# split charge relay, control by a zener diode 13.5 volts.

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#### Gerard58

##### New Member
:?: I want to fit a the split charge relay, control by a zener diode 13.5 volts on the main battery. this will charge the second battery only when the main battery is at 13.5 volts. my problem is the number for the zener diode and how to set it up. can anyone supply a Sketch.

Gerard58

#### TheOne

##### New Member
Here is a basic idea.

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#### Gerard58

##### New Member
I take it that the RL1 is the split relay

:?: First, Thanks. I am not an Expert in this field so bare with me. I take it that the RL1 is the split relay and the power that is 14V is from the generator. Can I instead link into the main power from the generator to the main battery and use the split relay just as a switch to the second battery. I do not want to disconnect the main battery as this might blow the generator when it switches to the second battery. In addition, what is the D1 BZV85C12 for, it is attached on to the battery 2. Is this the sensor for the voltage drop for the D2 Zener Diode? If so, can this be wire onto the main battery, as this must have priority because this is the starter battery for the vehicle? Again thanks for your time.

#### Gerard58

##### New Member
I take it that the RL1 is the split relay

:?: First, Thanks. I am not an Expert in this field so bare with me. I take it that the RL1 is the split relay and the power that is 14V is from the generator. Can I instead link into the main power from the generator to the main battery and use the split relay just as a switch to the second battery. I do not want to disconnect the main battery as this might blow the generator when it switches to the second battery. In addition, what is the D1 BZV85C12 for, it is attached on to the battery 2. Is this the sensor for the voltage drop for the D2 Zener Diode? If so, can this be wire onto the main battery, as this must have priority because this is the starter battery for the vehicle? Again thanks for your time.

#### TheOne

##### New Member
Here the GEN is connected to the main bat2. Once this reaches 13.5V the relay connects bat1 also to the charger. D1 is a 12V zener that together with the resitor and transistor Vb-e will switch the relay around 13.5V.
You can make a more accurate switching circuit by using a programmable zener or opamp circuit.

D2 is just a protection diode 1N4007 etc. to protect the switching transistor against spikes from the relay coil.

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#### Gerard58

##### New Member
What are the codes or ref numbers

:?: Thanks. As you can read, I have a lot to learn. What are the codes or ref numbers for the D1 12V zener diode, resistors, and transistor (In that D2 diode number is 1N4007)? If I had a lot more knowledge on circuitry, I would take you up on the programmable zener diode, but I have to start somewhere, and I think I will stick with the basic circuit first. Thanks again.

#### TheOne

##### New Member
That is actually a valid part number for the zener. The transistor should be selected based in the size of relay (coil resistance) that you will use. Something like a 2N2222 will work in most cases.

#### Gerard58

##### New Member
:?: Hello again. I have added an attachment in PDF file of the details of the 12-volt relay. On this information would it be possible to supply the ref numbers that are required for the transistor and so on. What I am looking for is a shopping list that I can give the supplier for parts that are required to make the circuit. Again thanks for your time.

Gerard.

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#### TheOne

##### New Member
The 2N2222 will be a bit small for the job. I will rather use a BD139. With this reduce R2 to 120 Ohms. This will give us about 7mA of base current and a bit more than 180mA through the relay coil at 65 Ohms.

Parts needed:
1 X 1N4007 diode
1 X BD139 transistor
1 X BZX55C12 (0.5W 12V zener)
1 X 120 Ohms resistor (1/4 W)
1 X 10k resistor (1/4 W)

use www.findchips.com to search for a supplier for your parts.

#### Gerard58

##### New Member
:?: Manny thanks again. It grates to be able to have someone like you, to ask. Now that I have all the part ref numbers and list, I look forward to making it and installing it. The only problem now is time.

Thanks again.

#### Gerard58

:?: I have almost completed the circuit. I am having problems with obtaining a 120 Ohm resistor. I have tried to place an order with the company you mention, but the minimum order they will take for overseas is $280.00. I have looked up a few catalogues with no results. Perhaps I am reading the Multimeter wrongly. To take the readings of a 120 Ohm resistor on a Multimeter. Would the readings be .120? As 1K is 1000. Again I am learning as I go. Can you help again. Thanks. TheOne said: The 2N2222 will be a bit small for the job. I will rather use a BD139. With this reduce R2 to 120 Ohms. This will give us about 7mA of base current and a bit more than 180mA through the relay coil at 65 Ohms. Parts needed: 1 X 1N4007 diode 1 X BD139 transistor 1 X BZX55C12 (0.5W 12V zener) 1 X 120 Ohms resistor (1/4 W) 1 X 10k resistor (1/4 W) plus your relay. use www.findchips.com to search for a supplier for your parts. #### Nigel Goodwin ##### Super Moderator Most Helpful Member Gerard58 said: :?: I have almost completed the circuit. I am having problems with obtaining a 120 ohm resistor. I have tried to place an order with the company you mention, but the minimum order they will take for overseas is$280.00. I have looked up a few catalogues with no results. Perhaps I am reading the Multimeter wrongly. To take the readings of a 120 ohm resistor on a Multimeter. Would the readings be .120? As 1K is 1000. Again I am learning as I go. Can you help again. Thanks.
120 ohms is a standard value resistor, you can buy them anywhere that sells resistors - there's no problem whatsoever finding them.

If you go and ask in your local TV service department, they should certainly have them in stock!.

As for your meter, on the rage where a 1K reads 1000, a 120 should read 120, 1K is 1000 ohms.

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