ATX PSU Modified Into a Car Battery Charger-Part 2
Click HERE to go to part 1
After thinking of all these parameters over again, I took in hand my good old Motorola manual (Linear/Switchmode Voltage Regulator manual, 1982 edition), located the TL494 IC data sheet and started working first with pencil and paper, trying to analyze the original design, in order to modify it later and bring it to the shape I needed.
Well, I will not enter here a boring for many readers mathematical analysis, but I will give you the basic information about what I did with it and first of all I think that some basic theory is important for a better understanding of the topic.
These PWM controllers (as their shortcut name implies, meaning Pulse Width Modulation) control the magnitude of the output voltage of a PSU by controlling the width of each pulse within the pulse train they produce at their output. This function breaks the rectified DC input voltage in pulses and the following width control of them corresponds finally to corrections the PW modulator does against any variations of the output voltage from its predetermined voltage level. These variations can be due either to load changes at the PSU output or to input (mains) voltage variations.
Voltage control is achieved by comparison of a sawtooth shaped, high frequency oscillatory signal, with a DC feedback voltage within an op amp. The op amp is working as a voltage comparator. The DC feedback voltage is a fragment (sample) of the output voltage of the PSU. If this voltage tends for any reason to decrease, the pulse width increases instantly and accordingly, in order to correct the output, bringing it to the predetermined level. This works vice-versa.
Current control is achieved in a similar manner. This time a second op amp is used, working as voltage comparator again. It compares a varying DC voltage feedback, with a stable DC reference voltage. The magnitude of the feedback voltage represents the magnitude of the output current of the PSU. Therefore it’s a load dependent voltage. The stable DC reference voltage on the other hand, represents the maximum permissible output current. As long as the output current is lower than the maximum permissible value, the output of the op amp remains zero volts and vice versa. The transformation of the output current in voltage is easily achieved by using a sub-Ohm current sensing resistor, connected in series with the DC output of the PSU.
Both of these op amps, along with the independent PWM comparator, are included in the TL494 and finally they control the PWM modulator, also included therein. Originally only one of them was used, regulating the output voltages. So I made the proper modifications in the PCB foil traces and employed the second op amp as well. After calculating the new resistor values of the feedback resistive divider for the voltage regulator, I inserted them in the empty places of the original resistors and drew a pair of thin cables for the voltage selector switch. These are the blue colored cables shown in the above photo. I also exploited a mains voltage selector switch (115/220V) which already was available. These two thin blue cables go directly to its terminals. Its 115V indication means now 13,2V DC at the output, whereas its 220V indication means boost charging voltage of 14,7V DC at the output.
All this switch does is to short out an additional (third) resistor which I connected in series with the first one one of the upper side of the resistive divider. This in turn results in 13,2V DC at the output. For boost charging voltage at the output the switch is thrown to its open contacts (220V) position. This action removes the previous short, the voltage at the divider’s node drops and this finally forces the output voltage to jump to 14,7V DC.
For the current limit circuit I used a 0,1Ω resistor (the big white one shown in the photo of the PCB in its final form) and worked applying Ohms Law once again. I set the maximum permissible current output level to six amps. This means 600mV stable reference at the current comparator. I chose this amperage just for safety, considering that the original rectifier diode was rated 10A. This amperage is enough for a decent car battery charger and for batteries up to 100Ah.
At the end I also replaced that current sensing resistor with one of higher wattage. I put there a 10W resistor instead of the 5W one, again for reliability reasons alone. The power consumption of it was calculated at 3,6W and the 5W resistor I originally had put there was getting remarkably heated when the PSU was working under current limit conditions. (This will be a normal condition from now on during the first (bulk) stage of the charging procedure of a flat battery). Although the wasted energy in form of heat within it, under current limit conditions, will remain unchanged, the upgraded replacement has nevertheless better thermal distribution on its (bigger) body and of course can tolerate this power more easily. Practically it works much cooler than its 5W counterpart under current limit conditions.
