I am stumped on this curcuit

volts

Yes, this isn't hard. Your scheme won't be very stable, in that if there is any fluctuations after the transformer (which there will be under load) the transistor will wobble around with them. A better way would be to use a Zener diode to set the voltage more precisely, rather than using a voltage divider. (Note that in your circuit, if you assume an amp gets drawn out of the transformer, your 2N3055 will be dissipating about 20W, so use a big heat sink.)

Also, the 2N3055 already is a diode, so you don't need the extra one; current won't flow in the other direction.

The best way to do this kind of stuff is to use a comparator and a voltage reference. Use a divider to get a percentage of Vbattery, and compare that to a voltage reference. Set the comparator to go high when the voltage gets below the reference by connecting the reference to the + input, and the divider to the - input. Then, set up the relay to stay closed unless the comparator goes high.

Use some positive feedback from the output to the V+ input to set hysteresis.

and

charge

Take a look at

There isn't any current limiting, so its not really a very good supply. However, it sketches out how to do the battery backup charging, and also how to do the cutout circuit.

Regards, Bob Monsen

Hi Scott,

Robert already hit most of my objections.

I wouldn't use a 28V trany, the full wave rectified voltage in the

There is nothing in your circuit which would limit the current, either, except the transformer winding resistance.

Figure that there are times when this thing will want to deliver 5 or more Amps. The dissipation in the 3055 will be (ThirtySomeThingV-12V)*5A = ~100W, which requires several square feet of heatsink...

Try to find a tranny that has a secondary voltage something like 15V. Even with loss in the diode bridge, it would charge the cap to a peak voltage of ~(1.414*15- 1.4) = ~18v which still leaves plenty to charge the battery to ~13.25V

Why did I say 13.25V? That is the required "float" voltage for a flooded cell lead acid storage battery.

You want your circuit to work like this:

Bat V Current

In other words, when the battery voltage is less than 13V, you want your charger to behave like a constant-current source such that for all voltages below

For battery voltages from 13 to 13.3V, you want it to taper from "full" to zero current. This means a high-gain control loop, which requires something more complex than your pot tied to the gate of the 3055. Robert has it right; this reqires a reference voltage source, and a feedback amplifier to drive the 3055. Since the

A crude way of limiting the current that your charger might deliver into a discharged battery is to put a "ballast" resistor in series with the primary (on the 120V line side). A 120V lamp can be used. Try 25 to 150W bulb to get your desired max "full" current. The lamp will glow dimmly; its brightness is an indicator of how much current the charger is putting in the battery.

MikeM, PhdEE

You might also want to add something to limit the current through the 3055 when a flat battery is connected?

volts

and

charge

your problem is that a relays holding current can be low compared to what it requires to pull in. what you need is to add a simple OP-amp to form as schmitt trigger. that is a Voltage comparator with a feed back to the + side to forward bias the comparator so that is off set.. this offset can give you the desired window you need for the turn on point and turn off point, this Op-amp will drive a 2n2222 or any switching transistor that can handle the current of your relay as long as you saturate the transistor. what your doing now is most like not switching the relay when you want due to relays wide windows and also q2 maybe getting alittle warm at times .. if you need more help on that idea i am sure you can get it from some one here. there are allot of schmart guys here! :)

Scott Wiper wrote:

Now watch it. i got kicked out of an Electronics Forum on IRC "Undernet:Electronics" for using the term "Trany" it seems that some take this field so seriously that they can't get their heads out of the clouds ,do a little R&R and throw in some slang phrases now and then! it sure saves on typing and getting the point across! Now i know that some would feel insulted getting caught communicating with such people using short cut slang phrases. so these people (some of which are channel Op) would rather Banned you from the rome then let you help some one on live chat that is understanding you very well. now if this OP thinks i am a moron then more power to him ;) How ever, i can tell you a little story of an individual years ago walked in my shop and asked me if i could replace the pills in his amplifier ?, well after thinking about it for a moment and then coming to the conclusion that he must of been talking about the ceramic RF power transistors i then ask him if he had a subscription. he looked at me for a moment thinking about my question verses his and said he changed his mind and walked out. i did get a kick out of it how ever! :))

MikeM wrote:

Hi Jamie,

I'm one of those "old farts" that has earned a special dispensation to call things what I want; you know, like

Transformer = Tranny Bush = Idiot Cheney = Crook

MikeM

Jamie wrote:

: Transformer = Tranny : Bush = Idiot : Cheney = Crook

: MikeM

No one "earns the right" to be a FOOL! It comes naturally! You sound like a Demon_Rat to me! Learn to use the Language or be ridiculed frequently!

-------------- Roger the Git, above, doesn't appear able to handle the Truth!

-Steve