Power supplies

[DAVEHALL]

Something I've never thought about before.
Simple 7805 regulator circuit; volts in, filter cap across input and ground.
Now I decide to have a multi supply with something like 5v, 9v,12v etc. Any design blokes comment on what the input filter capacitor needs to be with possibly some/all of the outputs used? Then again we see discrepancies - data sheets say input filter of 330nF - other sources say 100nF.

[cmjones01]

Do you mean separate regulators for each output? If so, then 100nF across the input of each one will be perfectly OK. There will be no problem with having too much capacitance there. If the regulators are really close together (less than an inch or so) you could easily get away with one single capacitor of 100nF-1uF but normal practice would be to give each one its own capacitor.

Usually the filter capacitor after the rectifier (if this is what you are using) is enough for the input capacitor if the regulators are within a few inches (of wire) to it. A 100nF (+/- lots) on the outputs won't do any harm either. Nice thing about the 78 series of regulators is that the tabs are 0V no bloomin' mica washers etc to deal with, and the current/thermal limiting is very good, I have yet to blow one up in use.

[paulsherwin]

I don't think the values of these caps are very critical.

Have you considered using a single LM317 class regulator? Then you just need a switch to select the appropriate control resistor to get the voltage you want. You can use presets if you want to adjust the voltages to be spot on.

Don't forget the protection diodes whatever regulator you use. They aren't strictly necessary, but given the cost (you can scavenge suitable diodes from any dead PSU) it's a false economy to omit them.

I'm not a design bloke though, just a tinkerer

[evingar]

Originally Posted by paulsherwin

Don't forget the protection diodes whatever regulator you use. They aren't strictly necessary, but given the cost (you can scavenge suitable diodes from any dead PSU) it's a false economy to omit them.

They are more important if you have a large values of post regulator capacitance - As Paul says though, it's daft not to fit them as standard given the cost.

The LM317 sounds like a more elegant solution - You also get 1.5 Amp capability from the standard TO220 package (VS the more usual 1 Amp from the 78x types).

[Brian R Pateman]

Originally Posted by evingar

The LM317 sounds like a more elegant solution.

Unless you want to use more than one voltage at a time.

I used to have a home made supply based on 78XX regulators to give +5V, +5V, +12V. +15V 0V -15V, +24V, and +30V 0V -30V.

It started as a way of using a transformer with lots of windings and ended up as the one of the most useful bits of kit I ever built.

[DAVEHALL]

Originally Posted by paulsherwin

I don't think the values of these caps are very critical.

Have you considered using a single LM317 class regulator? Then you just need a switch to select the appropriate control resistor to get the voltage you want. You can use presets if you want to adjust the voltages to be spot on.

Don't forget the protection diodes whatever regulator you use. They aren't strictly necessary, but given the cost (you can scavenge suitable diodes from any dead PSU) it's a false economy to omit them.

I'm not a design bloke though, just a tinkerer

It's more of a car project - I've got an RF module with secure logic to control my boot release .Makers suggest lots of filtering as module has low voltage limits and low spike threshold ,so found it would work OK ( and has done for few years) on a 7805( Maplin do a chepo with good noise figures an 2A capacity) dropping the 12 -13,8 car supply to 5v ,then 5 -12 module to up the voltage .( module input limits outside range of car charging limits).and both devices noise/anti spike limits combined seem to provie protection .
Decided to advance it to doors and decided that I' like some indication of lock -so a timer controlling a short ccross the hazard facility seemed way to go .This led to 555 1-2 sec timer ,but with crowbar problems thought -why not just stick in a second 7805 to get rid of problem .Problem was -no two sources give a set value for the input filter ,and certinly not for paralled evices -so thought I'd tp the vast stock of practical experience on here .
But ,again -I'm likewise not a design bloke,learnt mine mostly from tinkering ,,apart from bit of college stuff .So thought I'd ask if anyone had paralled 78xx devices with any problems encountered .( PS -Mods -this laptop misses letters occasionally -so my apologies if post seems out of sorts)

[davegsm82]

usually 100nF will be ok, I used a 78L05 in my KEELOQ car central locking/boot release controller that I built years ago with a 100nF either side, paralelled up with some 47uF's for the longer, larger spikes.

