green energy

Expanded Scale Analog Meter Circuit

February 25, 2012 Kirk Comments 0 Comment

I am putting in a small scale solar system with a 24VDC flooded lead acid battery bank for back up power. I want to make a low power consumption expanded scale analog meter circuit to monitor the battery bank voltage of my system. The expanded scale allows me see the voltage range that I am most concerned about between 21V to 30V and that gives me about three times the resolution on the meter face.

While searching for circuit ideas I found this website “The Back Shed” and I liked the simplicity of the circuit that they presented on their site (this is the circuit from their site). I tried to make the circuit but found that the response was not very linear.

The problem with this circuit is that the current going through the Zener diode changes as the voltage input to the circuit changes (I=V/R) because it uses a simple resistor current limit.

Zener diode voltages are specified at a set current in the datasheet. In this Zener diode curve, Izt is the current where the Zener voltage Vz is specified, if you change the current through the Zener diode the voltage changes.

I wanted an expanded meter with a fairly linear response. So after thinking about the linearity issue with the Back Shed circuit and how it was related to the current changing in the circuit, I decided to revisit our old friend the current limit circuit I blogged about previously.

I modified the current limit to pull about 1.5mA through the circuit and added Zener diodes in place of the LEDs.

(To get a larger image, click image, then click image on the following page) So here is how this circuit works: For voltages below 21V the Zener diodes are off and there is no current flowing in the meter path of the circuit. Somewhere around 21V the Zener diodes crack over their knee voltage then current starts flowing in the circuit. The current limit circuit turns on and starts to drop voltage across the 2N7000 NMOS FET. As the voltage going into the circuit rises the current limit circuit causes the NMOS to drop any voltage not going through the Zener and current sense resistor. The Zener diodes drop a fairly constant voltage in this circuit because the current limit circuit is holding the current at a fairly constant rate. Since the 10VDC panel meter is measuring the Voltage across the NMOS it displays any voltage over ~21VDC on the meter face. The battery bank voltage should not go over 30VDC , unless there is something wrong with the charge controller, so hopefully we never see the meter at 32V.

(Note: instead of the 400 Ohm resistor in the schematic I used a 1K trim pot in series with 200 Ohms) (Also for the meter I bought a cheap 10VDC panel meter from E-bay, $5.99 including shipping)

This circuit still has the drawback of the temperature coefficient (TC) of the Zener and also the temperature effect of the VBE on the transistor that will have an over all effect on the accuracy with temperature. The temperature error is a very small (around 0.08%/degree C, for a change in room temperature of 65F to 75F you would see an error of less than 0.1V) and that small of an error really does not matter to me. If you are concerned with having a more accurate meter over a large temperature range, you can use a combination of Zener diodes with a positive and negative TC. An example would be to use four 1N5222B (TC=-0.085%/C) and a 1N5242B (TC=+0.077%/C) .

I took the 10VDC panel meter apart and scanned the face in on a flat bed scanner at 600DPI then modified the the scan with GNU Image Manipulation Program (GIMP) and reprinted it at 600 DPI onto a shipping label sticker. Placed the sticker onto the meter face cut the edges of the sticker off then put the meter back together. The following picture was the first pass:

After I put the circuit to the meter I found it was a little off ( this is was really no surprise, since Zener diodes are +/-5%, but it was a lot closer than I expected) so I had to calibrate the meter with one more graphic spin in GIMP. To calibrate the meter face with the actual DC voltage reading. I put the expanded scale meter in parallel with a digital multimeter, made little tick marks on the meter face for the voltages.

I ended up with a graphic like this. ( Note: I highlighted the 28.8V mark since this is where my charge controller will push the battery bank to at the bulk charge phase) ( the 30VDC mark is also important for when I equalize the battery bank)

And here is picture of the meter hooked up with a power supply and a DMM to verify operation.

