I need a programmable-voltage linear regulator with a fixed but well-defined current limit. This would power an output switch that would drive a customer's 50 ohm load (which he might short) and if I output a max of 6 volts, I'd need about 120 mA.
LTC has some parts, but the prices are insane.
This is crude but cheap:
Any other ideas?
John Larkin Highland Technology, Inc
picosecond timing precision measurement
There are a lot of high-side switches around that have current limits, some programmable. A programmable LDO (switch FB resistors, perhaps) and a high-hide switch should do exactly what you want to do.
There are two basic ways to blow up a 317: input overvoltage and running a lot of current through it backwards, e.g. by attaching a big output cap and shorting the input to ground.
You'll never blow one up by power dissipation--they'll happily sit in therm al limiting from now to next Whitsun. One of George's more creative custome rs figured out how to make the thermal limit cause an overvoltage, but that took unusual ability even for a student. ;)
An almost as old design did much the same, but rather better.
In practice you had to put a trim-pot/select-on-test-resistor into the curr ent limit to cover the tolerance on the difference between the two Vbe's, b ut that worked fine.
I managed to build fold-back current limiting into the version we shipped w ith the S200 electron microscope. I used to get regular modification propos als from the shop floor - the select on test resistor would stay the same f or about six months as we worked through a batch of LM305's, and people kep t on wanting to skip the testing and fit whatever resistor value was workin g for the current batch ...
On a sunny day (Mon, 16 Oct 2017 17:56:14 -0700) it happened John Larkin wrote in :
Funny, I have a Chinese TV HD satellite receiver box (size of a packet of sigarettes), and it uses the LM317 both as variable controlled voltage regulator to the LNB (switches between 13 V and 18 V) as well as amlifier for 22 kHZ control signals. I have now replaced the six or so LM317 in it after the last lightning shower...
Anyways it is a classic and should work. Don't you want to compare to the output, and not against the control input of the second LM *for simplicity sake ;-) *?
Why not just use PNP forward regulator or NPN emittor follower?
Or better use a switcher (chip)?
There are a few cheap switcher chips with current sense input.. or LM2596 and pull voltage sense input up with current sense shunt opamp.
Modify it a bit ... Cannot beat price and shipping... unsolder chip, use parts, circuit diagram is somewhere on internet.
Many people have remarked about the poor quality of eBay parts. Some are counterfeit, some are salvaged from old equipment, some are floor sweepings.
Apparently parts salvage is a big business in China.
I don't think you save much money buying risky parts on eBay. You burn time waiting for delivery, which can take more than 60 days recently. You lose time checking the devices to find out which, if any, are good.
I buy from Digikey, Newark, and other reputable suppliers. They ship fast. My big beef is having to pay $7.00 flat shipping fee for a couple of 10 cent devices. I had to do this recently, but I needed the parts quickly for a friend's broken appliance. Fortunately, he didn't mind paying for them, but I never saw the money. I never bugged him about it. The appliance is still working.
On a sunny day (Tue, 17 Oct 2017 09:04:59 GMT) it happened Steve Wilson wrote in :
Yes, maybe they have garden shed chips fabs too?
It depends, paying 25 Euro). Shipping from Hongkong takes sometimes just a week, China a few weeks. Bigger things they may have in storage warehouses in the EU. Ordered a drone from China and received it 2 days later, no customs????
ON AVERAGE I have good deals from ebay, save tons of money. The few bad ones .. OK part of the game.
Some of these 'reputable' ones buy from China I am sure, that is where the profit margin is.
Have not used any of these you mention for a long time, we have Conrad here locally,, I have just as much bad stuff from them as from ebay. Using Bing finds best prices quickly.
There's an application note (I dimly recall) that used a PNP transistor to sniff input current, and activate the shutdown on LP2951. Can't find it, though (and it's no more accurate than the LM723 scheme).
This is a kitchen-sink-included solution, in surface mount, and modern.
but it looks a bit... overcomplicated. Not a part I'd keep in stock.
Cute, I always think about current limit on the other side of the Vreg. I've only done the circuit sensing the Vbe drop of a pnp(?) with a resistor and turning down/off a big pass transistor on the output.
a lot of current through it backwards, e.g. by attaching a big output cap a nd shorting the input to ground.
rmal limiting from now to next Whitsun. One of George's more creative custo mers figured out how to make the thermal limit cause an overvoltage, but th at took unusual ability even for a student. ;) Well an LM395. I should see if I can blow up an lm317 with an inductor on the output. Hey said 'inductor' is on my table, for B-field fun.
I've ordered from these folks before, I believe they're based in Thailand:
Shipping is naturally not as fast as ordering domestically, and prices are a little higher than the true bargain basement eBay stuff (but still half as much as say Mouser in small quantities), but as far as I can tell so far they're legit parts from the representative manufacturers that meet spec