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I'm building a simple 9v linear supply using a 7809. The transformer is capable of the 300mA that might be drawn (guitar effects supply). I'm looking for a a simple way of limiting the output current. The regulator has a inbuilt current limit of course but that leaves me with a choice of 500mA or 100mA - too much or too little. Any suggestions? Putting the regulators in parallel seems wrong as it relies on them being very closely matched as the load is variable, but maybe that's a workable solution?

Edit: Should have said that I'm thinking about a through hole device - My experience predates SMD. I'm just getting back into design after a few decades away and I'd like to avoid surface mount for this project, but realise it may not be possible.

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  • \$\begingroup\$ I would not know a simple solution, but then you started out with a regulator which offers very few options. Simplest solution is to switch to a different type. There are 300mA regulators an regulators with adjustable current limits. \$\endgroup\$ – Oldfart Apr 20 '18 at 14:34
  • \$\begingroup\$ Why do you want to limit the current? The power supply does not force its maximum rated current through the load - the load just draws the current it requires. \$\endgroup\$ – Peter Bennett Apr 20 '18 at 16:00
  • \$\begingroup\$ Which linear regulators have adjustable current limits? I'm not wedded to the 7809, just had no success searching for alternatives \$\endgroup\$ – Gobuddy Apr 20 '18 at 16:11
  • \$\begingroup\$ The current limit is just to protect the transformer. Most cases the draw will be just 10mA or so - but the possibility of short circuits, paralleling devices etc. needs to be taken into account. \$\endgroup\$ – Gobuddy Apr 20 '18 at 16:13
  • \$\begingroup\$ To protect the transformer secondary, a 20 mm glass 500 mA slow-blow (T for time delay) fuse will probably suffice. Or 315 or 400 mA if you want. \$\endgroup\$ – Andrew Morton Apr 20 '18 at 18:53
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You haven't given any reason you need deliberate current limiting at all. Just skip it.

It sounds like the transformer itself will limit the total delivered power. I don't get what exactly you are trying to protect against. The 7809 will protect itself because it has a thermal shutoff built in. You didn't give specs on the transformer, but there's a good chance that it can handle a shorted secondary indefinitely.

And no, the 7809 does not do current limiting. You are confusing the current capability of different 7809 variants with current limiting. The current capability is the max current the device is guaranteed to be able to output indefinitely, assuming you have ensured that all other parameters are met. Note that this almost certainly includes a heat sink or some kind of cooling.

The 78xx series usually needs 2 to 3 V of input headroom. At 2.5 V input headroom and 500 mA out, it will dissipate 1.25 W. Check the junction to ambient thermal resistance, but that probably means just sticking up a TO-220 package in free air isn't good enough.

Your real challenge will be designing the thermal system to deal with the heat produced in the 7809. A transformer followed by a full wave bridge and a cap isn't going to keep the 7809 input voltage right at the minimum threshold. The dissipation will therefore be higher than in the example above.

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You could use a PTC (Positive Temperature Coefficient) resistor in serie with your linear regulator output. PTC will prevent overcurrent by increasing its effective resistor value when it is heated (by an excessive current). You can find a wide variety of PTC with various tripping current depending on your desired specification.

A few things to note: 1) They tend to trip a bit before the rated tripping current 2) Once tripped, they may take a while to cooldown and reset 3) Alternatives: usage of a fuse or a real resetable fuse if you think that it will happen often (if it does, then you have a design problem somewhere else).

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By using a transformer that will withstand shorts on the secondaries there is no need for current limiting!

As this is a low current supply then just doing the maths on heat dissipation is all that's needed.

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