Desulfator for 12V Car Batteries, in an Altoids Tin

Step 4: Smoke Test 1 - pots instead of fixed resistors for R2 and R4

It's time to test your handiwork!

For those of you who used fixed resistors for R2 and R4, skip this step and go on to the next step, Smoke Test 2.

For those of you who used pots instead of fixed resistors for R2 and R4:

First, turn off S2, put a 555 chip in its socket and a 2A fuse in the fuse holder. Set the pots to their mid-range and attach the plus lead clip of your circuit to the plus terminal of a 12V battery. Attach the ground lead clip of your circuit to the minus probe of a multimeter, and set the multimeter to the 10A AC scale. Briefly touch the plus probe of the meter to the minus terminal of the battery. Check for smoke. No smoke? Good! Try it for 5 seconds, then 10 seconds. Still no smoke? Great!

Check the 555. Hang a scope probe (if you have one) on pin three of the chip and check for pulses. Adjust R4 for peak output at around 1000 Hz (the exact level isn't critical).

Now check the output stage. Turn on S2 and briefly touch the plus probe of the meter to the minus battery terminal. You should see a brief spark and hear a faint 1000 Hz tone come from the coils. The LED will turn on in the presence of output pulses. If it doesn't, but you hear the tone, then the LED may be mounted backwards. If you don't hear the tone, or see smoke, then something is wrong and you'll need to check your output stage wiring.

If the fuse blows try adjusting R2 down a bit (the direction of turn depends on how you have it wired). Smile when you get the meter reading below 0.8A -- you're almost there!

If all is good then adjust R2 so the meter shows no more than 0.7A on the AC scale. This should yield a good output into the battery without overheating the output stage. Finger test the coils, C4, FRED, and the FET. If all are no more than slightly warm after 30 minutes then you're in the clear. You can SLIGHTLY increase the pulse width and the current into the meter a little at a time until the circuit reaches about 1.0A but at this level my charger won't flip into trickle charge mode because the combined currents of the circuit and charger are beyond its trickle threshold. I therefore keep it at around 0.7A. Anything beyond 1.0A gets a bit too toasty after a night's use anyway. Also note that the circuit will tend to consume 0.2A to 0.3A more current and get hotter when the charger is on high charge rate. It's therefore best to stay at or below 0.7A to prevent the current from getting too high as the charger adjusts its charge rate from high to low. Be conservative, especially with an undercharged battery because as the lead sulphate crystals dissolve into the electrolyte the battery voltage climbs and this increases the current and the heat dissipated by the output components.

Comments

[rudyg]

Thank you for the tips! After verifying all the points you raised, I replaced R2 with a variable resistor and found that even at 20 us pulsewidth Q1 still got hot. So I installed a pair of FETs to give me a bit more time per power-on period. Low and behold they both got hot, even at 20us pulses. So I began to ponder my layout (parts placement) and first suspected that perhaps the chokes were causing some feedback to the power on the 555. I separated the power supply connections and separately supplied the 555 with its own power. While doing this I noticed that I could make the waveform on the FET drain look either clean (only pulses) or very noisy (no defined pulses), just by either applying power first to the 555 and then to the chokes and vise versa. This way I found the source of my problem. There seems to be a nasty incoherent mechanism going on where the FETs are just thrashing around non stop whenever power reaches the 555 just a bit later than the chokes. In this mode the power goes through the roof! This made me remember a piece of info where someone discovered that there was a power delay issue between the chokes and the 555 and the fix was to reduce C1 from 100uF to 10uF to shorten the delay for power stabilization on the 555. I will try that next (the 470uH choke I added in the same power feed might even contribute more to the delay...hmmm). Sure hope I don't have to sequence the power. Any thoughts/insights? Thanks for any feedback. Best Regards.

[kmpres]

Hmmm, I had none of the problems you described. I think you're onto it, though by cleaning up the waveform going into the gate. Any noise will make the FET output that noise in wasted power so if you can clean up the 555 waveform you'll have a happier FET. The 555 output should be very clean with nothing attached to it. That's why I added switch S2, so I could isolate and troubleshoot the 555 front end first, then switch in the output and see how the FET/coils/D2 behaved. By the way, did you add the 15V zener for the 555? It protects the 555 by clamping its supply voltage to 15V max and should be all the power regulating you'll need. Differences in chargers and variances in the batteries under test can cause the supply pin to receive more than 20v, which will blow the 555 without the zener there to protect it.

