How do I determine a fuse that I need for a circuit?

[EasyGoing1]

I have these very cheap but very useful 18650 battery chargers. However, if you accidentally put the battery in backwards, it immediately frys the chip on the charger and lets the smoke out. Someone at one time suggested that I use a fast acting resettable fuse inline between the battery and the charger and ive been looking at them but I don't know what kind of fuse to get.

I was thinking that I could use a variable power supply and connect it backwards to the charger then up the voltage while I monitor the current and see at what point the chip frys ... obviously ill be able to determine how many amps it takes to fry the chip, but since the fuse must be installed in series, I don't know what voltage rating to spec the fuse at ... in fact ... i'm not even sure why fuses have voltage ratings since the voltage drop across the fuse would seem to be a non-issue. I did get some 1 amp fuses or maybe they were 2 amp I cant remember but they didnt do anything ... but I haven't tried my variable voltage test yet either ...

So i'm looking for some advice on how to do this so I don't have to keep guessing.

Thanks,

Mike

[petehall347]

Re: How do I determine a fuse that I need for a circuit?

ultra fast very expensive fuses will do it .. or simply make it fool proof then make certain a fool cant break the fuse .

[budm]

Re: How do I determine a fuse that I need for a circuit?

One simple method is to put diode's cathode on the (+) and Anode on the (-) of the charger then put a fuse (must be higher current rating than the charging curent) than the between Cathode (output of the charger) and the (+) of the battery.
When you put the battery in backward, the diode will crowbar the battery so the fuse will blow which will disconnect the output from the battery that is put in reversed into the charger. Diode can be 5A or more, it will just conduct long enough to blow the fuse, but if the battery has really low Voltage the fuse may not blow.
There are more than one way of handling the situation:
https://www.maximintegrated.com/en/a...ex.mvp/id/4572

Not sure if some of the circuits has been verified or not if the battery has really low Voltage.
http://blog.deconinck.info/post/2017...ity-protection

[eccerr0r]

Re: How do I determine a fuse that I need for a circuit?

I wonder if a PTC thermistor would slow down charge rate enough or fast enough to protect the charger?

[Andrew F. Ali]

Re: How do I determine a fuse that I need for a circuit?

No. I don't think so. The best bet is as budm suggests with the diode across.

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

One simple method is to put diode's cathode on the (+) and Anode on the (-) of the charger then put a fuse (must be higher current rating than the charging curent) than the between Cathode (output of the charger) and the (+) of the battery.

So I decided to play a little, and I used an INA3221 that has a voltage input (which in this case would be the battery being charged) then on the monitoring terminals, I connected the output of the charger ... it's a nice chip because when the battery is plugged in properly and is being charged, the INA3221 will give me the current flowing to the battery plus the shunt voltage, which is actually the voltage that the charger has to elevate its output to in order to reach its programmed current (which is about 1 amp) but it will also give you the load voltage which is the voltage of the battery ... I don't know how it does, unless it disconnects very rapidly to read the battery's voltage then reconnects ... the load voltage and the source voltage (both are technically the battery) only differ by .01V which is more than acceptable in terms of margin of error ...

So anyways ... I wrote a java program that received data from arduino at a fairly rapid rate ... something like 20 reads a second ... and logged it to disk then I put a 5k pot in series with the monitoring terminal and the battery connected backwards and slowly turned down the pot until the chip fried (the chip on the charge circuit) ... heres the data that is relevant:

Current	  Shunt V
127.6 mA	864 mV
128.2 mA	856 mV
136.3 mA	941.3 mV
144.8 mA	1,008 mV
151.5 mA	994.7 mV
165.0 mA	1,136 mV
175.8 mA	1,205.3 mV
190.4 mA	1,309.3 mV
192.3 mA	1,306.7 mV
208.0 mA	1,466.7 mV
221.1 mA	1,445.3 mV
218.5 mA	1,496 mV
226.8 mA	1,437.3 mV
222.6 mA	1,584 mV
216.6 mA	1,301.3 mV
189.0 mA	1,264 mV
177.7 mA	1,122.7 mV
173.8 mA	1,173.3 mV
192.7 mA	1,400 mV
218.3 mA	1,536 mV
184.7 mA	1,136 mV
169.3 mA	1,138.7 mV
171.8 mA	1,149.3 mV
181.9 mA	1,245.3 mV
198.1 mA	1,304 mV
203.9 mA	1,418.7 mV
212.8 mA	1,429.3 mV
209.3 mA	1,333.3 mV
201.4 mA	1,346.7 mV
206.6 mA	1,386.7 mV
195.1 mA	1,304 mV
205.2 mA	1,437.3 mV
211.6 mA	1,370.7 mV
218.6 mA	1,458.7 mV
221.8 mA	1,490.7 mV
206.5 mA	1,373.3 mV
205.4 mA	1,362.7 mV
209.3 mA	1,410.7 mV
214.2 mA	1,416 mV
213.9 mA	1,426.7 mV
236.5 mA	1,629.3 mV
252.0 mA	1,762.7 mV
265.8 mA	1,813.3 mV
273.7 mA	1,904 mV
289.8 mA	1,936 mV
284.1 mA	1,930.7 mV
264.0 mA	1,784 mV
275.3 mA	1,912 mV
290.7 mA	1,962.7 mV
299.4 mA	1,997.3 mV
296.7 mA	1,965.3 mV
302.8 mA	2,104 mV
307.7 mA	2,002.7 mV
285.4 mA	1,858.7 mV
237.3 mA	1,373.3 mV
203.3 mA	1,368 mV
215.3 mA	1,402.7 mV
212.7 mA	1,474.7 mV
221.6 mA	1,546.7 mV
223.5 mA	1,442.7 mV
210.7 mA	1,378.7 mV
212.9 mA	1,381.3 mV
214.2 mA	1,496 mV
253.2 mA	1,816 mV
276.1 mA	1,938.7 mV
292.3 mA	1,938.7 mV
312.3 mA	2,197.3 mV
340.9 mA	2,317.3 mV
388.2 mA	2,744 mV
448.4 mA	3,176 mV
507.0 mA	3,520 mV
1,063 mA	10,920 mV <---Fried
1,638 mA	10,920 mV
1,638 mA	10,920 mV
1,638 mA	10,920 mV
1,638 mA	10,920 mV
1,638 mA	10,920 mV
1,638 mA	10,920 mV
1,638 mA	10,920 mV
1,638 mA	1,0920 mV