One Winding Joule Thief

February 19, 2012 Kirk Comments 0 Comment

We had some high permeability toroid cores shipped in. The permeability is so high that it can achieve somewhere between 3-5uH per each winding. This allows for us to make a High Power Joule Thief with just one winding on each side of the transformer.

The new one winding Joule Thief is very stable, starts up at below 600mV, and runs very bright at 1.5VDC leaving spots in your vision if you happen to glance at the LED while it is on. These units are assembled, tested, and can be purchased here.

Here it is running off of a single AA battery.

Introducing the Cree 1 Watt LED High Power Joule Thief Kit

December 4, 2011 Kirk Comments 0 Comment

We are very excited to introduce our newest Joule Thief kit. This kit is a higher power Joule Thief kit that includes a Cree 50 Lumen XLamp 1 Watt white LED. See our energy harvesting product section to purchase this kit .

You can also purchase these Cree XLamp XL LEDs as an individual unit by going to our components section.

Make a Joule Thief Battery Charger

January 19, 2011 Kirk Comments 0 Comment

Recover the last bit of energy from a “dead” alkaline battery. When your modern electronics gadget turns off because the alkaline batteries are “dead” it just means the voltage in the batteries has dropped below a usable level for that gadget, which depending on the electronics that voltage could be around 0.9 VDC to 1.2VDC per cell.

I found this nice graph on http://www.powerstream.com/AA-tests.htm that shows the discharge curve for alkaline batteries. You can see that when the alkaline battery is below 0.9VDC there is not much usable energy left, but if there is 1.2VDC left in the battery there is about 28% of the energy left in the battery.

AA Alkaline Battery Discharge Curve @ 100mA

So what can I do with this “dead” alkaline battery? I can use a Joule Thief to make a battery charger that depletes the remaining energy from the alkaline battery and recharges a NiMh battery.

To make a Joule Thief battery charger is a quick and easy project. Here is the Joule Thief battery charger schematic:(to view full size images click image then click image on following page)

I am in the process of building a battery charger this week and will put data about this project as I charge batteries.

Some notes about using the Joule Thief to charge NiMh batteries:

1) This probably is not the most efficient way to recover the energy, but hey it is quick, cheap, and easy to do. The batteries were going to the trash so I might as well try to recover the lost energy from them.

2) The LED in the schematic probably uses half of the energy that would be recovered, but it is the only good way to see if the circuit is still running. You could also modify the circuit and charge up to 4 NiMh batteries in series (of course this will reduce the charge current, since the boost voltage has to increase).

If you use a white LED the circuit can be used as a night light, but the white LED (3.5V forward voltage) will consume about 79% of the energy when you are charging one NiMh Cell. If you charge four NiMh batteries in series the white LED will consume about 41% of the charging energy, the LED will be dimmer since the current will drop.

If you use a standard red LED (1.7V forward voltage) the LED will consume about 57% of the charge energy when you charge one NiMh cell, with four series NiMh cells the red LED will consume about 25% of the charge energy.

3) This circuit, if built properly, will run the alkaline battery down to 350-400mV which will truly make it a dead battery.

4) As long as your NiMh battery has a high enough capacity you will not overcharge it with this circuit, provided you do not exceed its C/10 rating (capacity/10). I found this on http://www.powerstream.com/NiMH.htm
“The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mA/Hr battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature–at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.”

5) It can take several “dead” alkaline batteries to recharge a 1500mAH NiMh battery, before I experiment I am going to estimate that it will be somewhere between 6-15 batteries.

Update: I built the circuit to charge 4 batteries in series. The battery charger circuit was working great for several days until the charged batteries got up to around 5.4v then they started to discharge. I was really perplexed for a while as to why this was happening. I finally figured out what went wrong, the LED reverse breakdown voltage was somewhere around 5.4V and it ended up destroying the LED and discharging the NiMh batteries. I have a new circuit design that will be more efficient and will charge the batteries quicker.

I have ordered parts for the new higher power joule thief circuit. I will build some and try them out, if they work out well I will add them as a new high power joule thief kit.

Mad Scientist Hut

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