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Useful circuits
Small useful circuits that help your everyday electronic life.

Small fixed current limiter PDF Print E-mail
Thursday, 23 October 2008 21:29

This circuit can be used for driving capacitive loads from a DC/DC converter, or as a driver for opto coupler inputs.

When used as a DC/DC converter load controller it can prevent the DC/DC converter from failing to start because of a too high capacitive load (in which case it basically sees a short and won't start). This can easilly happen when a board with too many decoupling capacitors is supplied from a small DC/DC switch converter.

When used as an input protection it can be used as below, driving an opto coupler. This gives a high input voltage range, something like 5V...30VDC, making it possible to drive the input from a 5V logic source or e.g a 24V PLC.

The first PNP transistor starts to conduct when the VBE voltage is >0.6V. This happens when the current trough the 30ohm resistor is 20mA. When it conducts it will increase the voltage on the second transistors base, effectively blocking it until a steady state occurs. The last series diode is a reverse polarity protection for the opto coupler, as the LED itself can not withstand a high reverse polarity. 

Current limiter

Last Updated on Friday, 24 October 2008 07:33
Generic valve coil replacement PDF Print E-mail
Thursday, 23 October 2008 20:45

Some days ago the environmental chamber from Blue M borke down (again). This time it turned out to be the coil for a cooling media valve. The valve is made out of a pin with a coil around it. When the coil is energized (in any direction, mind) the middle pin will be "drawn" into the center, opening the valve.

Ok, so this valve, which was a MKC-1 type from Sporlan, was burned. Would probably not be a problem if it wasn't for the fact that it was an old one made for 120VAC which is quite uncommon in Europe. Parker in Sweden (who has this valve in their product line) was confused when we called and couldn't help us. Email to the international site resulted in a compact silence.

The machine was down and people started to panic. What to do? Well here is the quick, dirty but functional solution:

SolenoidWe bought a 24VDC solenoid from Kuhnke (ELFA 54-603-73, Kuhnke H-6206). The center hole is 13mm in diameter which fits our 0.5" pin of the valve. We put it on and also bought a small transformer with 2x 115VAC primary windings and 2x12VAC secondary windings. The primary side we parallel coupled and we put the secondary windings in series, feeding a rectifier and a small capacitor.

Seems to be a bit of a clumsy solution but it worked and we got the macine up within two days.


Last Updated on Friday, 24 October 2008 08:22
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