LED as reverse polarity protection and making a 15k pot

W9GFO · 2009-09-03 16:55

Two questions;

First, I am using an LED between VIN and a 555 circuit to indicate power and to provide reverse polarity protection. I am not using a current limiting resistor because the associated components are limiting the current anyway. Is there anything wrong with this approach?

Second, I really need a 15k pot but can only get 20k or 10k pots. Will a 100k trim pot (50k at center) in parallel with a 20k pot work to make a 15k pot with adjustable center? I think it will but I'd like some conformation.

Thanks,

Rich H

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HollyMinkowski · 2009-09-03 17:09

I keep thinking about it but I'm just not certain.
Maybe you could just hook them together and then test to see
what you get? That seems like the easy way to find out.

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W9GFO · 2009-09-03 17:15

Well the LED thing seems to be working just fine, just curious if it is "correct" to do it that way.

As for the pots, I tested two different setups, not with the values I need. On one, it worked as I expected but on the other it didn't. The one where it didn't work the pots were still in the circuit so my results may have been invalid due to the involvement of other components. I think it will work, but I've been wrong before. No, really!

Rich H

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Mike Green · 2009-09-03 17:23

LEDs make very poor reverse polarity protectors. Read any LED datasheet. I did a quick websearch and found a typical red high-intensity LED whose datasheet shows a reverse voltage of 5. Use a conventional diode with a rated reverse voltage greater than what you expect to protect against.

The question of how to make a pot with a particular resistance range given that you only have other values depends on what you need. What's the actual resistance range needed and what do you have to make it with? You need to learn the parallel resistance formula (Rt = 1/(1/R1 + 1/R2)) and maybe create a spreadsheet to show you the total resistance value for different pot positions for different values of the pot and any parallel resistor. You may find that it doesn't work quite the way you think.

Chris Savage · 2009-09-03 17:27

To add to what Mike said, in systems where an LED could be subjected to reverse voltage, such as an AC input, the LED has its own protection diode in reverse so that diode is biased during reverse voltage. Take care.

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Chris Savage

Parallax Engineering
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Mike Green · 2009-09-03 17:40

Here's one example of such a spreadsheet printed out. The forum software won't allow uploading of spreadsheets.

NetHog · 2009-09-03 17:48

Regards the Pot - you'll probably have more success making the circuit work with the different pot - why is 15K required and what is the circuit?

W9GFO · 2009-09-03 17:58

Thanks, that pdf shows just what I want for ranges, with a 100K trim pot I'll be able to tune it for just the range I need.

I looked at an LED data sheet, what I don't understand is if the polarity is reversed - say 6 volts - does that mean that 1 volt gets through or all 6?

I missed it before, there is one pin of the 555 going to ground so that means that the 555 is limiting the current to the LED but by how much? I don't see where it says that in the data sheet. The low brightness of the LED indicates that there is sufficient current limiting going on in the 555.

Rich H

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W9GFO · 2009-09-03 18:12

NetHog said...
Regards the Pot - you'll probably have more success making the circuit work with the different pot - why is 15K required and what is the circuit?

Yes, the plan is to use a 20k pot with a 100k trimpot in parallel. The pot is used to adjust a pulse width. Currently a 10k pot does not give me enough adjustment. A 20k pot would give plenty of range but the center detent of the pot would not match the center of the range that I want - the low end will remain the same no matter what value of pot is used. The 100k trimpot will allow me to tune it so that the center of the range I want will match the center detent of the 20k pot.

Hope that made sense.

Rich H

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The Servo Boss, a 12 channel servo tester kit from Gadget Gangster.

Mike Green · 2009-09-03 18:30

The Wikipedia is your friend! Do a websearch for "wiki reverse voltage". A diode will only leak a little bit of current until the reverse voltage is exceeded, then it will "break down" or "avalanche" and the current will increase markedly, sometimes with destruction of the diode. In your case, LEDs leak quite a bit (100uA or so) near the peak reverse voltage. Beyond that, both the LED and 555 are likely to be destroyed.

kwinn · 2009-09-04 00:29

Re:

led for reverse voltage protection - as everyone has said it is not a good idea.

putting a trim pot in parallel with 20K pot to get 15K will allow you to adjust for 15K between the 2 ends of the pot but will also affect linearity. This may or may not be a problem, it depends on what the circuit is to be used for.

Bob Lawrence (VE1RLL) · 2009-09-04 04:21

re:15k pot

How about trying 1 variable resistor with 1 fixed resistor to get the range you want.

W9GFO · 2009-09-04 04:40

That would work fine except that I wouldn't be able to make fine adjustments in order to bring the center of the range to coincide with the center detent of the pot.

There needs to be a trim pot. There was a 5k trim pot in series with the wiper of the 10k pot, but now, since I need a larger range I'll just use a 100k trim pot across the 20k pot. Adjustments to the trim pot will expand or contract the range but that's ok. Also, linearity is not an issue.

Rich H

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Carl Hayes · 2009-09-04 14:58

I'd just put a 60KΩ fixed resistor in parallel with the pot.· 60KΩ in parallel with 20KΩ gives you 15KΩ.· It also affects the linearity, which you say you don't care about.

If you are using the pot as a rheostat (variable resistance) rather than as a potentiometer (resistor with a variable tap), and you need the center to be precisely 7500Ω at the detent, use a 30KΩ resistor instead of 60KΩ.· That will make the whole thing into a 12KΩ rheostat instead of 15KΩ, but it will put the center at 7500Ω, right where you want it.

Added note to first paragraph:· it affects the linearity only if you're using the pot as a rheostat.·

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Post Edited (Carl Hayes) : 9/4/2009 4:50:34 PM GMT

LED as reverse polarity protection and making a 15k pot

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