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Ohm's Law Track and electrical/DCC
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Frequent poster 920 posts Bruce Petrarca Frequent poster 920 posts
Bruce Petrarca
Dave (with the moniker HVT) raised a question on another thread about resistor calculation. So as not to hijack the thread, I've moved that discussion over here. He posted: "A question concerning the Hybrid circuit you mentioned.  I have a wireless camera which operates at 9v powered by a battery eliminator circuit using an LM317 voltage regulator because 9v batteries only last about 45 minutes.  But it now has intermittent signal issues.  I am planning to add a 9v NiMH battery rated at 170mAh in parallel.  I did look at Stan Ames' website but am uncertain of the correct value for the resistor.  What value resistor would you recommend?" Stan's site relates to the use of back-up batteries in support of a DCC decoder. Dave's need is similar, but different. Here's how I'd calculate the resistor. First, you need to know how much voltage to drop. Charge your battery to the max. Measure its voltage. Measure the voltage coming out of your eliminator. If the eliminator voltage is not larger than the battery, it won't work. Subtract the two numbers. This is the steady-state voltage drop. If the battery charges to 9 volts and the eliminator puts out 10.7 volts, the drop is 1.7 volts. It is safe to apply a charging current of 1/10 the capacity to a NiMh or NiCd battery forever. So, in your case, Dave, you can apply 17 mA (1/10 of 170 mA) to your battery. Ohm's law is R=E/I. R = 1.7 volts / 17 mA = 1.7 volts / 0.017 amps = 100 ohms.

Bruce Petrarca, Mr. DCC; MMR #574
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HVT Dave Prolific poster 1,212 posts
HVT Dave

Thanks for the new thread

Sorry about hijacking the thread.  And thanks for the detailed explanation, it answers many questions.  

Also, isn't there a 1.7 volt drop across the diode when running off the battery, minus something for what current passes through the resistor?  Is the net drop appreciable?  Would a Schottky diode be a better (necessary?) choice?

HVT is the Hill Valley and Thistle Railroad, a freelanced layout which is the project of this grandfather and his grandsons.

Regards,

Dave

Dave

Member of the Four Amigos

 
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Bruce Petrarca Frequent poster 920 posts
Bruce Petrarca

Connect your camera to the

Connect your camera to the battery. Use the eliminator like a battery charger. Diode not necessary, as the battery isn't going to ever (hopefully) feed power to the charger.

Bruce Petrarca, Mr. DCC; MMR #574
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HVT Dave Prolific poster 1,212 posts
HVT Dave

Thanks Bruce

Quote:

Connect your camera to the battery.

That's what I'll do.

Dave

Dave

Member of the Four Amigos

 
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