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Re: Voltage Limiter Mod. Question
[Re: 70mopes]
#257855
03/18/09 11:35 AM
03/18/09 11:35 AM
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Joined: Oct 2007
Posts: 3,884
Oregon
hooziewhatsit
master
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master
Joined: Oct 2007
Posts: 3,884
Oregon
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I don't know what your end goal is, but adding a resistor in the ground leg of the regulator will change the output voltage. There is usually a formula for each regulator that will tell you the new output voltage.
-Dave
If you ever find yourself in a fair fight, your tactics suck.
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Re: Voltage Limiter Mod. Question
[Re: 70mopes]
#257858
03/18/09 03:15 PM
03/18/09 03:15 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
The end result is to get a solid 5 volts on the output leg of the volt limiter. I was just curious as to why the resistor is part of this mod. The 5VDC regulator maintains a steady 5V at the output leg with 12 going into the input leg and the center leg grounded to the can. Whatever the purpose of the resistor being added to output leg of reg. and to ground I have yet to figure out. 
Can you post the circuit, or a link?
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Re: Voltage Limiter Mod. Question
[Re: 6pkaar]
#257859
03/18/09 03:16 PM
03/18/09 03:16 PM
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Joined: Oct 2007
Posts: 3,884
Oregon
hooziewhatsit
master
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master
Joined: Oct 2007
Posts: 3,884
Oregon
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to smooth out spikes in the input voltage you'd want a capacitor between ground and the input pin.
Is that article online somewhere I can look at it?
-Dave
If you ever find yourself in a fair fight, your tactics suck.
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Re: Voltage Limiter Mod. Question
[Re: 70mopes]
#257863
03/18/09 04:27 PM
03/18/09 04:27 PM
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Joined: Oct 2007
Posts: 3,884
Oregon
hooziewhatsit
master
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master
Joined: Oct 2007
Posts: 3,884
Oregon
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Ok, I found the article referenced. It looks to me like the 270ohm resistor is supposed to go from the output to ground. What that will do is supply a small load (18mA) to the regulator to ensure that it outputs 5v. With no (or little) load regulators aren't guaranteed to output their rated voltage, like you noticed. Ideally the regulator would have capacitors (10uF to 100uF, or bigger) on both the input and output. This also helps the regulator work better. http://www.moparts.org/Tech/Archive/elec/11.htmlFor capacitors, the long lead is positive. They should also be marked with a vertical strip to indicate the negative pin. edit: I would also highly recommend heatsinking the regulator, either to the can, or a seperate heatsink. With all three gauges pegged, they will be drawing just over the rated current, which will make it get HOT, and cause the output voltage to turn off or otherwise act weird while it cools. -Dave
Last edited by hooziewhatsit; 03/18/09 04:43 PM.
If you ever find yourself in a fair fight, your tactics suck.
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Re: Voltage Limiter Mod. Question
[Re: hooziewhatsit]
#257864
03/18/09 06:43 PM
03/18/09 06:43 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
Ok, I found the article referenced. It looks to me like the 270ohm resistor is supposed to go from the output to ground. What that will do is supply a small load (18mA) to the regulator to ensure that it outputs 5v. With no (or little) load regulators aren't guaranteed to output their rated voltage, like you noticed.
Ideally the regulator would have capacitors (10uF to 100uF, or bigger) on both the input and output. This also helps the regulator work better.
http://www.moparts.org/Tech/Archive/elec/11.html
For capacitors, the long lead is positive. They should also be marked with a vertical strip to indicate the negative pin.
edit: I would also highly recommend heatsinking the regulator, either to the can, or a seperate heatsink. With all three gauges pegged, they will be drawing just over the rated current, which will make it get HOT, and cause the output voltage to turn off or otherwise act weird while it cools.
-Dave
Hoozie, I can't read those "picchhers" very well but it APPEARS they are hooking that reistor from the negative lead to ground, and the TAB is already grounded, meaning the same place as the neg. lead!!!!! In other words that resistor is hooked to ground on both ends!!!
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Re: Voltage Limiter Mod. Question
#257865
03/18/09 07:11 PM
03/18/09 07:11 PM
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Joined: Oct 2007
Posts: 3,884
Oregon
hooziewhatsit
master
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master
Joined: Oct 2007
Posts: 3,884
Oregon
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Here's a schematic I drew up real quick.
I mis-read the original question and went down a rabbit trail. This schematic should clarify how you want to build the circuit. Let me know if you have any questions.
-Dave
Last edited by hooziewhatsit; 03/18/09 07:12 PM.
If you ever find yourself in a fair fight, your tactics suck.
