Forklift Battery Charger

[ACY]

I have a 24 Volt Battery Charger, used for walk-behind forklifts. Its design consists of - transformer, capacitor (15uf), relay, diodes (x2), Ammeter (30A), and electronic circuit boards. I wish to place a manual radial timer in the circuit (8 hr MAX). Am I correct in assuming that the relay and circuit boards can be replaced by the manual timer? Why is the Cap copnnected to the transformer in the circuit? With the secondary transformer winding leads they run to the diodes (connected in parallel), Why?; off the diodes only the Positive (+) has a lead that runs to the output (also runs to the circuit board). The negative (-), come from the circuit board to the ammeter, then to the output. I dont understand how a negative is obtained, if not from the diodes. I have searched the net for explanations, and are yet to find any sites. I would like very much to gain an understanding of the whole battery charger concept. Thank you in advance.

 

[dpc]

http://www.powerdesigners.com/InfoWeb/design_center/Appnotes_Archive/1707.pdf

Probably looks something like that. It's hard to be more specific because there are a lot of different rectifier designs. Basically, the charger is 1)lowering the incoming line voltage with the transformer then 2) rectifying the ac to a pulsating dc with probably a little smoothing by the capacitor.

Why not put the timer on the ac power input to the charger? That would be a lot simpler - but maybe less interesting.

 

[buzzp]

Sounds like your not up to speed on electrical design. You may want to explain what your trying to accomplish a little better. If the goal is to only charge for 8 hours and shut-off then use an off the shelf timer and switch the power off to the charger. I see this is dpcs suggestion too.

 

[ACY]

Thanks for your responses. What you have suggested is exactly what i am looking at doing. What i am unsure of is ; Why does the circuit board exist? There must be a reason why it is there, otherwise its original design would have been as simple as we have suggested.
As for the capacitor, thankyou for your explanation.

 

[PowerfulStuff]

Most newer battery chargers have some smarts to control the charge into two phases. The first is 'constant current' where the current into the battery is kept at a constant level which charges the battery quickly but without risk of overheating it. Once it reaches a certain voltage it changes to 'trickle charge' where the charger puts out a much smaller current to keep the battery at a fixed voltage.

There must be at least two connections from the transformer to the circuit to work. In some rectifiers the negative can be connected to the transformer winding directly as the diodes on the positive side stop the current flowing the wrong way.

 

[buzzp]

You cant just hook up some power to the battery and watch it charge. The 120 is changed to a constant current source (automatically adjusted based on remaining battery capacity). The voltage is also changed to some other level (depends on the type of batteries). There are lots of tasks being handled by the board. Just hook your timer up to the 120 and the relay on the timer will switch the 120 to the charging circuit when it needs to charge (every 8 hours). Or if it has to charge for 8 hours then shut-off then you need an on delay timer. If you want charging every 8 hours then use a cycle timer.