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using a comparator, new to me. plz help

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tallyman

New Member
Hello folks,
New to the forum. Lots of great stuff on here. im teaching myself electronics. No other way to learn than to jump in, read, and experiment. I was hoping someone can help me with something ive been trying to figure out. I just recently learned what a comparator is. so it opened up a lot of info for me. Im trying to build a circuit using the comparator (as small as circuit as possible possible, weight is an issue.) to switch on and off another circuit that Ive been working with for a while now. A neat little thing known as the joule thief. I need to be able to switch off the joule thief once a supercap reaches its charged point (say 9 V) and on again once it comes down at about 7-8 V. Mostly for input battery preservation. I would like to keep the cap Voltage at no less than 7 volts or so. Is there a way to vary the on and off Voltages of the comparator circuit? Could I use small pots to do this? Any advice or scematics for this are welcomed. Thanks in advance
 

jta

New Member
You could use a voltage divider, where one resistor is a potentiometer. This would give you a variable reference voltage with which to compare your super-cap voltage with.

Of course, this would require you to adjust the potentiometer after every transition. Perhaps a better solution would be a comparator with hysteresis.

See: Adding Extra Hysteresis to Comparators - Maxim for details in how to do this. Figure 10 of this article gives the input/output relation of the circuit which should go far to explaining the hysteresis effect.
 

MikeMl

Well-Known Member
Most Helpful Member
Make a schmitt trigger out of the comparator by adding enough positive feedback to create about 2V of hysteresis such that the trip points are at 9 and 7V.
 

tallyman

New Member
Make a schmitt trigger out of the comparator by adding enough positive feedback to create about 2V of hysteresis such that the trip points are at 9 and 7V.

wow, i must be even worse off than I thought. I have no idea what you guys just tried telling me. I guess being an artists a picture would be worth a thousand words. Jeex I have a lot to learn. ANyone else have something that Barney could teach a kid because I think Im that bad off...lol
THanks
 

MikeMl

Well-Known Member
Most Helpful Member
One comment. The circuit to sense the capacitor voltage and turn the joule-thief will probably consume more power than the joule-thief itself. As a practical matter, if the goal is to extend the run time of the joule-thief, this solution might make the situation worse instead of better. Are you sure you want to proceed.

If you just want to learn about Schmitt triggers, follow this link.
 

tallyman

New Member
well...

well dang, didnt realize that. that takes the gem out of that one. :mad:
Well doesnt the Joule thief put out a pretty decent amount of energy in comparison to the input? It kind of acts like an amplifier doesn't it? Well is there another way of doing this? im trying to get a lightweight battery (like lightweight li-ion cell phone battery that has good capacity) to run for an extended amount of time. I figured charging the supercap to capacity and turning off the JT would help extend the battery out some. and if the Joule thief is constanly going without being switched off, and apparently the comparator circuit is consuming also then yeah I guess It would be defeating the purpose. What Im trying to do is get my hand held pistol coil gun to run on batteries, that last a long time. So a lightweight circuit that could help get 2-4 hours of runtime would be ideal. At least an hour and a half till I could charge a replacement battery. I can play around with just the joule thief and a 9 Volt walgreens battery ( i went for worst case scenario here...LOL) fo about 30 minutes before I cant charge all that well. Then I get shot. Im in the Marines so want to be able to beat my other guys in my unit. Were having a bit of a competition. (go figure) Who can build the smallest, lightweight and most powerful coilgun. They dont know what a JT is so i might have the edge if I can get this worked out. I really need this thing light and small!! Electronics is a new , but very rewarding, venture for me. Hey this might not even be possible but Ill never know till I try. Thansk guys let me know if anyone has any tips.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
well dang, didnt realize that. that takes the gem out of that one. :mad:
Well doesnt the Joule thief put out a pretty decent amount of energy in comparison to the input?

No, exactly the same as all circuits, it puts out less than it takes in.

The only advantage of a joule thief is that it works down to very low voltages, so it allows yoiu to use the battery for longer, and even batteries already too flat to work other items.
 

tallyman

New Member
the other advantage.

No, exactly the same as all circuits, it puts out less than it takes in.

The only advantage of a joule thief is that it works down to very low voltages, so it allows yoiu to use the battery for longer, and even batteries already too flat to work other items.

LIke i sad previously Im pretty new to this. But can someone then explain how the Joule thief can light a 3V LED off a 1.5V cell? Yeah it will run an item longer but it amplifies also doesnt it?
 

ericgibbs

Well-Known Member
Most Helpful Member
LIke i sad previously Im pretty new to this. But can someone then explain how the Joule thief can light a 3V LED off a 1.5V cell? Yeah it will run an item longer but it amplifies also doesnt it?

hi,
The small transformer in the JT is basically used as an Inductor and feedback winding to the base of the driving transistor.

You may know Lenz's Law.?

E = - di/dt, which means if you rapidly switch off the current flowing in an
inductor it will generate a 'back emf'.

This back emf can be a high voltage which enough to light an LED.

Do you follow OK.?:)
 

tallyman

New Member
hi,
The small transformer in the JT is basically used as an Inductor and feedback winding to the base of the driving transistor.

You may know Lenz's Law.?

E = - di/dt, which means if you rapidly switch off the current flowing in an
inductor it will generate a 'back emf'.

This back emf can be a high voltage which enough to light an LED.

Do you follow OK.?:)

yes, thank you im familiar with BEMF. I thought that was the concept, but from where i discovered the JT and the briliant people on here, are 2 different worlds. Well if people are saying NOT to use the comparator, can anyone detail to me how a disposable camera flash circuit charges a cap then shuts off without overcharging the cap? or does it not shut off? If it didnt, wouldn't it kill your regular cheap AA cell pretty quick ?
 

ericgibbs

Well-Known Member
Most Helpful Member
yes, thank you im familiar with BEMF. I thought that was the concept, but from where i discovered the JT and the briliant people on here, are 2 different worlds. Well if people are saying NOT to use the comparator, can anyone detail to me how a disposable camera flash circuit charges a cap then shuts off without overcharging the cap? or does it not shut off? If it didnt, wouldn't it kill your regular cheap AA cell pretty quick ?

hi,
Look at this link.

Disposable Camera Salvage/Disassembly, Ver 1.7
or Google: disposable camera charger
 

tallyman

New Member
hi,
Look at this link.

Disposable Camera Salvage/Disassembly, Ver 1.7
or Google: disposable camera charger

Thanks,
So basically the flashing is used to relieve the pressure on the cap? yeah I guess that would work. so could I hook up the JT circuit to input the charging into the camera cap charging circuit? Just the JT hooked up to 2- 30V 4000uF caps charges in about 3 seconds off a standard 9V cell. Can a standard camera cap charger handle the voltage from the output of the JT? IT outputs 13-15V from the 9V cell.
 
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