Mosfet as a diode , i donno is that gonna work?

[VidPro]

I keep going over this in my mind, seems right, but then it goes wrong, so i need mental assistance and math and science.

Here is the Idea, and it has been done before, with good results, but mabey not with my specs.

a 6Volt Solar cell, and a Li-Ion battery, Ignore the protection and charge alogrythm, i have that covered, its all about the solar cells discharge when dark.

When the solar cells are dark, they will drain the battery about 1/10th to 1/20th of what they could put in when they are fully sun lite.
so a diode is normally used to prevent this minor drain. but diodes have a voltage drop, and even the measley .3v voltage drop on a very low voltage system is drastic reduction in amperage when charging with full sun.
so .3v is nothing to a 12V system with 18-20V solar pannels, but in a single li-ion charge situation it is a LOT.

before i go on about the mosfet, one trick i did was put a RELAY in with a Photon resister, the relay took only 100ma in this particular system, and the whole (BIG) system (because of no diode) output 1/5th more total amperage to the battery. so this old stupid method was VERY efficient, light was required to hit the photo resister, so it was good for locked down application.

ok the mosfet, so instead of a .3v votlage drop, you toss in a mosfet, when there is power on the solar cell, the gate triggers, and the mosfet goes way low resistance, and its more efficinet than a diode, and there is a diode path in the mosfet.
But wait
ahh, and here is the mind bend, when the mosfet is at low resistance, then the battery is freely flowing in the other direction, so what is to stop the battery from keeping the mosfet gate open?
while the mosfet has diode properties, doesnt it go almost straight through connected (not one way street) when its closed?
so
how do i get the most straight through least resistance connection, and still have disconnection when the solar cells are dark and not outputting anything, but the battery is still existing and in the sence of having the least resistance, is very well connected to the solar cell?

 

[uk_caver]

How about a a light sensor (photodiode or photoresistor) driving a MOSFET to turn it on when there is light around?
It may be that it would be best to have some electronics (an op-amp as a comparator with a little hysteresis?) between the sensor and the FET, to avoid having the unit only gradually turning on or off as the light level increases or decreases.

However, due to the inbuilt diode in MOSFETs, if you connect one the obvious way (for a low-side switch with an N-channel MOSFET, solar panel ground to source, charger ground to drain), current can easily flow backwards through the diode even when the FET is off.

To avoid that, you can switch the FET source and drain connections round - see http://electronicdesign.com/Articles/ArticleID/9945/9945.html for the general idea of that (though those circuits are only for reverse-voltage protection, not panel/battery isolation).

 

[TranceAddict]

take Vgs and Vth into consideration, given the condition of sunlight just about to trigger on or trigger off, it will cause oscillation, i think need to deploy control system theory here.

second consideration is, assume you're going to connect solar panel positive to mosfet source, and drain connect to battery positive, potential should be very small. do a kirchhoff loop to see if the voltage is sufficient to trigger gate.

 

[ctom]

Probably not because most MOSFETs have a parasitic diode connected across the drain and source so that when the solar cell voltage falls below the battery voltage (less the diode's forward voltage drop - typically 1.3 volts), the battery will begin to discharge into the solar cell through the diode.

I think your solution with the relay is a better one because it provides complete isolation from the solar cell to the battery. The only downside, as you mentioned, is the constant relay current draw. If you are handy with electronics, use some CMOS comparators, a 4530 dual one shot, and a latching relay to construct a circuit to close the relay when the solar cell voltage is greater than the battery voltage and open the circuit when the voltage falls below battery voltage. The CMOS ICs draw microamps and the latching relay only require 40 mA (OMRON G6E series, as an example) briefly to turn on or off. Don't forget to add hysteresis to the comparator to prevent chattering if you try this!

 

[VidPro]

hmm using the photoresister just like the relay method, but in this situation the solar is portable, so the photoresister would have to be right up there with the solar pannel, which IS possible.
(in the other it didnt much matter, sun came up even shaded, and it poped on, it was hard to keep it off). relays dont go between they either are or they arent, it was so simple.

soo in ENGLISH, your saying that the trigger voltage of the Mosfet would be a large concideration, argg, i had me some low triggering mosfets, but today all i got is the one thing they sell at the rat shack.
which i managed to find this data sheet on.
http://www.datasheetpro.com/622952_download_IRF510_datasheet.html

the min low voltage on the battery will be about 3.5v (i think) cause the device (PNA) wont draw it all the way down.
so i have a max voltage hanging out around the solar cell of about 5V, a min of ~3.6V when the sun is low.

will it be triggered enough ?

then solar has a max amps of 2Amps, so i think that mosfet package will easily handle being partway on??? so if 3.6v would be part on, how partly on would it be for that thing? worse than a diode?

Solar cells change in AMPERAGE most when the lighting fluxuates, the voltage on the solar cell itself will be high as soon as it gets even clowdly light, even though the amperage will vary greatly, so the only voltage change will be due to the amperage change, not because the solar cell voltage varies much, of course the battery will hold it lower than its max when its connected.

hey there is only 3 Freaking wires out of the thing can it be done with it and nothing else, because if i gotta complicate it, i might as well put in the simple diode, there is no way to re-do everything once out in the feild if some complex stuff fails. it doesnt matter if it pulses throbs or fluxuates, what matters most is if i am loosing most valuable and limited power, otherwise the diode would work, the diodes only problem is the power loss.

