Electronic – Converting cordless to corded: Pulsing issue

conversionpower supplyswitch-mode-power-supply

So I am rather new at electronics and electrical engineering. But here is what I am trying to do and what I need help with.

Some years ago I had bought a cheap set of cordless power tools. Naturally the battery packs died much quicker than most. So I had the idea last night that I could just take some of the extra tablet power supplies laying around and convert these battery-less power tools to being corded. The power tools were labeled as 19.2 volts and the tablet power supplies were labeled as 19 volt output, but registered on the multimeter at about 19.3.

After disassembling the battery pack that went to the power tools, inside were 16 sub C batteries labeled as 1.2 volts and 1300 mAh and connected in series. So by my calculations, remember I am green at this, the battery pack had an output of 19.2 volts and 1.3 amps. The tablet power supply was labeled as 19 volts and 1.58 amps. So everything seemed rather straight forward. At least to me.

So I connected the tablet power supply to the leads in the power tool where the battery pack would normally connect and fired it up. That first half second was glorious. For a brief moment I was proud of myself. But only briefly. What ended up happening was that the power tool turned on and worked for a moment and then stopped. Then a moment later it would do the same thing. As I held the trigger down it continued this "pulsing".

After doing some research it seems the problem may be that the tablet power supply is a "switch mode" power supply and that this is causing the pulsing action? I am not sure. That is why I came here. Maybe someone could help to shed some light on this issue.

All the power requirements seem to match up so:

  1. Why would it pulse like this instead of being steady like normal?
  2. What's a good way to rectify the situation?
  3. If it is a "switch mode" power supply issue, can I somehow convert the power supply to work? Can I just add a capacitor or something somewhere down the line?

Thanks in advance for any assistance and for having patience with a new learner such as myself.

Tablet Power Supply
(from the box it came in)
Tablet Power Supply Specs

Best Answer

You are confusing amp-hours and amps. A 1.3 Ah cell can supply a LOT more than 1.3A - try 13A or even more if the screw is stuck - albeit for a lot less than a whole hour! In this case, theoretically 6 minutes (0.1 hour) but practically, probably a bit less, depending on the condition of the battery, the temperature and so on.

So the problem isn't that the PSU is switch mode - the problem is that it senses overcurrent, and shuts down, and then tries again to see if the overload has gone ... rinse and repeat.

  1. Find a good (or even marginal) battery pack. (Or dismantle two and replace dead cells with good cells from another).

  2. Rig the battery pack to run the tool externally via a cable, like you're using the PSU.

  3. Measure the actual current taken

  4. Use a PSU rated at that actual current.

  5. It may be possible to rig a lower current PSU as a trickle charger, continually replenishing a dodgy battery pack which provides the bursts of current you need.