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Author Topic: voltage monitor circuit with hysteresis  (Read 8301 times)
jhinshel
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« on: June 09, 2010, 10:44:22 PM »

Hi,
I am looking for a circuit design for a voltage monitor circuit with hysteresis for a 48v battery system ( 45-60) to control diversion loads for a wind/pv system.
Regards,
Justin
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tim_h
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« Reply #1 on: June 09, 2010, 11:12:15 PM »

We had the rep in from Nortonics the other day and he happened to mention some DC sensing relays they had added to their range. Is this one any good? http://www.nortonicsfoxtam.co.uk/pdf/76%20YW-8W-11WRTU.pdf/

I don't know about prices though.
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jhinshel
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« Reply #2 on: June 14, 2010, 10:40:45 PM »

I checked out the nortronics people - these dc sensing relays look pretty good. They are 50 each though. I am putting together a diversion controller that swithces in 3 separate dump loads. I was hoping to throw a control circuit together. Does anyone have a tried and tested circuit for this?
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Justme
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« Reply #3 on: June 14, 2010, 11:10:46 PM »

Some inverters & charge controllers have that function built in.
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jhinshel
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« Reply #4 on: June 17, 2010, 10:18:26 PM »

I am looking to balance the load. I can use a charge controller to switch all or nothing after the batteries are charged. It would be more usefult to be able to control a number of loads so that the batteries can continue charging at a lesser rate. the idea is to use 3 1kw immersion heaters. This would be best controlled with 3 seperate voltage controlled switches. I can buy these from the nortonics people, but I am more inclined to bung together a circuit that can do this. If anyone has a good suggestion for a circuit etc please let me know.
Regards.
Justin
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billi
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« Reply #5 on: June 17, 2010, 10:34:57 PM »

is your solar chargecontroller not the easiest to go for ?

I am sure you know that the MX 60 has a programmable relay

Billi
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stephendv
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« Reply #6 on: June 17, 2010, 11:39:55 PM »

Sounds like you will also need to measure the amount of available power.  The Pentametric battery monitor looks like it could be one piece of the puzzle: http://www.bogartengineering.com/pentametric.htm  it will measure 2 battery banks, including temperature compensated state of charge and 3 current shunts, so you could measure how much power is coming out of your PV and wind independantly. 
It only has 1 relay, so if you wanted to control more than 1 diversion load your best bet will be to use a microcontroller to interface to the pentametric over the serial port.  You could use a simple arduino for this with attached relays, which could then optionally connect to ethernet or internet over GPRS, etc.
 
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davebodger
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« Reply #7 on: June 17, 2010, 11:48:12 PM »

I am looking to balance the load. I can use a charge controller to switch all or nothing after the batteries are charged. It would be more useful to be able to control a number of loads so that the batteries can continue charging at a lesser rate. the idea is to use 3 1kw immersion heaters. This would be best controlled with 3 separate voltage controlled switches. I can buy these from the nortonics people, but I am more inclined to bung together a circuit that can do this. If anyone has a good suggestion for a circuit etc please let me know.
Regards.
Justin

Well if I was doing it I would throw together 3 small potentiometers, a 74C14 Hex Schmitt trigger chip and 3 logic-level power Mosfets on some Veroboard along with a small voltage regulator and a few additional capacitors and resistors.
Simple analog switching should do it.
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jhinshel
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« Reply #8 on: June 21, 2010, 09:07:25 PM »

Davebodger,
Your suggestion sounds like what I am on about. Could you suggest a circuit diagram for this?
Justin
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davebodger
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« Reply #9 on: June 21, 2010, 10:51:19 PM »

Davebodger,
Your suggestion sounds like what I am on about. Could you suggest a circuit diagram for this?
Justin

This diagram shows what I mean (only one circuit shown but you can build up to 3 per 74C14 chip) and I left off the voltage regulator for clarity.
The load will turn on when the battery voltage (sense voltage input) goes high and turn off when the battery goes low.
You would need to play with the resistor values to get the switching points where you want them.
And if you are going to run the voltage regulator for the chip from the battery bank you might want to pick a high-voltage low current regulator like the LR8.
The high-current parts of the circuit obviously need to be made from thick enough copper wires, and the power mosfet needs to be able to handle 25 amps continuous or better for a 1KW 48V load.
Running the chip at 12 volts means you don't need logic-level mosfets. Ordinary ones are cheaper.
Also you need to heatsink the power mosfet - even an IRFP4321PBF with 0.012 ohms Rds(on) at 20 amps will generate 5 watts; others with higher Rds values will generate even more.  whistlie


* simple dump load controller.gif (50.08 KB, 1691x1200 - viewed 2918 times.)
« Last Edit: June 21, 2010, 11:56:16 PM by davebodger » Logged

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jhinshel
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« Reply #10 on: June 22, 2010, 06:57:38 PM »

This diagram is interesting. Please could you show how to incorporate hysteresis into this?

I plan to use this as a diversion load controller to switch the ac side, so I think I would incorporate a 240v relay/switch instead of the power mosfet.

could you suggest a supplier for the LR8 in the uk?
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davebodger
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« Reply #11 on: June 22, 2010, 10:00:18 PM »

A google search for Supertex Semiconductors gives :-
United Kingdom - Clere Electronic Components Ltd.
Location: Kingsclere
Phone: +44 0 1635 291 666
info@clere.com
www.clere.com

Looking at the LR8 again, the LR12 might be a better bet, it's OK up to 100 volts and 50mA.

The circuit already has hysteresis - that is what a Schmitt trigger chip does.
Wheather it's got the right range for what you want to do I can't say (you never said what cut in/out voltages you wanted).
I suggest you look at the data sheet for the chip. http://www.fairchildsemi.com/ds/MM%2FMM74C14.pdf
Basically, when run on 12 volts, the output goes low when the input raises above roughly 7 volts and will not switch high again until the input goes below about 5 volts.
So the potential divider/resistor chain has to be set to achieve that voltage swing from your battery voltage changes.
If you need a narrower range and cannot achieve it by adjusting the resistor values then lower the operating voltage - the datasheet shows graphs of trip points vs supply voltage - but beware going below about 6 volts or the Mosfet won't turn on.
You will still need a mosfet if you want to operate a relay - the 74C14 chip can only sink/source 15mA at 12 volts supply, much less at lower supply voltages.
And don't forget a protection diode across the relay coil or it will likely all end in tears.  facepalm
« Last Edit: June 22, 2010, 10:06:52 PM by davebodger » Logged

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jhinshel
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« Reply #12 on: June 22, 2010, 10:27:45 PM »

Davebodger,
You're great - You make it all sound so easy. What do you do for a day job?
I am going to have a go at putting this together - thanks for your help
Justin
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davebodger
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« Reply #13 on: June 22, 2010, 11:28:44 PM »

Errr, I can't tell you what I do or else I have to kill you <JOKE>  whistlie
But I have been doing electronics as a hobby for at least the last 35 years, so you pick up a thing or two along the way.
When I started, TTL logic chips were only just beginning to be made and the home PC hadn't been invented.
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jhinshel
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« Reply #14 on: June 23, 2010, 06:40:22 AM »

It would be really useful to have adjustable hysteresis - could you suggest how to do this?
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