I have been trying to build a circuit that takes a nominal 12VDC input to energize a relay. The contact side of the relay will need to handle 15+ADC of current. So to attempt this I create a zener diode in parallel with the coil of the relay that is rated for the coil of the relay, 12V. I take a resistor in parallel and size it appropriate so that the relay will turn on correctly when 12VDC is applied. My problem is that when I apply 32VDC the bias resistor is either taking 15W of power or the relay begins to take to much power. I fill like this is an easy circuit and I am just over thinking all of it. Does anyone have any advice?

The goal is to have a relay at 12VDC and to make it withstand 32VDC normal and reverse polarity for up to one hour.

Thanks!

 

Welcome to AAC!

Post a schematic of your circuit.

 

This is a sample of what I was trying at 32VDC. In this sample the R4(225) is the resistance of the coil. And the part is rated for 30mA so it wouldn't work.



Here is the same circuit at 12VDC and this circuit works fine. I've tried different relay coils and what not, I get the same results.

 

Try putting the zener diode in series with the resistor.

EDIT: You actually need 2 zeners to drop the voltage 20V so the relay sees 12V. If the zeners can't take the power, you can use add a power transistor.

 

Try putting the zener diode in series with the resistor.

EDIT: You actually need 2 zeners to drop the voltage 20V so the relay sees 12V. If the zeners can't take the power, you can use add a power transistor.[/QUOTE

I've done that before, and from what I have found if it is in series in forward bias, it will drop .7VDC and act as a normal diode. This provides to much wattage for the coil of the relay. If it is in reverse bias, it will not break over till the zener voltage is reached and when it does it drops that voltage, so this only works for when I am at 32VDC and not 10 then. Or am I missing something?

 

I've done that before, and from what I have found if it is in series in forward bias, it will drop .7VDC and act as a normal diode. This provides to much wattage for the coil of the relay. If it is in reverse bias, it will not break over till the zener voltage is reached and when it does it drops that voltage, so this only works for when I am at 32VDC and not 10 then. Or am I missing something?

You are not missing anything.. But we are.. You are failing to give us information (or didn't make it clear)..
You did NOT state that you need the relay to work across an input range of 10V? to 32V

So please try to restate your problem again..
Like..
"I need to use a 12V relay and it will need to operate with a voltage as low as "X" and as high as "Y"
The circuit should also be protected from reverse polarity.."

 

Okay, sorry about that.

I need a 12VDC relay that has to be energized when the circuit has 10VDC-16VDC applied to it. The circuit must be able to withstand both normal and reverse polarity up to 32VDC for one hour without damaging the circuitry, but it does not have to work across the 16-32VDC range. The contact side of the relay must be able to withstand 15-20ADC of current. .

I hope this is easier to understand, thanks!

 

Okay, sorry about that.

I need a 12VDC relay that has to be energized when the circuit has 10VDC-16VDC applied to it. The circuit must be able to withstand both normal and reverse polarity up to 32VDC for one hour without damaging the circuitry, but it does not have to work across the 16-32VDC range. The contact side of the relay must be able to withstand 15-20ADC of current. .

I hope this is easier to understand, thanks!

Not really.

The relay operates at 10-16VDC, can withstand up to +/- 32VDC, "but it does not have to work across the 16-32VDC range"

So, when 16-32VDC is applied it does nothing?

 

I need a 12VDC relay that has to be energized when the circuit has 10VDC-16VDC applied to it.

What is the part number for the relay? We need to know the pick-up and maximum voltages for the coil. Coil resistance is 225 ohms?

Why do you require reverse voltage and over voltage protection?

 

Do you need the relay to operate when reverse voltage is applied?

 

I have been trying to build a circuit that takes a nominal 12VDC input to energize a relay. The contact side of the relay will need to handle 15+ADC of current. So to attempt this I create a zener diode in parallel with the coil of the relay that is rated for the coil of the relay, 12V. I take a resistor in parallel and size it appropriate so that the relay will turn on correctly when 12VDC is applied. My problem is that when I apply 32VDC the bias resistor is either taking 15W of power or the relay begins to take to much power. I fill like this is an easy circuit and I am just over thinking all of it. Does anyone have any advice?

The goal is to have a relay at 12VDC and to make it withstand 32VDC normal and reverse polarity for up to one hour.

Thanks!

Its the sort of job I'd chose a solid state relay - the inputs can be typically 5 - 24V.

Someone else posted Zener shunts and current limiting resistors that would work with a regular relay - it would be even easier with a SSR.

 

This gives 10.5V across the relay at 12V input and 12V across the relay with 32V input. The relay will not operate, but no damage with negative voltages.

 

This gives 10.5V across the relay at 12V input and 12V across the relay with 32V input. The relay will not operate, but no damage with negative voltages.

What gives 10.5v...?
I think you missed an attachment.

 

This gives 10.5V across the relay at 12V input and 12V across the relay with 32V input. The relay will not operate, but no damage with negative voltages.

Second attempt! (This is based on a 12V, 30mA coil relay)

 

I'm kin

Not really.

The relay operates at 10-16VDC, can withstand up to +/- 32VDC, "but it does not have to work across the 16-32VDC range"

So, when 16-32VDC is applied it does nothing?

It can do nothing or it can work. All it has to do is survive the 32VDC, but it must work at a voltage of 10-16VDC.

 

Second attempt! (This is based on a 12V, 30mA coil relay)
View attachment 112650

This looks good!I might need to look for different relay than I have now, but thanks everyone!

 

I might need to look for different relay

If you do this, get a solid state relay as suggested earlier. It will support a wider voltage range.

If you don't, you need to consider the pick-up voltage if you're going to operate at less than the recommended coil voltage.

 

If you do this, get a solid state relay as suggested earlier. It will support a wider voltage range.

If you don't, you need to consider the pick-up voltage if you're going to operate at less than the recommended coil voltage.

I would use a solid state relay, but the output is using a DC pulse current that can max out at +/-15A. I though solid states are used with AC outputs?

 

You might as well look at one hour as forever, if something gets hot it will be very hot.

Something that would work very well here is a simple low drop out voltage regulator, just select something carefully as you have about 1 watt at max input, and you want to keep the junction cool enough not self destruct.

The KA78L12 will thermally protect itself (finally found the ref in the data sheet) But has a large (1.7V) dropout voltage, meaning the low input end may be a problem.

An NCV8674120 may be a better choice, good dropout, good to +45 and -42 volts in, but it is only available in surface mount.

 

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