How obtain a constant voltage drop?

On a sunny day (Mon, 5 Sep 2011 08:40:39 -0700 (PDT)) it happened lionelgreenstreet wrote in :

Easy... use a TL431 plus a power PNP. ...Jim Thompson

-- [On the Road, in New York]

| James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

get a proper battery charger...

Thanks all for your reply...

@Jan

An ideal diode could be better...so i've thought about zener...

@Jim

How? Have you any link? I need a constant voltage, not a constant current sink...

@Fred

next time

Thanks

Sounds like you have no clue, but I'll draw it up... I need some distraction from Obama's union-thug speech :-(

...Jim Thompson

Assuming that's 4.8V drop at 3.7A, your device will need to handle

Like this...

...Jim Thompson

tolerances should be specified

Put a 4.8 battery in series, four Ni Cad cells is 4.8v

... and then call the fire department. The trucks should arrive just about in time.

There's two problems in the background, here. Firstly, some 'battery charger' devices have relatively high output compliance (i.e. the 'constant voltage drop' will have no effect except to waste power). Second, some 'battery charger' devices have low output impedance, and (in combination with a low impedance 'constant voltage drop' device and a charged battery) will cause a kind of race to see which gizmo blows up first.

The foreground problem, is that the battery charger is not INTENDED for this battery, and any engineering that went into making it a good charger is ... wasted. Are you certain you can re-engineer this safely?

I think I hear them now .

Jamie

Power transistor on heatink with 2 resistors, one c-b, one b-e. The Rs act as a voltage divider, the tr begins conducting at Vbe=3D0.6v, and the whole thing acts as a zener

NT

Where are you going to get an ideal diode, let alone an ideal Zener diode?

Michael, you need a sense of humor :)

Hi, i hava a little problem: i need a costant voltage drop between a source (a battery charger) and a load (battery to be charged). Voltage drop should be 4.8V; maximum current=3,7A. I thought about a zener diode, but nothing i've found with these features. Any suggestion? Thanks

---------

You can use a bjt on the low side of the battery(so the battery floats with - connected to the collector).

Linearize the bjt's base and program it so that when the V_CE > 4.8V it acts to close the base off and when V_CE < 4.8V it acts to drive the base(this will require a 4.8 zener or some combination of diodes or voltage reference).

Once you do that the bjt can easily take the 3.7A(on a heat sink as it is about 20W).

You could bias the bjt easier using a pot if your willing to monitor it. If you want to get fancy you could use some sort of feedback method such as a uC or what I described a little above.

The point is that a bjt can act like a constant voltage drop by driving it's base appropriately(the base current will generally follow inversely with the change in V_CE so it holds V_CE constant).

Another method is to use a switcher which is less wasteful but more complex.

If I didn't have a sense of humor, I would have told him what a fool he was, and in great detail. :)

Liberals absolutely hate people with a sense of humor... they know they're being laughed at. ...Jim Thompson

Radio Shack?

You give a lot of them too much credit. They don't even know if they are breathing. :(