Trying to find out the name of a type of component.

[09williamsad]

I have completely forgotten what a component that conducts current at or above a voltage value is called.

If any one knows what this is called please inform me.

Thank you in advance.

 

[Supercap2F]

A MOSFET maybe?

Or maybe a zener diode?

 

[hevans1944]

Zener diode is correct.

 

[09williamsad]

Zener diode is correct.

Ideally I am looking for a component that will not permit current to flow unless the voltage is at a certain value or higher.

I am not sure a zener would work as it permits current to flow in a direction and then allow the other direction when a voltage value is reached if I am not mistaken.

 

[BobK]

There isn't a single component that will do that.

Is this AC or DC?
What voltage range?
How much current?

Bob

 

[09williamsad]

There isn't a single component that will do that.

Is this AC or DC?
What voltage range?
How much current?

Bob

DC,
It would be low voltage, the trigger voltage would be around 1.5V,
Minimum current to be conducted would be 500uA.

 

[KrisBlueNZ]

There are several components and circuits that fit your description. Why don't you tell us what you're trying to achieve. Show us a schematic of the parts you have, with the mystery component drawn as a box, and describe what you want the circuit to do.

 

[09williamsad]

There are several components and circuits that fit your description. Why don't you tell us what you're trying to achieve. Show us a schematic of the parts you have, with the mystery component drawn as a box, and describe what you want the circuit to do.

Here is the circuit I have devised so far.
UH1 is a thermistor with 3.796k ohms being the resistance that I intend for D4 to illuminate at with D4 being illuminated constantly (provided that there is a supply).
The LEDs are the model that they are because of the low power consumption and the supply would need to be something small like a button cell.
Component X is what I am trying to figure out.

 

[KrisBlueNZ]

OK, so you have a 3V button cell and two low-current LEDs. You want one LED to be ON all the time, and the other one to illuminate only when a thermistor tells you that the temperature is above a certain threshold?

I think you need a voltage comparator. Probably one with a built-in voltage reference. There are a few options but I think the best is the Maxim MAX9064 (http://www.digikey.com/product-detail/en/MAX9064EUK+T/MAX9064EUK+TCT-ND/2234694), which has a built-in 0.2V reference that will allow you to operate the thermistor at a fairly low current - about 53 µA.

Have a look at the data sheet and see what you think.

 

[swagguy8]

or a transistor, itn't that something like if the one of the sides has to be a higher voltage than the other sides?

 

[KrisBlueNZ]

or a transistor, itn't that something like if the one of the sides has to be a higher voltage than the other sides?

A transistor isn't suitable if you want a clean on/off action or stability with temperature.

 

[swagguy8]

A transistor isn't suitable if you want a clean on/off action or stability with temperature.

very true, but mosfets and triacs are both based off of transistors.

 

[KrisBlueNZ]

swagguy8, please don't waste our time with irrelevant comments and poor advice. You've made another useless comment on another thread too. We haven't banned anyone for giving bad advice for a long time and I don't want to have to do it again.

 

[Bailey John]

Diode is a correct answer from my point of view.

 

[09williamsad]

OK, so you have a 3V button cell and two low-current LEDs. You want one LED to be ON all the time, and the other one to illuminate only when a thermistor tells you that the temperature is above a certain threshold?

I think you need a voltage comparator. Probably one with a built-in voltage reference. There are a few options but I think the best is the Maxim MAX9064 (http://www.digikey.com/product-detail/en/MAX9064EUK+T/MAX9064EUK+TCT-ND/2234694), which has a built-in 0.2V reference that will allow you to operate the thermistor at a fairly low current - about 53 µA.

Have a look at the data sheet and see what you think.

Based on the simulations I have done that seems to be working perfectly.
Thank you very much.

 

[Merlin3189]

Although it is probably not what is wanted here, the description, " a component that conducts current at or above a voltage value", made me immediately think MOV (metal oxide varistor) or voltage dependant resistor. These used to be routinely soldered across power supply inputs to protect against voltage spikes, though I gather they are not so common now. Since they are symmetrical they work for AC or DC.

 

[KrisBlueNZ]

Sorry, I've put you wrong there. You shouldn't use a comparator with a reference voltage for detecting the temperature. You need the voltages at both comparator inputs to be proportional to the battery voltage, because the battery voltage will vary as it discharges.

So you need a two-resistor voltage divider at one input, and at the other input, a voltage divider made from a resistor and your thermistor.

I thought you might also want to monitor the battery voltage, so your "always ON" LED turns OFF when the battery voltage is low. In that case you do need a reference voltage, and a MAX9043 (http://www.digikey.com/product-detail/en/MAX9043AEUB+/MAX9043AEUB+-ND/1495090) looks like a good option - it has two comparators and a separate reference voltage.

You'll want to minimise load current on the battery, so use high resistances (in the hundreds of kilohms) in all of your voltage dividers. Assuming you can't use a different thermistor that has a higher resistance, you can still minimise the current consumption in that leg of the circuit by using a high resistance in series with it, so you have a low voltage (e.g. 0.1V or so) across the thermistor. You will have to use a similar resistance ratio in the leg that feeds the other comparator.

 

[09williamsad]

Sorry, I've put you wrong there. You shouldn't use a comparator with a reference voltage for detecting the temperature. You need the voltages at both comparator inputs to be proportional to the battery voltage, because the battery voltage will vary as it discharges.

So you need a two-resistor voltage divider at one input, and at the other input, a voltage divider made from a resistor and your thermistor.

I thought you might also want to monitor the battery voltage, so your "always ON" LED turns OFF when the battery voltage is low. In that case you do need a reference voltage, and a MAX9043 (http://www.digikey.com/product-detail/en/MAX9043AEUB+/MAX9043AEUB+-ND/1495090) looks like a good option - it has two comparators and a separate reference voltage.

You'll want to minimise load current on the battery, so use high resistances (in the hundreds of kilohms) in all of your voltage dividers. Assuming you can't use a different thermistor that has a higher resistance, you can still minimise the current consumption in that leg of the circuit by using a high resistance in series with it, so you have a low voltage (e.g. 0.1V or so) across the thermistor. You will have to use a similar resistance ratio in the leg that feeds the other comparator.

This only needs to be theory based so the info I have for now will be but I will keep this in mind in case I need to revise the design.

 

[cjdelphi]

There isn't a single component that will do that.

Is this AC or DC?
What voltage range?
How much current?

Bob

a DIAC fits this description although I doubt it's what the OP is after..

 

[BobK]

a DIAC fits this description although I doubt it's what the OP is after..

Not really. It triggers at some specific voltage but then continues to conduct at a much lower voltage.

Bob