For the same (safety) reason I replaced the output filter capacitor with another one having kept the original capacitance but upgraded voltage rating. The original was a 1000μF/16V, but due to the increase of the output voltage after the modification it was working marginally, that is, very near to its nominal voltage. So I replaced it with another one keeping the same capacitance but upgrading its working voltage to 25V. The one with the green color shown in the modified PCB is the new replacement cap.
Finally I also changed the power supply connection of the cooling fan.
Originally the fan was supplied from the +12V output supply. After the modification this voltage was increased (ranging from 13,2V to 14,7V) and the cooling fan (which is permanently powered, without a thermostat to control it) was spinning crazily!! So, in order to protect it as much as possible, I connected it to the auxiliary (standby) +12V supply, which was independently stabilized, produced from the little standby transformer which also feeds the PWM IC from another secondary winding with a higher voltage of 22V. The power consumption of the fan was far below from being considered as dangerous for the tiny standby power supply. An additional advantage of this change, apart from the normal spinning of the fan, is the fact that the fan itself was now a direct indicator that the standby supply is working (no matter that this standby function does not exist any longer, because both main and auxiliary supplies are main switch depended after the modification and this fact is not likely to be changed for the rest of the life of this PSU). However I also connected a green LED in parallel to the fan power connections, for better control of its working state.
This photo below shows its inner side, at its final form.
You can see its output voltages below. The small variations are due to the tolerances of the resistors I used.
This photo above, is the output with the switch in its 115V position.
And this one below is the output with the switch in its 230V position.
You can also see below the current management of this PSU.
First shown is the short circuit current. It is just a little bit above the calculated 6A maximum, due to the resistors’ tolerances again.
And this below shows the current draw of a 12V/60W rated H4 halogen car head lamp. The filament used for the test is the high beam one.
Normally, in order for me to avoid these variations from the calculated values, I should use metal film resistors of tighter tolerances, say 1%, but I have plenty of carbon film resistors in stock (with 10% tolerances of course) and so I preferred to use these. Anyway the resultant slight variations of the output parameters mean nothing important in this application and I am sure that my brother will forgive me about that!
The only missing protection of this PSU is the one against accidental connection of the battery poles in reverse. I asked my brother if he wanted me to include a relevant additional circuit therein, but he was quite convincing me in that on one hand he didn’t need this protection, while on the other hand he already knew about the results of such a wrong connection and therefore he is cautious enough as regards this matter. Therefore I excluded this protection circuit keeping things as simple as possible.
Talking in terms of reliability, this charger has all those elements which can characterize it as a decent and bulletproof car battery charger, except that “battery in reverse polarity” protection. As functional tests, I short circuited its output about a hundred times repeatedly and additionally I used it to charge some batteries I have here to play with. After all these basic crash tests I informed my brother that he does not need to buy any car battery charger at all. His charger is already here waiting to serve him. And I believe that it will do so both flawlessly and for long time. So I intend to give it to him by the first chance I will visit him.
I believe that (if he inspects its blower for proper function regularly…I made that crystal clear to him) he will be very happy with it, enjoying its services possibly for many years to come!
I also hope that this modification was interesting for you and that you enjoyed this experience as well…
This article was prepared for you by Paris Azis from Athens-Greece. He is 59 years old and has more than 30 years’ experience in electronics repairs, both in consumer and industrial electronics. He started as a hobbyist at the age of 12 years and ended his professional carrier as a senior electronics technician. He has been a specialist in the entire range of consumer electronics repairs (: valve radio and BW TV receivers, transistorized color CRT TV, audio amps, reel and cassette tape recorders, telephone answering and telefax devices, electric irons, MW cooking devices e.t.c) working in his early stages at the official service departments of National-Panasonic first and JVC afterwards, at their premises in Athens.
Then he joined the telecoms industry, working for 20 years as field supporting technician in the sector of DMRs (: Digital Microwave Radio transmission stations), ending his carrier with this subject. Now he is a hobbyist again!
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