V AC produces a DC voltage that's higher than 24V. Typically aound 30V depending on how much current the charging circuit draws from it. This is because the voltage of the positive and negative peaks from a 24V AC supply are higher than 24V (by a factor of the square root of 2, which is 1.414) (and also because AC plugpacks are not regulated and the 24V AC specification applies under maximum load; under less load, the voltage will be higher).

When you feed 24V DC into the bridge rectifier, its output will be only around 22V because of voltage drop in the diodes. This probably won't be enough to charge the batteries.

So don't use your DC circuit. Use the adapter that came with the mower.

Also, the socket on the mower should be labelled 24V AC. That should make it clear that you're supposed to feed AC into it.

For more information, Google some of those keywords - bridge rectifier; AC RMS and peak voltage; diode voltage drop.

Another clear and concise explanation that makes the mysterious into something simple that anyone can understand.
Good answer for someone who didn't really understand why wall adapters are labelled ac or dc.