Using LM317T as a heating element

Thread Starter

weishan13

Joined May 12, 2020
12
Hi all,
I am tasked to design a heating circuit using LM317T that will be able to heat and stabilize at 37 degree.
I will require at least 6 LM317T.
So the connection that I had is as below:
Vin = 5.47V
Vadj = 5.05V
1ohm 1W resistor is connected from Vadj to Vout.
I tried connecting up all in series, my output temperature keeps rising and there is no stop to this.
How can I go about this?
 

Audioguru again

Joined Oct 21, 2019
1,743
You did not attach your schematic so I am just guessing how you connected it. I have no idea why or how you connected LM317 ICs in series.
Please attach your schematic, here is my guess:
 

Attachments

jpanhalt

Joined Jan 18, 2008
10,078
You need a way to sense temperature. At the required temperature, you need to stop heating and/or start a fan. We made make-shift incubators with a light bulb in one box (heat box) connected to another box (incubator box). As the temp reached the desired value in the incubator box, an exhaust fan started in the heat box . That was usually enough to give a pretty steady temperature.

BTW, 37°C is not the best temperature for growing most bacteria significant in human disease. Those organisms and viruses for which it is appropriate shouldn't be messed with outside a sufficiently safe laboratory. Ours was BSL3. For ordinary mesophilic bacteria, 35°C is preferred.
 

MrAl

Joined Jun 17, 2014
7,755
Hi all,
I am tasked to design a heating circuit using LM317T that will be able to heat and stabilize at 37 degree.
I will require at least 6 LM317T.
So the connection that I had is as below:
Vin = 5.47V
Vadj = 5.05V
1ohm 1W resistor is connected from Vadj to Vout.
I tried connecting up all in series, my output temperature keeps rising and there is no stop to this.
How can I go about this?
Do you have a constant ambient or can it vary?
 

Thread Starter

weishan13

Joined May 12, 2020
12
You need a way to sense temperature. At the required temperature, you need to stop heating and/or start a fan. We made make-shift incubators with a light bulb in one box (heat box) connected to another box (incubator box). As the temp reached the desired value in the incubator box, an exhaust fan started in the heat box . That was usually enough to give a pretty steady temperature.

BTW, 37°C is not the best temperature for growing most bacteria significant in human disease. Those organisms and viruses for which it is appropriate shouldn't be messed with outside a sufficiently safe laboratory. Ours was BSL3. For ordinary mesophilic bacteria, 35°C is preferred.
Thanks for sharing, I am using another transistor to sense the block temperature. But whenever I connect 2 or more LM317T, the temperature keeps fluctuating and it will not stabilizen
 

AlbertHall

Joined Jun 4, 2014
10,227
I still don't understand what you are doing with the LM317s nor how this temperature sensing transistor is connected.
Please show a schematic of your circuit otherwise we are just guessing here.
 

John P

Joined Oct 14, 2008
1,818
Something is wrong here. You say:
Vin = 5.47V
Vadj = 5.05V

But an LM317 will have a minimum of 1.25V from Vin to Vout, and another 1.25V from Vout to Vadj. I don't understand how your numbers can be right. How does Vadj get to be that close to Vin?

Is it your objective to drive the LM317's to the point where their internal power-limit feature is activated, or is current flow limited some other way?
 

Thread Starter

weishan13

Joined May 12, 2020
12
This is the layout of my connection. There is an external Temperature sensor LM35.
When the temperature reaches above 37 degree celsius, the voltage drop across the resistor becomes 0.
Is there a way for me to connect all LM317?
But if connected to only 1x LM317T, the circuit will work perfectly fine.
layout.PNG
 

atferrari

Joined Jan 6, 2004
4,069
The heating element, should be outside the circuit, that is, the Load as shown in post #2 by AG.

How do you control the LM317? Getting them all in parallel should be the last part to solve.

The more I look at this the more I feel you should show a block diagram and whatever full schematic of what you have implemented up to now.
 
Last edited:

andrewmm

Joined Feb 25, 2011
527
taking step back,

why do yo say "I will require at least 6 LM317T "

Its very normal to increase the current capacity of the LM317 with an external transistor.

https://makingcircuits.com/blog/increasing-lm317-current-with-a-transistor/

Lets call the 317 with a transistor the Vreg.

