Voltage regulators

[bobby]

I need to parallel 2 7808 to double the Amp required (each circuit takes 1 Amp ) 2 Amps are needed . Can I just parallel them or do I need a resistor between output pins for maybe a slight voltage difference ? I can also use a To3 transistor cage but space is a must ..
Thanks for taking the time .. Bobby

 

[dknguyen]

No, you cannot simply parallel them because of the slight voltage difference between them. Putting on a resistor would fix this...but then the voltage will drop as current increases.

What circuit is this? Even 1A is a lot of current for many circuits.

 

[bobby]

Its a power supply for a handheld tranceiver radio needing 7.V DC only at 1.5 amps max. power source is 12 V DC 18 amp battery . Maybe a good idea will be to use a single BUT bigger To3 with at least 3 Amp capacity to use wiyhout a heatsink ... Thanks dknguyen .......Bobby

 

[Roff]

Its a power supply for a handheld tranceiver radio needing 7.V DC only at 1.5 amps max. power source is 12 V DC 18 amp battery . Maybe a good idea will be to use a single BUT bigger To3 with at least 3 Amp capacity to use wiyhout a heatsink ... Thanks dknguyen .......Bobby

A TO-3 package without a heatsink has a die-to-ambient thermal resistance of about 39 deg C per Watt. With 12V in and 8V out, at 1.5A, the dissipation is 6W, so the die will try to rise about 240 deg above ambient. The internal thermal protection will quickly shut it down.

 

[crutschow]

Adding a power resistor of the appropriate size and power rating between the battery and regulator can reduce the regulator dissipation. You want to size the resistor so that the regulator voltage is just above the minimum regulator dropout voltage at maximum load current and minimum battery voltage.

Of course you haven't eliminated the heat, but a significant portion can be transferred from the regulator to the resistor where it's usually more easily dissipated.

 

[3v0]

**broken link removed**

I do not know if the OP has room for it or the ability to handle the heat. I was thinking about a switching regulator too.

 

[bobby]

less complicated and easy way to go looks like the TO3 and small heat sink.
Thanks you all for your support .. Bobby

 

[dknguyen]

THe heatsink is going to have to be very large. Even with a very very good 15C/W heatsink, it will still rise to 100C above ambient. You might need forced cooling since heatsinks have diminishing returns as they get bigger due to the contact area being fixed. Passive heatsinks with lower than better than 15C/W are huge giant honkers.

 

[Roff]

THe heatsink is going to have to be very large. Even with a very very good 15C/W heatsink, it will still rise to 100C above ambient. You might need forced cooling since heatsinks have diminishing returns as they get bigger due to the contact area being fixed. Passive heatsinks with lower than better than 15C/W are huge giant honkers.

Don't forget ΘJC, which I think as around 4C/W for the TO-3 package, and adds to the heat sink thermal resistance.

 

[Amrth]

how to calculate c3 capacitor of boot pin & inductor value in LM22670 switching regulator?