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Series circuit with diode?

27ace27

Junior Member
#1
Hello all, forum newbie here so perhaps I should introduce myself first.

My name is Paul, I Love flitetest (who doesn't) and have been into RC for some time. I now (mostly) know what I'm doing (at the expense of several planes) and like to dabble (what a strange word) in other things such as electronics.

Since RC and electronics kind of go together and since I always have a bunch of batteries lying around, I decided to build a charger for my laptop that uses two LiPos in series. Version 1 worked fine, but I didn't trust the ex-USSR military mini analog voltmeters that I picked up off of eBay, so it was rebuilt with smaller digital meters.

Unfortunately, the new meters do not like their polarity reversed (as I found out the hard way), so I tried adding diodes between the batteries and meters, but the voltage drop rendered the readings useless. The next option was to add a diode between the batteries as shown in the diagram (this also having the added benefit of preventing back-charging, which I also discovered the hard way), but this yields strange results.

When measured at the charger board, the voltage is 0 even though the batteries each read 12.4V. At the diode, the voltage is negative 24.8V. What gives? It seems that the diode is backwards, but current flows from V+ in to V- in, right?

While drawing the diagram, it became apparent that perhaps this was not the case within the pack arrangement. i.e. while current flows in one direction overall, it flows in the opposite direction between the batteries, but still from + to -.

I may have just answered my own question but just want to be sure before I go fry more meters.
Regards,
Paul

TL;DR: Diode probably backwards, please confirm!

battery.png
 
#2
Take the following with a grain of salt:

Diodes inhibit the flow of electrons in one direction. If you're getting a negative voltage reading at the two ends of the series, then the electrons have been forced to flow backwards to complete the circuit. If memory serves correctly, electrons flow from the cathode (positive) end of a power source to the anode (negative) end of a power source with a complete circuit under normal conditions.
 

27ace27

Junior Member
#3
Thanks for the tip, Mustang, and you're right but that's not my problem. I pretty sure that the diode is reversed and the reason I got a negative voltage was because I measured backwards thinking that the current flowed from overall positive to overall negative, and measured the potential voltage, not the voltage actually flowing. The current does indeed flow from + to - between the batteries.

The new problem is that reversing the diode will remove the reverse polarity protection. The new plan is to use low-impedance MOSFETs as detailed here. Problem solved! If only they were all that easy...
 

tramsgar

Senior Member
#4
Current flows from the positive (anode) pole to the negative (cathode) pole. Electrons actually flow the other way, but just imagine it's positrons and all will be fine.

The diode in the circuit above will prevent current from flowing from the left to the right cell, thus discharge is prevented. But it will allow current to flow backwards, as when they are charged. (All assuming that one cell's voltage is above the forward voltage drop and that the breakdown voltage of the diode is larger than one cell's maximum voltage.) You typically do not want the voltage drop between the cells that the diode will give you.

I was also taught to "never trust electricity".
 

vk2dxn

Senior Member
#5
Not sure that I completely follow your question however if you are worried about reverse polarity protection then you can run the charge through a bridge rectifier then it doesn't matter what way you connect the battery polarity and run your volt meter on the output (load side) of the bridge rectifier. The voltage drop of the rectifier will be minimal

If you are worried about voltage drop then you can run a parallel configuration to reduce the voltage drop
 
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chaydock

Junior Member
#6
Paul,

In the 30+ years of working with electronics I've personally never ran into a volt meter that's been fried by connecting it's polarity backwards (it seems especially odd on a digital meter). What I HAVE seen is someone thinking they are taking a voltage reading when in fact they have the meter set up to resistance or current measurement. In that case you effectively short the power source and as some would say "let all the magic smoke out of the box." Another case of killing equipment is using an oscilloscope as a volt meter and not paying attention to your polarity. Then you stand a chance of short circuiting the power source with the oscilloscopes ground lead. (but, you didn't say you were using an oscilloscope... your not right?). What make and model of meter are you using? :confused:

As for your circuit, I would personally highly advise you not add a diode into your battery system in series. I could go into a multitude of reasons that would make this a quite lengthy explanation... but I'll save you the reading unless you ask me for it. I see your new plan is to use a MOSFET. While more efficient than a diode, I still feel that adding one would be wasteful and unnecessarily complicates the circuit. FETs also have an observed polarity and current flow... so charging/discharging through a FET could prove problematic depending on your final design. I think you're better off using a polarized plug connector that mechanically prohibits reverse polarizing the circuit rather than unnecessarily complicating your electronics. Especially if your current draw is substantial or full of noise (MOSFETS don't usually like power spikes... you can get phenomenon such as "avalanche breakdown" and "punch through"). Small power FETs are not usually that cheap and connectors are. Unless absolutely necessary... stick to the KISS method.

In any case, just because a diode or MOSFET is placed in the circuit does not mean that you have protected yourself from smoking your meter. If you connect across the power supply in a resistance or current check (which is usually the culprit) then you'll kill your meter again. This is by far the most common way I've seen people kill multi-meters (I've personally killed one meter this way :black_eyed:). If you are in fact using an AC powered oscilloscope, then you can buy yourself some insurance by getting a ground isolation transformer (essentially a 1:1 transformer that has no ground connection). But, do NOT confuse an isolation transformer with an auto-transformer... you'll let the smoke out again and oscilloscopes are not cheap.

I apologize for the long post. I used to teach electronics so I sometimes get carried away.