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

Anyways, it looks like the chip will fry at 1.6 amps ... the most amps that it will throw out to a battery that it's charging is 1 amp ... so my fuse selection is going to have to be fairly precise so that it only blows when the current is about 1.1 or 1.2 amps but not 1.5 or 1.6 obviously ... does such a fuse exist or am I S.O.L.???

[petehall347]

Re: How do I determine a fuse that I need for a circuit?

take your pick ...https://www.google.co.uk/search?sour....0.c9zjrXuVtXw

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

take your pick ...

So the voltage rating of the fuse makes no difference at all in the results? They are strictly current sensing and a higher voltage fuse wont inadvertently NOT break at voltages this low?

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

Looking at resettable fuses ... the datasheets seem to indicate that it takes several SECONDS before the fuse trips ... are there extremely fast acting resettable fuses available?

[R_J]

Re: How do I determine a fuse that I need for a circuit?

Basically the voltage rating is so when the fuse does fail and open it will not continue to conduct or arc across the blown elements of the fuse
You can use a 250 volt rated fuse to protect a 12 volt circuit without a problem

What about using a low FV diode (shottkey) in between the charger and battery to block the reversing of the battery and protect the charger

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

What about using a low FV diode (shottkey) in between the charger and battery to block the reversing of the battery and protect the charger

I've considered this, but diodes have their own voltage drop, and there is no way for the charger to understand how the voltage is being used that it puts out ... so it technically thinks that the only thing attached to it is a battery but when you add the diode, then its output voltage MINUS the diode is actually being applied to the battery which will slow down the charge time ... although im not sure by how long ... I suppose its worth a test..

Then again ... if the chip is in cc mode, then it SHOULD up the voltage until the desired current is reached and at that point, it should push harder to compensate for the diode ... but when it switches to the end of the charge cycle, it goes into CV mode ... and THAT would slow down the charge time ... but again ... still worth a test I suppose ...

Which diodes have the smallest voltage drop? I remember .7 volts being the standard back when I took electronics, but im sure they have new stuff these days, no?

[budm]

Re: How do I determine a fuse that I need for a circuit?

Putting the diode is series with the output will not protect the IC when battery is put in backward, the diode will be forward bias by the battery! read the links I provided.

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

What about using a low FV diode (shottkey) in between the charger and battery to block the reversing of the battery and protect the charger

OK, I tried all of the diodes that I have, and most of them wouldn't even allow the charger to kick in and charge the battery ... a couple of them did let the charger start charging, but without a diode, it would be charging at 800mA and with the diode, it would only charge at like 200mA ... so thats a no go ... at least for these diodes.

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

the diode will be forward bias by the battery! read the links I provided.

What's the quick answer to "Why would the diode be forward biased?"

[EasyGoing1]

Re: How do I determine a fuse that I need for a circuit?

You know whats interesting ... there was a time when I somehow used two MOSFETs and I connected them in such a way that they protected the circuit when the battery was plugged in backwards... the problem with that config (the way I had it anyways) was that when the battery was connected normally and charging ... the MOSFETs got WAY TOO HOT ... so it didn't seem practical ... I didn't take any current readings either. But I thought it was interesting that I was able to protect the circuit with a couple of MOSFETs ... I still have the circuit around here somewhere...

[budm]

Re: How do I determine a fuse that I need for a circuit?

What's the quick answer to "Why would the diode be forward biased?"

The negative of the battery will be connected to cathode of the diode, the positive will be connected to GND when battery is put in backward.
BTW, that is how crowbar circuit works, it seems like you did not read the links to learn about the circuits.
May I ask how much understanding you have about diode, MOSFET, transistor, and circuit?

[budm]

Re: How do I determine a fuse that I need for a circuit?

You know whats interesting ... there was a time when I somehow used two MOSFETs and I connected them in such a way that they protected the circuit when the battery was plugged in backwards... the problem with that config (the way I had it anyways) was that when the battery was connected normally and charging ... the MOSFETs got WAY TOO HOT ... so it didn't seem practical ... I didn't take any current readings either. But I thought it was interesting that I was able to protect the circuit with a couple of MOSFETs ... I still have the circuit around here somewhere...

The heat has to do with Voltage drops on the MOSFET and current flowing through the MOSFET, if the MOFET is not fully turned on then its resistance will be at the minimum (mOhm range), simple Ohms law.

[R_J]

Re: How do I determine a fuse that I need for a circuit?

I looked at the second link and I see what they are talking about, I guess a diode with a charge circuit will not protect the charger.

[budm]

Re: How do I determine a fuse that I need for a circuit?

The diode in series is a cheap way of protecting the load from wrong battery polarity.
In this case we are dealing with charger.