[rudyg]

Yes, I have the zener installed. Mine is 14.1V.
I was able to solve my problem.
I determined that the timer consistently went into an uncontrolled oscillating state whenever power was applied simultaneously with the output stage (normal power scheme). When I separated power from L1 and only supplied power to the 555 section (open connection between R1 and L1), it always started up normally.
Concluded that some noise made its way from the output stage to the timer in my specific layout of the circuit.
As I said in the beginning, I additionally placed a 470uH choke for extra protection between L1 and R1. Now I had reason to think that this might contributed to the problems in my circuit, so I removed it.
Next, the reset pin on the 555 timer had to be separated from pin 8 and now has its own 100 ohm resistor with a 100uF capacitor.
Finally, I replaced C1 with a 10uF value.
This then altered the power up scheme of the timer such that the faster time constant via C1 = 10uF allowed pin 8 to stabilize while pin 4 was held low for a bit longer, with its longer time constant resulting from the new 100uF capacitor I added. So the chip is held in positive reset during power up.
No more oscillations. Circuit works very stable now and I was able to go back to just a single FET without it getting hot.
One curious thing, though. I had to reverse the polarity of my LED to work in the circuit. When I look at the voltages on D2, then I see +12V on the cathode and +12V with negative pulses on the Anode (FET switching on and off). So the cathode of my LED has to be placed toward the anode of D2.
Since I can not see very narrow voltage spikes on my small scope, is there any noteworthy effect of the LED circuit on the performance of the desulfator?
Thanks for all your comments. It helped me get on the right track.
Best Regards.

[kmpres]

Glad you found your problem. I congratulate you on your investigative skills in finding a workable solution! Since I had none of the instability problems you mentioned I can only surmise that maybe your choice of components or wiring layout had something to do with the oscillations, but as long as you got your circuit working that's all that matters. I'm not sure, exactly, about your LED wiring. Maybe you got a reverse wired LED, or maybe your FRED is leaky, or maybe the fact that in this application we're sort of tapping off a large diode to get a small one to conduct might have something to do with the problem you encountered. The LED is not important to the circuit's function -- the 1000 Hz tone from the coils should be enough to tell you it's working. However, you should probably try to borrow a fast scope to make sure that your output spike is of sufficient amplitude to do your battery some good.

[rudyg]

I am still trying to see what I can measure with my Velleman HPS10 scope. At $120 you can not get too demanding. Still trying to get used to it. It has a sample frequency of 10 MHz, giving me 100 ns resolution. I also use a 0.1 Ohm resistor in series with the negative lead for this measurement. Would you be able to let me know at what resolution you see the spike on your scope? On another note: I measured my AC current per your step 5 and it is 0.8 A. I assume this reflects the RMS value of whatever current the circuit puts out? Trouble is that converting this to peak value does not get you there, presumably due to the narrow pulse width the "spike" has. Also, just to clarify, no charger is connected for these readings, correct? My DC current draw is 95 mA. This is somewhat higher than I expected since the 555 should only draw about 16 mA max. Not sure if I am seeing some component of the circuit's AC action here? Any comments on this? As always, thanks for your thoughts! Best Regards.

[kmpres]

Per my response to a question on Feb 15, 09, my scope is set at .1us/div and 5v/div. The pictures show you what the spike is supposed to look like. The pulse width is .14us, or 140ns. Since this is close to the max resolution of your scope, it is probably not up to the task of viewing the spike. .8A is fine for current through the circuit, charger at trickle or off. Your FET, FRED, C1 and L2 should be quite warm but not hot. After a while, hours maybe, one or more of them may get hot enough to smell, which is my cue to turn down the heat. I keep mine at .7A, charger off (no smell and no burned fingers, even after days of use), but your circuit, due to using different components, may show a higher or lower value than this. When my charger is on the current through the circuit climbs to about 1.0A, then drops steadily as the charging current bleeds off. When my charger thinks my battery is at full charge it turns off the charge current entirely (no trickle charge), at which point the circuit current should be around .7A again. This usually takes 2 to 8 hours depending on the state of discharge. Your setup will probably produce different numbers so use your best judgement. 95ma DC does seem a little high. If you disconnect your output stage you could maybe see what the chip is drawing vs the rest of the circuit. As long as your circuit is working reliably without getting too hot you should be OK, however.

