sn't fry, and it doesn't need very much in the way of exotic features. In that event, it doesn't matter whether the diode is biased, or not. Obvious ly, in order to be stable and predictable, there needs to be a bias resisto r. It wouldn't hurt to put a large resistor from the negative input to gro und, as well.
op across the 33R. To get a diode's worth of drop you'd need 2 diodes on th e opamp input side not 1.
Hah, right, but that does take us back to the original transistor circuit.
oesn't fry, and it doesn't need very much in the way of exotic features. I n that event, it doesn't matter whether the diode is biased, or not. Obvio usly, in order to be stable and predictable, there needs to be a bias resis tor. It wouldn't hurt to put a large resistor from the negative input to g round, as well.
drop across the 33R. To get a diode's worth of drop you'd need 2 diodes on the opamp input side not 1.
.
Otherwise a fet/ srs / switchy thing as JL suggested right off. GH
I like the relay idea in general. I use them a fair amount myself--just today I was going over a diode laser driver design that has a NC relay contact across the laser to keep it safe during handling. There are maybe a couple of issues though:
Power consumption. Even small relays dissipate a few hundred milliwatts when the coil is active.
Speed. A chip can easily blow up before a relay contact has a chance to open.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
Every amateur "design" starts with an OpAmp. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
It's what you learn, after you know it all, that counts.
-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
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| 1962 | It's what you learn, after you know it all, that counts.
OK, with everyone's input, I think I have settled on a solution:
formatting link
If I haven't made any errors, this should allow for Vin to be up to 18V and it should work with Vin - Vbatt as low as 2V, or perhaps even a bit less. It shouldn't croak when mains power fails, although I might add a largish resistor from the (+) input of the op-amp to ground for safety.
The point I've been trying to make, which seems to be falling on totally deaf ears... you start with a gozinta/gozouta spec/performance definition, perhaps even a flow chart if there are "states" involved.
THEN: You can usually readily see some semblance of components needed.
Starting in the middle is bizarre.
But, now-a-days, school boys _always_ need an OpAmp :-( ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
It's what you learn, after you know it all, that counts.
Continuing to charge the battery at a constant current indefinitely even when fully charged will shorten its working life considerably (or mean that it takes the battery way too long to recover from being used).
I think what you actually want is a three terminal *voltage* regulator followed by a diode and a resistor from that to the reserve battery to limit the maximum charging current and allow it to naturally tail off as it approaches full charge. Schottky diode from the reserve battery to load so it can take over if the input power fails. The odd extra diode to protect the whole thing from reverse polarity idiot users.
A bit of cunning and you may be able to arrange it so that the load helps to protect the reserve battery from over charging.
oesn't fry, and it doesn't need very much in the way of exotic features. I n that event, it doesn't matter whether the diode is biased, or not. Obvio usly, in order to be stable and predictable, there needs to be a bias resis tor. It wouldn't hurt to put a large resistor from the negative input to g round, as well.
drop across the 33R. To get a diode's worth of drop you'd need 2 diodes on the opamp input side not 1.
) op-amp input, the emitter, and the top of the resistor are all at the sam e potential - they are tied together. Assuming the op-amp has a negligible input offset value, the (+) op-amp input is also at the same potential as the top of the resistor, which means the 1N4148 anode is also at the same p otential, since it is tied to the (+) input. During normal operation, the anode potential will be something like .7V above the cathode potential, and the cathode is tied to to the bottom of the resistor. Thus, there is roug hly a .7V, or 1 silicon diode potential difference across the resistor.
Ah, I missed that the -fb is from the tr emitter, not the opamp output
Nickel cadmium batteries can tolerate fairly substantial over-charging inde finitely. Once the cell is fully charged the current produces hydrogen gas rather than nickel metal, which diffuses through the cell and recombines wi t oxygen, warming the cell, but not doing any damage.
Smart chargers detect the temperature rise and throttle back the charging c urrent, but the data sheet species a charging current that can be sustained indefinitely - typically the current that would recharge a flat battery in ten hours (C/10).
NiMH cells work much the same way, but the manufacturers do suggest that pe rsistent charging current be limited to about C/30 to maximise cell life.
Oops! You're right. That one slipped by me. Good catch, and thanks!
The battery connection is not. I will be using a Deans connector, which is polarized. They are widely used in Radio Control devices. The main supply is. Que sera. The polarity will be clearly marked and prominent warnings will be displayed.
Not significantly for NiCd, NiMH, or Lead-acid batteries, if the charge cur rent is less than .01C. For legacy battery types, this has historically be en the a common means of charging consumer batteries.
).
What constitutes "too long"? Am I happy the batteries will take perhaps several days to fully recover from a power failure? Not really, but it is a compromise that allows for the flexibility I am seeking. All engineerin g is a compromise, and since this allows me to achieve the results I want, I am wiling to make it. The odds of having two back-to back extended power outages within the space of 3 or 4 days is pretty low. In the rare instan ce it happens, the batteries may not hold up as long as one would like duri ng the second outage. Such is a relatively minor impact that is not very l ikely to happen in the first place. I feel it is a reasonable compromise.
All of that with less than 2V charging headroom is a very tall order. I f you have a design with components that can handle it, I would be thrilled to see and implement it.
Since the load is a computer and an ADC is being employed to measure the operational voltages, it wouldn't require all that much cunning, I suppose , but I would be uncomfortable with having the charging under software cont rol by the load. Although a nice feature, I am not convinced it would be w orth the cost and effort just to shorten the charging time. If the batteri es were the primary power source, it would be another matter, but in this c ase the batteries provide secondary power. The charging cycle does not int errupt normal operation as it does for a primary battery power system.
definitely. Once the cell is fully charged the current produces hydrogen ga s rather than nickel metal, which diffuses through the cell and recombines wit oxygen, warming the cell, but not doing any damage.
current, but the data sheet species a charging current that can be sustain ed indefinitely - typically the current that would recharge a flat battery in ten hours (C/10).
persistent charging current be limited to about C/30 to maximise cell life.
Exactly, and I am recommending the user employ more like C/100. If they pu sh the charging rate too much, then it is on them.
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