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Basic Electronics » feeding a 7805 with 24VDC for <25mA output?

February 13, 2013
by 6ofhalfdozen
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Ok, I am hoping someone might be able to clarify my thoughts on this or just logic check my thinking. I looked around on here, but haven't found any answers close to what I need. My apologies if I just missed it in my search. Short version of question, if I feed a 7805 with 24VDC and am only sending out a small current (<25mA) on the 5VDC side, am I pushing the 7805 too hard and likely to overheat/burn it up??

Longer version of the question. I have a liquid pump on a vendor built system. The pump runs 24VDC main power and control signal of 0-5+VDC. I say 5+VDC, because it can handle up to 24VDC on the control input, but anything over 5VDC is full on. The pump spec sheet mentions that with control voltage of <6VDC, the control input impedance is 10KOhms and at >6VDC it is 30kOhms. The vendor put a 15 turn 10Kohm ceramet pot between the 24VDC and pump to control the pump speed. Anyhow, it is sitting in a bad spot inside of a very little box next. I have several 10Kohm potentiometers with long handles which would make it much easier to adjust, but they are single turn and not very precise over their range, especially when trying to split 24VDC into <5VDC. My thought was to feed a 7805 from the 24VDC, creating a 5VDC which is split through the potentiometer and into the pump with a much more controllable signal. The datasheets I have for 7805's have mention a 18VDC or 20VDC as their max input. I am not sure how that relates to pulling <25mA instead of the full 1A. Anyone ever try something like this or have any thoughts on how will the 7805 will behave??

February 13, 2013
by JimFrederickson
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The 78xx series of voltage regulators are "linear voltage regulators"...

While low cost, and readily available if you they are really designed for providing voltage regulation where the input voltage and the output voltage are relatively close. (Although the difference must ALWAYS be greater than the particular regulators "drop-out voltage".)

What is effectively happening is that the voltage regulator is "burning off as heat" the difference between 4v and 24v. So 19v is being used up in order to supply the 5v you really need.

Switching to a "switching voltage regulator" would be the best choice.

If you want to "make your own" then using a MC34063A with a few external components would work must better.

If you don't want to fool around too much, which I prefer NOT to do, you can go a little more expensive but MUCH MORE convenient route...

Murata Power Solutions - 7805SRH-C

Both are available from Mouser.com.
(I am sure other places have them as well.)

I have used the Murata parts, their efficiency is appreciated in situations where you are running on battery or have greatly disparate input and output voltages.

I haven't seen a "specific thread on exactly this", but I did start a thread awhile back where I was running LED's off of at 7805 and then later a 7803 from battery. (Power is amperage*voltage) There was virtually NO DIFFERENCE in run time until I switched to a switching regulator.

NOTE: In your case your 7805 is not being stressed by the "load you have", but is being stressed from "reducing your input voltage to the desired output voltage". If you do insist/continue to run it this way it will likely work okay, albeit wasting some power. I would DEFINITELY put a heat sink of some sort on it though if you still decide to use it. (Virtually ALL silicon based electronic parts work longer/better if you can keep excessive heat off of them.)

February 13, 2013
by 6ofhalfdozen
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thanks Jim! That was a great idea and something I hadn't thought of at all! I was trying to keep it simple ( <3 components), and that will definately fit the bill.

February 13, 2013
by BobaMosfet
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6ofhalfdozen-

<25mA on the LM7805 at 24V (which is okay for Vi) will generate 210mW or less on the LM7805 itself, so you have no worries. It might not even feel warm to the touch.

BM

February 15, 2013
by mongo
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Most 7805's will handle up to 30VDC but they will run warmer, having to sink off more voltage. I generally don't go above 12 or 15 Volts just for that reason. But, if you are limiting the current ahead of the circuit, it may work. It might also do some strange things like oscillating as the load changes.

February 16, 2013
by rajabalu21
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I suggest that you do not push 7805 beyond the maximum input voltage specified by the datasheet. After all you want to build a reliable solution and I think your question is not purely based on academic interest. You can consider a 5.1V Zener Diode along with a resistor to get 5.1V regulated output. Since your control circuitry's input maximum is 6V it should be in a position to work as desired. If not go for a combination of a Zener say 12V and then a use 7805 for final regulation. LM317's input voltage maximum is about 40V. You could consider using LM317 also for a simple reliable solution.