Also, yes diodes are essential, I've blown up expensive projects before by omitting a few pence worth of diodes.

Dave.

[fetteler]

I would second the fitting of a protection diode to the regulator to guard against damage if the input is shorted to ground or the output is (for some reason) raised above the input. Just connect a diode between the O/P (anode) and I/P (cathode) - Sorted!

Cheers,
Steve.

[paulsherwin]

If you look at any of the standard regulator datasheets they will have example circuits showing the use of protection diodes.

[davegsm82]

Is you Keeloq controller a ready built unit or are you making your own up from scratch?

I built one using the RF600 chipset and the RS ready made 3-button keyfob.

All the details are in the following thread if you want to have a look;

http://www.ukfiestanetwork.com/index...locking-system

You may need to sign up to view the thread but that shouldn't be an issue, its a free forum.

Dave.

[val33vo]

I made a multi voltage supply a few weeks ago and used a LM723 IC ( because I had repaired a supply for a friend a while back which used this IC ) . with a multipole switch connected up to a resistor network you can get any voltage desired ( obviously within certain bounds 2v to 37v) I used it to drive a 2n3055 transistor so its got quite a reasonable current output, its also got a current limiting feature so in theory if you short circuit the supply it should emerge ok, just download a data sheet for it off the net and study its features

works a dream

regards

val33vo

[val33vo]

Ooh I missed the point you really need a electrolytic at the input maybe 1000uf and also 100nf ( the big one is to prevent voltage ripple ( mains hum ) and the wee one to prevent instability ) same on the output as a rough guide

regards

val33vo

[DAVEHALL]

Originally Posted by davegsm82

Is you Keeloq controller a ready built unit or are you making your own up from scratch?


Dave.

Reay made item - from Maplin ( and the tech support from the maker is good) . The makers suggested that it didn't like spikes,(and since the input volts is +/- 1v at 12v) ,I came up with the idea of dropping car supply to 5v (7805) ( lowest comp count) and using a convertor to get back to 12v . Whether or not I'm lucky that car is deisel which means no spikes from ignition sytem ,I know not ,but with this PSU it's worked for over three years .Next problem was to add central locking to the boot opening function ( boot is opened by lever in drivers side ).To indicate lock -I looked at some form of timer operated from the lock relay and a 555 seemed to fit the bill, but might draw exces from the convertor , and raw 12v might cause faulse triggering -so back to atap on the 5v supply . Having looked at the hazard light side -it's a toggle function on the CPU -and at present some form of relay control look easiest .So question is now irrelavent ,and mods ,no longer exits .Thanks for the inputs ,guys

[murphyv310]

Hi.
Dependant on layout some regulators can "take off" and oscillate, the LM317 is very guilty of this phenomenon, I agree with the above but sometimes even 100n is too big a 1 or 10n can be quite sufficient. These caps must be fitted as close to the regulator as physically practical, and tied to one earth point also a ferrite bead should be slipped on each leg of the regulating device.



Crossed with Dave

[murphyv310]

Originally Posted by DAVEHALL

Whether or not I'm lucky that car is deisel which means no spikes from ignition sytem

Dave.
All modern cars are unbelievably noisy whether petrol or diesel! Properly suppressed a 40 year old car is incredibly clean!

[Ed_Dinning]

Hi Gents, please note that if you use a cap on the output of the regulator and turn off the input while drawing no output current, the regulator may suffer damage due to reverse voltage.
The cure is to fit a 1A diode in reverse (cathode to input on positive regulators) across the device (input to output).

Ed

[kalee20]

Definitely agree with Ed Dinning's advice.

And good practice to have another chunky rectifier, directly across the output, in a general purpose power supply, to stop the output ever being pulled below ground if someone uses it as, for example, a battery charger and connects it backwards.

[davegsm82]

One thing to considder because you have a diesel is that on starting the engine the battery voltage may slump to below what your system needs to function, 7-8 volts on a normal linear reg.

This shouldn't be too much of a problem but you need to ensure the system won't crash or do unpredictable things.

As Trevor has pointed out, all cars are still noisy, diesels can be worse than petrols as you've got massive inductive spikes from things like the starter and the glow-plugs.

Dave.

[Sean Williams]

Inductive spikes from glow plugs? How - they are a resistance, not an inductance......