zener diode with PNP regulator mod for battery charger

Thanks Bertku for the help, it is great. What forum is appropriate for this?

I redrew what I think your saying.


Your right that this ought to work fine. If I do it, then I have to readjust the big resistor, likely slide slider up? Or could it stay as is?

Can you tell me why this wont work also?
This way I could leave the big resistor adjustment alone.

 

Someone suggested trying this idea.
Would it work? Eliminates the adjustable 400 ohm resistor completely.

If so, what wattage for the 330 ohm resistors? I have two 1/4 watts here.
would 330 ohms work or adjust the ohms or watts on those two, lower or higher?

Right now the 400 ohm lower side is adjusted to about 200 ohms and this circuit could only go to 330 ohms if you put minipot all the way down to zero.

 

Hi sdowney. Look it all boils down to what kind of Voltage are we talking about. I have assumed a 12 Volt charger with a 16 to 18 Volt average Voltage input. If you have a 24 Volt battery bank, it is a totally different story.

I stay on a 12,8 Volt system i.e. at 14.4 Volt charging + 0,5 Volt drop for the thyristor = 15 Volt pulsed DC. minimum, but most likely 16 to 18 Volt.
Your zenerdiode goggles up 6.8 Volt + 0,5 Volt for the base Voltage of the PNP. ( I assume silicon PNP and not a Germanium) which leaves you 18 Volt - 7,3 Volt = 10.7 Volt over the 15 Ohm base resistor + 47 Ohm for the "ON" collector resistor = maximum 173 milli Ampere. As soon you turn your new potentiometer higher, you may have 200 Ohm in total in that circuitry.
Thus sufficient for your low HFE old fashioned 2N525 PNP transistor of HFE 44 to drive the voltage for the electrolytic capacitor and the base of the PNP transistor. A 330 Ohm at both side is only a good idea , if you work at 24 Volt or a 36 Volt battery bank. But for 12.8 Volt battery bank, a small resistor is sufficient or none if your new 500 Ohm potentiometer can handle the current. What does the spec says of the new potentiometer, what is the max current. I thought you mentioned somewhere 500 milliWatt, i.e. max 30 miliiAmpere.
It means that if your pot meter is turned down to 5 % , you also will burn your potmeter out. Thus a small resistor in series on the lower side of the new potentiometer may not be a bad idea, in case you accidental turn the potmeter nearly totally down. If you have 16 Volt instead of 18 Volt, you may get away without any extra resistor. But please calculate it careful or give us some voltages to work from. At the other side it is not necessary. With a HFE of 44 the total circuitry can be up to a few kilo Ohm. Bert
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OOOPs it was really a late night, last night. Only now I discover that it is for a 12.8 Volt system. Thus my calculations are the correct ones for 12.8 Volt.

A small resistor whereby you limit the total base current to the "OFF" transistor to 30 Milli Ampere is indeed wise. But I assume you are clever and don't turn the potmeter totally down, while the charger is switched on. You only wanted to have some fine adjusting's somewhere in the middle part.

18 Volt - 7.3 = 10.7 Volt at 30 milliampere the total resistance should be not less than 356 Ohm minus 47 Ohm and 15 Ohm = 294 Ohm ( either 300 or 330 1/4 watt)
At 16 Volt you will have 8.7 at 30 milliAmpere = 290 Total resistance - 47 + 15 Ohm = 228 Ohm ( either 220 Ohm or 270 Ohm resistor)
Please give us some real Voltages to work from.

Bert

 

Any change to get a more robust potmeter? like a 2 watt or a 1 watt. If you are sick and tired with your old potmeter place 5 x 81 Ohm 3 watt resistors in series and forget the old potmeter totally. Now you take your red wire from 81 Ohm, or 162 Ohm or 243 Ohm etc. with the 500 Ohm potmeter as suggested to any of those 4 points. You then regulate only with the 500 Ohm potmeter.
Bert

 

Yes it is a 12 vdc charger.
If I measure transformer output before the two rectifier diodes I get 15.5vac, so I suppose that is the max it can go if shore input volts are 120vac going into the transformer.

Great and Interesting idea you have of stringing together the resistors and using one of those position to attach the minipot. Good thinking.

I bought the minipot (10 of them for $1.79) pretty cheap off Ebay and they are 1/2 watt rated. They wont arrive for likely another month.

The end of the zener attaching to the adjustable resistor, it is the connection towards the Q1 2n525 (germanium) transistor which feeds the SCR that is relevant to adjusting the voltages? The battery negative is not the true ground to me for this circuit? I think of true ground regarding the flow of current as the connection back to the centertap on the transformer? So does the zener drain down through those two 300 ohm resistors back to the transformer, when the volts rise too high?

The person suggesting the last diagram with two 330 ohms resistors, it seems the resistance will be too high for it to regulate the range i need.
Because as of now, the lower leg of the 400 ohm adjustable resistor is only about 200 ohms and 330 ohms is a lot more.
Does that make any sense?

