I am rejuvenating an old cordless battery powered iso tip vintage soldering iron. The charger's transformer is bad. The original charger is supposed to supply 2.4vdc @ 375ma. Question, I have many wall warts and want to use an old phone charger rated at 5vdc and 1a. What is the simplest way to reduce the current so that it's not so hard on the batteries? The batteries are (2) 1.2vdc, 1300ma in series. The tool is meant to be left on the charger when not in use.

 

Below is the LTspice sim of a simple circuit using the common LM317 regulator, that should work for you:
(CBattery is just for emulating the battery load as it charges.)
The green trace is the charging current, and the yellow trace is the battery voltage.

 

A 1.2V battery requires a voltage higher than 1.2V to charge. NiCd's are more tolerant of improper charging. Other chemistries are likely to result in fire if sufficiently abused.

A simple way to drop the voltage is to use diodes in series. Picking a combination that will give you 2.4V will be problematic. A better solution would be to use an LDO regulator.

 

You can likely use a Buck Converter like these or similar. The link is merely an example, they even have them with voltage and current meters built in. What is the battery chemistry? That or the LM317 circuit crutschow posted above. Then as mentioned a few 1N4002 diodes in series, each diode will drop about 0.6 to 0.7 volts.

Ron

 

Place 4 diodes in series with the 5V output, reducing the charging voltage to 2.4V and add a 2.7V zener diode across the battery pack to prevent the charge voltage increasing above this as the charge current drops towards zero.

 

A series resistor is all you really need, based in what you said.

For a trickle charge you want about 1/20 of the Ah rating, which is 1300/20 or 65mA. The power supply is V and the battery voltage is 2.4V, so the resistor has to drop 2.6V at 65mA.

From Ohm’s law:

V = IR

2.6 = 0.065R

R = 2.6/0.065 = 40 Ohms

 

A 1.2V battery requires a voltage higher than 1.2V to charge. NiCd's are more tolerant of improper charging. Other chemistries are likely to result in fire if sufficiently abused.

A simple way to drop the voltage is to use diodes in series. Picking a combination that will give you 2.4V will be problematic. A better solution would be to use an LDO regulator.

I agree and am not sure why it is designed that way. It is a w00044 transformer with no other electronics involved so those batteries get 2.4v 24/7. Maybe to be kind to the batteries?

 

those batteries get 2.4v 24/7. Maybe to be kind to the batteries?

If the batteries are NiCd, they'll never get fully charged.

 

If the batteries are NiCd, they'll never get fully charged.

They are NiCd . Sorry I missed that on the post.

 

You can likely use a Buck Converter like these or similar. The link is merely an example, they even have them with voltage and current meters built in. What is the battery chemistry? That or the LM317 circuit crutschow posted above. Then as mentioned a few 1N4002 diodes in series, each diode will drop about 0.6 to 0.7 volts.

Ron

NiCd sorry I missed that.

 

Below is the LTspice sim of a simple circuit using the common LM317 regulator, that should work for you:
(CBattery is just for emulating the battery load as it charges.)
The green trace is the charging current, and the yellow trace is the battery voltage.

View attachment 318435

Clever circuit but how effective is this vs two LM317 in series?

 

Place 4 diodes in series with the 5V output, reducing the charging voltage to 2.4V and add a 2.7V zener diode across the battery pack to prevent the charge voltage increasing above this as the charge current drops towards zero.

So, how much current will be drawn through the zener when the batteries are either fully charged or removed from the charger?

 

Clever circuit but how effective is this vs two LM317 in series?

Why would you put two LM317s in series?

Even using one is going to be problematic, since the recommended minimum dropout is 3 V, which means you may not be able to get more than 2 V out with a 5 V supply. You can 'probably' get 2.4 V out, since parts are almost always capable of operating a bit outside their specs, but it is very poor design to rely on that.

 

Even using one is going to be problematic, since the recommended minimum dropout is 3 V,

This from the LM317 data sheet would indicate that the dropout is low enough to work readily at the low current used for this charger.

 

This from the LM317 data sheet would indicate that the dropout is low enough to work readily at the low current used for this charger.

View attachment 318459

Which data sheet?

Those are (almost certainly) typical values. Designing to typical values is not good practice when min/max values are provided.

https://www.ti.com/lit/ds/symlink/lm317.pdf



What are the min/max values from the data sheet you are referencing?

 

Place 4 diodes in series with the 5V output, reducing the charging voltage to 2.4V and add a 2.7V zener diode across the battery pack to prevent the charge voltage increasing above this as the charge current drops towards zero.