[kmpres]

I meant C4 in the above post, not C1. C4 is the cap that parallels L2 and the FET in the output section of the schematic.

[rudyg]

Hello again, Thanks for the clarification, I realized what you meant. As for my project: I reduced the pulse width from 60 us to 50 us, which lowered the current some. I also replaced C4 with two 50uF caps in parallel and placed a second diode in parallel with D2 (I am using RHRP3060's). This gives me enough wattage capacity to avoid overheating when engine temps max out in 120F summer weather. I have the circuit running now for two weeks on a bad battery and monitoring the "peak voltage" every other day or so to see what happens. In retrospect, since I am using the larger toroids (J.W. Miller series 2300), along with the faster diode RHRP3060, my instantaneous current is likely larger and thus no surprise that D2/C4 were getting a bit warmer. (note: although not a perfect current sharing solution, paralleling these diodes will still provide some offloading as compared to a single device) So, for now I am in the monitoring stage to see what happens with my dead battery (was only holding some 10.5 volts when I started). Had to do some scrambling to find a circuit for measuring the so-called peak voltage. Still not sure if it is the right approach, but going by what others have suggested (www.frontiersprings.com). Not sure what the "correct" peak voltage is that would indicate a healthy battery. Any input? Thanks as always, and Best Regards

[kmpres]

I'm traveling in the US on holiday so don't have my equipment with me. You sound like you're well on your way. I'm not sure what the best peak volage is either as it may be different from battery to battery. From what I've read online, the battery will begin to bubble when it reaches the last 24 hrs of its recovery. This assumes you use a trickle charger in parallel with the desulfator. The bubbling is good as it churns the electrolyte some and helps to even out the concentration. Just don't let it boil away with too much charge current. Good luck with your battery recovery!

[rudyg]

Hi, I wonder if you can share some insight into a problem I'm having. I built this circuit from a schematic a fellow by the name of Ron Ingraham published some time ago. It is identical except it does not have your switches and other useful notes attached. I am pretty good at building stuff like this, but from the first moment I turned it on, my FET Q1 blew instantly. I deviated on this build as follows: For the inductors I used the J.W. Miller series 2300 toroids. L1- 1000uH, 2.4 IDC, 0.3 Ohm L2- 220uH, 5.8 IDC, 0.054 Ohm I added a 470uH inductor between R1 and the point where L1 is connected as additional protection for the 555. After re-checking the cicuit and replacing Q1, I disconnected the output stage and verified that my timer portion was fully functional, nice 60us pulse on the gate of Q1, with 940us between next pulse. Temporarily re-connected the output stage while holding my finger on Q1 and within 1/2 sec it was too hot to touch, aborted. I would sincerely appreciate if you could help with most likely "suspects". Best Regards.

[kmpres]

The most likely suspects are that your pulse width is too wide or the FET isn't turning off properly at the end of each pulse. For the former you'll need to narrow the pulse coming from the 555 by adjusting R2 down, per the instructible. You can easily see the digital pulses coming from the 555 on a scope but the pulses at the source of the FET are harder to see because it's hard to get a scope to trigger on it. It's at the very front of the waveform and may require you to delay trigger and/or boost brightness a bit to see it. My Tektronics 475 is pretty fast (200mhz) but it takes a bit of knob fenageling to get the pulse to show itself (see my reply to an earlier comment for pictures of how mine look). Keep adjusting R2 until Q1 feels just warm to the touch. You can also watch the pulse amplitude go up and down in response to pulse width by putting your probe on either side of the fuse and adjusting R2. I get around 55v p-p at the point where things start to heat up. Any more than that and you're forcing the FET to dissipate too much power. It drops to 36 volts p-p at the battery due to various losses along the way which is still pretty good. For the latter you'll need to check that the Turn-off Enhancement Circuit (in the schematic) is working properly. This circuit helps the FET turn off completely after each pulse thereby keeping the FET from overheating and improving efficiency.