How about going with 100 ohm and 3 watts, here is good prices
http://www.ebay.com/itm/3w-Watt-100...895?pt=LH_DefaultDomain_0&hash=item2a4200a0f7

a little better feedback. a little more money, but only 5 resistors
http://www.ebay.com/itm/5pcs-100-oh...342?pt=LH_DefaultDomain_0&hash=item2ed702f6ee

here are 5 watt, would more watts be better or superfluous?
http://www.ebay.com/itm/5-Watt-100-...611?pt=LH_DefaultDomain_0&hash=item337c604b4b

 

Pity, you may have a too high value selected, with a too low power dissipation for your application.

The purpose of the sliding resistor and your potentiometer is to shift the Voltage level up or down. But there is a level which will be too low and there will be a level which is too high. I would have taken the 2 x 330 Ohm resistors parallel and have tested what influence it has on the level of the charging Voltage. I would have made a table and then concluded what potentiometer to buy or to use. Alternative I would have calculated all parallel circuits and behavior.

You miss the point. The zener is for the creation of a window ON/OFF. It is the Voltage what shuts the transistors down or open up. Your sliding resistor is regulating this and now additional with your 500 Ohm Potmeter with a small resistor is serial. Whether the 500 Ohm potmeter is going to be a success is to be seen. Too much resistance in the base of the "OFF" transistor, may have a bad influence on the output to the electrolytic Cap.

Maybe yes, maybe not. The total resistance you will then have increased from 47 + 15 Ohm + 1/2 of the sliding resistor to 330 + half of the 500 Ohm. It may be too much. But maybe you have a good transistor which survived the 30 or 40 years this circuit is old. But maybe your HFE has dropped and then too much resistance in the base is no good.

Cannot comment as I don't know what you would like to do with the 100 Ohm. It is not a potmeter. If it was a 3 Watt potmeter, I would say, a better choice than your 500 Ohm potmeter 0,5 watt.

Cannot get to that page, I get an error.

Also not a potmeter. Just experiment with parallel connecting of a few resistors you have at hand. You cannot blow any thing other than under or overcharging. But that is very quickly stopped.
Bert

 

I forgot. Yes, your center point of the transformer is your real earth. Thus your battery is floating. If you use the negative of the battery bank as your earth/ground/mass, you have to make sure that the circuit is floating and does not make contact with anything. Bert

 

Bertku, I thought this was the idea?
You mentioned 81 ohm used in series, I could not find, but I found 100 ohm.
And set minipot at about 100 ohms

 

No you regulating the current flow and not the voltage level. This is what I would have done. Thread is now closed for me. I am on my way to overseas. Bert

 

I appreciated your help Bertku, have a good trip.
I have some good ideas now to try out.

 

The original schematic was simple enough but became complicated. Keep it simple.

The supply voltage divides in the 400 ohm potentiometer throught the wiper arm. The fixed 500 ohm further limit the voltage so as not to turn on the zener diode. when the desired voltage is reached, the zener fires (conducts) and turns on or off the 2N525 transistor.

If the 500 ohm is replaced with a trim pot and the circuit still would not work at any settings, the zener diode has shorted and conducts at very low voltage settings. A good one will conduct only on the rated voltage. Measure the voltage across the zener terminal. It should be the rated voltage. If you are getting less than 2 it is shorted.

I still have my Raritan charger and it is as big as a truck battery.

 

His original problem was, that the sliding resistor is not fine tuning enough. Your solution is again more of a current regulation solution. The only way he will be able to adjust and do fine tuning the voltage is by his solution of a 25 turn potmeter placed at a position which regulate the voltage. He just needed some help where to place the 25 turn potmeter. Bert

 

The alternative he could do is as per below solution. Your and my solution will burn the potmeter, if he slides it down too much. The real solution may still be that what somebody else suggested with the 2 x 330 Ohm resistors on both side of the 500 Ohm potmeter. In that case, whatever way he turns the 500 Ohm potmeter or sliding resistor, he will never burn the 500 Ohm potmeter out. Even with your solution, he could burn the potmeter out. Although it is not a voltage regulated solution. In the end, if the germanium PNP transistor has not lost some of its HFE, he may get away with the 2 x 330 Ohm solution and we have to salute that person.
Bert

 

The normal range of resistors are: 10,12,15,18,22,27,33,39,47,56,68, EIGHTY ONE, 100 or a multiple of 10 or 1/10 of the above.
You also have a further, more in depth range, but not many shops do stock them.
Yes, you could do 5 x 100 Ohm (instead of 4 x 100) and the extra 100 Ohm is to compensate for the parallel resistance of the 500 Ohm potmeter and to avoid burning your 500 Ohm potmeter out if turned fully. But your potmeter must be acting as a voltage regulator and not a current regulator. I think your HFE of your Germanium transistor 2N525 can handle the extra resistance in the base. The 2N525 has to drive a 220 Ohm + 100 Ohm resistor only. Even if the HFE has dropped to half, your base resistance could be maximum
about 5000 Ohm. Your 500 Ohm with the 2 x 330 Ohm resistors would be fine. I would try that first. If it does not work, you have to look at replacing the 225 milliwatt Geranium transistor with a Silicon BC327-40 or just a BC327 transistor. Or you have to consider to buy a lower value and more robust potmeter. However your 25 turn potmeter will fine tune your charger beautifully. Just try that solution with 2 x 330 Ohm 1/4 watt resistors out. You will be happy.
Bert