-Raja

February 16, 2013
by JimFrederickson
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rajabula21 - I am not sure what you are talking about regarding "push the 7805 beyond the maximum input voltage"...

NO 7805 part I have purchased in the last 10yrs or so has been rated below 30v maximum input.

Are the 7805's you are using rated below that?
What brand are they?

6ofhalfdozen - If you are going to use a separate "pre-power supply" it should be a complete power supply. (With the appropriate filter capacitors...)

For me though, when I need to regulate a voltage larger than 12 to 5v, or any ongoing 5v battery powered device, I am VERY HAPPY with the switching 7805 replacements that I had mentioned.

February 16, 2013
by BobaMosfet
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The datasheet for LM78XX clearly states that for output voltages of 5V to 18V (this determines which regulator to use), an input voltage of up to 35V is acceptable. For an output voltage of 24V, an input voltage of up to 40V is acceptable. I looked at 3 different manufacturer datasheets, and they all concur.

A word to the wise-- it isn't the input voltage or current that determines thermal dissipation in the regulator.

BM

February 16, 2013
by rajabalu21
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JimFrederickson - "am I pushing the 7805 too hard and likely to overheat/burn it up??" is the phrase used by 6ofhalfdozen. So that is where I got "push 7805" comment.

"The datasheets I have for 7805's have mention a 18VDC or 20VDC as their max input." is what 6ofhalfdozen states as part of the original question. If this statement is true, then, 6ofhalfdozen should not go beyond what the datasheet states.

You are talking about the device datasheets you have looked up. I am not even sure which device 6ofhalfdozen is referring to. We do not know the exact part number or the supplier name. So I am just going by what 6ofhalfdozen reads from the datasheet as the maximum input voltage.

Since you asked, take a look at this 7805's spec and pricing , the maximum supply voltage is 25V.

Here is another, the exact same part I am using. It is from TI but, I got it from sparkfun here is the datasheet and pricing. Look up Page #2, it states that the Absolute Maximum is 35V. Scroll down to Page #3, Recommended operating conditions for 5V regulator is 7 to 25V. So I will never select this component based on Absolute Maximum criteria, but, only based on suggested operating conditions. However, this is perfectly ok for 6ofhalfdozen's 24V scenario. But, he does not seem to have the part I have.

There you have it, you can now see that there are parts that have the maximum operating conditions less that 30V. I have also included the pricing links to show that these are not some obsolete fake devices. They are very much active and available and one can buy them if the want.

I never challenged your suggestions. Again, I only answered based on 6ofhalfdozen's statement that the datasheet of the part on hand states that the maximum input voltage is 18V to 20V range. I believe that there could be a device that has those specifications. I do not have it. That does not mean that no such parts exists. If the datasheet states that the maximum input voltage is > 24V, then, 6ofhalfdozen will not even ask this question in the first place.

The take home is, one should not design a circuit based on Absolute Maximum Ratings (AMR) but based on Recommended Operating Conditions (ROC).

-Raja

February 17, 2013
by JimFrederickson
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rajabalu21 - I did indeed miss/overlook 6ofhalfdozen statement of "The datasheets I have for 7805's have mention a 18VDC or 20VDC as their max input."

That does indeed explain where you came up with your statements.

February 28, 2013
by BobaMosfet
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rajabalu21--

6ofhalfdozen--

I think it's useful to understand that since the LM78XX is such a ubiquitous part, the datasheets are usually for a series of regulators. The XX tells you what voltage difference the regulator maintains between it's output terminals for any current up to its max rated current (on the referenced datasheet, that's 1.5A).

As such, using the datasheet that rajabalu21 referenced on page #2 (The page #2 reference is a generalized absolute maximum for all 5-18 (35V) or 24 (40V).), it's actually more useful to look on page #3, at the top under 'recommended operating conditions'. The recommended maximums for each regulator LMXX is shown, individually. Whatever datasheet you refer to for your specific LMXX, should have a similar table/reference.

BM

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