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Re: Voltage Limiter Mod. Question
[Re: 70mopes]
#257866
03/18/09 10:55 PM
03/18/09 10:55 PM
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Joined: Jun 2008
Posts: 1,822
Colorado
denfireguy
top fuel
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top fuel
Joined: Jun 2008
Posts: 1,822
Colorado
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As has been pointed out in many other threads, the three terminal regulator does NOT have the capacity to power all the gauges. It is rated at 1Amp with a heavy enough heat sink to keep the device cool to 25C (about 77 degrees F). Your gauges can draw up to 3 Amps depending on whether they are all at full scale. When this regulator fails, it will either open up, a good thing. Or it will short 12 volts to your gauges, a very bad thing. There is a pass transistor inside the regulator that can develop a emitter to collector short causing just a little less than the input voltage to appear on the output, way more than the designers of the gauges anticipated. As far as I can tell, the output shunt resistor is not needed at all. There will always be a load present so the regulator will be within its 5ma quiescent current range at all times. Next draw back of the circuit is the three terminal regulator puts out a pure, solid 5V DC. The factory limiter put out pulsating voltage that averaged 5 V. This is not the same and the gauges will not read exactly right. You can see the same effect measuring AC voltage with a DC meter. Close but no cigar. Third drawback: The factory limiter put out a higher voltage on turn on to heat up the bi metal strips that move the gauges up and down. That is why when you turn the key on, the gauges all pin for a second and then settle down to where they belong. The three terminal regulator does not duplicate this and therefore on turn on, the accuracy of the indications on the gauges are not as exact until they "warm up" To get rid of the factory limiter (good idea) and have peace of mind about the safety of your 40 year old gauges, I would suggest the limiter that Greg on this board sells. You can find them here: http://rt-eng.com/mediawiki/index.php/Main_PageI know this is pretty technical and I will be glad to answer any questions. Craig
2014 Ram 1500 Laramie, 73 Cuda
Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango
Only previous Non-Mopar: Schwinn Tornado
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Re: Voltage Limiter Mod. Question
[Re: hooziewhatsit]
#257867
03/19/09 10:00 AM
03/19/09 10:00 AM
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Joined: Jun 2006
Posts: 821
NW Ohio
6pkaar
super stock
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super stock
Joined: Jun 2006
Posts: 821
NW Ohio
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Quote:
For capacitors, the long lead is positive. They should also be marked with a vertical strip to indicate the negative pin.
The capacitors I got from Radio Shack have one lead going in each side, not two in one end. The leads are the same length. Also no verticle lines, but there is a > marking pointing to one end, does this indicate positive or negative?
Also, if there is a question of having enough output power, would ot work to "double up" using two 7805 regulators wired parallel?
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Re: Voltage Limiter Mod. Question
[Re: hooziewhatsit]
#257870
03/19/09 12:12 PM
03/19/09 12:12 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
Quote:
Quote:
For capacitors, the long lead is positive. They should also be marked with a vertical strip to indicate the negative pin.
The capacitors I got from Radio Shack have one lead going in each side, not two in one end. The leads are the same length. Also no verticle lines, but there is a > marking pointing to one end, does this indicate positive or negative?
Ahh, for that kind of capacitor the 'dimpled' end is the positive end. The arrow points to the negative lead.
Arrow==neg
http://saint.419.removed.us/pics/clapcap4.jpg
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Re: Voltage Limiter Mod. Question
[Re: groundpoint6]
#257874
03/20/09 12:58 AM
03/20/09 12:58 AM
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Joined: Jan 2005
Posts: 1,647
IL
71383beep
top fuel
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top fuel
Joined: Jan 2005
Posts: 1,647
IL
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I just put in the rt engineering voltage regulator and so far it works great. It's 50 bucks but it is no hassle and it seems to do the job quite well.
'73 GK6 Challenger Rallye - 340 4-Speed
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Re: Voltage Limiter Mod. Question
[Re: dgc333]
#257876
03/20/09 11:37 AM
03/20/09 11:37 AM
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Joined: Jun 2008
Posts: 1,822
Colorado
denfireguy
top fuel
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top fuel
Joined: Jun 2008
Posts: 1,822
Colorado
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You are right, it is rated at 125C junction temperature. The old sheet I used specified 25C for case temperature for rated output (National Semiconductor). I looked it up on the web and now only Fairchild has a spec sheet and has no rating for case temperature but does say 125C for junction temperature, both on TO-220 cases.
TO-3 cases (no longer being made) were good for 3A with heavy heat sink.
The radio capacitor in the dash would be adequate for keeping noise on the radio down. The output capacitor in the application notes was also for suppressing transient pulses that could damage the pass transistor in the regulator. Using the small capacitor in parallel with the radio capacitor would be highly recommended.
Craig
2014 Ram 1500 Laramie, 73 Cuda
Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango
Only previous Non-Mopar: Schwinn Tornado
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Re: Voltage Limiter Mod. Question
[Re: groundpoint6]
#257877
03/20/09 11:56 AM
03/20/09 11:56 AM
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Joined: Jun 2008
Posts: 1,822
Colorado
denfireguy
top fuel
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top fuel
Joined: Jun 2008
Posts: 1,822
Colorado
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Quote:
I`m confused, why do you not just use a new voltage limiter
Yes, they are still available but the technology is 1950's era. It uses a mechanical system to limit the voltage. It always bothered me that mother Mopar was ahead of the curve on things electronic like voltage regulators and alternators and yet these dinosaurs lurked in the dashboards of 1970s cars. The limiters were very reliable and very simple but the consequence of failure were three burned up gauges if the bi-metal strip stuck on.
If you want my old limiter, you are welcome to it. It still works well. I replaced it with the RT Engineering regulator.
Craig
2014 Ram 1500 Laramie, 73 Cuda
Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango
Only previous Non-Mopar: Schwinn Tornado
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I have also found that it is hard to get the true 5V out of the regulator with the resistor hooked up. I get around 4.7 volts.
