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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Simple Constant Current Device or Circuit
Hi,
I searched the web for a simple constant current circuit so I can charge a 19V rechargeable "power pack" that contains NMH cells after they "run down" using a timer. My DC source will be 24V. The simple circuits I found are for lighting LEDs, anything requiring more current involves complex circuitry. Does anyone know of a device like a simple 3 lead voltage regulator, but for current, or a simple circuit? Thank You in advance, John PS, Remove "ine" from my email address if you have an image file. |
#2
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Simple Constant Current Device or Circuit
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#3
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Simple Constant Current Device or Circuit
Let's look at some basic electronics theory.
A junction transistor can be modeled as having a constant-current output. You can use this to produce a constant-current charger. It should be obvious how, but I'll explain it. Suppose you want a constant current of 100mA, and the transistor has a current gain (beta) of 20. Put 5mA into the base and the output will be 100mA, /regardless of how much voltage is dropped across the load/. Of course, the power supply has to be at least a couple of volts higher than the battery's maximum voltage. You should be okay with 24V. The charging current can be varied by varying the bias current. If you don't know how to bias transistors, now is a good time to learn. |
#4
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Simple Constant Current Device or Circuit
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#5
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Simple Constant Current Device or Circuit
wrote:
Hi, I searched the web for a simple constant current circuit so I can charge a 19V rechargeable "power pack" that contains NMH cells after they "run down" using a timer. My DC source will be 24V. The simple circuits I found are for lighting LEDs, anything requiring more current involves complex circuitry. Does anyone know of a device like a simple 3 lead voltage regulator, but for current, or a simple circuit? Thank You in advance, John PS, Remove "ine" from my email address if you have an image file. With a small differential like that, a resistor would work. Not the best way to charge that battery. Greg |
#6
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Simple Constant Current Device or Circuit
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#7
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Simple Constant Current Device or Circuit
wrote in message ... Hi, I searched the web for a simple constant current circuit so I can charge a 19V rechargeable "power pack" that contains NMH cells after they "run down" using a timer. My DC source will be 24V. The simple circuits I found are for lighting LEDs, anything requiring more current involves complex circuitry. Does anyone know of a device like a simple 3 lead voltage regulator, but for current, or a simple circuit? Thank You in advance, John PS, Remove "ine" from my email address if you have an image file. The very simple answer is the LM317T. Just Google "LM317T as a constant current source." Just needs one resistor to set the required current. I have been using these adjustable voltage regulators as constant current sources for probably 30 years without issue. Other more powerful three terminal adjustable voltage regulators can be used in exactly the same way if you need more current. Arfa |
#8
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Simple Constant Current Device or Circuit
wrote in message ... I consider myself to be smart and experienced.. And the result has never been as simple as a lm317 and a resistor. http://www.hqew.net/product-data/LM317T Why the link to the data sheet I wonder ? That only gives details of its original designed purpose as a simple-to-use 3 terminal adjustable voltage regulator. As far as the solution "never being as simple as an LM317 and a resistor", it depends entirely on your requirements. The OP merely requested details of a 'simple' constant current source to charge a battery pack, and the LM317T, suitably configured and connected, will do just that - provide a constant current at any value you like with the addition of one resistor. No more, no less. I don't think anyone, least of all me, suggested that this would be the best or kindest way to charge a pack of NiMH batteries, but with the charging current correctly dimensioned, it would be unlikely in it's 'raw' form to cause any likelihood of explosion or serious long term damage. For sure, the circuit could be refined. A simple timer could be added for instance, to just turn the whole affair off after a time that would ensure that the pack is fully charged. Or a multistage timer could be incorporated to change the charge current during the charge cycle. Or you could start monitoring the terminal voltage and so on and so on. But then, of course, you are getting away from the OP's original requirement for a 'simple constant current source'. Whilst I wouldn't disagree with anyone that a degree of care should be taken over the charging of NiCd and NiMH cells and packs, I also think that sometimes, you can get a bit paranoid over it all. Many 'simple' chargers provided by equipment manufacturers to charge their items, are nothing more than a basic power supply with a current limiting resistor on the end. Cordless phones used to be a good example of this, and the battery packs used to last a long a time on them. The cordless drill that I use every day in my workshop and charge once every week or ten days is another example. Arfa |
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