truck battery to 12vdc regulated supply

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positronicle
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Re: truck battery to 12vdc regulated supply

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Chris Smith
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Re: truck battery to 12vdc regulated supply

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I would use “any” regulator rated 50% bigger than your actual requirments. <p>Buck boost is fine for the bench supply where power is an issue, but because your battery and alternator are not at issue, use either the NTE970 at three amps or the LH1605 [c] 5 amps or the LM 138, 238 338 at five amps,... adjustable voltage with a 7 amp peak.<p>Also use big heat sinks and a battery of at least two amp-hour rating. This will give you a unwavering voltage for almost one quarter hour independent of the main battery. Use a five amp diode to isolate the two batteries on the pos to pos leads. Tap off your regulator from battery #2+ le3d and into the regulator for a smooth 12volts uninterrupted. . <p>If you go with a cap, you will need almost a half farad [500,000] if you want to run at two amps, actual draw. Even this value will drop the voltage after just a few seconds.<p>[ September 14, 2005: Message edited by: Chris Smith ]</p>
Robert Reed
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Re: truck battery to 12vdc regulated supply

Post by Robert Reed »

Forget the posted shematics.Even with the 120 ohm zener-base resistor, you would have to handpick a 3055 transistor (dozens) for the Hfe spec to source 2 + amperes. If you could even come up with one at all. I don't see how that circuit could work. Example- 3055 at 2 amperes Ic and a an Hfe of 30. base current required=2/30 or 66 ma. 120 ohms (the value chosen in place of the 1K) would only allow 8 ma of drive current (13.7v -12.7v =1V, 1V/120 ohm=0.008) far short of what you need. Adding follwers or darlinngtons to boost drive current will reduce available voltage by virtue of their added B-E drop.
positronicle
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Re: truck battery to 12vdc regulated supply

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Colinr
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Re: truck battery to 12vdc regulated supply

Post by Colinr »

look at useing a DC-DC converter expencive but will save the lcd screen one example is the <p>TEN 30-2412WI
9V to 40 volt input 12V out at 2.5A<p>Colin
Robert Reed
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Re: truck battery to 12vdc regulated supply

Post by Robert Reed »

Unless I am missing something here, your 970 or similar type regulator wont work here, as you need 2-3 volt margin minimum (Reg. input must be higher than out put by that margin). Trying to drive a power transistor with it even worsens that problem. The spec sheet on this device is breif and a little vague on some of the device parameters.
positronicle
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Re: truck battery to 12vdc regulated supply

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positronicle
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Re: truck battery to 12vdc regulated supply

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Chris Smith
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Re: truck battery to 12vdc regulated supply

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These regulators have a 1.2 volt drop. <p>The lead acid battery runs at 14.7 [max] when running, and a average of 13.2 fully charged. <p>Minus 1.2v leaves you a 12 volt charge, in *ideal conditions*.
Robert Reed
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Re: truck battery to 12vdc regulated supply

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Chris
I think NTE is an "after market" company like Phillips ECG used to be. Copycat devices at atroscios prices. This regulator looks suspiciosly like Nationals and others. They have higher minimum differential reqirements. Where did you get your 1.2V spec?
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Chris Smith
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Re: truck battery to 12vdc regulated supply

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Robert<p>Im not sure about the NTE series, that’s why I prefer the National LM series which goes down to 1.2 volts.<p>Most regulators that are knock off [that I have dealt with in the past] also do this, but I only have the zillion page National Linear Data book handy, and the NTE one under a pile of dust. <p>The NTE "Quick Search Cross Reference Manual" in my computer shows the NTE970 as a 1.2 to 33 volt, ... meaning that this is usually also the DROP OFF voltage difference.
dyarker
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Re: truck battery to 12vdc regulated supply

Post by dyarker »

A small battery followed by a low dropout regulator.<p>Digikey has LD1085V12, a 3A 12V regulator, under $2 in quantity one. A more thorough search at Digikey may turn up one with an even lower dropout.
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Re: truck battery to 12vdc regulated supply

Post by rshayes »

If the current drain is 2 amps, the voltage on a 1 farad capacitor would fall at a rate of 2 volts/second. The 5 percent tolerance on the supply would be crossed in about .6 seconds. Several farads of capacity would be necessary to hold the voltage up for even a few seconds.<p>The simple solution might be to simply leave the display turned off until the engine is running.<p>Alternately, a DC/DC converter could be used to provide regulated power under all engine conditions. It would also provide isolation from the vehicle power system and its ground.
Robert Reed
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Re: truck battery to 12vdc regulated supply

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Chris
The 1.2V you refer to is the chips internal reference voltage. This limits the lowest output attainable (cannot output lower than reference). These chips are of similar design to the 78xx & 79xx familys of fixed voltage regulators, and as a matter of fact, they can be wired adjustable with a few passives. The minimum in/out differential voltage is 2.5 volts, give or take for load, junction temp,etc. I had this in my mind, but had to dig out the Data books to confifm(is that legal ? Forgive me if I was cheating ;) So rearmed with this info, it looks like this chip won't work for the situation at hand. I like the battery-diode(s) or lo dropout regs. as has been mentioned on this post.
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Chris Smith
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Re: truck battery to 12vdc regulated supply

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Robert
All the ones I worked with were a 1.2 volt difference.<p>If your regulator was a 1.2 to 35 volts range you had to add in a min of 36.2 to attain that voltage range. <p>1.2 v is the cut off where the regulator works in its envelope to control.<p>All the VRs that I looked up say the drop out voltage [stops working] is at 1.2 volts in, and 2 volts for the wider range regulators like the LM350. <p>The reference voltage is centered around 1.25 volts [+/- .2v], and the feed back starts to regulate at or above this.<p> The simplistic LM78 series doesn’t work this way and in some cases require 7 volts or high before they starts to regulate across the board. <p>I remember ramping up some of the better regulators and they started as low as 1.2 to 1.5 volts, before they warmed up and started passing current.<p>[ September 16, 2005: Message edited by: Chris Smith ]</p>
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