Second up ,
is it the temperature of the Vreg you want to control ? Where is the current the vreg is passing going to ?

As a system, if you have a Vrge like this and a resistor for Vreg to drive, the total power dissipated for a constant input V is going to be a constant, adjusting the Vreg is only moving the dissipation between the load and the Vreg ,


The normal way of looking at this sort of problem is that the Vreg is controlling a "load / heater" , its the temperature of the heater you want to measure and control.

Also remember , that if the load is insulated, then when you stop heating it , it will not cool down,
so you end up having to put a heat sink on the load !
 

jpanhalt

Joined Jan 18, 2008
10,078
Ar first you said:
Hi all,
I am tasked to design a heating circuit using LM317T that will be able to heat and stabilize at 37 degree.
I will require at least 6 LM317T.
So the connection that I had is as below:
Vin = 5.47V
Vadj = 5.05V
1ohm 1W resistor is connected from Vadj to Vout.
I tried connecting up all in series, my output temperature keeps rising and there is no stop to this.
How can I go about this?
You have never said where the negative ("ground") from the DC supply is attached. Nothing should happen without that, as Audioguru again said. YOu do not show a heating element. That made it appear that the LM317 was being used as a heating element.

Then we got into a discussion about how you were regulating the temperature. At that point, you said:
This is the layout of my connection. There is an external Temperature sensor LM35.
When the temperature reaches above 37 degree celsius, the voltage drop across the resistor becomes 0.
Is there a way for me to connect all LM317?
But if connected to only 1x LM317T, the circuit will work perfectly fine.
Which resistor? The ones shown in your diagram or the one used for the LM35? Where in the datasheet is that shown? Here is the datasheet: https://www.ti.com/lit/ds/symlink/lm35.pdf?&ts=1589781663381

And now you report:
Ok. Thank you everyone for the input, i had solved my issue, i changed the resistor to a higher wattage.
OK, so are we to assume this circuit (from Audioguru again) gives a stable temperature at 37°C so long as an LM35 is sitting somewhere in the vicinity and you use a resistor rated for more the 1 W?

1589796786017.png

NONE of that makes sense. Since a lot of us have spent time trying to help you, it would be polite for you to at least show us your COMPLETE working circuit, including ground connections, any heating element not mentioned by you, , how many LM317's are used, and the LM35 connections.
 

Attachments

Thread Starter

weishan13

Joined May 12, 2020
12
Ar first you said:


You have never said where the negative ("ground") from the DC supply is attached. Nothing should happen without that, as Audioguru again said. YOu do not show a heating element. That made it appear that the LM317 was being used as a heating element.

Then we got into a discussion about how you were regulating the temperature. At that point, you said:


Which resistor? The ones shown in your diagram or the one used for the LM35? Where in the datasheet is that shown? Here is the datasheet: https://www.ti.com/lit/ds/symlink/lm35.pdf?&ts=1589781663381

And now you report:


OK, so are we to assume this circuit (from Audioguru again) gives a stable temperature at 37°C so long as an LM35 is sitting somewhere in the vicinity and you use a resistor rated for more the 1 W?

View attachment 207470

NONE of that makes sense. Since a lot of us have spent time trying to help you, it would be polite for you to at least show us your COMPLETE working circuit, including ground connections, any heating element not mentioned by you, , how many LM317's are used, and the LM35 connections.
I tried including the capacitor as shown in the diagram but it doesnt work. So I changed the resistor to a higher ratings Wattage.
The 5.47V which is the Vin comes from a control box which was from a manufacturer which has a temperature feedback circuit built on it. There is a NDP6060 Mosfet transistor used to supply the 5.05V to connect to the LM317 Vadj. Thus the 5.47V and 5.05V will be supplied to all 6 LM317 to connecting all Vin together and Vadj together.
Instead of using 1ohm 1 watt resistor which cannot hold the power, I increased to a 1 ohm 10watt resistor which able to contain the power.

I am not very good with electronics. So I hope the explanation helps.
 

AlbertHall

Joined Jun 4, 2014
10,227
So the 5.47V changes in response to temperature and is controlled by a NDP6060 MOSFET which is rated for 48A.
I don't see what the LM371s do to help. Can you not just the heater/resistors dorectly to the controller?
 
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