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Forum Index : Electronics : current sensing switch
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Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
I have a 24V battery bank that has its charging current monitored by a 100A/100mv shunt in the negative lead from the MPPT to the battery bank. This works fine with the LCD panel meter connected to it to display Amps directly. Now, can I use this shunt voltage (from 1mV up) to somehow drive a small 12V relay which in turn switches something else on - but only when the charging current is about 1A or greater? The relay should be off at other times. If anybody has a simple circuit I'm interested. I do want to keep the shunt where it is in the negative lead. I just spent an hour surfing Google to see what's available, from that I gather I need a low side sensing current switch. Are the wire through hole Hall effect current sensors suitable to drive a small relay (~1000 ohm coil)? These sensors are about $40.- so I want to be sure before purchasing. Thanks Klaus |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Hi Klaus, You could use a "Shunt monitor" chip and have something like a picaxe reading the shunt monitor, then the picaxe would switch the relay on/off as per programmed, over all a simple circuit. Should you not find another solution then i have some monitor chips here, but they are SMD components and very tiny, so more than likely would need to mount one on a PCB for you unless you are good with small SMD stuff. I will need to check if the chips will work as low side monitors as i have only ever used them as high side monitors. Here is the data sheet for the monitor chips 2012-07-28_163745_ina195.pdf Pete. Sometimes it just works |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks for that info Pete, never came across 'shunt monitor' chips before so I learned something new. It looks simple enough, what's shown as "load" in the circuit would be the battery bank in my case. I doubt my eyes are good enough for SMD chips these days, thanks for the offer. I did try using a comparator (LM311) but the few mV across the shunt were too low for it I presume. Just had another thought, when the battery bank is charging there is always a considerable voltage difference across the MPPT regulator. For example, solar panels at 34V, battery bank at 26V. With no solar input that difference disappears. Perhaps I could use that to feed the comparator input? Don't know if that causes some common mode problems but a quick breadboard hook up should find out  . Have to be careful though, a wrong connection here can easily release the magic smoke  .Klaus |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Should you want to monitor the shunt I think you would be better off with a single rail opamp like a LM358 (compared to the comparator) and might need to add some feed back to amplify the signal. Monitoring the MPPT voltage should be easy enough using a voltage divider to set the threshold voltage from the mppt. If you held the inverting input to a fixed voltage (say 5v) and used a 10k trimpot set up as a voltage divider going to the non-inverting input, and dial the trimpot in to give >5v out at say 32 volts at the mppt, then you would have your switch. Again some feed back might be needed to give a little hysteresis. If you want to use the shunt then here is a circuit you could try with a LM358 opamp. I have not tried it myself but it would be a start, the output would need to be feed back to the second 1/2 of the opamp (dual opamp in a single package) and a voltage reference used to set the relay trip point. The top of the circuit is missing (not needed) but the opamp was supplied by 5v i think, and the other line not marked was the V+ side of the shunt. Note:- R1 and R2 are your shunt and not needed to be added to the circuit. Pete. 
Sometimes it just works |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Trying to reliably measure a one millivolt signal will be difficult but not impossible. Doing it magnetically has a few advantages. How about a Hall effect sensor? Some of those can switch 50mA on and off directly, which will be more than enough to drive a LED or a relay. Make up a magnetic circuit out of some scrap steel pieces, and place the Hall sensor into a suitable narrow air gap. Then loop a few turns of high current cable through the hole. With a bit of fiddling you will be able to set the threshold current to whatever you wish. Cheers, Tony. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Your hall sensor method would work for a while but not that reliable at the low end and would likly keep the relay latched after the current fell away, see Glenns Piclog thread for his home made hall current sensor and why he changed it out to a shunt. Tinker only needs to monitor 1mv and above, so a circuit with suitable gain should allow it to be done even if its not exactly linear would not matter. Pete. Sometimes it just works |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
A linear analog Hall effect current sensor is one thing, they can be VERY tricky to apply. Hall effect on off switching control is entirely different. Placing a large air gap in the magnetic path ensures there will be absolutely minimal remanent flux remaining when the dc current falls to zero. These switching Hall sensors also have very effective inbuilt hysteresis to give positive noise free switching at quite low flux levels. For only a few dollars for the bare Hall sensor (and absolute three pin simplicity) well worth trying OMHO. Cheers, Tony. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Tony, I understand what you are saying, but it still comes back to the same problem of reading the very low current end, and the metal will become polarised and likely to hold the relay latched after the current has returned to zero. Otherwise i would agree fully with using a hall sensor. Pete. Sometimes it just works |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
