7
\$\begingroup\$

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

\$\endgroup\$
  • \$\begingroup\$ The power supply may be a switch mode, but the output should be roughly DC equivalent. I would wonder if the tool is doing some of the battery management internally, thus it sees it's getting power but shuts off the second it sees no sensing data. Did the battery contain other outputs besides the 2 primary? Did it have a multistage output (outputs from different stages of cell contacts for different voltage output)? Any sensing leads? \$\endgroup\$ – Jarrod Christman Apr 28 '15 at 16:44
  • 2
    \$\begingroup\$ Most tools like that have a universal motor on them, so their inrush current is pretty high. Batteries can handle that, but the Switching supply was probably shutting down on some protection circuit and re-starting. \$\endgroup\$ – R Drast Apr 28 '15 at 16:48
  • \$\begingroup\$ @RDrast could be right, but we will need more details on the exact type of "tablet" power supply you are using. \$\endgroup\$ – Sada93 Apr 28 '15 at 16:52
  • \$\begingroup\$ Added a picture of the power supply specs from the box. \$\endgroup\$ – The Duke Of Marshall שלום Apr 28 '15 at 16:59
12
\$\begingroup\$

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.

\$\endgroup\$
  • 1
    \$\begingroup\$ 4b. It will be a big power supply and expensive. The only time this is normally worth doing is for 12-14.4V tools where you can run them off a car battery or desktop PC PSU. \$\endgroup\$ – Chris H Apr 28 '15 at 19:17
4
\$\begingroup\$

The 1300 mAh rating of the batteries is a measure of the energy they can store, and has very little to do with the maximum current they can deliver.

I expect that the tool draws much more than 1.3 Amps, so the tablet power supply shuts down to protect itself when the tool demands a high current. After the tool stops, the supply recovers, and tries again, etc... This action is not necessarily related to the fact the tablet supply is a switch mode type.

\$\endgroup\$
0
\$\begingroup\$

DC motors usually require continuous high current. Power supplies that cannot provide the current demanded, will not be able to hold the voltage required and if it has protection circuits, it will shut down. Your idea of adding a capacitor should help, but a better idea is to use rechargeable batteries (five 3.7v Li will do) and use the power supply to keep them charged. You can also use a "better" supply. I would recommend one that can supply 10A peak and 3A continuous. Good luck!

\$\endgroup\$
0
\$\begingroup\$

Cordless drills that have dead batteries are plentiful.Most of the time the motor,gearbox and controller are fine . What a waste .My first conversion was to place a parallel combination of low ESR electros and ceremic caps inside the drill housing .Then I installed a reverse polarity diode for protection.I then put 2 meters of 10 Amp DC cable on it and used it for working on the car and the boat .My next conversion used a 12VAC 40 watt mains halogen lamp transformer .I then made a bridge from 4 TO220 shottky diodes and used lots of junkbox electros totaling about 20 ooo microfarad at 25V .This worked well on my old drill .The lamp transformer is ideal because its thermaly protected and can stand big overloads .It runs cold on the casual use I give it .If you wanted to use a SMPS you would need at least 10 Amps to avoid too much nuisence tripping so the old school PSU is best here .

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.