I’ve been experimenting with powering a small 5.5” cordless circular saw with laptop power supplies.
The nominal voltage of the saw is 19.2VDC. It draws about 5 amps on startup then settles at about 3 amps, no load.
Some of the PSUs work fine and power the saw well enough to cut 2x boards while others with about the same or even higher ratings do not.
All PSUs are rated around 19-20VDC and over 4.5 amp.

The puzzling ones are all Lenovo-type with 7.9mm OD barrel connectors with a center pin.
I’m using a tip adapter which ignores the center pin and which is plugged into a 2.5mm female barrel jack wired to alligator clips, which then connect to the motor leads.
When the switch is pressed the Lenovo-type adapters will run the saw for a second or so before the charger output voltage drops from around 20VDC to 0. Even stranger, one of the chargers seems to cycle its power in very short bursts, so if the the trigger is held closed, the blade turns briefly, stops, turns again, etc.

I have tested the chargers with an electronic load and they deliver their rated current at 19-20 VDC, so I’m baffled.
Can anyone tell me why the voltage is cutting off with certain chargers ?

Thank you

 

Welcome to AAC!

The battery in a cordless appliance can put out high current for a short period. Laptop supplies won't go much above their continuous ratings because they weren't designed to do that. If they were, they'd have the higher rating.

How they react to overloads depends on how they were designed.

 

Welcome to AAC!

The battery in a cordless appliance can put out high current for a short period. Laptop supplies won't go much above their continuous ratings because they weren't designed to do that. If they were, they'd have the higher rating.

How they react to overloads depends on how they were designed.

Dl,
Thanks for your reply.
So if I understand, you're suggesting an overcurrent protection circuit is being activated ? Correct ?

 

So if I understand, you're suggesting an overcurrent protection circuit is being activated ? Correct ?

That, and the saw may be drawing significantly more than the 5A you mentioned.

 

I’ve been experimenting with powering a small 5.5” cordless circular saw with laptop power supplies.
The nominal voltage of the saw is 19.2VDC. It draws about 5 amps on startup then settles at about 3 amps, no load.
Some of the PSUs work fine and power the saw well enough to cut 2x boards while others with about the same or even higher ratings do not.
All PSUs are rated around 19-20VDC and over 4.5 amp.

The puzzling ones are all Lenovo-type with 7.9mm OD barrel connectors with a center pin.
I’m using a tip adapter which ignores the center pin and which is plugged into a 2.5mm female barrel jack wired to alligator clips, which then connect to the motor leads.
When the switch is pressed the Lenovo-type adapters will run the saw for a second or so before the charger output voltage drops from around 20VDC to 0. Even stranger, one of the chargers seems to cycle its power in very short bursts, so if the the trigger is held closed, the blade turns briefly, stops, turns again, etc.

I have tested the chargers with an electronic load and they deliver their rated current at 19-20 VDC, so I’m baffled.
Can anyone tell me why the voltage is cutting off with certain chargers ?

Thank you

I considered it a potential option for a cordless drill with a knackered battery - but a very suitable lithium pack turned up in recycle bin salvage, so the idea got dropped.

 

Your laptop charger is rated at 5A, and it will be going into over current mode, to limit it at switch on, ideally a battery across the output or a large Electrolytic capacitor would help with the initial surge...