AC Adapter amperage

[flinx]

I have a nicd powered 18 volt pipe cutter. I found it in the street one day (in its box with 2 batts and a charger) and though I do not have much use for it, I do use it occasionally. However the batteries died and I did not want to spend the money to get new batteries for something that I hardly use. So I decided to hard wire it.

I have a PILE of AC adapters and found one for an HP laptop that is 19 volts and (supposedly) 7.1 amps. However when I hard wired it to the pipe cutter it barely could move it and the light would go out on the AC adapter. Is this because it is not running at 21.75 volts that a fully charged battery pack puts out? or is the AC adapter lying about its amp rating?

correct me if I am wrong but most nicd battery packs are are only around 1.3 to 2.2 amp hours.

[tom66]

Re: AC Adapter amperage

Doesn't matter how many amp hours the battery has, it could potentially provide a lot more than 7.1 amps. Also, the stall or startup current of that motor is likely very high; you may be able to get it to run better by giving it a little push.

[flinx]

Re: AC Adapter amperage

Doesn't matter how many amp hours the battery has, it could potentially provide a lot more than 7.1 amps. Also, the stall or startup current of that motor is likely very high; you may be able to get it to run better by giving it a little push.

Hmm... well it does turn by itself but in a pulsing way. About a tenth of a turn every half second (give or take).

[budm]

Re: AC Adapter amperage

Start-up and Stalled current of motor can be as high as 20 to 40 times running current, switch mode power supply is not made for high in-rush current, especially the laptop one, it will trip the over current protection circuits.

[flinx]

Re: AC Adapter amperage

sigh™ OK, so my options are:
Purchase a battery pack $61.00
rebuild the battery pack $39.00
Build a power supply from scrounged parts $??

I have 2 or 3 full wave bridge rectifiers (unknown specs).
A large collection of various capacitors (from badcaps.net.
The thing I need is a decent transformer. I have a small collection. What size transformer would I need to get a dc output of 21-22 volts?

[budm]

Re: AC Adapter amperage

16V about 5A feeding bridge rectifier with about 10,000 uF to supply surge current.

[flinx]

Re: AC Adapter amperage

16V about 5A feeding bridge rectifier with about 10,000 uF to supply surge current.

I located a transformer from an old APC UPS. The output from it is 16.25 volts AC, Once ran through the Rectifier the voltage is now about 15.25 DC. I have not found a 10000uf cap. Any Ideas where I might scrounge one? I do have several 3300uf 16v caps I can run in parallel.

[mariushm]

Re: AC Adapter amperage

Use 3 x 3300 uF /25-35v in parallel. 16v is awfully close to 15.25v DC.

[flinx]

Re: AC Adapter amperage

Use 3 x 3300 uF /25-35v in parallel. 16v is awfully close to 15.25v DC.

that was my original idea but I do not have any, also I do not know the amerage rating of the rectifier. There are several numbers on it, one side says GI 212-152-04 the other side says 502b.

[flinx]

Re: AC Adapter amperage

I googled the rectifier and all the pages that came up were in chinese, but I think the rectifier is able to handle 20 amps.

[budm]

Re: AC Adapter amperage

Are you getting 15.25VDC without the filter caps connected, if you do have the caps connected, the voltage should be around 22VDC.

[flinx]

Re: AC Adapter amperage

Are you getting 15.25VDC without the filter caps connected, if you do have the caps connected, the voltage should be around 22VDC.

no filter caps connected as I do not have any of the right voltage. But that answered a question. I checked all my scrounge and everything is either too low of a voltage or too low capacitance.

[mariushm]

Re: AC Adapter amperage

Thanks for making me look again at his posts budm...

The output from it is 16.25 volts AC, Once ran through the Rectifier the voltage is now about 15.25 DC

That's not right. Are you sure that rectifier is good? Did you connect the wires properly?

When rectified, voltage should be Vac x ( square root of 2) = 1.414 x Vac

If it's 16.25v AC, then rectified it would indeed be 16.25 x 1.41 = ~ 23v . Though under load, your DC output will actually probably be about 21v so 25v rated capacitors would be the lowest you should use.

[b700029]

Re: AC Adapter amperage

I'm not familiar with pipe cutters but sounds like it's similar to an electric drill, which is also very power-hungry as it uses a brushed DC motor. Stall currents can be over 100A.

[flinx]

Re: AC Adapter amperage

Thanks for making me look again at his posts budm...



That's not right. Are you sure that rectifier is good? Did you connect the wires properly?

When rectified, voltage should be Vac x ( square root of 2) = 1.414 x Vac

If it's 16.25v AC, then rectified it would indeed be 16.25 x 1.41 = ~ 23v . Though under load, your DC output will actually probably be about 21v so 25v rated capacitors would be the lowest you should use.

When I put my multimeter on it it briefly reads around 23 volts but then drops almost instantly to 15.48.

Is it good? dunno I pulled it from an electric wheel chair many years ago and never used it till now.

[flinx]

Re: AC Adapter amperage

I'm not familiar with pipe cutters but sounds like it's similar to an electric drill, which is also very power-hungry as it uses a brushed DC motor. Stall currents can be over 100A.

I call it a pipe cutter but it is a portable band saw.
http://www.stouttool.com/productdetails.asp?ProductID=1

I also plan on using the Power supply for a full set of SKIL cordless devices I acquired (also missing the batteries).

[mariushm]

Re: AC Adapter amperage

On your bridge rectifier you need to connect the leads from the transformer to the pins labeled with a ~ ... or to the two pins that don't have a + or - sign.

The DC is obviously measured with the meter on DC.

Alternatively, just search for 4 diodes and do the bridge rectifier circuit yourself :

http://en.wikipedia.org/wiki/Diode_bridge

The picture under "Basic operation" is self explanatory.

For a simple test without load, any 4 regular diodes will do.

[flinx]

Re: AC Adapter amperage

On your bridge rectifier you need to connect the leads from the transformer to the pins labeled with a ~ ... or to the two pins that don't have a + or - sign.

The DC is obviously measured with the meter on DC.

Alternatively, just search for 4 diodes and do the bridge rectifier circuit yourself :

http://en.wikipedia.org/wiki/Diode_bridge

The picture under "Basic operation" is self explanatory.

For a simple test without load, any 4 regular diodes will do.

Yes the AC is connected to the pins marked ~ voltage is measured from the pins marked + -. I have another rectifier marked d3sba60 it comes back as a 4A 600v.

I may have to order the capacitor as I am not having much luck finding a source locally. Most electronic component stores have closed or move very far from where I live.

[flinx]

Re: AC Adapter amperage

I tested the other rectifier and it did the exact same thing. when I checked the voltage it read 23 volts then instantly dropped to 15.

Edit: I temporarily installed a 470uf 100v cap across the dc output and the voltage read 21.78 which is right about where I want it. So now I just need to find that capacitor.

[budm]

Re: AC Adapter amperage

4A Diode will be too low of the current handling. I use this kind for my heavy duty project.
http://www.mouser.com/ProductDetail/...GHhHT7GhpbM%3d