Boost Current Upgrade Your USB Hub

Problems can arise with USB hubs that are powered from a PC when gadgets plugged into them draw too much current. This is often the case with devices fitted with USB cables that are too long or too thin, causing voltage drop. There’s no need to scrap your old USB hub, however, if you upgrade it using this little circuit and an external power supply.

Just cut the 5-V power wire of the USB cable inside the hub and solder a diode (D1) in the pass-through direction. Now connect the 5 V wire from the external power supply to the cathode of this diode. D1 prevents any current from the power supply from flowing back into the PC.

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Simple Current to voltage converter circuit Diagram

A filter removes the dc component of the rectified ac, which is then scaled to RMS

 Simple Current-to-voltage converter Circuit  Diagram

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Ampere or Current Booster

Small and very useful circuit, Can be built on a veroboard

Volt regulators such as the LM708, and LM317 series (and others) sometimes need to provide a little bit more current then they actually can handle. If that is the case, this little circuit can help out. A power transistor such as the 2N3772 or similar can be used.

The power transistor is used to boost the extra needed current above the maximum allowable current provided via the regulator.
Current up to 1500mA(1.5amp) will flow through the regulator, anything above that makes the regulator conduct and adding the extra needed current to the output load. It is no problem stacking power transistors for even more current. (see diagram). Both regulator and power transistor must be mounted on an adequate heatsink.

Circuit diagram:

Ampere or Current Booster Circuit Diagram

Parts:

R1 = 1R-2W
R2 = 10R-2W
C1 = 35v-470uF
C2 = 35v-470uF
Q1 = TIP2955
IC1 = 78xx Regulator

Source :http://www.ecircuitslab.com/2011/06/ampere-or-current-booster.html

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Low Current Relay

Low current relay
This low current relay circuit is designed to be used in battery operated electronic devices. Its operating current is in micro amperes (µA). This is done by using a bistable relay and adding some components to force the relay to behave like a monostable relay.



A bistable relay stays at its last state when the power is turned off but consume at least 50mA trigger current. A monostable relay switches back to its original state when the power is turned off.

How does the low current relay works?

When the power is turned ON, the C1 charges via D1 and the relay coil and this current activates the relay. D1 ensures that the base of T1 is always more positive than its emitter and because of this T1 and T2 are always blocked.

Once the power is turned OFF, the emitter of T1 is coupled to the charge voltage at the positive pole of C1. Its base and the relay coil on the other hand are coupled to the negative pole of C1 and now T1 and T2 conduct, C1 can discharge through T2 and relay. The current flows to the relay coil but in reverse order so is activated to its other state.

It has the advantage of consuming little current, around 150 µA. For a reliable operation, select the relay’s operating voltage as 2/3 to 3/4 of the main power supply. For example is using a 12V power supply select a 9V relay.

Transistors replacements:
T1 = 2SA499 = BC557, BC558
T2 = 2SC734 = BC547, BC548
Low current relay switch

source:http://electroschematics.com/6546/low-current-relay/ 

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