latching circuit problem

b-bassett

Newly Enlightened
Joined
Nov 22, 2008
Messages
180
Location
hertfordshire, uk
hi all

I'm hoping someone can help with this problem

a year or so ago a built a quad xre , mag based, diving torch, with a 6 x D cell 6000MaH Nimh umbilical battery pack

i used a KD 2.8 p7 driver with the LED's in 2p2s and switched the power using a piezo momentary switch and a small latching circuit.

all worked well although i didn't seem to be getting full output (due to a 0.7V drop in the latching circuit.

and then i flooded it
ohgeez.gif



so.. i rebuilt with better seals and an extra battery

now iv got problems...

when fully charged i cannot get high or medium power, all i get is a quick flash then the light goes out. low power works fine however.

removing the extra battery solves this problem, as does letting them partially drain down.

testing with a bench PS replicates the fault under certain conditions

with an input voltage below 8.3V all works fine.

increasing above 8.3V and the problem arises, unless i also increase the input current above 80mA @ 8.4V then continue to increase until i get to 3A @ 12V (PS max 3A)

the only faults i can think of are:

a) the input voltage is too high for the components (although both the driver and latch circuit are designed for up to 12V)
b) the initial rush of current does not increase rapidly enough causing the circuit to de-latch
c) the extra voltage is causing the switch to bounce ( although why would increasing the current solve this?)

I'm hoping someone here can help me as i would like to have my torch running at full power when down in a wreck at 50M

here's the circuit I'm using

420960873971b557214d67daec775dd58482a0302d0ad3151440c2397406cad27473d570.jpg


and here's the original ''Relay Toggle Circuit Using a 555 Timer''

http://www.bowdenshobbycircuits.info/page9.htm#555-T.gif

teh only changes i made were a darlington pair transistor for higher current capabilities, i omitted the resistor+capacitor filter combo and swapped the relay for an LED driver
 
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I think the problem is in the biasing of the TIP142. You want it to act as a switch, and therefore you need to drive it into saturation, but the 1k resistor isn't allowing that - therefore you're dropping voltage across the collector-emitter junction, and that's probably why you've never quite achieved full output.

The base current with the circuit as it stands is 7mA - that's 8.4V (from pin 3 of the 555) minus 1.4V (across the base-emitter junctions of the darlington pair) equals 7V / 1K = 7mA.

If you change the base resistor from 1K to 330R, that will give a base current of around 21mA, which should allow the transistor to saturate and switch properly at full load - you didn't say what the full current is, but I'm assuming it would be around 4 or 5 amps. If you don't have a 330 ohm resistor handy but if you do have a few more 1K's, then three 1k resistors in parallel would do the job nicely if you have enough room for them. I might be wrong, but I'd be fairly confident that that's what the problem is.
 
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thanks
Selectron
for the advice.

ill give it a go tomorrow and see what happens.

it seams odd that the circuit latched
every time
with a lower voltage, but wouldn't with this extra battery.


iv also found another latching circuit online which uses a pair of inverter gates and a
fet
. problem is i know even less about them
lol

as soon as the parts turn up ill build it
as well
, hopefully it will end up more
efficient
and take up less space

thanks again

jon
 
well i replaced the 1k resistor with a 330 ohm one, but unfortunatly the problem is still there.
sigh.gif


im wondering if iv got a high resistance somwhere in the battery pack? maby im getting a voltage drop when the circuit tries to pull higher currents.

the main reason i wasn't getting full output with only 6 cells, was that i only had 6.5V available to the driver ( 7.2V-0.7V drop) and with the led's in 2p2s i needed around 7.4V with the KD driver outputting 2.8A on high.

the transistor was only switching about 3A max and should now be switching a little less so i don't think its the transistor at fault

i think its something to do with the timer not latching when the driver tries to pull higher amps. possibly the voltage dropping and shutting down the latch circuit?

I'm using the torch tomorrow so will have another look at it next week

on a seperate note, anyone know why the pic i uploaded only shows for a day then dissaperes?
 
