Cheap 12V to 9V Adapter

[brscanning]

I headed to Radio Shack for parts to build a 12V to 9V adapter for my PSR-500. I always check the closeout section and found a Car Power Adapter (part #21-100) for $0.97. It is listed for use with Radio Shack GMRS/FRS radios and is rated for 9V and 1000mA.

I bought the adapter and changed the DC power connector to a 4.75mm OD and 1.7mm ID plug and now have my adapter. Right now I am using it in the house with my 12V power supply that supplies a lot of my scanner equipment but it will work in my vehicle's light sockets.

It you attempt to do this, please make sure you have the polarity correct before plugging it into your scanner. I will not be responsible if you toast your scanner.

 

[gmclam]

Well, there are lots of ways to drop 12 volts down to 9 volts; when you only need 300 ma. But I certainly wouldn't be looking for the CHEAPEST way to do it. Hell you can use a resistor if you want cheap. One thing to consider is that automotive power is not 12.0 volts, but can be over 14 volts when the engine is running (and auto battery being charged).

So on one hand you have to consider the highest possible voltage (I'd use 15 minimum, 18 suggested) and the lowest (12 ok, 11 to be safe). Even at only 15 volts you have to drop 6 volts to get to 9 volts. If actually passing 300ma that would 1.8 watts!

They actually make a TO-220 style regulator that is a switching regulator. Unlike linear regulators it won't get as hot. You could use one of those and have 9 volts regulated for hardly any increase in cost. But those are not available at RS.

 

[brscanning]

Well, there are lots of ways to drop 12 volts down to 9 volts; when you only need 300 ma. But I certainly wouldn't be looking for the CHEAPEST way to do it. Hell you can use a resistor if you want cheap. One thing to consider is that automotive power is not 12.0 volts, but can be over 14 volts when the engine is running (and auto battery being charged).

So on one hand you have to consider the highest possible voltage (I'd use 15 minimum, 18 suggested) and the lowest (12 ok, 11 to be safe). Even at only 15 volts you have to drop 6 volts to get to 9 volts. If actually passing 300ma that would 1.8 watts!

They actually make a TO-220 style regulator that is a switching regulator. Unlike linear regulators it won't get as hot. You could use one of those and have 9 volts regulated for hardly any increase in cost. But those are not available at RS.

I was not looking for the cheapest way to provide 9 volts. It just turned out that way and it came in a nice package. I was originally looking for a project box to build my own circuit.

I understand all about the voltages, spikes, etc in an automotive system. I took the adapter apart and it seems to have a decent voltage regulator circuit using the MC34063A chip with an external power tansistor.

 

[KC0QNB]

Does it have crowbar OVP?, does it have LVD?, both could be handy.
Let me explain crowbar OVP is a type of over voltage protection, if the voltage rises above a set value, it simply blows a fuse, thereby saving your equipment.
LVD is a low voltage disconnect, if the voltage drops below a set value, it will simply shut off thereby again protecting your gear.
Commercial grade power supplies have these features, when you buy a bigger power supply, or maybe a auto backup unit these features are something to look for, these aren't generally available on RS type supplies.