Hymie- full disclosure, I am not an electronic wiz-just a guy that enjoys tinkering with this stuff as a hobby. I am learning allot from you guys along the way. After reviewing how Zener's work compared to resistors and your method regarding 4 diodes in series to drop the voltage to 2.4 from 5, I am wondering why you wouldn't use a 2.4v zener on the 5v input rather than 4 1N4002's? Regarding the zener across the battery are you assembling this in reverse bias mode?

 

Diodes in series will not limit the current when the battery voltage is low, I would stay away from that. A 2.4V zener in parallel would require a series resistor as well, and is a poor substitute for a proper voltage regulator, having no benefit above the resistor only.

If you want something better than a simple trickle charger, go with @crutschow’s suggestion in post #2.

 

Reducing current in an electrical circuit can be achieved through various methods depending on the specific context and requirements of the circuit. Here are some general techniques:

1. Resistance: Introducing resistors into the circuit can limit the flow of current. The relationship between voltage (V), current (I), and resistance (R) is defined by Ohm's Law (V = IR). By increasing the resistance, you can reduce the current.
2. Series and Parallel Resistors: Connecting resistors in series increases the overall resistance, thereby reducing the current. Conversely, connecting resistors in parallel decreases the overall resistance, increasing the current.
3. Voltage Regulation: Using voltage regulators or transformers, you can control the voltage supplied to the circuit. Since current is directly proportional to voltage (I = V/R), reducing the voltage will reduce the current, provided the resistance remains constant.
4. Current Limiting Components: Components like current-limiting diodes or current-limiting resistors can be used to restrict the maximum current flowing through a circuit.
5. Feedback Control: Implementing feedback control mechanisms such as using operational amplifiers or transistors in feedback configurations can help regulate and limit current.
6. PWM (Pulse Width Modulation): In applications like motor control or LED dimming, PWM can be used to effectively control the average current by modulating the duty cycle of the signal.
7. Current-Limiting Circuits: Specialized circuits such as current mirrors or current limiters can be employed to restrict the current within desired limits.
8. Current-Shunting Devices: Devices like zener diodes or voltage-dependent resistors (VDRs) can be used to shunt excess current away from sensitive components.
9. Power Supply Adjustments: If the circuit is powered by a variable power supply, simply reducing the supply voltage or current limit setting can reduce the current.

Remember, the specific method you choose depends on factors such as the requirements of the circuit, the components available, and the desired level of control over the current. Always ensure that any modifications made to the circuit comply with safety standards and do not compromise the functionality of the overall system.

 

Reducing current in an electrical circuit can be achieved through various methods depending on the specific context and requirements of the circuit. Here are some general techniques:

1. Resistance: Introducing resistors into the circuit can limit the flow of current. The relationship between voltage (V), current (I), and resistance (R) is defined by Ohm's Law (V = IR). By increasing the resistance, you can reduce the current.
2. Series and Parallel Resistors: Connecting resistors in series increases the overall resistance, thereby reducing the current. Conversely, connecting resistors in parallel decreases the overall resistance, increasing the current.
3. Voltage Regulation: Using voltage regulators or transformers, you can control the voltage supplied to the circuit. Since current is directly proportional to voltage (I = V/R), reducing the voltage will reduce the current, provided the resistance remains constant.
4. Current Limiting Components: Components like current-limiting diodes or current-limiting resistors can be used to restrict the maximum current flowing through a circuit.
5. Feedback Control: Implementing feedback control mechanisms such as using operational amplifiers or transistors in feedback configurations can help regulate and limit current.
6. PWM (Pulse Width Modulation): In applications like motor control or LED dimming, PWM can be used to effectively control the average current by modulating the duty cycle of the signal.
7. Current-Limiting Circuits: Specialized circuits such as current mirrors or current limiters can be employed to restrict the current within desired limits.
8. Current-Shunting Devices: Devices like zener diodes or voltage-dependent resistors (VDRs) can be used to shunt excess current away from sensitive components.
9. Power Supply Adjustments: If the circuit is powered by a variable power supply, simply reducing the supply voltage or current limit setting can reduce the current.

Remember, the specific method you choose depends on factors such as the requirements of the circuit, the components available, and the desired level of control over the current. Always ensure that any modifications made to the circuit comply with safety standards and do not compromise the functionality of the overall system.

are you a Chat GPT bot? Some of the moderators think this might be so.

 

are you a Chat GPT bot? Some of the moderators think this might be so.

I think non native English speakers are using the AI bots for translation. The results are easy to detect due to the very formal and detailed with proper grammar reply not usually seen in traditional human created posts.

Too correct, means AI or Vulcan.