Point and Laugh ( and then maybe help )

[PeterDove]

HI All,

I have been attempting to make a temperature based controller of current flow. ie something starts getting hot we reduce current. I include a schematic below.

**broken link removed**

V1 is a variable voltage representing the output of the centigrade temperature sensor LM35DZ in which each degC is 10mV. Here at 750mV ( 75degC) the current is higher.

Below you can see the effect of reducing the voltage at V1

**broken link removed**

The problem is... I actually want the effect reversed.. I want a way to INCREASE the current as the voltage at V1 is decreased.. What sort of transistor/semiconductor will allow me to do that?

Thanks

Peter

 

[karthik16]

this can b done with the help of thermistor.thermistor in NTC negative temperatur quotient will do the same as u have done..if u use it in positive temperatur quotient u can reach where u wanted..for any other clarification mail me at gkarthik16@gmail.com with subject "ELECTRONICS"

 

[audioguru]

The opamp now is non-inverting. Make it inverting then the output direction will be reversed.

 

[Hero999]

Also why are you using a linear amplifier?

This is normally achieved by connecting the op-amp as a comparator with some hysteresis so the current is stopped completely when it overheats.

 

[Nigel Goodwin]

And why not use the circuit I already gave you the link for?.

 

[PeterDove]

And why not use the circuit I already gave you the link for?.

Because I wanted to use the LM35, looks like I will have to use a thermister.

Peter

 

[Nigel Goodwin]

Because I wanted to use the LM35

Use that in the comparator circuit I posted!.

 

[Hero999]

Don't just use a componant simply because you want to, use it beasue it's suited for your application.

 

[evandude]

There is absolutely no reason you can't do what you are trying to do with an LM35DZ instead of a thermistor; the problem is not with your choice of sensor, the problem is with the circuit...

At first glance at least, it seems odd to me that you would be using an NPN transistor but having it sourcing current to the load... and having the feedback be the base voltage of the transistor, rather than (for example) from a current sensing resistor... a BJT is essentially a current-controlled device, so driving it with a gate voltage proportional to temperature is not a great way to control output current... Granted, once you get the amplifier polarity right it will at least be relatively proportional, though not linearly... It seems to me this current control circuit is not going to be very consistent, since it's effectively open-loop with respect to the current going to the load... And if you need a particular operating current, it'll require tweaking the pots just right, and probably drift with time, temperature, phase of the moon, etc...

 

[PeterDove]

Use that in the comparator circuit I posted!.

OK, I shall explain more clearly - I dont want a simple on/off which is what you keep pointing me at.

What I wanted was something that would reduce the current in proportion to the voltage/temperature from the LM35DZ. I almost have something in the schematic except it increases current with voltage.

I am sure I will work it out - frankly a PTC thermistor seems to be the thing - looks like I will have to find a source for them - the whole idea of the LM35DZ was its linear scale and of course the fact that I had some already.

Peter

 

[evandude]

Like I said, there's no reason you can't get a linear scale with the LM35. You just need to re-think your op-amp circuit to be controlling the current directly, not controlling it indirectly by controlling the base voltage of a BJT and crossing your fingers for a linear relationship to current from there.

For starters, if you switch out your NPN for a PNP, you should at least get the current changing in the right direction with temperature, with the polarity of your current op-amp circuit...

 

[Hero999]

Whay do you want to decrease the current linearly with temperature?

Whay is it not acceptable to just turn it off?

 

[PeterDove]

Whay do you want to decrease the current linearly with temperature?

Whay is it not acceptable to just turn it off?

THe idea was that I would still end up with a trickle charge, as the temp would rise pretty quickly and the current would be dropped too but not entirely just off.

Peter

 

[Hero999]

Switching operation is still acceptable, if it gets too hot the charge will stop, then start again when it cools back down again (which shouldn't take too long given a small hysteresis band).

 

[PeterDove]

Switching operation is still acceptable, if it gets too hot the charge will stop, then start again when it cools back down again (which shouldn't take too long given a small hysteresis band).

OK, so you used that word 'hysteresis' too many times for me not to look it up - yes you are right - the battery wont cool back down instantly so a comparitor is making a lot more sense now. In fact maybe I can use two opamp comparitors to choose which level of charge I apply, full charge/trickle charge.Or even base it a combination of voltage levels in batteries plus thermal. Three levels of charge, where voltage < normal level, batteries are not hot, batteries are hot. So perhaps a boost charge of 300mA when voltage is less than the batteries standard, 150mA when batteries are at required level, and 50mA when they are starting to get hot.

Now I just have to find an opamp supplier in England that accepts Paypal!

Thanks again for your help.

Peter

 

[eblc1388]

What I wanted was something that would reduce the current in proportion to the voltage/temperature from the LM35DZ. I almost have something in the schematic except it increases current with voltage.

That's easy to correct.

If you refers to the datasheet of LM35DZ, you will see a load resistor connecting the LM35DZ to V+ as the usual application.

However, you can swap the places of the resistor and LM35DZ, with the resistor now going to 0V instead. Now if you monitor the voltage across the resistor, its voltage will decrease with increase of temperature.

There is a slight problem with this arrangement in practice as the voltage across the resistor is now very near V+ and could be outside of the common mode range of the opamp. To take care of this limitation, I would split the value of the single load resistor into 20% and 80% resistors and use the signal at the resistor junction for the opamp input. The larger value resistor is the bottom one going to 0V.

 

[Nigel Goodwin]

Switching operation is still acceptable, if it gets too hot the charge will stop, then start again when it cools back down again (which shouldn't take too long given a small hysteresis band).

You don't even need to think that hard - simply place a resistor across the transistor, so when the transistor turns off the resistor provides a trickle charge to the battery!.

 

[PeterDove]

You don't even need to think that hard - simply place a resistor across the transistor, so when the transistor turns off the resistor provides a trickle charge to the battery!.

I like it!

Cheers

Peter

 

[Turk2045]

Turk2045

HI All,

I have been attempting to make a temperature based controller of current flow. ie something starts getting hot we reduce current. I include a schematic below.

**broken link removed**

V1 is a variable voltage representing the output of the centigrade temperature sensor LM35DZ in which each degC is 10mV. Here at 750mV ( 75degC) the current is higher.

Below you can see the effect of reducing the voltage at V1

**broken link removed**

The problem is... I actually want the effect reversed.. I want a way to INCREASE the current as the voltage at V1 is decreased.. What sort of transistor/semiconductor will allow me to do that?

Thanks

Peter

Instead of using Thermistor try a ThermoStat in place of V1. U would then have control over the current.