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Hi & thanks for the response, in the circuit provided by you I do not understand the negative bus, kindly guide, I shall be building this circuit shortly
Ah,.. oops! Sorry, I forgot to check that side of the schematic didn’t I?
The centre-tap of the transformer connects to the 0 volt line and I would suggest that the charger (low volts) circuit is ISOLATED from the case entirely. This ensures that accidentally dropping the live lead on the box does not burn something out or frighten the life out of you with the spark.
Certainly connect an earth point to the case to prevent any mains failure from making the case live.
So, the circuit should now look like this,… with your 18-0-18 transformer you should then have a positive line of 25 volts DC, in which case C1 should have a working voltage higher than that and I tend to allow for transients 10 volts above so it would be 470uF, 35 volt working at least.
Ah,.. Sorry, I forgot to check that side of the schematic didn’t I?
The centre-tap of the transformer connects to the 0 volt line and I would suggest that the charger (low volts) circuit is ISOLATED from the case entirely. This ensures that accidentally dropping the live lead on the box does not burn something out or frighten the life out of you with the spark.
Certainly connect an earth point to the case to prevent any mains failure from making the case live.
So, the circuit should now look like this,… with your 18-0-18 transformer you should then have a positive line of 25 volts DC, in which case C1 should have a working voltage higher than that and I tend to allow for transients 10 volts above so it would be 470uF, 35 volt working at least.
Ah,.. Sorry, I forgot to check that side of the schematic didn’t I?
The centre-tap of the transformer connects to the 0 volt line and I would suggest that the charger (low volts) circuit is ISOLATED from the case entirely. This ensures that accidentally dropping the live lead on the box does not burn something out.
Certainly connect an earth point to the case to prevent any mains failure from making the case live.
So, the circuit should now look like this,… with your 18-0-18 transformer you should then have a positive line of 25 volts DC, in which case C1 should have a working voltage higher than that and I tend to allow for transients 10 volts above so it would be 470uF, 35 volt working at least.
hey Pete thanks for your response and the corrections, I shall be implementing this circuit shortly and update on the functionality, I just hope that it trips the way I want it to.Thanks again
Thanks for your circuit Pete, I shall be implementing the same shortly and updating you accordingly, I am sure this will work, but again wish to check it
Hi Pete, have tried pout this circuit and sob sob sob sob, it doesnt work, I have made it exactly the way you denoted but the RED Led comes on and cuts off the charging, the charger voltage is set to 13.8v without the zener connected, and the moment the zener is connected it drops off!!!! what do I do now, also the funny part that I noticed is if the zener is not connected and I touch it with my finger the RED Led comes on and stops charging, funny but true, so what is it that you can please help me with now, the circuit for sure did look very promising but hasnt yet worked, also some points I would like to tell you the batter used was a AMPTEK 12V7.6 SLA and transformer was 18-0-18 1.5 Amp, the battery post discharge showed voltage as 12.42 and on full charge generally shows around 13.10. Please guide your help is utmost required.
Hi Irfan. Ah,.. oops!
I did say that I thought the voltage of the zener used here might be wrong as it looks like the original schematic was generated by a computer program. In my long experience (40 years), I have found that all the theory in the world develops some very strange habits when we let it lose in the wild.
I have a looked at this schematic again and as the forward voltage drop on a red LED is approx 1.6 volts and the voltage drop from base to emitter on the transistor will be 0.6 volts, (due to the minute current the resistor is not doing much) then the zener then should be,.. 13.8 – 0.6 – 1.6 = 11.6 volts. Now that is a very odd value and I suggest making the zener 10 volts with the resistor R4 a 5K preset potentiometer, then adjust it till the circuit shuts off at the suggested ‘float charge’ level of 13.6 volts which is standard for all lead-acid bateries, or just a little higher than that. It is possible to charge to a higher voltage but that may also cause the battery to heat up and shorten its working life-span. I’m not saying that the circuit goes into a float state, it shuts off completely at the max value you set.
You also mention that putting your finger on the end of the zener causes the circuit to switch off. Yes it will.
This is because our bodies operate with electrical impules in the nerves that are actually a lot higher than some might think. Add to that the fact that we also pick up any mains interference like an aerial, thus what is happening is that our bodies generate quite enough voltage to interfere with a simple base junction in a transistor! This is the buzzing noise you get if you touch the input lead to an audio amplifier. It is also why sensitive electronic components are packaged in anti-static bags and foam.
If there is a lot of electrical noise in the envoronment (remember there is a mains transformer very close), adding a capacitor of around 100nF from the bottom of the zener to 0 volts should dump that noise and stop EMI (electro magnetic interference) from triggering the transistor at random.
Just to be certain, as I have just had a delivery of 10 x LM317 chips, I will dig out some bits tomorrow and actually put this circuit together myself to pinpoint where the computer that originally designed it, got it so wrong. I will update the schematic accordingly.
Sob Sob, Pete tried everything you had guided, changed zener to 10V and then 13V and also R4 to 5K Vario, but alas seems lady luck aint happy with me at all, now if I connect the 10V zener voltage drops down to 11.96, and with the 13V zener nothing happens, if I change the preset(adjust) then nothing for 13v zener but in 10V zener fluctuates only a bit from 11.88 to 11.96, which I guess is not suffice. What do I do now and is there any other way to contact you via mail or somethng
Hi Irfan.
I notice your comment here, “.. but in 10V zener fluctuates only a bit from 11.88 to 11.96” so if we look at that result, then the voltage drop across the transistor and LED is 1.88 volts isn’t it? That’s the voltage with R4 at a zero value or close as make no difference.
As we are aiming at getting a cut-off at 13.6 volts than we need to arrive at a value for the zener/resistor combination of 11.72 volts.
As we have found that making R4 a 5K variable and that gives just over 1 volt adjustment, we could then make up the right voltage drop from a 10 volt zener by adding two diodes (1N4001,.. anything silicon really) in series. Each diode has a forward voltage drop of 0.6 volts (actually between 0.6 and 0.7 but always give as the ‘worst case’) and that would then make the effective zener+diodes a value of 11.2 volts, the 5K can then be used to trim the extra 0.7 volts and you should be in business.
In every case such as these where circuits don’t work quite as expected, it is neccessary to take a step back and look at what causes the error, then tweak the smallest number of component values till it does work. Adding series diodes to a zener reference is quite common practice because it gets expensive for equipment manufacturers to specify odd values which are not produced in such large numbers.
I would also add the 100nF capacitor from the junction of the diodes and R4 to 0 volts to be certain you will not get random variations due to noise. Zener diodes are electronically very noisy devices as they approach the point where they start to operate.
Oh yeah, if you were to search the internet for my name (this is my real name too), there is only one of me and you will find the website with contact details, and all kinds of other stuff I get up to!
The circuit should now look more like this with the diodes added to a 10.0 volt zener and the capacitor added to eliminate spurious triggering.
Ah,.. just re-reading your comments again, that 5K isn’t doing much is it?! This is because the zener ‘can’ handle 1 Watt but in this case it’s nowhere near that, so the actual current flowing is very small. That means that where I have suggested a 5K preset, to get enough adjustment it maybe should be increased to as much as 50K or more, or it’s still going to be pretty ineffective.
I think maybe I should get to bed earlier so that my brain is actually awake when I post replies in here! Haha.
Dear Pete , lady luck sure aint happy with me, this is what I have made, tested and failed, somehow am just doubtful on the preset value, shouldnt it be lower rather than so high, please lemme know if you have been able to make and implement tthis circuitwysiwyg_imageupload:10057:height=700,width=700
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