I build a robot using 2 dc motors from scraped video machines. The motors are for 12V and were used as loader motors. I made a picaxe control a L293B. Now I only got room for a 9V battery. That not quite enough. The robot drives, but when I provide 12 volt from an unregulated power supply it's much better. I want to make a 555 voltage doubler as I got one such chip already.
Question is: Will a 555 voltage doubler provide enough current? I've been googling for a day to find 555 voltage doubler circuits that will double my 9V to 18 volt. Can anyone help me?

Thank you
Allan Bertelsen

 

It won't provide anywhere near enough current, you really need a switch-mode converter, and not just a low current doubler. I'm presuming you're using a small 9V battery? (MN1604, PP3), these are also only really suitable for low current.

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Yes it's just a PP3 (6LF22) 9V battery.
Will six 1,2 NiMH AA batteries (1750mAh) do it. And if, could the voltage be doubled by 555 or is ICL7660 the answer for my problem?

 

AA's would provide far more power than a PP3, but you still can't use simple capacitive doublers to drive motors (you need amps, not milli-amps).

However, try it with AA's, don't use 6 (that's only 7.2V), use at least 8 of them - it should be a lot better than just a PP3. If you can find space for ten AA's, even better!.

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You could get a small lead acid battery or a gel battery(the gel battery is better, it is sealed), they provide around 13V when fully charged, you can get them rather small 100mm x 45mm x 50mm( I have one this size) it is rated at 1.2Ah but you could parallel two of them for 2.4Ah, the only disadvantage is that they weigh about 1.25 pounds each.

hope this helps

 

Remember also that when you change one voltage into another, the power that you get out of the circuit will always be less than the power that you put in. For example, lets say that you have a 12V 1A motor that controls a drive wheel. That is 12 Watts = 12 V * 1 A of output power. Suppose you want to run this motor from a 6V lantern battery, and that your SMPS is 80% efficient.
That 2.5 A drawn from the battery will typically be one half the peak current in the inductor of a boost regulator. Now designing a circuit that will handle 5A peak current requires some planning and forethought. My advice is to buy one and use it before trying to build one as your first project. Once you get a feel for how a SMPS works with a motor you will be in a much better position to articulate your requirements.

You could also just put two 6V batteries in series and be done with it. Less fuss, no muss.

 

Why don't you just use 10 NiMH cells?

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Hi
Thanks for all the replies. My robot is not that big. The 9V fits right under it. It would be a simple solution if a 555 voltage doubler could do the job.

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You'll have a heart attack when you see the size inductor you will need to make a boost regulator with 9V in and 18V out. The diode capacitor doubler just won't provide the current you need. If you demand that current the voltage will just collapse. You need a higher energy density solution like a camcorder battery, or an LiPoly battery that RC modelers use to start their engines. Keep looking.

 

OK
I will see if I can find such a battery. Or else I just have to reconstruct. Thanks for the answers. Now I know what not to do! ;o)

 

Two 9V bateries in series?

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Yeah - I'm just trying to do that right now...

Wow - it's alive!

Now it can drive on my carpet. Before it could only drive on the wooden floor. It's just going a little to fast. Hm… it makes strange sound when blocked. And it makes marks in the baseboard when smashing into it. – hope my wife doesn't notice.

 

I connected the enable pins on L293 to +5V. Should I use a free output pin on my picaxe to control the enable pins? In stead of using PMW on the input on L293, could PMV on the enable pins be a little more motor friendly. I just want to slow it a little. And I don't like the strange sounds it makes.

Ah – here might be another opportunity to use my 555. I will make a little pulse generator for the enable pins. Stop me if this is too silly.

 

What frequency are you using for PWM?

If it's below 20kHz then no wonder you can hear it causing the motors to vibrate!

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Have you considered that the higher you go, the more the inductance of the motor reduces the power available?. This is why motors are usually fed from low frequency PWM, too high and power drops off!.

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