Detecting down to 25mA-150mA on 0-20A line?

I'd like to use a 20A CC/CV power supply to charge LiPo cells and want to terminate the charge when the charging current drops to 25mA-150mA (selectable).

Sense resistors are out, IMHO, because of the 20A max charging current (for multiple parallel cells). Any value that can give me a decently measureable voltage would burn up at 20A. And I can't find any Hall-Effect sensors (allegor, etc.) that can work down to those current levels.

I don't need to measure the current, just flip a bit when the current drops below my 25mA-150mA setpoint. Or, I can monitor a voltage and just ignore anything over a certain level.

Any recommendations for a part/method that could measure such low current levels while not burning up when the charge current is as high as 20A? Anything to +/-10% would be OK, +/-1% would be perfect.

Thanks!

-- remove SPAMMENOT for e-mail responses --

Another approach would be to use a sense resistor bypassed with a diode, so the diode takes most of the load at the higher currents.

Sense resistors are great and give a good signal with low impedance. A 10mOhm resistor gives 200mV at 20A and 250uV at 25mA. Easily with reach after a times 20 amplifier. The other option would be a LEM, to make the 200mV burden vanish.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

You could use a sense resistor bypassed by a high current diode. This will clamp the voltage across the sense resistor to VF of the diode with a power dissipation of VF^2/Rsense. The way to proceed would be to set the Rsense voltage drop at "maximum Imin" well below the forward bias of the diode. So trying 0.4V at 150mA gives Rs~2.7 ohms. Then even a weenie 1/2W Rsense can handle sqrt(0.5W*2.7)=1.2V for VF, diode at 20A. Going with say a 2 Watt 2.7 ohm, the VF,max becomes 2.3V etc..There are ample precision sense resistors available in the range with very small tempcos like 50ppm, usually in a TO-220 type of package. Now all you need to do is find a 20A diode, and that's nothing special, you can manufacture one by paralleling lesser rated diodes with ballast resistors. Getting back to the original problem, using 2.7 ohms, gives a voltage detection threshold of 0.4V at 150mA to 0.05V at 20mA.

Consider avoiding the problem!. Just have a normal current sense resistor, of perhaps 2R, and bypass this with a large Schottky diode. For currents in your required reading range, the voltage across the resistor will drop below the point where the diode starts to conduct, and you have a voltage output that can be read by a simple ADC. At higher currents, the diode conducts, limiting the maximum voltage across the sense resistor. Chose a diode, like the 52CPQ030, and the unit will withstand up to 50A, yet will give a nice linear voltage in the sort of region you require.

Best Wishes

Some of the performance specifications for some of today's op amps are pretty impressive.

It may be possible simply to use something like a one milliohm sense resistor (perhaps a "dog bone" style resistor built onto your PCB using the copper trace(s) as the resistive element, with larger pads on the ends for heatsinking and kelvin connections), and use some insanely low offset op-amp like the MAX4238 to amplify your 25uV (at 25mA) signal to something more useful.

Obviously layout/routing/attention to parasitics and noise reduction in the rest of the circuit will be crucial in order to make this method succeed.

As for the other approaches... The shottky diode in parallel with sense resistor may not work so well. At 20A, the shottky diode will still need to dissipate large quantities of power, but at 25mA a diode that large will have very tiny (likely off the datasheet) forward voltage drop, quite possibly well under 100mV depending upon temperature. You would still need a decently precise op-amp for this case, but you'll have the additional disadvantages of an expensive shottky diode, heatsink, and substantial wasted power. Attention to routing/layout/noise reduction would still be important, although nowhere near so as the above mentioned method. The characteristics of a silicon diode would probably be more ideal, but with the serious disadvantage of substantially more power waste.

Why not shunt your sense resistor with a power-MOS (with control circuitry) that kicks in over-range?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

[snip]

What is the value of "CV"?

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

or if you don't want to bur any power in the diode use a low Rdson resistance FET instead to bypass the resistor. Switch the FET off briefly to take the measurement every 10 seconds or so.

Whether you use the diode arrangement or a very low value resistor (= short thick wire), you will still need some circuit to do the switching. While the diode solution suggested is a neat one if you need a higher sense voltage, the other solution suggested (eg 741 op amp as a comparator) means that a low voltage can be detected. One problem with the diode (even a Schottky at 0.4 v drop), is that is will still dissipate 8 watts at 20 amps, and will need some heatsinking. Granted you could use a simple transistor or darlington pair with the diode arrangement, but using a 741 with the thick wire would mean no problems with things getting hot!

Just my tuppence worth.

Eric Sears

News==----

Newsgroups

You're preaching to the choir Eric . . . I wouldn't use a diode in that application for the power dissipation reason.

The only drawback is that trying to use a single 741 op amp might require a separate power supply. The poster didn't mention what voltage this thing was running at - but if it were me, I'd find a way to make it work, or put in another supply just for the op amp if necessary - 8 watts of heat is too wasteful IMO.

