Post by thread:[OT]: 78M05 Regulator. Please help!

Hi everyone, I think most of you here are familiar with the 5V regulator 78M05. You know that a single regulator can only supply 0.5A max. My problem is that I need to have 2A+ of continunous current at 5V to drive a DC motor. Questions: 1. Can I put them in parallel to boost up the supply current? If yes, how? 2. Is there another way to get this kind of current? 2. I remember seeing people use an PNP transistor in conjunction with the regulator. Can't remember how to connect them together now... Best regards, Thomas _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

At 12:55 AM 3/29/01 +0000, Thomas N wrote: >Hi everyone, > >I think most of you here are familiar with the 5V regulator 78M05. You know >that a single regulator can only supply 0.5A max. My problem is that I need >to have 2A+ of continunous current at 5V to drive a DC motor. It's worse than that.. You have to supply "locked rotor" current at start, or you're going to have a very "droopy" motor. -- Dave's Engineering Page: http://www.dvanhorn.org Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Thomas wrote: >Hi everyone, > >I think most of you here are familiar with the 5V regulator 78M05. You know >that a single regulator can only supply 0.5A max. My problem is that I need >to have 2A+ of continunous current at 5V to drive a DC motor. > >Questions: > >1. Can I put them in parallel to boost up the supply current? If yes, how? >2. Is there another way to get this kind of current? > >2. I remember seeing people use an PNP transistor in conjunction with the >regulator. Can't remember how to connect them together now... Check out the National Semiconductor site: http://www.national.com/ds/LM/LM340.pdf It show how to do this. Regards... -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Finding 2A+ from an off-the-shelf transformer will be next to impossible (or very expensive). But the motor supply doesn't have to be that well regulated. Try this - Use a 6.3V AC, 0.5A transformer (gives 8.95V DC), then a full wave regulator and a 2200uF 'tank' capacitor on the +ve line (gives smoothed ~8.3V DC). From this point on you use 2 separate power circuits. 1] Run the 8.3V DC +ve through a 3A diode to the motor (gives 7.6V DC), then to a transistor (gives 7V DC) or fet (gives 7.5V DC) and back to the capacitor. Put another 3A diode *BACKWARDS* across the DC motor. 2] Also take the +8.3V to a 100uF capacitor, then the 78M05 and on to your logic circuits as normal. Running your 5V (nominal) motor at 7V should not be a problem. The 2200uF 'tank' capacitor can supply more than 2 amps for 1/3rd of a second at startup, but after that you're back to 0.5A. (Bigger tanks are *DANGEROUS* - they hold a hell of a punch *AFTER* the power is turned off). If your motor really requires more than 0.5A when running then you need to use a 100R 10W resistor and a 6V rechargeable gel cell battery after the 3A diode on the motor power circuit. This continuously pumps 70mA @ 8V into the battery while ever the circuit is on, and outputs lots of amps @ about 6.8V when the motor is on. (If you use a very small gel cell battery then adjust the size of the resistor to match the maximum charge rate for the battery or it'll overheat, dry out and die). The tank and the diode are *ESSENTIAL* isolators when motors and logic share the same power source. Bye. {Original Message removed}

Bruce wrote: >Finding 2A+ from an off-the-shelf transformer will be next to impossible (or >very expensive). But the motor supply doesn't have to be that well >regulated. Eh? Come on, we're not that badly off here in OZ are we? Jaycar, Altronics or Dick Smith should have them readily available. {Quote hidden}>Try this - >Use a 6.3V AC, 0.5A transformer (gives 8.95V DC), then a full wave regulator >and a 2200uF 'tank' capacitor on the +ve line (gives smoothed ~8.3V DC). > > >From this point on you use 2 separate power circuits. >1] Run the 8.3V DC +ve through a 3A diode to the motor (gives 7.6V DC), then >to a transistor (gives 7V DC) or fet (gives 7.5V DC) and back to the >capacitor. Put another 3A diode *BACKWARDS* across the DC motor. >2] Also take the +8.3V to a 100uF capacitor, then the 78M05 and on to your >logic circuits as normal. Careful here, 8.3V is low for a standard 5V regulator which need up to 4V headroom to maintain regulation. You could use a low dropout regulator like the LM2940T-5 (1A) or LM2931AZ-5 (100mA) to solve that. >Running your 5V (nominal) motor at 7V should not be a problem. The 2200uF >'tank' capacitor can supply more than 2 amps for 1/3rd of a second at >startup, but after that you're back to 0.5A. (Bigger tanks are *DANGEROUS* - >they hold a hell of a punch *AFTER* the power is turned off). You can put a 470R resistor across the cap to discharge it after power-off if you want to avoid the residual voltage and potential sparks through mishaps! <VBG> {Quote hidden}>If your motor really requires more than 0.5A when running then you need to >use a 100R 10W resistor and a 6V rechargeable gel cell battery after the 3A >diode on the motor power circuit. This continuously pumps 70mA @ 8V into the >battery while ever the circuit is on, and outputs lots of amps @ about 6.8V >when the motor is on. (If you use a very small gel cell battery then adjust >the size of the resistor to match the maximum charge rate for the battery or >it'll overheat, dry out and die). > >The tank and the diode are *ESSENTIAL* isolators when motors and logic share >the same power source. > >Bye. > > >{Original Message removed}