High side switch

[mmmalmberg]

Can you give me any other numbers for a mosfet that I can look for? Particularly 10-15A? I truly appreciate (and need) the help.

 

[mmmalmberg]

About to head off to Fry's, will check this thread from there to look for updates. Can I go with 1/4W for the resistors?

 

[MikeMl]

About to head off to Fry's, will check this thread from there to look for updates. Can I go with 1/4W for the resistors?

Yes. I recomputed the resistor values for a 12V SLA battery. Cutin at 13.2V, Cutout at 11.5V. I tried to use 5% resistor values. See attached:

 

[MikeMl]

Can you give me any other numbers for a mosfet that I can look for? Particularly 10-15A? I truly appreciate (and need) the help.

Try this one, the NTE2371

 

[mmmalmberg]

Thank you!! How would I adjust if I want to cut in at a lower voltage, say between 12 and 12.5?

I need to be able to cut in earlier so that my pieces can angle themselves for charging once they do start to get some light in the morning. Fry's doesn't have any p-channel mosfets so I will buy everything i can and look for those at the one other electronics shop in town tomorrow.

Thanks again.
-Mark

 

[mmmalmberg]

Would I lower R9 to lower the reconnect threshold?

 

[MikeMl]

Would I lower R9 to lower the reconnect threshold?

Reducing R8 will reduce the both Cutin and Cutout together. Increasing R9 will decrease the hysteresis between Cutin and Cutout. I did that in the following sim. Starting from standard resistor values, setting the 1kpot at 42% of the way from the bottom gets the trip points to 12.5 and 11.5, respectively.

 

[mmmalmberg]

Fantastic. Bought most of the parts last night and found the mosfets this a.m. I should be able to fit the whole thing on a little board the size of a postage stamp... Haven't used mosfets before, very impressed they can pass 20 amps or whatever in a little TO220 package. I have some small heatsinks that will fit these. Most of the time these won't be handling more than 3 or 4 amps and occasionally ~8A or so for maybe 3 seconds, so these will be I think pretty robust in my application.

I sure hope these work I will be building and installing these in the sculptures tonight and shipping tomorrow, not much time to test. Thanks so much for your help.

 

[MikeMl]

Grab hold of the tab on the TO220 with the motor running. If you burn your fingers, you need a bigger heat sink.

 

[mmmalmberg]

Sounds like a good "rule of thumb"...

 

[mmmalmberg]

https://www.flickr.com/photos/40089219@N08/4545140676/sizes/o/
They even seem to work!

Thank you. Sculptures shipped out tonight with about 1 hour testing time Truly grateful for your kind help with this.

 

[mmmalmberg]

Found it! Was looking for this thread..... Mike, if you're still around - I've been using the circuit you provided and it's been working for two years or so now. I'm re-doing the whole sculpture, improving the power supply circuitry and replacing the SLA with a 58F 16V capacitor. I'm making pcboards for everything and would like to use this low voltage disconnect circuit to save the charge on the supercap overnight. I wondered if you could give me new resistor values to drop the low and high voltages to 7 and 9? I remain grateful for your help last time around!
thanks/best
Mark M.

 

[audioguru]

The voltage of a zener diode is at a certain current that is listed in its datasheet.
YOUR zener diode has no part number and the resistor value that sets its current is not shown so maybe the zener is barely regulating so its voltage varies a lot.

 

[mmmalmberg]

I wonder if you're looking at the same drawing as I, that being the last posted by ML using a TL431 regulator? All the parts appear to be named and valued, no?

Reading back through the thread I think I just need to reduce R8 but I'm not sure by how much. Can always use trial and error. Or "Guess and Check" as they taught it to my kids in grade school

 

[ronv]

Haven't heard from Mike in a while (few months). Your cap will only run the 4 amps for maybe 4 minutes. Is that okay?

 

[ChrisP58]

Haven't heard from Mike in a while (few months). Your cap will only run the 4 amps for maybe 4 minutes. Is that okay?

Actually it won't even do that.

Starting with 13V on the cap, and modeling the 4A load as a 3 ohm resistor:
At one minute the voltage drops to 9.2V
At two minutes the voltage drops to 6.5V

 

[mmmalmberg]

Hi Chris and Ron; thanks for your replies.

I have three motors that each run intermittently; one I'll current-limit to 4 amps, one to 2 and the third is probably a half amp or less. Each has a duty cycle of probably not more than 10%. The system is run by an Arduino, and as the voltage drops I reduce the use of the biggest motor, then the others. I have a couple of small solar panels that put out a total of about 1.8A in full sun so overall it should be fine as long as there's good sun. What the capacitor won't do that the SLA's did is store up a bunch of extra juice for the late afternoon. On the batteries these pieces would keep running for an hour or two after losing direct sun. Problem with the batteries is they've been dying after about 6 months. I think it's from being charged hard (their only 1.2AH batteries) and spending every night in a partially discharged state.

Anyway I've already bought the caps so I'm going to try them. If I could figure an easy way to combine them I probably would, especially if I knew that even if the batteries petered out they could still run on the caps...

The first version of the sculpture is on my website; it's two separate pieces that communicate wirelessly and interact in sound and motion. http://www.markmalmberg.com Thanks again for your help and comments

 

[ronv]

Cool.
LiPo might be the way to go, but they are hard to charge with solar. I'll tweek Mikes circuit in the am.

 

[mmmalmberg]

Thanks Ron, much appreciated.

You guys have me thinking more now about integrating the 12.5V SLA with the 15V capacitor. I'm using a buck/boost switching regulator to charge the capacitors. They're not cheap but maybe I could use another from the battery to the capacitors set to same voltage. Then I'd just need a way to charge both the battery and the cap from the solar cells. The thing is I'd like to charge the cap first then use the battery to soak up energy whenever the caps are full...

 

[mmmalmberg]

Or maybe simpler to just have the cap and battery in parallel with diodes so that whichever offers higher voltage powers things. I could use a low voltage cutoff on each, with a lower cutoff voltage for the caps. In that way, when they were between 15V and 12.5V the caps would provide power. Then the battery would power until it drops to 12V or whatever its cutoff voltage is. Then the caps would kick back in and run down to 8V. I like that...

Still leaves me needing a way to charge the cap and the battery both from the solar panels and preferably with priority to the caps.