Then he's talking about using a hall effect sensor . . . that might be more complicated, expensive and bulkier, than an op amp, and harder to adjust - assuming he wants to adjust the set point..

If his source is a switcher and he's able to get at the high frequency output a current transformer might be the best choice.

--

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Thanks for the responses!

The CV value will be 4.20V or multiple thereof, depending on the number of LiPo cells being charged. This is considered the full-charge value for LiPo cells. But, I'd like to be able to do use other voltages from 3.70V to 4.5V per cell for testing purposes.

I also have available regulated 9.3VDC-9.5VDC and 5.0VDC.

One reason to avoid a sense resistor is that its voltage drop has to be added to the voltage of the battery at full charge to arrive at the power supply voltage to set. For each new end-of-charge current value (from 25mA to 150mA), I'll need to have a different power supply voltage for the LiPo cell/pack to come up to. I'd love to avoid this if possible. Especially since a LiPo cell is very easily damaged above full-charge voltage values of 4.235V to 4.250V/cell (depending on the manufacturer).

This is why I was wondering if there was a hall-effect or other method to sense the current?

I could start a timer when the voltage reached 4.19V or 4.20V (whatever) and just let the battery get charged until the timer stops but I want to graph the charging curve of these cells and don't want to guess at what the timer value should be. I could either waste a lot of time or end a charge early which would affect the capacity of the cell during the discharge testing.

-- remove SPAMMENOT for e-mail responses --

It should be obvious to the most casual observer ;-)

How about amplifier offset voltage?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Would this be the same sense resistor that's supposed to tell when it is over range? And how does it recover? Presumably the thing starts off drawing tons of current so the MOSFET shunts the sense - what mechanism do you propose to enable the shunt to measure current after the current drops if the sense is shunted?

My idea is to just amplify the piss out of the Vdrop across the sense resistor so a very low value can be used. I do it all the time with high current supplies and it works very well.

--

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Not so. It is common practice when sensing voltage for regulation purposes to sense voltage after the current sense resistor. That's the way it is done in linear supplies.

And if the current sense resistor is .0012 ohms that amounts to a 24 millivolt drop, you'd have that much in the on-board or off board wiring and could be using that wiring as you sense resistor.

It works very well . . . and is a lot easier than hall effect. For the hall effect system, you'd need a linear hall effect sensor or would have to settle for a single fixed current with no easy adjustment. To get enough flux down at 25 milliamps you'd need several turns of wire that would drop more than the amplified sense resistor scheme - and/or you'd have to worry about magnetic shielding down at low flux levels.

Seems like a no-brainer to me. If you have the 9 volt supply it is easy to get an op amp functioning single supply for that app.

--

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

That would work. Have a timer kick out the MOSFET for a short period of time on a regular basis and read the sense resistor. We used schemes like that for recovery on foldback current limiting supplies

It is more hassle and complicated than just making the sense resistor smaller than the lowest R MOSFET you're likely to find.

I used the open loop gain of a single op amp for an illustration. Use a dual amp and you'd lower the drop further. Low impedance across the input so there's little noise to pick up - get too carried away with amplification and it might require a integrating cap to slow the response and keep it from oscillating in the RF range.

Interesting project.

--

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Not a problem. I haven't encountered an op amp with specs so poor that it has any significant offset when the differential input is shunted. I've used a single 747 (dual 741) without offset compensation. One section regulates the pass transistor(s) the other works as the current limit. The 741(7) is a relatively primitive op amp and it works.

It was a scheme we used for multiple linear outputs on made to order logic supplies in the 70's. If there is a problem with the scheme it was when we had high inductance wire wound resistor for sensing. On the low current supplies it was never an issue - on the high current

10-20 amp 5V supplies we used a two turn phosphor bronze wire wrapped around a one watt resistor for a coil form. That two turns would have it oscillating in the MHZ region if the leads to the load were longer than a few feet - for that 100 pf cap from the inverting input to the output would neutralize it.

That wasn't really a problem, but more a matter of the variable load we used for testing. Our spec was 0-20 amps and at an amp or two, with long leads, and the active transistor load, it could oscillate if the op amp was fast enough. Most of the op amps were too slow to oscillate, but we started using the neutralizing caps just to be safe.

In the real load application they had no tendency to oscillate because of the bypass caps used in the load. Our dummy load was a low value, high wattage resistor with a self-biased NPN transistor with pot to control current. That load could be unstable.

This thing is probably going to be working as a comparator with some small amount of positive feedback to lend hysteresis, or just latch the output off when the current falls. He doesn't say what he wants full charge condition to be.

--

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

The fundamentals are shown at...

I'll leave (as an exercise for the student ;-) where to manipulate ground placement and add your various sensing and control circuits to you.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Sounds like you're doing a manual adjust on a power supply with adjustable current limit. Since you are over your head here, just buy a different power supply with a current readout.