This is a multi-part message in MIME format. ------=_NextPart_000_00F4_01C57F42.5AEAE600 Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=response Content-Transfer-Encoding: 7bit Some good comments on bridges so far. Here''s what I came up with. I'm sure people will want to offer constructive comments ;-) I've drawn half the bridge. The other half is identical except motor top and bottom connections are swapped. (THis is standard for an H bridge - just not a usual way of drawing it). I've shown Vbattery as 3v - 7v which is otside original spec. At 3v anything is on the verge of dying. 3V is not as unrelaistic as it may sound in practice as, while a cell has an endpoint of 1v or maybe 0.9 or certainly 0.8 the internal resistance also rises. if the battery id drained slowly by standby current and lifetime degradation and then the motor is run the terminal voltage will fall well below its lightly loaded valuie. _____ Parts count is lowish BUT there are some options. Version 1 is as shown with a link at point A. For 5v drive R1 = 1k5 and R2 = 1K. This establishes *about* 2 volts at point X and about 1.25 volts at point Y and ABOUT 0.5 volts at Q3 emitter (call that point Z). Make R5 = 2r2 (not 3r3 as shown). That means Q3 current is about 0.5/3.3R =~~~~ 230 mA max. ie Q3 is a constant currenht source using the 5v drive voltage as its reference. Temperature drift of the two Vbes and changes in Vbe with load on Q3 especially will affect the current's constancy. Drive input low. All off. Drive input = 5v, Q1 on. Q2 and Q3 on as R4 links 2 bases. Set R4 for stiff base current for selected collector current. Something like 220r to 470r. Note when supply = say 3V5 there will be about 0.5v drop in R5 and 2 x Vbe drop in Q2 & Q3 = say 1v5 so available voltage across R4 = 3.5-0.5-1.5 = 1.5v. Fror Beta Q2/Q3 = 80 say and 240 mA colelctor/motor current we need 3 m+ base current. 1.5/3mA = 500r so somethinhg somewhat smaller is wise - say 220r? This gives oodles of drive at higher supply voltages. May wanty to ioncrease R4 and suffer som epossible drive loss at low Vbattery. Actual beta should be double this for original transistors so higher R4 OK. R3 is simply a pullof for leakage current when off. OPTIONS: 0. As above 1. Diode as reference. Remove R2. At B place 2 diodes (1N4148 in seriesm cathode down). Increase R1 if desired to say 10k.This hold B at 2 diode drops when driven and notionally should give sameish result as before. In practice it seems worse and rather temperature sensitive. 2. Zener as reference. As above but uuse 1v2 or similar zener at B. No better than diodes. Not marvellously constant current. 3. Place Lm385-1v2 at B. This holds X at a very good 1v2ish when driven. Result is good but maybe not as good as version using resistor divider. LM385 about $US0.10 in volume anmd we need 2 here (2 halves) Could possibly use 1 only and a diode to couple them to bases. Would add problems too. Goodness of resultys seems so far to be 0, 3, (1,2 draw) Still haven't fully tested. A single R5 can probably be shared by both halves fwiw. RESULTS. Constantish on current due to Q3 constant current action. Zeroish off current. Much less drop across cct than original. Cost over original is a few extra resistors in case 0. Notes: This satisfies Olins ideas re on transistors - which i generally concur with AND is not subject to reasonable variations in device parameters. Q1 & Q2 are saturated when on. Q3 CAN be saturated when Vbattery is too low but usually will be held out of saturatuion to provide current source operation. I can discuss this more anaon as required. Seems to work well but still needs fully testing - main concern is temperature variations. Supply variations work wellish. 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