Still not latching at the higher power settings eh? That's disappointing. I was going to suggest feeding the 555 off a 7805 voltage regulator to give it its own stable supply rail, but the 7805 needs an input voltage which is at least 2 volts higher than the output, so you would be right on the threshold and chances are it wouldn't prove to be successful. You could put a transistorised voltage regulator circuit ahead of the 555, running at 5 volts, with the driver and LEDs still running at the full supply voltage, but unless you have a specific reason for using the timer IC as the basis for the latch, I'd just bin it and use a different circuit. You did say you'd never had full output from the circuit even in its original form, so it never really was fit for purpose; I'd start afresh with a new latch, and preferably one which somebody has already demonstrated to be stable and reliable at your required current and voltage.
 
hi

yes it is dissapointing that your suggestion didn't work, thanks anyway

theres no real reason for me to use that particular circuit, just that i more or less understood how it worked, and i could fit it into a very small space.

with the previous set up the latch worked fine, was just that the driver was a buck driver. so was never going to give full output. (6.5V input < 7.4V needed for led's)

the system probably would have been fine with the origional input voltage and a boost driver, but i thought it would be easier to just increase the cells count as i couldn't find a boost driver for 4 xre's @ 1.4A

ill be building a different latching circuit next week so hopefully ill be able to use that instead

otherwise i can re-use the origional circuit with different led configuration, maby a sst-50

thanks for the help so-far

ill try the other circuit and let yo know how i get on

cheers

jon
 
and here's the original ''Relay Toggle Circuit Using a 555 Timer''

http://www.bowdenshobbycircuits.info/page9.htm#555-T.gif

teh only changes i made were a darlington pair transistor for higher current capabilities, i omitted the resistor+capacitor filter combo and swapped the relay for an LED driver

I don't know why your image disappears, but I'd like to see it. Maybe you can e-mail me? dlburdette "at" juno "dot" com.

If you eliminated the 100 ohm/100 uF filter, that might be your problem. The sudden switch-on of the transistor may cause a brief drop in voltage at the 555 chip, which could make it misbehave. I normally like to have a 0.1uF ceramic capacitor right next to the power pins of any IC in a circuit like this. The 100 ohms might help, and the 100 uF cap might also help. I'd add them in that order.

If you want the lowest voltage drop, you should definitely use a FET instead of the darlington. 0.7V is actually low for a darlington. I usually figure on 0.8 - 1.0V. With a FET you could get down to 0.05V or less. A IPP096N03LG would be a good choice, though there are many others. The FET can be a direct replacement for the darlington (base->gate, emitter->source, collector->drain), but you want to reduce the 1K resistor to 10 ohms, or even zero (just wire pin 3 directly to the gate). The main difference between a regular (BJT, or bipolar) transistor and a FET is that where the BJT has output current controlled by input current, the FET has output resistance controlled by input voltage. Mathematically,
Ice = f(Ibe) for the BJT
Ids = f(Vgs) for the FET

or
Current from collector to emitter is a function of current from base to emitter (BJT)
Current from drain to source is a function of voltage from gate to source (FET).


D
 
thanks for the advice, hopefully ill get a chance to test them during the week.

i suspect you may be right about the brief voltage drop, i just cant really see why i didn't have the same problem with the lower voltage pack.

after doing some research and seeing how small a fet actually is, i think it may well be the best idea (both for efficiency and space saving).
i believe ill need an N channel fet to replace the NPN transistor? i cant seam to find any details or supplier for the one you mentioned.

ill have to have another play with that pic i posted, wasn't anything special though just the modified circuit diagram.

thanks again

jon
 
Re: latching circuit problem solved http://www.candlepowerforums.com/vb/images/sm

:) :) :)

thanks DIWdiver problem solved :)

iv put the 100 Ohm resistor back in on the supply to the 555 and it all works fine now.

the test circuit will now switch on and stay on with as little as 20mA at any voltage above 6V

all iv got to do now is swap the transistor for a FET (when i work out which one) and things will be perfect.

being as iv got to open up the torch head to add the resistor ill snap some pics of its construction, then when i work out how to post images for more then 24 hours, ill put them up in case anyone is interested.

thanks again DIWdiver :grin2:

thanks to Selectron also for your advice.

jon c
 
Re: latching circuit problem solved http://www.candlepowerforums.com/vb/images/sm

all iv got to do now is swap the transistor for a FET (when i work out which one) and things will be perfect.

jon c

The FET I recommended is available from www.digikey.com.

There are many other parts that would work fine, in various packages, some much smaller than the one I recommended.

Let me know if you need any other help selecting a FET or converting the circuit over to a FET.

D
 
cheers DIWdiver

i managed to pick up a few of these last week from RS, i think they will be ok

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=543-0866

i hooked one up to a test circuit and it seemd to work with the 1K resistor removed, although i think i had to increase the resistor on the supply to 200 ohms.

depending on how work goes today, ill put it all together in the torch, but it might have to be tomorro, as iv already been driving since 4 AM and dont fancy another 16 hour day.


i also found another circuit which might work but it seems to need a + 6v and a -6V supply.

this is the only link i can find for it, it near the bottom.

http://www.divematrix.com/showthread.php?3668-Compact-500-Lumen-LED
 
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cheers DIWdiver

i managed to pick up a few of these last week from RS, i think they will be ok...

