LM7824CT replacement

It makes sense to get a larger unit if it's not going to put you in the poor house.

Jamie

The 1.5A part may have a somewhat beefier onboard series pass transistor than the 1.0A part. However a big difference in the two parts is the point that internal current limiting kicks in.

On the other hand keep in mind that the primary driver on how much current that a particular 7824 regulator installation can handle is limited by the power being dissipated in the device. Input to output voltage differential and type/amount of cooling will be major factors to consider.

We used to use that LM78xx family in the 80's, a lot of them. The 05, 08,

I don't remember a 1.5A version in the TO220 case, but technology marches on as they say.

One hint, they aren't rocket science. It's a cheap regulator that does the job simply, but I'm unsure if better ratings would give better results.

The main problem we found with those is the source voltage, I think they are all rated with Vcc max at 36v, even the 5v ones. There is a min of something like 3v over the regulation voltage (7805 needs min 8v in, the 7812, 15v and so forth). The closer you can get to that, the better the life span.

The heat sink is also important, it needs one. I think the data sheets from National said something about 100ma-250ma "free air", and the full 1A draw with some minimum postage-stamp sink, but don't beleive it.

They are/were supposed to be over-current and short circuit proof, but they do pop like firecrackers when they let loose.

One handy trick with them, if you lift the ground pin and insert a pot or some diodes, you can make it regulate at different voltages, keeping all the ripple rejection and other specs within specs. So a 7805 with a 30v source, a few caps and like a 500 ohm pot makes a quick and dirty 5v-24v variable regulated power supply, 1A max as usual.

I don't know what you found with the 1.5A rating but it would just be cost considerations, if there is a 10 cent difference, fine, if it's $10, forget it. This is a 30 year old comment that probably doesn't apply but we did find differences depending who made them. Both the National and Motorola versions were fine, the ones by SGS stunk. There was another by Fairchild but I think they were SGS's with a re-ink job on them.

-bruce snipped-for-privacy@ripco.com

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That's very interesting about the 500 ohm pot. I've never heard of doing that before. I would have thought that the regulated voltage would have been dictated by some internal zener diode or something. But in any case I have a bag of 7805's and I'm going to experiment with them. As far as the capacitors go are you talking about using any additional from the usual input and output caps? Lenny.

Here is the datasheet on that family of regulators. the first page shows 'Adjustable Output Regulator' and that you need a fixed resistor from output to common, along with the pot from common to ground.

Texas Instruments UA7824CKC is 1.5A

Mouser has several 1.5A types in stock:

starting at 47 cents each, which is only seven cents more than the 1A types. They are $4.56 for ten or $43.70 per hundred. this is for the

The description column list the output of each part.

Yeah but I think that is specific to the LM340, on the 78xx series you just need a resistor in the path to ground.

This is more what I was talking about:

It's a bit messy with the 741 in circuit (fig 13), but allows you to use higher value pots. The single resistor method works but may require a multi-turn pot to dial it in with precision or a need for odd values to hit the target.

Just play around with it, use a 7805 and Vcc around 24-28v for the best range and just experiment with different values to ground. Just tie the wiper to one of the ends, one end to ground and the other to pin 2.

You need that cap on the output to ground to snuff oscillation and something on the input for ripple but thats about it. That 78xx family is forgiving but if it's too hot to touch, you are doing something wrong.

-bruce snipped-for-privacy@ripco.com

I did it over 30 years ago for a high current bench power supply to service business radios. It provided 5.5 V to 16 V at up to 90 amps, and was used once to test a car starter I'd rebuilt for one of my service trucks.

If you read the datasheet, LM340 is the National Semiconductor name for the 78** family. It is followed by a dash & the output voltage.

The reason for the second resistor is to make the output voltage more stable. The current consumed by the regulator varies with temperature, so the lower value fixed resistor across the output to the common terminal makes it a lot stiffer.

WTH would you spend money on a precision variable resistor just to let it drift, as the die temperature changes with the output current? A pair of resistors is a better choice for a fixed output, since no one can screw with it and the output won't go high if the rheostat opens.

It's still cheaper to buy the common values 10 or more at a time and use the right part. You waste time & parts to kluge the wrong part into the circuit, and both cost you money. In some types of repairs, you are not allowed to kluge in the wrong part without a huge pipe of paperwork, and I did a lot of that kind of work, from '72 until I was sidelined by my health a few years ago.

I keep 5, 8,9,12,15 & 24 positive regulators and 5,12,15 V negative regulators in stock. Most TO-220 are under 25 cents each, in single quantities. I still have some TO-3 metal cans in stock, along with some long obsolete Motorola ten pin regulators that were common in early external 300 baud modems.

There was an early EPROM programmer project in BYTE magazine that used separate transistors to ground different resistors to set the programming voltage on various pins, since there were so many combinations used in the '70s.

Maybe not. The temperature where most folks say "too hot to touch" (too hot to keep your finger on) is around 130 F; the maximum junction temperature rating for those regulators is around 300 F.

Isaac

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Thanks for all the replies guys. While I can certainly appreciate not having to screw around klugeing a circuit it is interesting to know that I can play with this basic design as it was suggested if I should ever need to. And many times over the years I've had to do things like this to get the job done. When you're 75 miles from home on a Friday afternoon and "Miller time" is fast approaching it's nice to be able to make something work that otherwise might require another service call on Monday. Not to mention the added inconvenience this saves the customer. BTW, I was always taught that good design practice dictates that if a semiconductor is too hot to keep your finger on, (greater than around 55.0 degrees C), then it's running too hot. However I remember some military aircraft power supplies I worked on in the 70's that ran a lot hotter then that on a regular basis. It always seemed like poor practice to me but I never heard of an F15 going down because of one, so I guess it was OK. Lenny