 

[VidPro]

Probably not because most MOSFETs have a parasitic diode connected across the drain and source so that when the solar cell voltage falls below the battery voltage (less the diode's forward voltage drop - typically 1.3 volts), the battery will begin to discharge into the solar cell through the diode.
!

ahh so it wont work because the diode itself in the mosfet, normally going the wrong direction for what i want, and that is why Uk_caver was pointing out to hook it up backwards?
and because that diode is between the input and the output, the photo resister being off doesnt help nothing.

so when it comes to "switching" what really is backwards? its just the diode properties in the mosfet? or am i gonna have trouble hooking it up backwards? why i am actually pretty good at hooking stuff up backwards most the time

 

[ctom]

yes, the diode is part of the MOSFET - I should have been more concise. I tend to agree that simplicity is better and your original idea of using a low forward voltage drop schottky diode is the way to go...

 

[VidPro]

ok i tested the reality, and observe the quandry again.

i have a relay here (that would work) takes only 60ma, or i got an even smaller one here , 5v relay. the relay will often switch at much lower voltages (than speced), so the coil isnt taking to much power. because the relay triggers before battery connect, the voltage is from the solar pannel, and is high, once it triggers, as the voltage drops the relay stays triggered . because it takes less power to keep it locked than to trigger it initally, so even with a photo resister, it can work, but i really would like to get even less than the loss of 30-60ma, by using mosfet.

average power is say 1amp at 3.6v, a .3v drop is about 1/10th the power , or 100ma?

i ended up with the relay before in a similar application, but it was 200ma of 9amps total. that saved me lots of power losses.

and here is the real test just now, sun happens to be out.
charging 1.11A without diode, .90 with diode, real test i just did, using shokey diode. that leaves some 200ma wasted? the test was done with clipleads and all hanging around and meter resistance too. tested V-drop over the diode without the ampmeter in, .36v

ahhhh now you see, when it comes to Low voltage and some usable amps, the slightest drop and things go to heck fast, and with a 6V solar, it should be sufficient voltage, but reduce the so minor voltage differential (between battery and charge), and that is my problem.

last time i checked this, and ended up with this same feeling about it, i actually thought about accepting the dark losses, and the discharge through the dark pannel, because it isnt as bad as the loss from the diode, depending on the ammount of sun vrses dark. tiny voltage drop, but really HUGE efficency losses , when measured its unbelievable how big the losses are, specially when they got this kinda stuff going on all around us everyday. doing a fireside chat by the computer

 

[ctom]

Another option is to use a low saturation bipolar transistor like the Zetex ZTX851 in place of a MOSFET. It has a Vce(sat) of 50 mV typical (100 mV max) at 1 amp. One potential downside is that the base current drive is 50 mA.

 

[mpf]

www.linear.com Application Note AN53

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1142,C1041,P1385,D4142

provides a fully isolated "switch" using back to back mosfets. I use a similar arrangement in my torches to allow them to be powered as a standard lamp, from the mains, while not draining the battery.

matthew

 

[VidPro]

so that is a mosfet "driver"?
and the back to back would be the diodes are going the reverse direction, so when the mosfet is off there is no passage through either diode in either direction because of back to back diodes.

but do i need back to back when solar to the battery is good, battery back to the dark solar is bad?

and do i need a driver too, because of mosfet gate voltage? or does exist low voltage mosfets, its just that "Logic" or lower voltage ones just dont switch high voltage things?

one of the issues i have with the low voltage mosfets is they often are little flea sized parts so although they only cost 63c in bundles of 1000, i need $20 of work from somone who has assembled it on a board i can connect it to without Flea Circus tools .

heck i lost a resister yesterday, and i couldnt find it from the Dust on the bench

 

[VidPro]

now i see a reason for the mosfet driver thing, gate triggering was a beech to get concise.

well here is what it ended up with, and it was a lot harder than it seemed like it was going to be, but it worked awesome.

i got a "hexfet" now i dont even know what one is, but it had good specs for a mosfet, it was cheap as heck 20 of them for some $5 shipped, and it went really low resistance. i cant believe the spec sheet was so full of specs , it didnt have the gate source drain thingee marked:shrug: , but i managed to just find out by hooking it up to the power supply.

it was in a BIG package, not that tiny stuff, but the trigger voltage was very low still.

as a DIODE it Already dropped less voltage than the diode .5v (ooops the diode before wasnt a shotkey, that is why my loss was so high) , so once i had it put in right (yes it takes me a while) it just started working as a diode without even pushing the gate.
working already actually made it HARDER to figure out when it was switched or not, so i tracked the .5v voltage drop to determine if it was closed or not.

NPN
once i got the gate triggered, as long as i pulled ALL the power off, i could touch it with my finger (to ground) and get it to turn off, but the tiniest ammount of power still on the gate, and it was still on.
so to turn it off i had to pull it to ground (of course) but the slightest ammount of power and bang it was back on again.
it was terrible to try and get it working with a cadmium light resister, with it also resistered to the gate, because the "range" on those isnt very wide, so i switched to a Photon Diode, which switched to practally 0 resistance (then i had to get That in the correct direction too).
so after i got a good range from the photo reactive part, then i balanced out the pulldown reisistance with the added resister from the photodiode, and in the end it uses about 2-4Ma of power (probably to much, but i was through with it).
i really had to manually set the resistance balance, to very tight parameters. (or the gate pull-down/pull-up fighting would use a lot of power, passing through the resistances)

so now it locks on with almost 0 voltage droop anywhere, and only uses tiny ammounts of power to function that way , much better than the relay.

anyways thanks for all the knowlege, and discussion, i still dont know one thing, why i would hook it up backwards, or did I already? and what is the human differences between a mosfet and a hexfet? like where would one be applied over the other?

 

[HarryN]

Hi, the info here is just crazy enough to be useful.

I will be asking similar questions about how to get really low Vf from a diode for a voltage regulator in a few days. (after I figure out what some of you have posted)