| Electronics > Beginners |
| LM7805 unstable |
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| bogdant:
Do you think, replacing the LM7805 with a LM317 would be an option? I am thinking 3k//1k on the adjust pin and a load of 1kohm rezistor, 1uF to input and 0.1uF to output capacitor for stabilization. The Vin=35V and the target is to have Vout=5V. As consummers I have a micro + LCD, should be no more then 50mA. |
| bd139:
LM317 will do the job. Buy a TI branded one, make sure it has a 10mA minimum load current either from the load or the voltage setting network and make sure it is adequately heatsinked. It'll still kick out over a watt there. A small clip on will probably be enough heat sink for that. |
| mariush:
You should also look into AP7381 or LM2931. LM2931 is available on * Farnell : https://ro.farnell.com/search/prl/results?st=lm2931&sort=P_PRICE * RS-Components: https://ro.rsdelivers.com/productlist/search?query=lm2931&tag=&family=9492#PI-41|PS-99|viewtype-2|sortBy-Price|sortOrder-asc AP7381 is available on * RS-Components : https://ro.rsdelivers.com/productlist/search?query=ap7381&tag=&family=9492#PI-1|PS-21|viewtype-2|sortBy-Price|sortOrder-asc Note that the cheapest on RS-Components require buying certain quantities. Digikey has a larger selection but being in US would be more expensive to ship : https://www.digikey.com/products/en?keywords=ap7381 |
| bd139:
LM2931 discounted because max input voltage is too low (26V) AP7381 outside SOA ... user needs 1.6W dissipation. High dropout voltages are a special problem space. |
| Ian.M:
A LM317 *NEEDS* that 10mA minimum load current to guarantee regulation. If the real load current can drop to zero, as the regulator acts to maintain Out 1.25V above Adj, the upper resistor of its potential divider should always be under 125 ohms to guarantee 10mA minimum load current for the regulator. Use 120R and calculate the lower resistor for the desired voltage (96 ohms per volt at Adj). References to a 5mA minimum load current and a 240 ohm upper resistor only apply to the LM117 which is on the same National Semiconductor datasheet as the LM317. If there's any chance of spikes or ringing on the 35V supply, get a LM317HV which can withstand up to 60V input to output differental. |
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