Hi all

I have a circuit powered by 3xAA 1,5V batteries, so the maximum input will be 4,5V and i want to add overvoltage protection to my circuit. The overvoltage circuit must deal with up to 1.2A of current and has to be small (SMD components).
I saw a simple circuit with a resistor and a zenner, but the input current will allways flow through the resistor, even Vin<4,5V, thus wasting power. I was looking for a more efficient circuit, since it will be powered by batteries.

Any help?
Thanks

 

If it will ALWAYS be powered by 3 AA batteries why do you need over voltage protection?

 

If it will ALWAYS be powered by 3 AA batteries why do you need over voltage protection?

Because my circuit has 2 pins for Vin, so anywone can power it with more than 4,5V... Besides that, with the overvoltage protection i can guarantee that works with batteries > 4,5V, like 7,2 NiMh batteries...

 

Simple enough. Use a 4.7V zener to ground and put a 1.5A or so polyswitch fuse before it. If the input voltage exceeds 4.7V the polyswitch will trip and not reset until the offending input voltage is removed.

You might want to make that a 5.1V zener, new Alkaline battteries usually put out around 1.63V each.

 

It sounds more like you want a regulator if you want to use 7.2V batteries as well. You need something that will output about 5V with varying inputs up to about 12V. Does that sound right?

 

Simple enough. Use a 4.7V zener to ground and put a 1.5A or so polyswitch fuse before it. If the input voltage exceeds 4.7V the polyswitch will trip and not reset until the offending input voltage is removed.

You might want to make that a 5.1V zener, new Alkaline battteries usually put out around 1.63V each.

Hum... it sounds simple... I would prefer a solution that keeps the voltage out, not shuts down, but it is so simple that i will try it.
Thanks!

 

It sounds more like you want a regulator if you want to use 7.2V batteries as well. You need something that will output about 5V with varying inputs up to about 12V. Does that sound right?

Yes, that's it, but i don't want to use a regulator, just a simple overvoltage/clamping circuit that allows to keep working with Vin>4,5V

 

If you want it to be efficient, you'll have to go to a buck/boost type switching regulator. A linear solution just won't cut the mustard; you'll have terrible efficiency.

What's the maximum and average current drawn by your circuit, and what are it's voltage requirements?

 

Because my circuit has 2 pins for Vin, so anywone can power it with more than 4,5V... Besides that, with the overvoltage protection i can guarantee that works with batteries > 4,5V, like 7,2 NiMh batteries...

I hate to say this but why? If some idiot puts in more than 4.5V it's their own fault; it's pretty difficult to do this accidently. There is a saying: "If you make something idiot proof, the universe will make a better idiot." So you protect it against >4.5V but then someone reverses the input defeating your protection. Or, they decide to run it off the mains.

If you want to keep the output at 4.5V, you *will* waste power with a higher input. There is no avoiding that.

 

A regulator is one 3 pin device, a few caps, and a few resistors (If that).

I think you are thinking of a regulator as something different.

You can easily get a LDO regulator that will hold your voltage at a variety of input voltages.

 

If you want it to be efficient, you'll have to go to a buck/boost type switching regulator. A linear solution just won't cut the mustard; you'll have terrible efficiency.

What's the maximum and average current drawn by your circuit, and what are it's voltage requirements?

My circuit has already a switching regulator. I built the circuit to be powered by 4,5V @ 1,2A but the switching regulator can handle up to 6V. Later i realize that it would be nice that it could accept different types of batteries like those 7,2V Nimh, so i was looking for a simple solution to clamp Vin to 4,5V if Vin > 4,5V. I don't mind wasting power in that situation, just don't want to waste power if Vin=4,5V.

My goal was to add overvoltage protection and (if possible) clamp Vin if Vin>4,5V. I realise the second is difficult without a switching regulator or wasting power, so i think i will just keep the first one, and try the circuit suggested by marshallf3.

 

An LDO regulator might be an option, depending upon the OP's circuit's power consumption.

However, if they want to regulate the input at 4.5v and someone uses 9v to power it with, about 1/2 of the total power dissipation will be in the regulator itself. Also, no LDO regulator is perfect; there will be some drop across the regulator when powered by 4.5v.

 

My circuit has already a switching regulator. I built the circuit to be powered by 4,5V @ 1,2A but the switching regulator can handle up to 6V. Later i realize that it would be nice that it could accept different types of batteries like those 7,2V Nimh, so i was looking for a simple solution to clamp Vin to 4,5V if Vin > 4,5V. I don't mind wasting power in that situation, just don't want to waste power if Vin=4,5V.

My goal was to add overvoltage protection and (if possible) clamp Vin if Vin>4,5V. I realise the second is difficult without a switching regulator or wasting power, so i think i will just keep the first one, and try the circuit suggested by marshallf3.

We cross-posted.

If you can find a 5.6v Zener that's rated for >8.4 Watts, and can figure out a way to get rid of that heat, marshallf3's suggestion might be do-able.

However, it certainly would be more simple and far more efficient if you would replace your switching regulator with a unit that is rated for your desired input supply range.

 

I think we have some confusion here regarding the way the term "overvoltage protection" is being used.

 

Agreed. Overvoltage protection is typically used for handling spikes.

redes, upgrading your regulator should be all you need to do.

As the sarge said, there are LDO (Low Drop Out) Regulators that will give you 4.5 volts from anywhere from 5.5v to upwards of 30v.

That would allow you to use a wide variety of power sources.

 

Agreed. Overvoltage protection is typically used for handling spikes.

redes, upgrading your regulator should be all you need to do.

As the sarge said, there are LDO (Low Drop Out) Regulators that will give you 4.5 volts from anywhere from 5.5v to upwards of 30v.

That would allow you to use a wide variety of power sources.

Well, upgrade the regulator is not an option since the PCB is already made... It has to be something that i can add to my current solution...

 

Can you post the shematic of your regulator? If you have already made a switching regulator it "MAY" be possible to replace some components to solve your problem. Beats working blind.

 

Can you post the shematic of your regulator? If you have already made a switching regulator it "MAY" be possible to replace some components to solve your problem. Beats working blind.

I'm using LM3410. They have a solution on their datasheet on page 26(www.national.com/ds/LM/LM3410.pdf) but it doesn't work to me since i set Vout to 5,9V, so if Vin>5,9, the load will see the entire Vin.