Heating Element Question

[camino75080]

I'm sorry I ask so many questions, I dont know as much as would like to think. Sorry for the long post, this is a very puzzling promblem for me so I have explained as best I could. Thanks for any help.<p>A bird store I goto uses a tool consisting of a battery switch, and Ni-Chrome element to trim the nails of birds. The problem is that the batteries only last about 5 minutes before they need to be recharged for foir hours. I was trying to come up with a circuit using a 10F supercap as a power source and a simple regulator where's what I've com up with:<p>The regualtor uses to transistors one acting as a series pass regulator and the of watching the voltage drop of a sampling resistor so that when the voltage drop is greater that .6 volts the transistor switchs on and diverts the base drive from the series pass transistor and thus lowers the voltage and current. Whew.<p>My problem that the element was made to run from 3 volts, but the supercap's surge volatge is three volts (I'd hate to blow up a $7 cap), so when it is charged to 2.5 volts the voltage getting to the element is only about 600 millivolts at 50 milliamps. Also the elemnt has a room tempuator resistance of 1 ohm so it pulls about 3 amps, I guess. So after all that should I look for a totaly different circuit, for efficant transistor, or something else?<p>Sorry again for the long post, and thanks again for any help.

[dyarker]

How are you charging the super cap (which is made for long time low current, not short time high current)? <p>What size batteries were they using? AA I bet, from the 5 minutes you describe. They need to be C at least, D better.<p>Yes, the start current is 3A with 1 Ohm cold and 3V. As wire gets hot it's resistance increases. So running current is probably 1A to 2A.<p>Later -

[rshayes]

A 10 farad capacitor charged to 2.5 volts holds about 31 joules. The element requires about 3 watts. Even under ideal conditions, the capacitor would only last about 10 seconds.<p>An AA nicad battery will supply about .5 ampere for 1 hour at 1.1 volts average. The energy stored in the battery is about 2000 joules, or about 60 times the amount stored in the capacitor. A battery is definitely the best bet. Even then, a single battery would last less than 6 minutes.<p>The temperature could be regulated by varying the duty cycle of a series switch. This could be a power transistor. This will require a fair amount of base drive current. A power MOSFET would not even turn on with 1 volt on the gate. A linear regulator will be quite inefficient, and only result in even shorter battery life.<p>Two or three batteries in series would extend the operating time and help the efficiency. A large power transistor, operated well below its maximum current, should only drop a couple of tenths of a volt. The base current will need to be about one tenth of the collector current. This is still a hundred milliamps or so. A further stage would bring the drive signal down to a reasonable level. This could be generated by a variable duty cycle multivibrator.<p>Overall, it would probably take 4 or 5 transistors and associated components. Efficiency would porbably about 70% or so at full load. Three "AA" nicads might give about 10 minutes of operation on a full charge. Three "C" cells might give about 20 to 30 miinutes and, with a high rate charger, could be recharged in about the same time.

[Mike]

I'm sure this is a dumb question, but why not build a power supply so it is plugged in all the time?

[toejam]

Get an ammeter put it in series with the nichrome wire and measure the amount of current it uses. You should be able to parallel a couple of wall warts to come up tith a suitable supply.

[camino75080]

Thanks for the ideas here are my answers:<p>Dale Y: The super cap will be charged from a charging base in about 5 min. The batteries were AA rechargable alkine because, you have to hold the tool like a pencil to trimm the bird's nails.<p>stephen: I'm wasn't trying to control the temperature, I was trying to etend the charge on the super cap by controling current draw.<p>Mike and Toejam: I already suggested that but the owner was afaird that if a bird go scared it might bite though the wire.<p>Thanks again for the help, the more ideas the better!

[jwax]

Assuming you're using top-of-the-line rechargeables, how about making the pen assembly twice as long, and add two more AA's in parallel?
Either that or making the battery assembly a snap-lock affair to facilitate easy swapping with a charged set.
Sorry, but there's not a lot of options to put more energy in a limited space!
John

[camino75080]

Okay, guys new plan, what is I puts 2 supercaps in series, and then used a simple current mode ps. Could that work?

[dyarker]

Nope, the caps don't have near the energy density of batteries.<p>If you can't change to C size; get two sets of high rate AA NiCads. Like 40 minutes to recharge, one set in tool, other set in charger, swap as needed.<p>Cheers,

[perfectbite]

Try modifying a 2 'D' cell size flashlight body. Lots of space, lots of energy. No frequencies though or the birds really will get agitated.