Can someone please tell me where I have gone wrong? I have a battery powered impact screwdriver, the battery is rated at 12vdc 2amp. The battery was exhausted so I built a power supply for it. I used a ring transformer with an output of 12v and 50va. The output was fed into a bridge rated at 400v and 4 amperes. Across the dc output I connected a 3500mfd 35vdc electrolytic capacitor. When the drill was connected across the capacitor output it worked fine in both directions. However when I tried to screw a screw into a plastic wallplug the screw would only go in about halfway - it was also difficult to unscrew it. Can anyone give me an explanation and how, if possible I can correct it?

 

What does

However when I tried to screw a screw into a plastic wallplug the screw would only go in about halfway - it was also difficult to unscrew it

that have to do with the power supply you described?

 

Under load it isn't preforming the way you feel it should? The cap may be under powered. Without the cap, do you notice normal power?

 

Can someone please tell me where I have gone wrong? I have a battery powered impact screwdriver, the battery is rated at 12vdc 2amp.

Are you sure that's not 2AH (Amp hours)?

 

Are you sure that's not 2AH (Amp hours)?

Thank you and the others for replying. You are right, the battery does say 2 AH. So what is the difference if I can take at least 2 amps via the power supply for as long as I wish and not being limited to a certain amount in one hour. Should the capacitor value be increased? I understand that the function of the electrolytic capacitor across the output terminals is to smooth out the rectified voltage. Anyway any practical advice would be appreciated.

 

The drill motor takes far more current momentary than the transformer can supplied. The peak current can be more than 10 amps for a brief period when the most torque is being called for.

A battery has no problem supplying such amperage for a few seconds but not a transformer.

You would need a bigger rating transformer, along with higher rating rectifier and higher value filtering capacitor.

 

Ebic, thanks for the information. It hadn't occuured to me that the drill motor would take a much heavier current. What would you suggest as a practical rating for the transformer, the bridge and the capacitor? Could I use a power supply from a computer as a reliable current source for 12vdc?

 

It is hard to say without knowing the particulars of the drill. Trying it out using a PC power supply is certainly a good idea.

However, I'm not so sure the usual ATX PC power can output a large current on the 12V rail if there is no load on the +5V rail.

IF you have to 'buy' a PC supply in order to find out, you better wait until someone here have done a test with a real drill to tell you the performance.

Maybe others have already done that and have experience to share.

 

I have several old computer power supplies and I once tried using a 10 ohm 10 watt resistor as a load across the 5 volt output but it didn't work. I was able to use the drill without a load, but with a load it didn't seem to have any power. What value resistor would I need across the 5 v output to be able to draw 10 amps or more from the 12 volt?

 

There are hundreds of posts about how to hack an ATX power supply. Besides the load on the 5V rail, there are power good and possibly feedback lines that need to be connected. See this Instructable, steps 2,3,and 4.

http://www.instructables.com/id/Eas...upply/step1/WHY-ARE-THERE-SO-MANY-DARN-WIRES/

John

 

I have several old computer power supplies and I once tried using a 10 ohm 10 watt resistor as a load across the 5 volt output but it didn't work.

With a 10Ω resistor the 5V load is only 500mA so it will keep the power supply alive only. It is not much of a loading.

I would say you will need at least 5A loading on the 5V side. That translates to a 1Ω resistor with 25W heat dissipation.

You will have to use at least a 30W or higher rating resistor mounted on a heatsink for this.

 

Here is what I would do to get some working data for your drill. First, beg or borrow a variable transformer (Powerstat) capable of at least 10 amperes of current. Next, connect the FWB rectifier (heatsinked) and Capacitor. Now dial up your Powerstat to get 12Vdc at the drill, check full speed and torque (driven screw). If you have a current meter or Amprobe, check the full load current when driving a screw load. Now Vac x Iac x 1.11 will give you the minimum transformer VA that you will need for your drill.

Cheers, DPW [ Everything has limitations...and I hate limitations.]