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=543-0866


i also found another circuit which might work but it seems to need a + 6v and a -6V supply...

http://www.divematrix.com/showthread.php?3668-Compact-500-Lumen-LED

Yes, that FET should work. At 2.8A it shouldn't even get warm.

The circuit doesn't require two supplies, those are just the + and - of a single 6V supply. I don't know which version of the chip you have, but the LM556 should work from 4.5V to 16V inputs.

D
 
ahh ok i thought it might have just been a 6v supply as there wasn't a 0v rail, but wasnt sure.

iv only got 555's here which i think will be interchangable.

i tried that Fet with the origional circuit but the origional problem re-apperd, ill give the other one a go and see if it performs better

not suer how much ill get done this week, i didnt get in till midnight last night, and looks like ill be out a few hours tonight
 
well i managed to spare an hour for some more testing. unfortunatly with no luck. :(

i tried the fet on the original circuit, but had to increase the input resistor to a point at which there wasn't sufficient voltage to drive the fet

so i re-used the transistor with a 100 ohm resistor on the circuit supply, but that wouldn't work either.

it seems my previous testing did not solve the initial problem.



iv had another look at the other circuit i mentioned, but ill have to order a 556 chip as its being used as an inverter pair. (or possibly use 2 separate 555's)
 
There is no good reason why this circuit would work at low voltage and fail at high voltage. I can't help but think it's the current surge when the FET turns on that causes the circuit to switch off.

If that were so, then the way to fix it is usually not to change the circuit, but to change the way it's built.

Please forgive me if I'm preaching to the choir, but I wonder if it's wired in such a way that some connections between elements of the control circuit (resistors, capacitors, IC, switch) also carry the high current from the LED. The connections between the control circuit elements should not carry high current. The high current should only go from the battery to the driver board/LED, through the FET, and back to the battery. The rest of the circuit should connect at only 3 points: +, -, and the FET gate.

In other words you should not connect, for example, from the battery - to the IC, then to the resistors and capacitors, then to the FET. This would cause high current to flow through the connections between the IC and the other components. The high current should flow from the FET directly to the battery.

This idea is hard to put into words. I hope it is understandable.

If you can post a photo of your project, it might help to see if this is the case.

D
 
I just had another idea.

I once had a problem with a device that had two large DC motors that were run off a 24V battery. When the motors were turned on, they would draw a surge of 60A each before dropping back to a lower current. This surge caused the battery voltage to drop low enough to cause problems in the control circuit.

The way I fixed this was to put a diode where you have the 100 ohm resistor, between the power supply and the control circuit. This way (in your circuit) when you have a surge from the FET turning on, the diode will allow the battery voltage to drop while the the 100uF capacitor will keep the supply voltage constant to the IC and other control elements.

In order for this to help in your case, you MUST have the circuit wired as in my previous post, so all the control elements get power through the diode, but the LED current does not pass through the diode. The diode would be in series with the 100 ohm resistor.

I would recommend either a signal or schottky diode, not a normal rectifier diode, for this application. 1N914, 1N4148, 1N5819 are common and all good, while there are many others that would work too.

Hope this helps.

D
 
thanks for the help DIWdiver

iv managed to get it working with the transistor for now as a freind is using the torch this weekend.

i agree its probably the high surge current causing a voltage drop as the problem happens on high and med power, but not low.


i cant see how the high current could running through the control system but ill double check when i get it back.


as it stands, the +ve lead splits into 2, one direct to the driver, and the other to the circuit.
the -ve goes from the batt canister direct to the fet then a lead comes form the fet to the control circuit -ve rail. (is this a prob?)

i could move the circuits -ve to the batt lead, but i cant see how that would effect things ?


im goign to try that 556 timer circuit nxt as its got a few less componants and im already runnign out of room.

cheers
jon
 
Sounds like you've got it wired correctly.

If the driver has a large capacitor on the input, then turning on the FET might cause the battery voltage to drop quite a bit, very briefly. The diode in series with the 100 ohm resistor might help if this is what's happening.

Good luck.

D
 
cheers, ill have a play with it when i get it back and see what happens

iv got sum 556 Ic's now so i can try the other circuit, which hopefully will be more reliable and smaller

ill let you know how i get on

cheers

jon c
 
just an update

i fiddled around with the other circuit based on the 556 chip and can report that it works extreamly well :)

iv managed to get it onto a peice of vero board aprox 1'' square, but am hoping to make it smaller (possibly 2 boards back to back)

thanks to all for their help

jon c
 
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