AC Adapter amperage

[flinx]

Re: AC Adapter amperage

I just realized I will probably need a bleeder resistor. How do I calculate that?

I have some 270 ohm resistors.

[budm]

Re: AC Adapter amperage

You can use 500 Ohms 2 Watts (@22V and 500 Ohms the power dissipation will be 0.968 Watt, doulble that for safety, so it will be 2 watts)
@500 Ohms and 10,000 uf, it will take 5 second for the cap to discharge down by 63% from full charge (500 Ohms x 10,000 uF = 5 seconds for 1 TC (Time Constant), it will take 5 TC for the cap to fully charge and dsicharge the cap. The Charge and Discharge time is not linear, @first TC the voltage will change by about 63%. so after 5 second the voltage on the cap will be about 22V x 0.63 = 13.86V, after 5TC (25 seconds the cap should be fully discharged.

[flinx]

Re: AC Adapter amperage

I went and picked up 2 - 10,000uf caps and since I was there picked up 2 - 25 amp rectifiers. So instead of being mostly free it cost me about $20 to build.

It works perfectly. Voltage is 22vdc +/- before load, 18vdc with load.

I temporarily mounted the parts to a board. At some point I will mount it in a box with fuse, cooling fan, power switch and power indicator light.

I appreciate all the help.

[budm]

Re: AC Adapter amperage

That is great, now you can also use it to charge you lead acid batteries by using the Variac to feed the transformer and adjust the DC output to be around 13.8V (2.30V per cell) to 14.70V (2.45V per cell), or other 18V tools. You can mount that bridge on Aluminium plate with bent up edges to use as a heat sink.
By the way, you may want to attach that safety ground of the power cord to the transformer mounting frame in case the transformer has insulation failure.

[flinx]

Re: AC Adapter amperage

Spent most of the day finishing the power supply. I could not find a box to mount it in, so I re-used the UPS case. I was able to re-use the original circuit breaker, and switch. I also added a 20 amp fuse on the DC side, and an LED on the front of the case.

The rectifier is mounted to a piece of aluminum (with heat sink compound) that I had to install to properly brace the front and rear panels, since the original circuit board was used for that.

The LED is connected to a 270 ohm 5 watt resistor. It bleeds the capacitor down to a safe voltage in about 25 seconds when power is turned off. The LED will stay dimly lit for some time after that. Pressing the switch on the band saw will make the LED go out if power is off. The LED does NOT dim when the band saw is in use.

I may mount a fan in it at a later time if I find there are any heat issues. I doubt there will be as I will not be using it for continuous duty.

Yes it is internally grounded.

[budm]

Re: AC Adapter amperage

That is great job, I use the similar APC case for my Surge protection/Over voltage board assembly.
May be you can add terminal strip for easy connections for other project. I am glad it works out great for you.

[cadiman]

Re: AC Adapter amperage

It would be a lot easier to just run it off a real battery (car battery)Thats what I did to cut pipe out on the golf course.

I just hooked it to the utility cart.

These take the current of a good battery to cut pipe and is hard to duplicate with a power supply and you don`t need 120v line feed.

The amps is more important than the voltage as long as it`s close.

A charging 12v system runs about 14.5 volts.

The slower blade speed might be better anyway.

That is the best way to use what you have and not need to buy anything special.