# Problem with transistor H Bridge

#### Nealieboyee

Joined May 24, 2017
56
Hi all,
I have an H bridge circuit here that refuses to work. This is not powering a motor. Its for a 6V latching solenoid, with resistance of 2.5 ohms. It will happily energise at 600mA. However using this circuit, I can hear a faint click from the solenoid, so it is trying to energise it, but failing. I've measured collector emitter voltage on Q6 and its sitting at 0.5V, which tells me it's not saturated enough to drive 1A, but even if I drop R10 down to below 50 ohms, its still not enough.

I thought I'd done the calculations correctly for voltage drop through EB of Q6, CE of Q1, and BE of Q3. I've calculated for a 1A collector current at Q6 with a gain of 10 to be safe, so I'll need around 80-100mA through Q1 to give Q6 enough oomph.

Inputs are at R8 and R9. For the moment I'm touching them directly to the positive 6V rail to turn on Q1 and Q4, though NOT together. Just one at a time.

Am I missing something obvious here?
Thanks
Neal

#### ElectricSpidey

Joined Dec 2, 2017
2,783
Did you calculate knowing that Q1 cannot saturate?

#### Nealieboyee

Joined May 24, 2017
56
Did you calculate knowing that Q1 cannot saturate?
No I calculated for saturation. I don't know why it isn't.

#### Beau Schwabe

Joined Nov 7, 2019
156
While I think the ZTX651 is adequate, the 2n2907 has a maximum rating of 500mA and a C-E saturation voltage of 1.6V ... between the two your loosing about 2V right off the bat, so that 6V supply to the solenoid is now about 4V ... That 60% reduction may not be enough pulling force for the Solenoid.

#### Nealieboyee

Joined May 24, 2017
56
While I think the ZTX651 is adequate, the 2n2907 has a maximum rating of 500mA and a C-E saturation voltage of 1.6V ... between the two your loosing about 2V right off the bat, so that 6V supply to the solenoid is now about 4V ... That 60% reduction may not be enough pulling force for the Solenoid.
Sorry, it's actually a ZTX751 for the PNP transistors, not a 2N2907. I forgot to change the tag. It has the same rating as a ZTX651.

#### Beau Schwabe

Joined Nov 7, 2019
156
So R10 and R11 should be in the range of 22 Ohms.

Between the B-E junction of each 'drive transistor' you lose about 0.6V and a few mV across the 'control transistors'.

So, since there are two drive transistors and one control transistor in the series chain you have about 1.3V loss. To provide 200mV to the drive transistors the value of R should be ...

R = ( 6V - 0.6V + 0.6V + 100mV ) / 200mV = 23.5 Ohms

Now, that said, if your power supply is not adequate and it "droops" under the load of the Solenoid, then the 200mV supply to the drive transistors may not be adequate.

Note Reference:
drive transistors = Q2, 3, 6, 10
control transistors = Q1, 4

Q1 and Q4 have an upper current limit around 200mA so there is a potential issue there as well.

#### crutschow

Joined Mar 14, 2008
34,459
Below is the LTspice simulation of your circuit, which seems to work.
Note that the solenoid current is over 2A so the bridge transistors should be rated for a least 3A.

#### Nealieboyee

Joined May 24, 2017
56
So R10 and R11 should be in the range of 22 Ohms.

Between the B-E junction of each 'drive transistor' you lose about 0.6V and a few mV across the 'control transistors'.

So, since there are two drive transistors and one control transistor in the series chain you have about 1.3V loss. To provide 200mV to the drive transistors the value of R should be ...

R = ( 6V - 0.6V + 0.6V + 100mV ) / 200mV = 23.5 Ohms

Now, that said, if your power supply is not adequate and it "droops" under the load of the Solenoid, then the 200mV supply to the drive transistors may not be adequate.

Note Reference:
drive transistors = Q2, 3, 6, 10
control transistors = Q1, 4

Q1 and Q4 have an upper current limit around 200mA so there is a potential issue there as well.
Thanks Beau. You mean 200mA instead of 200mV correct?
So perhaps I just wasn't saturating the drive transistors completely then, not giving me enough current to actuate the solenoid.
There will probably be a bit of droop in the battery supply (4xAAA batteries), but it can trigger the solenoid when connected directly. I won't rule that out though, and will try it with a higher current capacity supply.

#### Nealieboyee

Joined May 24, 2017
56
Below is the LTspice simulation of your circuit, which seems to work.
Note that the solenoid current is over 2A so the bridge transistors should be rated for a least 3A.

View attachment 244642
Thank you for that Crutschow! I'm only actuating the solenoid for about 200-500ms. Do you see a problem with that while keeping the same drive transistors?

#### LowQCab

Joined Nov 6, 2012
4,075
Keep the Parts-Count Low ..................
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#### crutschow

Joined Mar 14, 2008
34,459
Do you see a problem with that while keeping the same drive transistors?
Only that their ON saturation voltage will be higher.

You might consider a MOSFET bridge, as LowQCab suggested.
You can readily get ones with a low ON-resistance that would drop less than a tenth of a volt @ 2A.

AAA batteries are pretty marginal for that amount of current.
Suggest you go to AA's or larger.