One amp is hardly very low current !! Especially if there are several turns of wire through the core. Ordinary relays work in exactly the same way. They never lock up solid and fail to release when you remove the power. The very small amount of residual magnetism might only be a problem if there were zero air gap in the magnetic circuit. The Hall device will typically require about a 3mm air gap slot to fit, which is absolutely huge in magnetic terms. Cheers, Tony. |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks guys, you give me lots of ideas to pursue here. Basically, what I want to do is use a small relay to switch something on - but only while the solar panels are actually charging my battery bank. Since I have already a 100A shunt in the charging circuit (negative lead - my LCD current monitoring meter works better that way) I thought this would be an obvious place to sense charging but that looks too hard now. I did a few measurements today and think monitoring the MPPT voltage difference might be a much easier way to trigger the above relay. I will let you know how successful it is shortly. The neat thing about using the solar panel output voltage is that I could also power the trigger circuit from it so, when the relay is off, like overnight, there is also no standby current drain. Klaus |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
OK , I have found a workable solution. And its so simple its embarrassing to admit I had not thought of it in the first place. But somehow my thoughts got stuck with the current metering solution at first, until I realised that in order for a charging current to flow there has to be an associated voltage.... Anyway, what I did is connect a 12V and a 15V zener diode in series with my relay coil and this lot across the solar panel terminals. The relay coil has about 1000 Ohm resistance so it only needs a small current to operate. But this current won't flow until the panel voltage goes above 27V, at which level the batteries are also receiving charge from the MPPT output. Panel voltage below 27V means no battery charging and the relay is off, just what I want. Klaus |
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CraziestOzzy Senior Member  Joined: 11/07/2008 Location: AustraliaPosts: 135 |
...you could use the comparator as a voltage follower and then use another as an amplifier to increase the signal to your desired voltage output level. ...just a suggestion for later reference http://cr4.globalspec.com/member?u=25757 http://www.instructables.com/member/OzzyRoo/ |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
The home made temperature controller for my refrigerator uses a platinum RTD 100 sensor. The voltage across this sensor only changes 1.56 millivolts for a four degree Celsius change of temperature. A very similar problem to monitoring a current shunt. Using a voltage comparator directly is pretty hopeless. Just microvolt noise voltages and motor switching spikes can create some pretty erratic switching. The solution is to first amplify the dc millivolt signal using a high gain amplifier with a very restricted high frequency response. Once you have a noise free signal voltage with several volts of swing, either a voltage comparator, or Schmidt trigger will then give excellent stable repeatable results. Cheers, Tony. |
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Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi Tinker, One of the problems with using an op-amp to measure or amplify a small voltage so that you can switch your relay is that few are happy working down or at the zero volt value of their power supply (or up at the positive value for that matter). This even applies to my old favourite IC, the LM324, although you can get close. You can get around this by operating the op amp with a negative supply line that is more negative than the battery negative line, but this needs a DC-DC converter of some sort such as a diode pump. Possible, but becoming complicated. Why not look at one of the current to voltage ICs such as are described in the general pages of this web site? They have a built in hall effect device, no magnetic hysteresis, and a reasonable voltage output. You would probably still need to use an op-amp or comparator to set your switching point, but at least you would have a reasonable voltage level to work with. Regards Don B |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
LM324 and LM358 will work very well down to zero input volts, and actually slightly beyond into negative volt territory, with only a +ve supply rail. If you want to work right up at the positive supply rail, try a JFET input op amp. These can monitor a current shunt at the full supply rail voltage without any problem. It isn't difficult, the only trick is finding a suitable op amp with the input common mode voltage range you need. Cheers, Tony. |
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| sjh7132 Newbie  Joined: 27/12/2011 Location: United StatesPosts: 13 |
Comparators don't necessarily make good op-amps. One spec to watch when dealing with little voltages is the input offset voltage. For the 311 it's 2 to 10mv over the full temperature range. And the kicker is it's not a constant, it can vary with temperature, power supply voltage, etc. I've been burnt by that on other projects. Steve |
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