Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Unknown Voltage drop on a simple circuit with LM317T

Status
Not open for further replies.
can i not use a power resistor of about 50w, i could go out and buy a 1.5v battery to do this seperately or even a starter but i wanted to try a few things, learn a few things aswell, if its impossible ill admit defeat but i would of thaught there was a way somehow :(

You 'could' use a 50W resistor - although such a resistor would require mounting on a VERY substantial heatsink, and run hot enough to burn you (or make toast).

This of course assumes you're not running it off 4xAA's - which will be unable to supply a fraction of the required power.
 
Cheers for the persistance guys it must be a killer to have this conversation,

i was going to look at getting better batteries to run the complete circuit but its more hassel then its worth i think.

but on a positive note i set up my first regulator and it worked and this opened my eyes to whats available on the market interms of power 1000W resistors, wow, and i learnt abit about batteries.

All is not lost :D
 
Cheers for the persistance guys it must be a killer to have this conversation,

i was going to look at getting better batteries to run the complete circuit but its more hassel then its worth i think.

but on a positive note i set up my first regulator and it worked and this opened my eyes to whats available on the market interms of power 1000W resistors, wow, and i learnt abit about batteries.

All is not lost :D

I just googled 'glowplug battery', and there are plenty of low cost options - a single Alkaline C cell for example - or for £7.76 (inc. postage) a unit that works off 6V-24V (presumably a switchmode regulator).
 
yer no worries, cheers for the search. i was just hoping to get a working engine with my own electrics on it but hey ho ill just buy myself one for xmas haha

Cheers again guys
 
ive been thinking,

would it be possible to use a capacitor in any way, have a high charge cap charge it from the 6v batteries then discharge via the glow plug.

this is just an idea and i may be completely off, but i believe caps can release current very quickly so in theory 10Amps wouldnt be so much of a problem..........would it?

cheers
 
would it be possible to use a capacitor in any way, have a high charge cap charge it from the 6v batteries then discharge via the glow plug.
The quick answer is yes.
But once you had bought sufficient capacitors to do this task, you could have bought a whole new aircraft, radio control, engine, and the correct battery for the glow plug and still have sufficient money left for a bacon sandwich before you go out to fly the thing.

JimB
 
ive been thinking,

would it be possible to use a capacitor in any way, have a high charge cap charge it from the 6v batteries then discharge via the glow plug.

this is just an idea and i may be completely off, but i believe caps can release current very quickly so in theory 10Amps wouldnt be so much of a problem..........would it?

cheers

A HUGE capacitor could store enough energy, but you still require a high enough power source to charge it - you also have the problem that capacitors discharge exponentially.
 
haha thats a very informative answer Jim :D i didnt think of the expo discharge no worries looks like im getting a bacon sandwich :D
 
Just use a separate 1.5V battery that can deliver the current (such as an alkaline D cell) for the glow plug. Requires just one added part (no more than adding a resistor).
Would that be a problem?
 
There seem to be two problems here: 1) Appropriate current for a model glow plug; and 2) Getting the voltage regulator to work as you expect.

For the glow plug, you need to limit the current to 1.5 to 2 A, say for O.S. plugs. The voltage in a sense is irrelevant. As pointed out, the internal resistance of your source will limit the current. You can use a resistor to limit the current, just as you do with LED's, a purpose-made glow plug driver, or use a lower-voltage battery. It is extremely common to use a NiCd (1.2 V nominal) to power a glow plug. Batteries with other chemistries can also be used, but be sure they are not damaged by that supplying that level of current.

As for the regulator problem, this thread by MrAI might be of interest: https://www.electro-tech-online.com/threads/challenge-2-find-the-circuit-flaw.142632/#post-1197401

The relevant citation can be found in the TI datasheet:

Source: https://www.ti.com/lit/ds/symlink/lm117.pdf
Load Regulation
The LM117 is capable of providing extremely good load regulation but a few precautions are needed to obtain maximum performance. The current set resistor connected between the adjustment terminal and the output terminal (usually 240Ω) should be tied directly to the output (case) of the regulator rather than near the load. This eliminates line drops from appearing effectively in series with the reference and degrading regulation. For example, a 15V regulator with 0.05Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05Ω × IL. If the set resistor is connected near the load the effective line resistance will be 0.05Ω (1 + R2/R1) or in this case, 11.5 times worse.

Sorry it took so long to find.
John
 
Last edited:
55 years ago I used a #6 dry cell for the glow plugs on my model airplanes. The #6 dry cell battery was carbon-zinc, was huge and weighed a lot.
How can you measure such a low resistance on a multimeter? The leads on my multimeter measure 0.3 ohms when they are directly connected together.

The more expensive LM117 uses a 240 ohm resistor between the adjustment terminal and the output terminal but the cheaper LM317 needs 120 ohms to prevent the output voltage from rising without a load.

This thread scares me. It might use the 6V to power a timer or radio receiver and the glow plug to make some chemicals go BOOM!
 
hello,

i need to start looking at this when im at home, the resistance muct be 15 ohms thinking about it, im honestly not sure, i break up today :D

i will get home and check all this and try to let you all know

cheers guys
 
If you want to stay with the 6V power source, then the better way to do it is by pulse width modulation. Ideally a buck converter with an inductor but, since the load is fundamentally a resistive heater, it can be just a simple 555 driving a mosfet.

If your input is 6V and you want an effective 1.5V, you could just run the 555 at 25%. However, since the 'heat' in the load is a function of the square of the current, 25% will be way too high. Probably something around 5-10%.

The current pulses drawn from the 6V source will be 4 times what they would be at 1.5V, but of low duty cycle. You may need a good low ESR cap in parallel with the voltage source.

The big advantage of this method is that nearly all of the input power ends up in the load, instead of only 25%.
 
Why not use a ni-cad as John suggested? Or several in parallel? The glow plug only needs power for a few seconds in normal use, so the battery could power it for that time, and be re-charged at a moderate current from your 6v supply for the rest of the time. With a 6v supply you would only need a resistor in series with it to get adequate current regulation - it's what the makers of cheap battery chargers used to do, apparently. If you used NiMh cells you wouldn't even have a significant problem with cell memory.
 
You are forgetting that your battery ESR must be at least load/10 to maintain voltage.

For in other words be capable of proving the current you need.

I suggest several D cells or a LiPo battery. with a current limit R= of 0.1Ω to divide voltage in half from 3.6V

Fuggetabout the LM317
 
I dont think the op will be back then.
Welcome anyway Cengiz.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top